Full Engine Tune-Up

Complete tune-up procedure for the M127 engine with Bosch mechanical fuel injection, covering compression testing, ignition system service, valve adjustment, fuel injection system checks, and final calibration.

Warnings

Tools required

Parts

• Bosch W7AC or Beru 14-7DU spark plugs × 6 — Bosch W7AC or equivalent NGK B6ES
• Ignition point set × 1 — Bosch 1 237 013 028 or equivalent
• Condenser × 1 — Bosch 1 237 330 077 or equivalent
• Distributor cap × 1 — Bosch 1 235 522 207 or equivalent
• Distributor rotor × 1 — Bosch 1 234 332 302 or equivalent
• Spark plug wire set (if deteriorated) × 1 — Beru or Bosch suppression wire set for M127
• Valve cover gasket × 1 — Mercedes 127 016 05 80 or equivalent
• Valve cover grommet seals × 10 — Mercedes 110 016 01 80
• Fuel injection system o-rings (assorted) × 1 — Viton o-rings for injectors and fuel lines

Fluids

• SAE 30 or 10W-40 engine oil (for valve cover reinstallation if needed)

Preparation

1. Ensure the engine is cold (below 100°F) for accurate compression testing. If the car has just been driven, wait at least 2 hours.
2. Gather all tools and parts. Verify that replacement ignition parts match the original specifications—incorrect point gap or dwell will cause misfiring.
3. Disconnect the negative battery terminal and set it aside, ensuring it cannot contact the battery post during initial work.
4. Clean the engine exterior, especially around the valve cover, distributor, and injection pump to prevent dirt from entering during service.
5. Obtain a clear workspace with good lighting. The valve adjustment requires precision work in tight quarters.
6. Have the factory service manual or reliable specification sheet available for reference. Specifications vary between early and late 220SEb production.
7. Label all spark plug wires with their cylinder number (1-6, front to rear) before removal to ensure correct reinstallation.
8. Photograph the distributor cap wire positions if this is your first time servicing this system.

Procedure

1. 1

   Perform compression test on all cylinders
   With the engine cold, remove all six spark plugs using the 14mm spark plug socket. Label and store them in order for inspection. Disable the ignition system by disconnecting the low-tension wire from the coil negative terminal (small terminal marked "1" or "-"). Disable fuel injection by removing the main power wire from the injection pump's electrical connector (small brown wire on the pump housing). Install the compression tester into cylinder #1 (front of engine). Have an assistant hold the throttle fully open by pressing the accelerator pedal to the floor. Crank the engine for 5-6 compression strokes (about 3 seconds) and record the peak reading. Repeat for all six cylinders. Acceptable readings are 135-170 psi with no more than 15 psi variation between cylinders. Variation greater than 20 psi or any cylinder below 120 psi indicates worn rings, valves, or head gasket issues that must be addressed before proceeding. Low compression in adjacent cylinders suggests head gasket failure. Low compression in all cylinders may indicate timing chain wear or incorrect valve adjustment. Record all readings for baseline documentation.
2. 2

   Inspect removed spark plugs
   Examine each spark plug carefully as they provide diagnostic information about combustion and fuel mixture. Normal appearance is light tan to gray deposits on the insulator with minimal electrode wear. Black sooty deposits indicate rich mixture or oil consumption. White or blistered insulators indicate lean mixture or overheating. Oil-fouled plugs (wet black deposits) suggest worn valve guides or rings in that cylinder. Fuel-fouled plugs (wet with gasoline smell) indicate injection system problems or ignition misfire. Note any plug showing significantly different condition than others—this points to cylinder-specific problems. Check electrode gap on all plugs; excessive wear (gap wider than 0.7mm) indicates they're overdue for replacement. The M127 is not tolerant of worn plugs due to the mechanical injection system's precise fuel delivery.
3. 3

