1991 Chevy Cavalier Car Runs Rough

Are you getting a Check engine light? Is this fuel injected or carburetor?
You say it has a new catalytic converter. So I wouldn't think that would be your problem. Do I understand you right? This is not happening all the time it is intermittent. Get back with me and let me know if you are getting a check engine light.

Good Luck.

The check engine light does not come on. There is one fuel injector, description follows: It's a single injector with a sealed fuel pressure regulator mounted over a relatively large throttle plate (like a carburator butterfly plate). The throttle body is mounted on top of the intake manifold which branches out like a four piece candelabra. How would you know if the catalytic converter was not defective to begin with? It is probably okay, but companies ship bad parts all the time.

You make a good point. You can check for exhaust restriction with a vacuum gauge or do what many others do and unhook your exhaust pipe from the manifold. I wouldn't recommend the latter. Hook a vacuum gage up to intake manifold vacuum you should read a steady 17 to 21 in. Increase the RPM slightly above idle and watch the gauge if the vacuum starts dropping gradually you have a restricted exhaust. When the throttle is opened and closed quickly the needle should drop to approximately 2 in. And rebound to 25 in. Or more if the pistons and rings are in good shape. If rings and or pistons are bad it will not rebound to 25 when the throttle is closed. A low steady reading (10 in) indicates late ignition timing. An even lower reading ( 3 or 4in) is an indication of an intake air leak. A leaking or sticking valve will cause the needle to drop several in. At regular intervals. A very eradicate reading over a wide range and increase with higher engine speeds is a weak or broken valve spring.

Good luck.

Was the catalytic converter changed because the vehicle was running rough? As well as the other parts you mentioned was this done to try and resolve the current issue or did this issue start after all of this was done? Have you checked all of your vacuum lines to make sure everything was put back on?

The catalytic converter was replaced because there was a hole in it. After that the car began the intermittent rough idle. If a vacuum line had been left off at the muffler shop, the mechanic at the engine repair shop would have seen that, wouldn't he? We had an independent mechanic go over the engine after that, and he didn't see anything amiss.

A diagnostic check at a quality shop showed that one ignition coil was bad. After it was replaced, the car ran better for a very short time, then went back to the intermittent rough idle. The second time in the shop, the mechanic noticed that a vacuum hose was cracked and replaced it. No diagnostic was done that day because it ran perfect the entire time they did a test drive. During another previous check by another mechanic, the EGR valve didn't appear to be working correctly; it was recommended we replace it. It was full of carbon. All these things were replaced because of the rough idle. It does not have the pep and power that it used to have, but has not stalled. Instead of driving in fourth gear at 30 MPH, I usually leave it in third because of the lack of power.
Thank you.

Has the fuel filter ever been changed? If not it is a cheap and simple item to help trouble shoot the problem it may even be the problem. Do you know if someone intentionally made the hole in the old catalytic converter?

No, we are making a list of small and/or inexpensive items to replace this weekend, plus replacing the spark plug wires, and possibly the O2 sensor. We will also be spraying down the vacuum hoses with carb cleaner while the car is running to check for leaks there.

The hole in the catalytic converter was from age. In early December, the exhaust system was getting loud. A check at the muffler shop found a hole in the pipe (from rust) at the catalytic converter. They were going to replace the converter and pipe at that time, but were able to flange in a new pipe. It ran fine after that. In early January, it got loud again. A recheck at the muffler shop found a hole in the converter at which time it was replaced.
Thank you.

Well sounds like you have a plan. The reason I ask about the catalytic converter was because of the low mileage on this vehicle for its age. I figure about 4000 miles per year. If that is stop and go city miles instead of highway. There could be and excessive amount of carbon built up on the heads. The thought being if you purchased the car from a used car dealer or an individual for that matter they may have knocked a hole in it to relieve some of the pressure. Then the new converter is installed causing the type of problems you are having. Was you able to hook a vacuum gauge to it? How about a compression test? High compression on a cylinder would indicate carbon build up in the compression chamber.

To thetoolman,

I just wanted to let you know that we replaced the second ignition coil for the other two cylinders, and the car has been running normal for five weeks now. That was our original thought, but felt that the shop would have found that out when they checked it. It just goes to show, when there are two of something and one goes bad, you should probably replace both at once.

Thank you for your help and suggestions.