P0174 on Chevrolet: How to Diagnose & Fix

The Vortec 4.3L V6 and 5.0L/5.7L V8 engines in these vans use lower intake manifold gaskets made of a plastic composite. The gasket sits between the intake manifold and the cylinder heads. Every time the engine heats up and cools down, those metal parts expand and contract. Over many miles, that movement and the coolant flowing through the gasket wear away and tear the gasket at its coolant ports. One repair guide lists the 1996-2002 Express (and its Savana twin) among the affected GM trucks and vans. The leak can show on the outside, low down where the lower intake sits on the block. Owners report that a leaking intake gasket usually sends coolant into the crankcase instead, where it mixes with the oil. Coolant in engine oil has been shown to speed up wear of the bearings' surface coating, so a leak that is ignored can lead to engine damage.

The cast-iron exhaust manifolds on the Vortec small-block V8s bolt to aluminum cylinder heads. The two materials expand at very different rates, and after thousands of heat cycles the manifold repeatedly loads the outer bolts until they snap - almost always the front-most and rear-most bolt on each head. Express fleets see this constantly: one operator running roughly seventeen 15-passenger Express vans, all with the 6.0L, reported the same manifolds breaking bolts at the front and rear of the head on both banks across the whole fleet. The tell is a sharp exhaust tick at cold start that fades or disappears once the engine is fully warm and the metal expands enough to seal the gap - which is exactly how you separate this from a genuine lifter or rocker problem. Left alone the leak burns the manifold gasket, can cook nearby wiring and oxygen sensor leads, and can throw catalyst-efficiency or lean codes.

Early Express vans ordered with the 4.3L V6 use GM's Central Sequential Fuel Injection - a single injector body in the intake plenum feeding six plastic tubes that end in simple mechanical poppet nozzles at each port. The plastic tubes crack and the poppets stick or fail to seal, dumping raw fuel into the intake after shutdown. The system is extremely sensitive to residual fuel pressure: a drop of only a few psi produces a long-crank hard start, and a leaking poppet floods the manifold overnight. Owners report extended cranking (worst after sitting), rough idle that smooths out once running, misfire codes, a fuel smell, poor economy and noticeably down power. This is a 1996-2002 Express issue and does not apply to the 2003 and later vans, which use individual multiport injectors.

GM 3.1L V6 intake gaskets use a plastic carrier that warps and leaks coolant into the oil or externally. One of the most common failures on this engine across all GM platforms.

The 4.3L Vortec V6 uses a central port fuel injection system (nicknamed "spider" for its appearance) that is prone to leaking fuel internally into the intake manifold. The fuel pressure regulator and poppet nozzles develop leaks, causing hard starting, rough idle, and a fuel smell. This is a fire hazard.

The 3.5L and 3.9L V6 develop coolant leaks at the coolant crossover pipe gaskets on top of the engine, continuing the GM V6 gasket legacy; GM bulletin 07-06-02-006E covers this leak on 2006-2007 Uplanders with the 3.5L or 3.9L. Coolant leaks externally.

The 5.7L Vortec V8 in 1996-1998 C/K 1500 trucks uses a Central Port Injection (CPI) spider assembly with poppet nozzles that clog and leak fuel. The plastic fuel lines become brittle and crack, causing hard starts, rough idle, and fuel odor.

The Tahoe's 5.7L Vortec V8 (L31), like other CSFI Vortec engines of the period, 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 policy 99066 (revision 99066F) for CSFI trucks, and under a California Air Resources Board agreement coverage of the fuel injector system on about 700,000 1996-2001 GM vehicles across several models was extended to 10 years/200,000 miles.

GM bulletin 10-06-01-008M covers oil consumption on 2007–2014 Tahoe V8s with Active Fuel Management, treating more than 1 quart per 2,000–3,000 miles as excessive. The bulletin names two causes: (1) oil pulled into the intake through the PCV system at the left valve cover; and (2) oil spray from the AFM pressure relief valve in the crankcase, which under certain drive cycles can cause excessive deposit formation in the piston ring grooves. Owners may see the low-oil light between changes, and the bulletin notes cracked or fouled spark plugs on cylinders 1 and/or 7 and a possible service engine soon light. Distinct from AFM lifter collapse — this is oil burning, not a lifter failure.

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.

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 4.8L, 5.3L and 6.0L V8s in these trucks use a plastic intake manifold sealed to the cylinder heads with molded rubber-bead gaskets. Over time the original (orange) gasket material degrades and stops sealing, and the plastic manifold can warp. Unmetered air then leaks in at the intake ports, usually around the bottom edge of the ports. On these engines the problem is usually a vacuum leak rather than a coolant leak, so it causes a lean condition and misfire. The classic sign is a rough idle or misfire on a cold engine that fades as it warms up, often with codes P0171, P0174 or P0300. GM documented this in technician bulletin PIP3232C, which covers 1999-2005 Silverado and Sierra trucks along with related SUVs and vans. One owner of a 2002 Silverado 5.3L reported the same pattern on a forum: below about 50°F the engine revved high until it warmed up, and spraying around the intake caused a reaction near one cylinder.

GM PIP5681C documents a possible exhaust bracket/weld failure on specified 2018-2022 LFY Traverse applications. Noise and mixture or catalyst codes can accompany the leak, but do not establish it without inspection. The adjacent Bank 1 pipe can also be damaged.

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.

GM bulletin 00-06-04-003B covers 1995-2002 Chevrolet S/T models with the 4.3L CSFI engine (except 2002 VIN X RPO LU3): deposit build-up on a CSFI poppet valve ball or seat can make the poppet stick open or closed, mis-fueling that cylinder and causing a misfire, typically as a rough idle after start-up. Similar symptoms can arise from other fuel, air, ignition or mechanical faults. Confirm the engine RPO and whether the original CSFI system remains installed; later MFI hardware and earlier CPI are not interchangeable.