   Install new spark plugs with correct gap
   Using the wire-type gap tool, set the gap on all six new spark plugs to exactly 0.6mm (0.024 inches). This is critical—the M127 ignition system is designed for this precise gap. Too wide causes misfiring under load; too narrow causes weak spark and fouling. Check each plug even if they come pre-gapped, as shipping can alter the gap. Apply a small amount of anti-seize compound to the threads (avoiding the electrode area) to prevent seizure in the aluminum head. Install plugs by hand first, threading them carefully to avoid cross-threading the aluminum threads. Once finger-tight, use the spark plug socket to tighten to 22-25 ft-lb. Do not overtighten—aluminum heads are easily damaged. Install plugs in order from front to back (cylinders 1-6). Leave spark plug wires disconnected for now to prevent accidental starting during ignition system work.
4. 4

   Remove and inspect distributor cap and rotor
   Reconnect the battery negative terminal briefly to note the position of the rotor before removal (mark the distributor body at rotor tip location for reference during reinstallation). Disconnect battery again. Remove the two spring clips or screws holding the distributor cap. Lift the cap straight up with plug wires attached and set aside. Inspect the cap interior for carbon tracking (thin black lines between terminals), cracks, or corroded terminals. Any carbon tracking or cracks requires replacement—these cause crossfire and misfiring. Check the center carbon button for wear; it should have at least 3mm of spring-loaded travel. Inspect the rotor for burning on the contact tip or cracks in the body. The rotor on the M127 distributor is keyed and can only install one way. Remove the rotor by pulling straight up. Inspect the distributor shaft for side play (indicates worn bushing) and up-down play (indicates worn thrust bearing)—either condition requires distributor rebuild. Clean the distributor base plate area with electrical contact cleaner and a lint-free cloth.
5. 5

   Replace ignition points and condenser
   Locate the ignition points inside the distributor—they are mounted on the movable breaker plate. Note the position of the wire connections before removal. Remove the screw holding the condenser wire and primary wire to the points set. Remove the condenser mounting screw and remove the old condenser. Install the new condenser, routing the wire away from moving parts, and tighten the mounting screw snugly. Remove the two small screws holding the points assembly to the breaker plate. The M127 distributor uses a specific point set with an integral mounting bracket—aftermarket points may not fit correctly. Clean the breaker plate contact surface with electrical contact cleaner. Install the new points assembly, ensuring the pivot post engages the hole in the points. Loosely install both mounting screws. The ground wire from the points must make solid contact with the breaker plate—verify no paint or corrosion is interfering. Connect the primary wire and condenser wire to the terminal on the points and tighten the screw firmly. Before adjusting gap, apply a tiny amount of distributor cam lubricant (usually included with points) to the rubbing block—this prevents rapid wear.
6. 6

   Set ignition points gap (preliminary)
   Rotate the engine by hand using a wrench on the crankshaft pulley bolt (27mm) in the normal direction of rotation (clockwise viewed from front) until the rubbing block on the points rests on a peak of the distributor cam. There are six lobes on the M127 distributor cam. The points should be at maximum opening. Insert a clean 0.40mm (0.016 inch) feeler gauge between the point contacts. The specification for the M127 is 0.40mm. Loosen the mounting screws slightly and adjust the eccentric adjusting screw or move the points base plate using a screwdriver in the adjustment slot until the feeler gauge has a slight drag—you should feel resistance but be able to move it without forcing. The gap must be consistent across all six cam lobes. Rotate the crankshaft to each lobe and check—variation indicates a worn distributor shaft or bent cam. Tighten the mounting screws while holding the adjustment, then recheck the gap as tightening often changes it. This preliminary gap setting will be verified with a dwell meter in a later step. Inspect the point contact surfaces—they should be flat and parallel. Any pitting or material transfer indicates a defective condenser or excessive voltage; new points may burn quickly if the condenser is faulty.
7. 7

