Warm-Up Regulator Service

Service or replace the warm-up regulator on the fuel injection system to restore proper cold-start fuel enrichment and eliminate rough running or hesitation during engine warm-up.

Warnings

Tools required

Parts

• Warm-up regulator assembly × 1 — Use OEM specification
• Coolant hose O-rings or gaskets × 2 — Use OEM specification
• Fuel line O-rings × 2 — Use OEM specification

Fluids

• Ethylene glycol green 50/50 — 1 qt

Preparation

1. Ensure engine is completely cold to the touch
2. Park vehicle on level ground and engage parking brake
3. Disconnect negative battery cable to prevent electrical shorts
4. Place drain pan under engine to catch coolant and fuel spillage
5. Relieve fuel system pressure by loosening fuel line at fuel distributor slightly, then retightening
6. Locate warm-up regulator on passenger side of engine, mounted to intake manifold or cylinder head

Procedure

1. 1

   Drain coolant from warm-up regulator circuit
   Place drain pan beneath the warm-up regulator. Using hose clamp pliers or flat-head screwdriver, loosen the 2 coolant hose clamps securing the coolant inlet and outlet hoses to the warm-up regulator body. Carefully twist and pull the hoses off the regulator nipples. Allow coolant to drain into pan. Expect approximately 1 quart of coolant loss.
2. 2

   Document electrical connections
   The warm-up regulator has a single electrical connector for the bi-metal heating element. Note the connector orientation and routing. If a warm-up regulator were present (it is not on a stock 1972 F-100 302), any electrical resistance checks would need to be confirmed against the specific Bosch part number's service data rather than an assumed value. Carefully pull the electrical connector straight off the regulator terminal.
3. 3

   Disconnect fuel supply line
   Using a 10mm wrench, loosen the banjo bolt securing the fuel supply line to the warm-up regulator inlet port. Remove the banjo bolt completely and pull the fuel line away from the regulator. Catch any fuel spillage with shop towels. Note the position of the copper or aluminum crush washers on both sides of the banjo fitting.
4. 4

   Disconnect fuel return line
   Using a 10mm wrench, loosen the banjo bolt or fitting nut securing the fuel return line to the warm-up regulator outlet port. Remove the fastener completely and separate the return line from the regulator. Again, note the position and condition of any sealing washers. Allow residual fuel to drain into the pan.
5. 5

   Remove warm-up regulator mounting fasteners
   Locate the 2 mounting bolts securing the warm-up regulator to its bracket on the engine. Using an 8mm socket or wrench, remove both mounting bolts completely. Support the regulator with one hand while removing the second bolt to prevent it from falling.
6. 6

   Remove warm-up regulator assembly
   Carefully pull the warm-up regulator away from the mounting bracket and engine. Maneuver it clear of surrounding components, taking care not to damage fuel lines or wiring harnesses. Inspect the mounting location for corrosion or debris.
7. 7

   Inspect and clean mounting surfaces
   Use a wire brush to clean the mounting bracket and surrounding area of rust, corrosion, and old gasket material. Inspect the coolant hose nipples on the new or rebuilt regulator for damage. Check that all sealing surfaces are clean and smooth.
8. 8

   Install replacement warm-up regulator
   Position the new or rebuilt warm-up regulator into the mounting bracket, aligning the mounting holes. Thread in the 2 mounting bolts by hand to prevent cross-threading. Using an 8mm socket, tighten both mounting bolts evenly in an alternating pattern until snug. Do not overtighten as the housing is aluminum or pot metal and can crack.
9. 9

   Reconnect fuel supply line
   Install new copper or aluminum crush washers on both sides of the fuel supply line banjo fitting. Position the banjo fitting onto the warm-up regulator inlet port and thread in the banjo bolt by hand. Using a 10mm wrench, tighten the banjo bolt snug. Typical tightening is firm snug, not extreme force, to avoid crushing the washers excessively or cracking the regulator body.
10. 10

    Reconnect fuel return line
    Install new sealing washers on the fuel return line banjo fitting or flare fitting. Position the return line onto the warm-up regulator outlet port and thread in the fastener by hand. Using a 10mm wrench, tighten snug following the same approach as the supply line.
11. 11

    Reconnect coolant hoses
    Slide the coolant inlet and outlet hoses onto the warm-up regulator nipples, pushing firmly until they bottom out against the regulator body. Position the hose clamps over the hose ends and tighten securely using hose clamp pliers or screwdriver. Ensure clamps are positioned at least 1/4 inch from the hose end to prevent splitting.
12. 12

    Reconnect electrical connector
    Push the electrical connector firmly onto the warm-up regulator heating element terminal until it clicks or seats fully. Verify the connector is secure by gently tugging on it. Ensure wiring is routed away from hot exhaust components and sharp edges.
13. 13

    Refill cooling system
    Refill the cooling system with approximately 1 quart of conventional ethylene glycol green 50/50 coolant mixture through the radiator filler neck. Start the engine and allow it to reach operating temperature while monitoring coolant level. Add additional coolant as needed to reach proper level. Check for leaks around the warm-up regulator coolant connections.

Reassembly

1. Reconnect negative battery cable after all connections are verified secure
2. Double-check all fuel line connections for tightness before starting engine
3. Ensure no tools or shop towels are left in engine compartment

Verification

• Start engine and immediately inspect all fuel and coolant connections at the warm-up regulator for leaks
• Allow engine to idle for 5 minutes while monitoring for fuel or coolant seepage
• With engine cold, verify that fuel pressure at idle is within specification (typically 28-32 psi for D-Jetronic system)
• After engine reaches operating temperature, verify warm-up regulator body is warm to the touch, indicating coolant flow
• Test drive vehicle and verify smooth cold-start operation and proper warm-up behavior without hesitation or stumbling
• Recheck coolant level after test drive and top off if necessary

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