P0171 on Chevrolet

Generation IV 5.3L Vortec engines burn an abnormal amount of oil — many owners report a quart or more every 1,000-2,000 miles. The defect is a combination of low-tension piston rings that fail to control oil, oil spray from the Active Fuel Management (AFM) system during cylinder deactivation, and a PCV path that draws oil mist into the intake. The oil fouls and prematurely wears spark plugs, causing rough idle, misfires, and a check-engine light. This was the subject of the Siqueiros v. GM class action (N.D. Cal., No. 3:16-cv-07244), which reached a final settlement in October 2025.

The Gen III LS truck engines use a plastic intake manifold with molded-bead gaskets. After many heat cycles the sealing beads flatten and crack, letting unmetered air in. Because the LS intake is 'dry,' the result is a vacuum leak that causes a lean condition, rough/unstable idle (worst on cold start), stumble, and lean/misfire codes rather than a coolant leak. Very common driveability complaint on high-mileage 4.8/5.3/6.0 trucks.

The Central Sequential/Port Fuel Injection ('spider') assembly used on Vortec 4.3L/5.0L/5.7L engines feeds nylon lines to poppet or MPFI nozzles inside the intake. Over time the poppet valves gum up and stick, O-rings and the pressure regulator leak, and the nylon lines crack. The result is misfires, hard starting, rough idle, a noticeable drop in fuel economy, and often a raw-fuel/rich condition. A leaking regulator can even flood a cylinder (hydrolock risk). Very high-demand failure on these older trucks.

The 1.4L turbocharged Ecotec engine in the Sonic shares the notorious PCV system failure with the Chevrolet Cruze. The PCV regulator diaphragm integrated into the valve cover ruptures, causing massive vacuum leaks. Additionally, the intake manifold check valve (small red/orange valve) fails, compounding the issue. This affects virtually all 1.4T Sonics eventually, typically between 50,000-100,000 miles.

GMT400-era Vortec V8s use a Central Sequential Fuel Injection (CSFI) 'spider' assembly with poppet-valve nozzles inside the intake. The poppet valves clog with carbon or the nozzle seats fail, so they leak/dribble fuel instead of atomizing. This causes hard starting, misfires, rough idle, hesitation, poor fuel economy, and often a raw-gasoline smell; failing poppets can dump fuel into a cylinder. A very high-demand repair for high-mileage 1996-2000 Suburbans.

The Gen III/IV LS-based Vortec V8s use a plastic (nylon) intake manifold with molded silicone/rubber gaskets. With heat cycling and age the gaskets harden and crack, creating an unmetered vacuum leak that leans out the mixture and can weep oil at the manifold-to-head sealing surface. Owners report rough/lean idle, hesitation, and lean-bank misfire codes. Not to be confused with the earlier 5.7 CSFI issue - this is the plastic-manifold sealing failure common across the GMT800 run.

The exhaust manifold-to-head bolts on GM small-block V8s fatigue and snap after years of heat cycling, most often the front and rear bolts on each manifold. The result is a distinctive tick/puff that is loudest on a cold start and quiets as the engine warms and the manifold expands to seal. It is extremely common on higher-mileage trucks and worse in road-salt regions where the bolt heads corrode. Left alone it causes an exhaust leak, can burn the manifold gasket, and may set O2/fuel-trim codes.

The 1996-2000 5.7L Vortec (L31) uses a Central Sequential Fuel Injection (CSFI) 'spider' assembly: one central housing feeds six nylon poppet spray nozzles via tubes. The poppet valves gum up with carbon and either stick open (raw fuel dumps, rich condition, hard hot/cold starting, fuel smell) or the plastic tubes crack/leak causing a lean misfire. This is one of the defining driveability failures of the GMT400 Tahoe. GM issued bulletin 87-65-07 and later covered many trucks under an extended emissions warranty for the poppet nozzles.

The 5.7L, 5.0L and 4.3L Vortec engines used a Central Sequential Fuel Injection (CSFI) "spider" assembly with nylon fuel tubes ending in spring-loaded poppet nozzles inside the intake. The poppets clog with carbon or leak/stick open, causing raw fuel intrusion, hard starting/flooding, and misfires. GM acknowledged the problem with special coverage 99066F on 1996-2003 CSFI systems (emissions-related, extending injector warranty to 10 years/200,000 miles for cleaning or replacement).

The 1996-1999 5.7L Vortec (L31) and 5.0L Vortec (L30) used a plastic-carrier composite lower intake manifold gasket that degrades and fails, typically between 60,000-120,000 miles. Failures commonly occur at the coolant ports and near the valley/distributor area, letting coolant seep externally, into the oil, or into the intake, and creating vacuum leaks. GM's light inch-pound torque spec contributed to uneven clamping and premature failure. This is one of the most widely reported problems on this generation truck.

Gen IV 5.3L V8s burn oil at an abnormal rate, frequently far worse than GM's own 1 qt / 2,000 mi 'acceptable' threshold. Three compounding root causes: (1) low-tension piston rings that fail to scrape oil from the cylinder walls; (2) the Active Fuel Management (AFM) oil-pressure relief valve in the valley that sprays a jet of oil directly at the deactivated cylinders' piston skirts, overloading the already-leaking rings; and (3) an early PCV routing in the valve cover that pulls liquid oil into the intake to be burned. Owners see the low-oil light between changes, fouled spark plugs on AFM cylinders (commonly cyl 1 and 7), and eventual carbon-clogged rings. Distinct from AFM lifter collapse — this is oil burning, not a lifter failure.

The small-displacement turbo engines in the Trailblazer can consume oil between changes, particularly the 1.2L 3-cylinder. Some owners report needing to add oil between the 7,500-mile change intervals.

The 3.5L and 3.9L V6 develop intake manifold gasket leaks, continuing the GM V6 gasket legacy. Coolant leaks externally or into engine oil.