   Install new distributor cap and rotor
   Place the new rotor onto the distributor shaft, aligning the keyed slot. Press down firmly until fully seated—a loose rotor will cause erratic timing. Install the new distributor cap, aligning the tabs or screw holes. The cap only fits one way on the M127 distributor due to the offset mounting. Secure with spring clips or screws. Do not overtighten screws as the cap material is brittle. Before connecting plug wires, inspect the terminals inside the cap to ensure the rotor will make proper contact. The M127 firing order is 1-5-3-6-2-4. Cylinder #1 is at the front of the engine. The distributor rotates clockwise viewed from above. Starting at the cylinder #1 terminal on the cap (usually marked), install plug wires in firing order sequence clockwise around the cap. Ensure each wire seats fully into the cap terminal with an audible click. Route wires away from hot exhaust manifolds and sharp edges. Keep wires in the factory wire separators to prevent crossfire from inductive coupling between adjacent wires.
8. 8

   Check and set ignition dwell angle
   Reconnect the battery negative terminal. Connect the dwell meter to the coil negative terminal (where the points wire connects) and ground. Follow the dwell meter manufacturer's instructions for setup. Start the engine and allow it to reach idle. The M127 idle speed is approximately 700-750 RPM, but it may be rough at this point if timing and carburetion are off. Observe the dwell reading on the meter. The specification for the M127 six-cylinder engine is 34-40 degrees, with 37 degrees being optimal. If dwell is too low (below 34 degrees), the point gap is too wide; if too high (above 40 degrees), the gap is too narrow. Stop the engine. Remove the distributor cap and adjust the points gap as needed—closing the gap increases dwell, opening it decreases dwell. Make small adjustments (0.05mm at a time). Reinstall the cap, restart the engine, and recheck. Continue until dwell is within specification. Correct dwell is more important than point gap for proper ignition performance. Dwell that varies more than 3 degrees indicates a worn distributor shaft, worn cam, or weak breaker spring. Once dwell is set correctly, the points will provide maximum coil saturation time and strongest spark.
9. 9

   Remove valve cover
   Ensure the engine is off and cool enough to touch comfortably. Remove the air cleaner assembly and set aside. Disconnect the crankcase breather hoses from the valve cover—note their routing for reinstallation. The M127 valve cover is held by ten nuts (10mm) around its perimeter. Loosen all nuts in a crisscross pattern to avoid warping the cover. Remove the nuts and their sealing grommets—inspect grommets for cracking or compression set. Carefully lift the valve cover straight up. It may be stuck due to gasket sealing; gently tap the cover sides with a rubber mallet if needed, but never pry against the sealing surfaces as this will cause leaks. Once removed, place the cover on a clean surface with the gasket side facing up. Remove the old gasket material from both the cover and cylinder head using a plastic scraper. Do not use metal scrapers or abrasive pads on the aluminum surfaces. Clean both surfaces with solvent and wipe dry. Inspect the valve cover for cracks or damage, particularly around the bolt holes. Inspect the cylinder head surface for warping or damage. The M127 head is aluminum and easily damaged by overtightening or corrosion.
10. 10

    Rotate engine to TDC cylinder #1 firing position
    Valve adjustment on the M127 must be performed with specific cylinders at top dead center (TDC) on their firing stroke. Rotate the engine using a wrench on the crankshaft pulley bolt (27mm) in the clockwise direction (viewed from front). There are timing marks on the crankshaft pulley and a pointer on the timing cover. Rotate until the TDC mark (0 degrees) aligns with the pointer. At this position, cylinder #1 (front cylinder) should be at TDC on its firing stroke. To verify this is the firing stroke and not the exhaust stroke, observe the valves on cylinder #1—both intake and exhaust valves should be closed (rocker arms should have clearance with no pressure on the valve stems). Alternatively, remove the distributor cap and verify the rotor points toward the #1 cylinder terminal. If both valves on cylinder #1 are tight (rocker arms pressing valves), rotate the engine one complete revolution (360 degrees) to reach the firing stroke TDC. At this position, you can adjust valves on cylinders 1, 2, and 4 (the rule is: when a cylinder is at TDC firing, adjust the valves on that cylinder).
11. 11

