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Uneven idle

The car is a '92 Seville. There is a roughness at idle that becomes more pronounced with a load such as in Drive or with the AC on. The higher the engine temp the more pronounced it becomes also. Interestingly, on increasingly rare occasions, at all temps, it is barely present at all. There is seemingly no pattern to this variable. The following parts have been replaced, cleaned, inspected, or tested per the FSM recently... ISC motor, plugs, cap, rotor, wires, module, cam position sensor, pickup coil, EGR valve, fuel filter, fuel pump, PCV valve, injectors, FPR, vacuum lines, motor mounts, brake booster, and all accessible electrical connections. The vacuum reserve tank holds vacuum after shut down. Ideas?

Reply to
NickySantoro
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Suggest a vacuum leak between the intake manifold and the block, the vacuum lines and brake booster having already been replaced. OUCH! This hypothesis will have greater validity if the intake manifold was ever removed.

Such a leak can be found at considerable personal risk by applying some propane to the joints of the intake manifold and the engine block. Alternatively, carburetor spray can be used but at similar personal danger - that it ignites and flashes in one's face. So if you try this keep your face back. This isn't kid stuff so be serious.

If there is a vacuum leak in the intake manifold - cylinderhead gasket the propane or carburetor cleaner spray will "fuel" the leak and so roughly restore the air / fuel ratio so the engine idles as it should rather than stumble from a too lean mixture caused by the vacuum leak.

Reply to
T.G. Lambach

"NickySantoro" wrote

Good suggestion.

You may also have bad rings or a burnt valve on one (or more) cylinders. If you haven't already, a compression and leak-down test should be done. That probably should have been done before replacing all those other parts. Good luck!

Reply to
Ken Abrams

Many thanks for the reply. I tested the booster for vacuum but did not replace it. It is fine. BTDT on the carb cleaner and propane. Manifold has not been off , AFAIK. I've had the car since '97. Got it with 35K, now at 70K. It sure feels like a vac leak, but I've been over every hose, port, connection, and device and have come up empty in that area.

Reply to
NickySantoro

Thanks for the reply. Compression is with spec per the FSM. Did it last year when I changed the plugs. Wrote the numbers down somewhere. Only 70K on the clock. I'm thinking that if I had a burnt valve the problem wouldn't be transient. It really seems to be a vacuum leak but I'm stumped as to where to look next.

Reply to
NickySantoro

It sure feels like a vac leak, but I've been over every hose, port, connection, and device and have come up empty in that area.

Consider a defective fuel injector seal - between the manifold and the nozzle.

Reply to
T.G. Lambach

Thanks for responding

Reply to
NickySantoro

No one else? I'll donate a brand new cupholder latch and a spare to whomever figures this one out.

Reply to
NickySantoro

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