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1991 olds got a miss

91 olds cutlas supreme with about 70,000 miles has a miss - when cold rungs fine warms up then does not want to idl3e and has a surge feeling but at highway speed do not notice it. Plugs are clean, spark wires changed 8,000 miles back . Could it be a coil pack failing?

The problem might be with a lazy oxygen sensor.

When the engine is cold, the engine management system is in what is called the open loop mode. The computer then takes the information from the throttle position sensor, the crank position sensor, the coolant temp sensor, and the manifold absolute pressure sensor to determine how much fuel to add to the engine. Once the engine comes up to operating temperature the engine management system goes into what is called the closed loop mode. The computer then takes the signal from the O2 sensor to determine how much fuel should be added to the engine.

On a vehicle with the OBDI engine management system, which your’s has, you can have a lazy O2 sensor but the Check Engine light doesn’t come on.


Hi gave up and took it to a local shop. The mechanic checked and no codes and all the sensors are working correctly so is ERG valvue. The man said it has an internal vacum leak. No exterior leak. The intatke or manifold…
The car runs fine at highway speed and when first started. But when engine warms up Per the mechanic the idle vacuum drops and this causes the system to run rich and have a miss/ stall at idle.
So Does this sound correct?
Would a weak oxygen sensor show up when the engine tested?

so drove it about 20 miles and now wen going at a constant spped upto 45 mph without pressing on the gas the car jerks/ lurches.
Its a great old car but rock and rooling down the street is getting a bit much. Ideas???

You’d have to say what you mean (in precise terms) about the guy saying the O2 sensors are fine. How - exactly - were they checked?

An intake leak is plausible, partly b/c your description matches up to what a vacuum leak would do (rough running at idle and/or with throttle closed). Some leaks can be temperature sensitive - you can have some that only leak cold & seal up as heat expands things. Or it can go the other way 'round.

You said the vacuum drops after the engine warms up - was this observed on an actual vacuum gauge? That would fit with a hot-only vacuum leak. The intake manifold job is not a cheap one, however. I’d want to know for certain that it is the problem before doing anything with it.

Howdy, the guy didn’t tell me how he tested just that all the sensors tested ok. The vacuum checked with gauge- when warm drops to about 0 from 20 at idle. But its odd yesterday drove it about 120 miles and it didn’t act up about 1/3 of the time when coming to stop lights …

If it drops all the way to zero then you have a fairly large vacuum leak, and it shouldn’t be all that hard to verify. Spraying something like carb cleaner or feeding unlit propane around vacuum connections & the intake should be able to pinpoint where it is.

The EGR can also act like a bug vacuum leak if it is open when it shouldn’t be. Do you know how the EGR was tested? Disconnect the valve & see whether that makes any difference.