    Adjust valves on cylinders 1, 2, and 4
    The M127 valve clearance specification is 0.10mm (0.004 inch) for both intake and exhaust valves when cold. Valves are adjusted at the rocker arm adjusting screw, which has a locknut. Starting with cylinder #1 (front), locate the two rocker arms. The front rocker operates the exhaust valve, the rear operates the intake valve. Select a clean 0.10mm feeler gauge. Loosen the locknut on the rocker arm adjusting screw (typically 11mm wrench on locknut, 9mm on adjuster). Insert the feeler gauge between the rocker arm tip and the valve stem. Adjust the screw until the gauge has a slight drag—you should feel resistance when pulling the gauge, but it should not be tight enough to lift the rocker arm. The correct feel is critical and comes with experience. Hold the adjusting screw with one wrench while tightening the locknut with another. After tightening, recheck the clearance as the locknut often changes the adjustment. Repeat for both valves on cylinder #1, then repeat the entire process for cylinders #2 and #4. Take your time—incorrect valve lash causes poor performance, valve burning (too tight), or valve train noise (too loose). Record which cylinders you've adjusted to avoid confusion.
12. 12

    Rotate engine 360 degrees and adjust remaining valves
    After adjusting cylinders 1, 2, and 4, rotate the crankshaft exactly one complete revolution (360 degrees) clockwise using the wrench on the crankshaft pulley bolt. The TDC mark will align with the pointer again, but now cylinder #6 will be at TDC on its firing stroke. You can verify this by observing that both rocker arms on cylinder #6 now have clearance. At this position, adjust the valves on cylinders 3, 5, and 6 using the identical procedure described in the previous step. Use the 0.10mm feeler gauge and aim for a slight drag. Work systematically from front to rear or in any consistent order to avoid missing a valve. After completing all twelve valve adjustments (six cylinders, two valves each), perform a final verification by rotating the engine through two complete revolutions and rechecking several valves—they should all maintain the correct clearance. Any valve that changes indicates a worn cam lobe, bent pushrod (not applicable to M127—it's overhead cam), or loose adjuster locknut. The M127 uses a single overhead camshaft (SOHC) with rocker arms, making this adjustment straightforward but requiring precision.
13. 13

    Install new valve cover gasket and reinstall cover
    Place the new valve cover gasket onto the cylinder head (not onto the cover). The gasket should fit into a small groove or register in the head. Install new rubber grommets onto each of the ten valve cover studs. These grommets seal around the nuts and are critical for preventing oil leaks. Position the valve cover onto the cylinder head, aligning all ten stud holes. Install the nuts finger-tight first, ensuring the cover seats evenly. Using a torque wrench, tighten the nuts in a crisscross pattern starting from the center and working outward. The torque specification for M127 valve cover nuts is 7-8 ft-lb (approximately 10 Nm)—this is very light. Overtightening will crack the aluminum cover or distort it, causing leaks. Make two passes: first to 5 ft-lb, then to final 7-8 ft-lb. Reconnect the crankcase breather hoses in their original positions. The M127 has a closed crankcase ventilation system, so proper connection is important for emissions and to prevent oil leaks from pressure buildup. Allow the gasket to seat for 15-20 minutes, then recheck torque on all nuts—cork-type gaskets may compress slightly.
14. 14

    Set ignition timing with timing light
    Reconnect the air cleaner assembly temporarily. Start the engine and allow it to reach normal operating temperature (coolant temperature approximately 180°F, which takes 10-15 minutes). Connect the timing light to cylinder #1 spark plug wire using the inductive pickup clamp. Connect the timing light's power leads to the battery. Aim the timing light at the timing marks on the crankshaft pulley while the engine idles. Locate the timing pointer on the timing cover and observe which mark aligns when the light flashes. The M127 220SEb ignition timing specification is 4 degrees BTDC (before top dead center) at idle with vacuum advance disconnected and plugged. To adjust timing, loosen the distributor clamp bolt (10mm nut on the side of the distributor base) just enough to allow the distributor body to rotate. Rotate the distributor body slightly—rotating counterclockwise advances timing (mark moves toward higher BTDC numbers), rotating clockwise retards timing. Make small adjustments and observe the timing marks. When the correct timing is achieved, tighten the distributor clamp bolt while holding the distributor body in position. Recheck timing after tightening as the distributor may shift. Once set correctly, increase engine speed to 2000-2500 RPM and verify the vacuum advance is functioning—timing should advance significantly (15-20 additional degrees). Reconnect vacuum line if it was disconnected.
15. 15

