The 1991 S10 Blazer with the 4.3L V6 is a simple, body-on-frame truck that suffers from three core weaknesses: the 700-R4/4L60 transmission cooler circuit fails catastrophically, the lower intake manifold gaskets leak coolant into the crankcase, and piston ring wear leads to oil consumption and eventual bottom-end failure.
Transmission Oil Cooler Line Failure / Internal Radiator Cooler Leak
Common · high severity
Typical onset: 80,000-150,000 mi
Symptoms: Pink or strawberry milkshake appearance in transmission dipstick, Transmission slipping or delayed engagement after coolant intrusion, Sudden transmission failure after radiator or cooler line rupture
Fix: Steel cooler lines rust through at frame crossmember or radiator connections. Internal radiator cooler can also rupture, mixing coolant and ATF. Requires replacement of lines (or full radiator if internal failure), complete transmission fluid flush, often followed by transmission rebuild if coolant contamination occurred. 4-6 hours labor for lines/radiator, add 12-16 hours if transmission rebuild needed.
Estimated cost: $400-800 for lines/radiator only, $1,800-3,200 if transmission damaged
Lower Intake Manifold Gasket Failure (Central Port Injection)
Common · high severity
Typical onset: 100,000-160,000 mi
Symptoms: Coolant mixing with oil - milky appearance on dipstick or oil cap, White smoke from exhaust on cold start, Persistent coolant loss with no external leaks, Overheating or erratic temperature gauge
Fix: The plastic and composite lower intake gaskets degrade and allow coolant into the valley and crankcase. Requires intake removal, gasket replacement with updated design, often oil change and flush. If driven long after coolant intrusion, bearings suffer damage. 5-7 hours labor for gasket job alone.
Estimated cost: $600-1,200 for gaskets and intake work, $2,500-4,500 if bearings or short block needed
Piston Ring Wear and Oil Consumption
Common · medium severity
Typical onset: 150,000-220,000 mi
Symptoms: Excessive oil consumption - quart every 500-1,000 miles, Blue smoke on deceleration or startup, Fouled spark plugs, Low compression on cylinder leak-down test
Fix: The 4.3L V6 has soft piston rings that wear into the bores, especially if oil changes were neglected. Ring replacement requires full teardown - many opt for short block or full rebuild at this point. If bores are scored, machine work or oversize pistons needed. 18-24 hours labor for full rebuild.
Estimated cost: $2,800-5,000 for short block or rebuild
Fuel Pump Failure
Occasional · medium severity
Typical onset: 120,000-180,000 mi
Symptoms: No-start or long crank time when hot, Stumbling or stalling under load, Fuel pressure below spec (9-13 psi TBI spec)
Fix: In-tank electric pump fails from age and heat cycling. Requires dropping the fuel tank for access. 2-3 hours labor.
Estimated cost: $350-650
Transfer Case Encoder Motor / Shift Motor Failure (4WD models)
Occasional · low severity
Typical onset: 90,000-150,000 mi
Symptoms: Service 4WD light illuminated, Transfer case won't shift between 2WD/4WD modes, Grinding or clicking from transfer case area when shifting
Fix: Electronic shift motor on NP233 transfer case fails or encoder position sensor goes out. Motor replacement is straightforward but requires raising vehicle for access. 1.5-2.5 hours labor.
Estimated cost: $250-500
Body Rust - Rocker Panels, Cab Corners, Wheel Wells
Common · low severity
Symptoms: Visible rust perforation below doors and rear quarters, Holes in floor pans or cargo area, Weakened body structure at jack points
Fix: These trucks rust from the inside out in salt states. Rocker panels and cab corners are typical rot zones. Structural if left unchecked. Repair requires welding in patch panels or replacement sections. 6-12 hours labor depending on extent.
Estimated cost: $800-2,500 for typical cab corner and rocker repair
Owner tips
Replace transmission cooler lines preemptively or install external cooler to bypass internal radiator circuit - cheap insurance
Watch intake gaskets closely - do NOT drive if oil looks milky; you'll destroy bearings in 50 miles
Run quality oil and change every 3,000 miles to maximize ring life; synthetic helps if consumption hasn't started
Inspect frame and body for rust before purchase - rust repair often exceeds vehicle value
Buy only if the intake gaskets and transmission cooler circuit have been addressed, oil consumption is minimal, and the body is solid - budget $2,000-4,000 for deferred maintenance on any high-mileage example.
