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Rough Idle and CEL

I have a 1995 Buick Regal, with a 3.8L engine. When I start the car sometimes it rough idles and I have to give it a lot of gas and then it runs fine. This is completely random. Sometimes it runs just a little rough, other times really rough. The rough idling when starting the car happens about 1 out of 25 times. Sometimes when this happens I get a check engine light, most of the time not. When I am driving around I also will randomly get a check engine light, but I can’t tell anything common to it. It has come on while braking, while coasting with foot off the gas and brake, normal city driving, highway driving. Also my CEL always turns off when I turn off the car and doesn’t come back when I restart the car. Always.

I haven’t been able to figure out what is wrong and since my car is transition OBD1 to OBD2 I can’t use a OBD2 code scanner (it is an OBD1 port), nor will shorting the pins in my OBD1 type port give me codes. Being this the case I took it to a local mechanic and they scanned the computer and said they got 16 error codes. They reset the codes but couldn’t get the CEL to come back on. I took it drove it for a couple of days the light came back on so I took it back to the shop and they got 18 codes out of it, but they were mostly different, with some of them giving the opposite readings of before (O2 sensor high one time, then low the next check). They said they didn’t know what was going on but wanted 3-4 hours to diagnose the PCM (Powertrain control module) and would probably replace the PCM (~500 on top of the diagnostic fee). That was a bit much for me, so I took the car back.

The things I have checked are fuel pressure (seems fine at idle and revving the car in park)
Vacuum is 19 and when I rev quick it drops to zero or so then up to 25 and slowly goes back to 19 (I think that means vacuum is ok)
I put the gage in the O2 sensor before the CAT and I didn’t read any pressure, so I think the CAT is ok.
I put on a new fuel pressure regulator, and a new fuel filter.
I got the same PCM at a Pick-n-pull place and put it is my car 3 days ago. Yesterday I got a check engine light while driving again. The mechanic will charge me to check the codes, but I think it is just doing the same thing as always.

I am sort of at a loss as what to do. I though it I was to dive around with a scanner hooked up and the light went on then I could see what the sensors were doing when all the trouble codes fire. I was looking at the Actron CP9190 for $312 on Amazon, but it doesn’t log the data stream for OBD1 cars, only for OBD2. The Snap On Ethos does, but that is like 2000 dollars (more than my car is worth). Does anyone know of some software or a non 2000 dollar scanner that would work for me, or perhaps some advice on what I should do next. Thanks a ton!

Oh, also last year I replaced the fuel pump.
The spark plugs look fine, replaced them two years ago.
I used some sea foam last year sucked in the vacuum lines.
Each oil change I put in some sort of gas additive.

Honestly, I don’t think you need to get weird and crazy about this until you’ve done a few very basic and non-computer related things. It might help to give some more specificity about all of the codes. But I would just start with checking out all of my main power connections and power harnesses, with special attention to grounding points. Clean the main power cables at both ends and make sure they end up good and tight. Inspect under the insulation for hidden corrosion. Locate and clean/inspect all of the ground points for engine & transmission. Follow all of the main wire bundles to their connection points, certainly including the primary PCM bundle. Get a close inspection of the main junction box under the hood (with all of those fuses & relays). You need to get a look underneath - where the corrosion will be building up. Do stuff like that before you go crazy on time and money.

Are you sure you can’t pull the codes by doing something like this: ? What happens when you try?

I can’t pull codes from your description, there is no connection at the B pin, so nothing to short out. This is one of the stupid GM transition vehicles between OBD1 and OBD2. I looked at my main ground off the battery on Monday and it looked good, but I will follow your suggestion to carefully follow all the wire harnesses, and also clean my main power cables.

The codes that I get were checked three times and each time it was a bunch of codes (16,18,20) and they were mostly different each time.

Thanks for your help.

I would also check for vacuum leaks,
including the plumbing between the MAF sensor (if it has one) and the throttle. Poke and prod.
Clean the MAF sensor (if it has one).
Do a compression test.
Does it lose any coolant? Could be head gasket seepage.
Could be a flaky ignition module or crank/cam position senso or sticky EGR valve.

I also was going to ask about coolent loss, but I was thinking intake. My 98 Lesabre would run rough on some morning, not all… I was getting a missfire on #6… Turns out I had an internal intake manifold coolent leak (VERY common on 3.8’s) and my #6 was flooding with coolent sometimes.

Its been fixed for about 5 months and runs great now.

Ok, so I did a compression test and I got 188-179-185-182-189-185 so between 5 and 2 there is a 10 psi difference, or about 6%. So I think the compression is pretty good. I didn’t do a test putting oil in there or anything.

I measured the resistance of my spark plugs wires and got in K_Ohm/feet: 2.7-2.3-3.4-2.9-2.7-2.4. I am a little worried about wire 3, and wire four had a bunch of white powder on the boot under the head shield, do you think I should replace my wires (they are the original wires). The factory service manual said to replace wires if the resistance is over 30K on any one wire. (If I was getting wires I was just going to go with the autozone ones since they have a lifetime guarantee, as opposed to performance ones or something like that)

I cleaned the MAF sensor.

@gsragtop - how do I check my internal intake manifold. Do I do a pressure test on my radiator?

I still need to follow the suggestion @cigroller told me to check my ground circuits.

@circuitsmith if my vacuum gauge is steady at 19 psi does that not tell me my vacuum is good? When you say check for vacuum leaks, are you suggesting getting some propane and release it around?

Thanks everyone and I will keep you posted.

My car still would pass the pressure test. Are you losing coolant??

I do seem to add coolant every now and then, but it is very very little (like 1/20 gallon per 3 months). How did you find out you had an had an internal intake manifold coolent leak?

Was vacuum 19" while idling rough? The leak could also be intermittent.
Propane, ether or even carb spray can locate a leak.

I wasn’t able to get it to rough idle while I had the vacuum gauge on it. I will try that. I also tried the carb cleaner but ran out really quick (the can was almost empty) I will try again with a new can of carb cleaner or propane. Thanks for you help.

I’m still on something like a poor ground connection or two. In terms of normal problems like fuel delivery or spark or vacuum, I can’t think of anything that would explain the PCM spitting out something on the order of 18 error codes.

To the list, add a load test of your battery & charging system. How old is your battery?