    Check fuel injection system pressure
    The Bosch mechanical fuel injection system on the M127 operates at a continuous regulated pressure of 28-35 psi. With the engine at operating temperature and idling, you will verify fuel pressure at the injection pump. The fuel pressure test point is located at the pressure regulator mounted on or near the injection pump. It has a small banjo bolt or test port. With the engine running, carefully loosen the connection slightly while holding a rag to catch fuel spray—only loosen enough to insert the pressure gauge adapter (M10x1.0 thread typically). Connect a fuel pressure gauge rated to at least 100 psi. Observe the pressure reading at idle—it should be steady at 28-35 psi. Rev the engine to 3000 RPM and observe—pressure should remain constant. Fluctuating pressure indicates a weak fuel pump, restricted fuel filter, or failing pressure regulator. Pressure below 25 psi will cause lean running and hesitation; pressure above 38 psi will cause rich running and poor fuel economy. If pressure is out of specification, check the fuel filter first (located along the frame rail or in the engine compartment), then check fuel pump output pressure and volume (pump should deliver 1.5 liters/minute at 35 psi minimum). The pressure regulator is adjustable but requires special tools and knowledge—do not adjust unless you have experience with these systems.
16. 16

    Inspect fuel injector spray pattern
    With the engine running at idle, visually inspect each of the six fuel injectors for proper spray pattern. The injectors are located in the intake manifold runners, one per cylinder. Each injector should produce a fine, even cone-shaped spray pattern. Using a flashlight and mirror, observe each injector while the engine idles. A proper spray pattern is a 30-degree cone that atomizes fuel into fine mist. Defective patterns include streaming (liquid stream instead of mist), dripping (fuel dripping when engine stops), uneven spray (cone is lopsided), or no spray (clogged or failed injector). The M127 injection system sprays continuously while the engine runs—there are no pulses as in electronic injection. Any injector showing abnormal spray must be removed and cleaned or replaced. To test for dripping, stop the engine and immediately observe each injector—they should stop spraying within 1-2 seconds. Dripping injectors cause hard starting when hot, rough idle, and rich mixture. The M127 injectors are Bosch part number 0 280 150 xxx series and can be professionally cleaned and flow-tested. Do not attempt to clean injectors with carburetor cleaner spray—this damages internal components.
17. 17

    Check throttle linkage and return spring operation
    With the engine off, inspect the mechanical throttle linkage from the accelerator pedal to the injection pump throttle shaft. The M127 uses a complex linkage system with multiple pivot points and return springs. All pivot points should move freely without binding. Worn linkage causes throttle response problems and makes idle adjustment impossible. Open the throttle manually at the pump lever and release—it should snap back to idle position immediately. If return is sluggish, clean and lubricate linkage pivots with light machine oil (not heavy grease, which collects dirt). Check for worn linkage bushings—excessive play causes poor throttle response. The throttle shaft on the injection pump should rotate smoothly without tight spots. Inspect the fast idle cam mechanism (used for cold starting)—it should raise idle speed when cold and progressively lower it as the engine warms. Verify the kickdown switch (for automatic transmission cars) operates properly when the throttle is pressed fully to the floor. Any binding, corrosion, or wear in the linkage must be corrected before proceeding to idle adjustment, as faulty linkage makes proper adjustment impossible and causes the idle to drift or stick.
18. 18