AI-assisted summary drawn from NHTSA recall data, our labor-times database, and platform knowledge. Not a substitute for a pre-purchase inspection on a specific vehicle.
Fitment notes: Side post terminals standard on GM vehicles; battery located in engine compartment
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Every control module on the 1984-1994 Chevrolet S10 Blazer — where it lives, replacement time, and what it takes to program a replacement. Modules marked dealer / factory tool won't work after a part swap alone — budget for programming.
Electronic Brake Control Module / ABS Control Module (EBCM)1.5 hr R&Rrelearn only +0.2 hrwith ABS (optional, standard on some late models)▸ programming details
📍 left frame rail near master cylinder, or integrated with BPMV (Brake Pressure Modulator Valve) under hood driver side
🔧 Tech 1 or drive cycle
⚠️ Kelsey-Hayes 4WAL (4-Wheel Anti-Lock) system common; requires bleeding and sensor relearn after replacement. Module often integrated with hydraulic modulator assembly.
⚠️ 1984-1991 use replaceable PROM chips for calibration; 1992-1994 can use Tech 1 for diagnostics but PROM replacement still required for recalibration. 1984-1985 use 160-baud ALDL; 1986+ use 8,192-baud ALDL with 12-pin connector.
Transfer Case Control Module (TCCM)1.2 hr R&Rno coding
📍 Behind dash near steering column or on transfer case (electronic shift models only, 1991-1994)
⚠️ Only on models with electronic push-button 4WD shift-on-the-fly; manual shift transfer cases have no module
⚠️ Most S10 Blazers use analog clusters with no module; digital clusters with electronic modules rare and typically found on higher trim levels. Analog clusters require no coding.
Electronic Climate Control Module (ECCP)1.0 hr R&Rno coding
📍 Behind center dash HVAC control panel (electronic climate control models only)
⚠️ Only on models with electronic automatic climate control; most S10 Blazers have manual HVAC with no module
📍 passenger side interior, behind kick panel or under seat
🔧 PROM chip replacement or Tech 1 (1992+)
⚠️ Only present on electronically-controlled 4L60-E automatic; earlier 700R4 and TH350/400 transmissions use hydraulic control only. TCM may be integrated with ECM on some 1992-1993 models.
Sensing Diagnostic Module / Diagnostic Energy Reserve Module (SDM/DERM)0.8 hr R&Rno codingwith airbag (1992-1994, optional then standard)
📍 center console area under dash, or behind center trim panel
⚠️ Driver airbag introduced 1992 model year; passenger airbag added 1994. Battery must be disconnected 10+ minutes before service. No programming required but crash data stored.
Vehicle Anti-Theft System / Pass-Key Module (VATS)0.5 hr R&Rrelearn only +0.3 hrwith VATS (optional, late 1990s introduction)▸ programming details
📍 integrated with ignition lock cylinder and ECM; separate decoder module behind dash on some models
🔧 Tech 1 or 10-minute relearn procedure
⚠️ VATS (Vehicle Anti-Theft System) uses resistor pellet in key; if present, requires resistance matching and relearn. Not common on S10 Blazer until very late in generation; more typical on 1995+ models.
Radio / Entertainment System (Radio)0.5 hr R&Rrelearn only +0.1 hr▸ programming details
📍 center dash, DIN or double-DIN slot
🔧 Theftlock code entry via radio buttons
⚠️ Delco radios with Theftlock feature (1990+) require code entry after battery disconnect or replacement. Code typically on card with owner's manual or retrievable from dealer with VIN.
Aftermarket tool coverage varies by software version and vehicle build — treat "aftermarket tool" rows as "usually possible" and verify against your tool maker's coverage list before promising a customer. Spot a wrong location or hour? Tell us — corrections ship fast here.
Size-standard part numbers — verify your connector type before buying. Rear blades are model-specific; check the package's vehicle list.
Fuel economy figures are EPA data via fueleconomy.gov (median across matching trims). Performance figures are compiled estimates for the 1991 Chevrolet S10 Blazer 4.3L V6 Vortec and can vary by trim.
🔧 Database maintained under the daily editorial review of Chris Hackleman · Master Technician · 20+ years and Jeff Moore · Master Lexus & Toyota Mechanic · 20+ years.