Engine surging rough idle 1995 Olds Cutlass

I have a 1995 Olds Cutlass Supreme with appx 185,000 miles. It has the 3.1 liter V6 engine in it. I recently had a bunch of maintenance work done and now I am left with a car that surges on the highway at speeds over about 60 mph and also has a rough idle and slight shakiness to it like it will cut out, it also does this on the freeway. The onboard diagnostic computer is not spitting out any codes even when the check engine light is on and is not working properly. In the last month we have done a complete tune-up, wires, plugs, oil change, gas treatment. Installed new oxygen sensor, map sensor, fuel system and idle air control valve, not sure what to try next..any ideas??

Welcome to af, did this start when the work was done, is it a dead misfire, tough to help from here. Why were all these parts replaced?

It is not a dead misfire and it is sporadic. A lot of the work was performed for maintenance purposes but the rough idle and power issues were going on for awhile. The fuel system was done about 5 months ago and we did another fuel filter within the last few days. The power surging and engine cutting out has been going on for the past two weeks which is when we tried wires, map sensor, which gave it back the power we lost and then the idle air control valve. It does have a very small vacuum leak that it has had for awhile but that is not something I can fix today. I was told to try the tps sensor sense a lot of the acting up is on the freeway and to have the battery checked..

All this work done and nothing is done to fix a vac leak?

Time to stop putting parts in and try to pinpoint the problem, a full scan looking at sensor inputs is a good start. If the cel is on there is a stored code, make sure you are using a scanner that will work on obd1.5

It is not that I am not fixing the vacuum leak that has been there for awhile and it is not as affordable to fix. Even with my own mechanics that is going to cost several hundreds of dollars in labor. Not even sure if its worth fixing, we are looking into it. As for a cel, it won't stay on and we have tried everything to get the codes. We are probably going to take it into the shop and pay the $100 to get it done on their machine. Its just the last time I did this they came up with several codes, paid the $100 and we were still in the same boat diagnosing it. Now it appears to be running quite a bit better other than when you first start the engine and its cold, cel isn't even coming on at all but when you try to run it cold it runs terrible for the first few minutes. I am really confused. The surging is still there on the highway but not as bad as in the past. I appreciate all of the comments and suggestions..:confused:

Surging, low fuel pressure, dirty injectors, bad o2 sensor are some common areas, clogged fuel filter another. Mis fire counts, o2 activity fuel mapping, are all available on a good scanner.

I have replaced the entire fuel system and the fuel filters twice. Oxygen sensor, Map sensor, replaced pcv valve and plugs and wires. I am being told to check out the TPS sensor and have it tested. As I said before the last time I tried the shop only method and scanning it came back with several codes and cost several hundred dollars only to end up being plugs that weren't diagnosed by any of the mechanics. The onboard computer is not working correctly and even when tripped isn't spitting out codes we have tried that. It's just frustrating. That was our first venture. Vacuum leak is being fixed within the next two weeks due to cost..

What is the vacuum leak? Lower intake?

I believe it is on the top area somewhere. It is going to be expensive to fix but has been there for quite some time. Problems didn't start until we did the maintenance things and that is what happened from there. The car died three times at the light on the way to work this morning and cut out twice on the freeway so now it does it at highway speeds and when you are stopped at a light. We have no codes and no absolute diagnosis, we are running out of parts to check.

We have no codes and no absolute diagnosis, we are running out of parts to check

If the CE light is on, there is a code set.....just that your tech can't read it with the equiptment he has....I agree you are running out of parts to check......that's what people who don't know how to diagnose do....they replace parts until they get lucky....at your cost....

Im sorry you feel that way. There is not a cel at all, no codes. Nothing, we have tried a shop scanner there is nothing, we are replacing parts that are worn and need to be replaced anyway and it is getting better. They are wonderful mechanics and are reasonable. They have tested all of the sensors and we only replaced the ones that were bad. We will figure it out, but thanks

When it stalls/dies does it start right back up? if not someone has to check for sapark and fuel pressure/injector pulse when this happens. Also how do you know it has a " small" vacuum leak what are the symptoms? I think other folks here with a good diagnostic backround look at the parts replaced and wonder how these components could all check out bad and have no meaningful effect on the car when replaced. IF the pcm is not showing codes with the cel on then the wrong scanner is being used or the pcm is bad, that is not normal.

We suspect that the pcm may be bad if you mean the diagnostic on board computer. The people I have taken it to and used are experienced mechanics as well. I didn't say there was no improvement just still some symptoms that concern me. We are going to pull the egr valve out today, I think we overlooked it. The MAP sensor and IAC valve helped tremendously as did the wire set as one of the wires was coming apart. It is just frustrating and time consuming. But for the most part it does get better each time. I have been at the shop and at home when they tried to retrieve the codes and except at the beginning weeks ago it never had one, the one code that came on stayed for a very brief period and after we changed out the 02 sensor it disappeared permanently. The vacuum leak is on the intake manifold gasket I believe and is being fixed this week. Due to the fact that I have spent a lot of money getting it into better running order I couldn't do everything at once. I am trying to be as specific as I can. I was told a good scanner could only do so much if your onboard computer either isn't working or registering codes but I appreciate the advice.

Oh and yes it starts right back up. When on the highway it stumbles but doesn't cut all the way out if that info helps.

Ok when the cel is on and no codes there is an internal issue with the pcm or as I said the scanner is a basic one that cannot communicate with obd1.5, a sticking or bad egr can cause rough idle, poor low speed performance and possible stalling when slowing down, but it would be there almost all the time. Try disconnecting it and see what happens, that should set a code also, stalling intermittently can be caused by a bad crank sensor, both wiring to the sensor or a pcm, but again all this would be guess work without having the condition stay long enough to diagnose.

The egr is next and we have tested the crankshaft and cam sensors if that is what you mean. We haven't had a solid check engine light for a very long time and it didn't stay. As soon as we changed out the o2 sensor it disappeared. The shop itself is even a little stumped but they also suggested that egr because it would cause those very symptoms. To be fair in the last day or so since we did the last repairs it runs a lot better. My question is that I heard there is some sort of re learn procedure we need to do when you install those sensors, the iac valve and the throttle position sensor, is that true??

No......

Ok when the cel is on and no codes there is an internal issue with the pcm or as I said the scanner is a basic one that cannot communicate with obd1.5, a sticking or bad egr can cause rough idle, poor low speed performance and possible stalling when slowing down, but it would be there almost all the time. Try disconnecting it and see what happens, that should set a code also, stalling intermittently can be caused by a bad crank sensor, both wiring to the sensor or a pcm, but again all this would be guess work without having the condition stay long enough to diagnose.


Well now it has started up again and it has some sort of smoke/exhaust coming out the tailpipe when we start it up. It still stumbles a little at stoplights and has a rough idle. I am getting really confused and frustrated but still no cel at all

What color smoke? You mentioned several times that your on board computer isn't working correctly. If it is bad why not replace it? If the computer is bad it could be the cause of some of these problems.

I guess I am confused replace what?? The egr valve is being pulled out and replaced tonight if necessary and we are being told to check the ecm. Everything on that car is really old I am finding out and little or no maintenance was done. So its not a matter of not doing things, its just doing the things first to try and correct the driveability problems and go from there. It is super expensive but I do have good mechanics and the two shops couldn't figure it out exactly either without doing some of the same things we are. Just frustrating.. But I do appreciate the comments and the suggestions. I am looking into the egr now, sounds like it could cause a lot of what is still happening, each part seems to improve the gas mileage and running condition of the car overall