    Adjust idle speed screw
    With the engine at full operating temperature, transmission in neutral (manual) or Drive (automatic—set parking brake and chock wheels), and all electrical loads off (headlights, blower, rear defrost), adjust the idle speed. Connect the tachometer to the coil negative terminal or use a handheld optical/inductive tachometer. The M127 220SEb idle speed specification is 700-750 RPM. Locate the idle speed screw on the injection pump throttle linkage—this is a large screw that sets the minimum throttle opening. Turning the screw clockwise increases idle speed, counterclockwise decreases it. Make small adjustments (1/4 turn at a time) and allow the engine to stabilize for 15-20 seconds before checking results. If you cannot achieve the specified idle speed, check for vacuum leaks (listen for hissing), intake manifold gasket leaks, or incorrect valve adjustment causing low compression. Once idle speed is set, verify it remains stable for 2-3 minutes—hunting or surging indicates vacuum leaks, linkage problems, or mixture issues. The idle should be smooth with minimal vibration. Rough idle despite correct speed indicates misfiring, worn engine mounts, or mixture problems that must be addressed.
19. 19

    Adjust idle mixture (CO adjustment)
    The idle mixture on the M127 mechanical injection system is controlled by a screw on the fuel distributor section of the injection pump. This is not a simple carburetor mixture screw—it adjusts the fuel distribution at idle only. The adjustment screw is located on the pump body and may be sealed with a cap or tamper-proof plug. Proper adjustment requires a CO (carbon monoxide) meter or exhaust gas analyzer. The specification for the M127 is 2.0-3.5% CO at idle. Without a CO meter, you can make a preliminary adjustment by ear and observation: turn the mixture screw slowly clockwise (leaner) until the engine begins to stumble or lose RPM, then turn counterclockwise (richer) until RPM peaks and begins to drop off from being too rich. Set the screw midway between these two points—this is approximately correct. After any mixture adjustment, readjust idle speed as they interact. A proper mixture adjustment results in smooth idle, quick throttle response, and no black smoke from the exhaust. Too rich causes black smoke, poor fuel economy, and fouled plugs. Too lean causes hesitation, stumbling, overheating, and potential valve damage. If you cannot achieve smooth idle with mixture adjustment, suspect injector problems, air leaks, or internal pump wear.
20. 20

    Check and verify ignition timing under load
    With all adjustments completed, perform a final timing check under running conditions. With the engine at operating temperature and idling at correct speed (700-750 RPM), verify base timing is still 4 degrees BTDC with the timing light. Disconnect and plug the vacuum advance line at the distributor and recheck—timing should remain at 4 degrees BTDC. This confirms the mechanical advance is at rest position at idle. Reconnect the vacuum line and verify timing advances with engine speed. Rev the engine to 2000 RPM and hold steady—timing should advance to approximately 20-24 degrees BTDC total (16-20 degrees advance beyond base). At 3000 RPM, timing should reach maximum advance of approximately 28-32 degrees BTDC total. If advance is insufficient, check vacuum advance diaphragm function by applying vacuum directly with a hand pump—timing should advance immediately. Check mechanical advance by removing distributor cap and verifying the advance weights move freely on their pivot pins. Sluggish advance causes poor performance and overheating. Excessive advance causes detonation (pinging under load). The M127 is sensitive to timing—incorrect advance curves cause flat spots, hesitation, and detonation.
21. 21

    Perform road test for drivability verification
    Take the vehicle on a comprehensive road test covering various operating conditions. First, allow the engine to warm fully from cold start to verify cold start enrichment and warm-up progression work correctly. The engine should start easily when cold, idle smoothly after 30 seconds, and idle speed should gradually decrease as the engine warms. During warm-up, there should be no excessive black smoke (indicates too rich) or hesitation (indicates too lean). Once warm, accelerate gently from idle through the entire RPM range—throttle response should be immediate with no flat spots, stumbling, or hesitation. Perform moderate acceleration tests (30-60% throttle)—the engine should pull smoothly without surging or hunting. Perform full-throttle acceleration from 2000 RPM in third gear (manual) or by forcing a downshift (automatic)—the engine should accelerate strongly with no detonation (metallic pinging sound). If detonation occurs, timing is too advanced or fuel octane is too low. Test steady-speed cruise at 40, 50, and 60 MPH—the engine should maintain speed smoothly without surging. Test light acceleration at cruise speed—response should be immediate. Any flat spots, hesitation, or detonation indicates incomplete tuning that must be corrected.
22. 22

