Question'66 Oldsmobile Delta 88
The problem occurs when the car is idling. When the engine is turned off, it continues to diesel and one to three bangs happen at the end of the tailpipe before the engine stops. A mechanic back in November replace several vacuum hoses. The backfiring is a recent problem.
AnswerThe engine is dieseling, because there is too much fuel getting into the cylinders after the ignition is shut off.
Most likely there is a serious vacuum leak, or the idle speed is set too high. If the engine is in really poor condition it may have to idle fast to keep it from stalling.
Try putting the car in gear when you turn off the key - that should stop the dieseling until you can fix the root cause.
If the idle speed is too high when the engine is cold, try adjusting the fast idle cam down so the cold idle is slower. If someone has fiddled with the automatic choke, he may have mistakenly set the cold idle speed too high. If it creeps strongly in gear when it is cold, this is a sure sign that the idle speed is too high. It should be about 550 RPM once the engine is warmed up.
Check to make sure that the idle solenoid has not become disconnected. This is a device with a wire on it that sits near the accelerator linkage on the carburetor. When the key is turned off, this is supposed to retract the idle stop so that the idle speed drops to zero. If someone has removed this device, or disabled it, that could also cause this problem. If no one has been fiddling with it, then maybe the wire just fell off, or somehow the accelerator linkage is sticking on high idle - try cleaning the choke linkage with carburetor spray type cleaner - make sure everything moves freely and easily.
Using higher octane gas will help also.
Good luck, I hope this is helpful.
Dick.