    Verify smooth deceleration and idle return
    During the road test, specifically test deceleration behavior. From highway speed (60+ MPH), lift off the throttle completely and observe the deceleration. The engine should slow the car smoothly using engine braking without backfiring, popping through the intake, or excessive exhaust popping. Some light exhaust popping on deceleration is normal for this era, but excessive popping indicates lean mixture or exhaust leaks. As the vehicle slows and RPM decreases through 2000-1500 RPM, there should be no surging or hunting. When coming to a stop, the idle should return smoothly to 700-750 RPM without stalling or running-on (dieseling). If the engine tends to stall on deceleration, check for vacuum leaks, excessive throttle plate closure, or incorrect idle mixture. If the engine runs-on after turning off the key, timing is too advanced, idle speed is too high, or carbon buildup in the combustion chambers is causing hot spots. The throttle return spring must bring the throttle to closed position instantly—sluggish return causes run-on. Perform several complete stop-and-go cycles to verify consistent idle return behavior.

Reassembly

1. Ensure all ignition wiring is properly routed and secured away from heat sources, sharp edges, and moving parts
2. Verify spark plug wires are fully seated in distributor cap terminals and on spark plugs with audible clicks
3. Double-check that valve cover nuts are torqued to specification (7-8 ft-lb) in proper sequence to avoid leaks
4. Verify all crankcase breather hoses are connected properly to prevent oil leaks and maintain proper crankcase ventilation
5. Check that all fuel connections are tight and no fuel leaks are present around injectors, fuel lines, or pump connections
6. Ensure the throttle linkage returns to idle position smoothly without binding when released
7. Verify the air cleaner assembly is properly installed and all intake connections are sealed to prevent unmetered air entry
8. Check that all tools and rags are removed from engine compartment before final operation
9. Clean any spilled fuel from engine or chassis components before driving

Verification

• Cold start the engine—it should fire within 3-5 seconds of cranking and settle to smooth fast idle (1200-1400 RPM when fully cold)
• Verify idle speed drops progressively as engine warms, reaching 700-750 RPM at full operating temperature within 10 minutes
• Check for oil leaks around valve cover perimeter after 20 minutes of operation—retorque valve cover nuts if seepage appears
• Verify no fuel leaks at any connection points, particularly at injectors and fuel pump connections
• Listen for valve train noise—properly adjusted valves should be nearly silent. Clicking or tapping indicates incorrect adjustment or worn components
• Check ignition timing with timing light at operating temperature—should be stable at 4 degrees BTDC at idle
• Verify fuel pressure remains constant at 28-35 psi at idle and under acceleration with gauge connected
• Observe exhaust color—should be nearly invisible when warm. Black smoke indicates rich mixture; blue smoke indicates oil consumption; white smoke indicates coolant entry
• Verify all six cylinders are firing by briefly disconnecting each spark plug wire individually at idle—each cylinder should cause noticeable RPM drop and roughness
• Test throttle response—engine should respond immediately to throttle inputs with no flat spots or hesitation throughout the RPM range
• Check for detonation under load—with timing set correctly and proper fuel octane, there should be no metallic pinging during acceleration
• Monitor engine temperature during 15-20 minute drive—should stabilize at normal operating temperature (180-195°F) without overheating. Overheating may indicate lean mixture or incorrect timing
• Verify smooth transition between idle and off-idle acceleration with no stumble or bog—common problem area for mechanical injection systems
• Document baseline specifications including compression readings, dwell, timing, idle speed, and fuel pressure for future reference and troubleshooting

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