1995 Bonneville: Bad idle, stalling, shaking, backfire

[95naSTA]

I'm not sure unplugging the MAF will do what it normally does on a S2 car since 95 L36s were MAF only. No MAP sensor on that car.

[Twystedhemp2]

It does not. But, one of the hot wires in the MAF is slightly discolored.

[Twystedhemp2]

I'm not sure unplugging the MAF will do what it normally does on a S2 car since 95 L36s were MAF only. No MAP sensor on that car.

Right, no MAP on this car. I will replace the MAF first and see if that is it. Everyone that has looked at it has tried unplugging it. This last mechanic with the fancy diagnostic machine was very surprised that the codes showed the MAF circuit and suggested the throttle body cleaning and EGR, but he unplugged the ERG too and it made no difference. You're the first to suggest the MAF disconnect might not be telling.

[95naSTA]

I'm not necessarily saying that it won't be telling. I honestly don't know if it will or not.

But, in all other 96+ Series 2 3800 cars, if you disconnect the MAF the ECU still has a MAP reference and table to run on. It'll still have some idea how much air is coming in. If you unplug the MAF in this car, it won't.

[MattStrike]

My recollection with series 1 is that unplugging the MAF will make it run not good - however it will be consistent. It runs a little rough at idle, and can't compensate for fuel properly so doesn't make much power and continues a little rough. That was on a '92/'93 car though so it might be different because the mixed bag the '95 is (series 2 on OBD1). The '96+ cars being OBD2 the unplug trick works, all the OBD 1 cars it doesn't work the same way - but it unplugging it will still tell you if it's the MAF because it will run bad but if the other problem goes away in addition.

[Twystedhemp2]

So, we changed the MAF and tried it, no difference. Changed the EGR and WOW! Definitely the source of the stalling. It runs and idles now, but it's still not perfect. It cranks fine, but then does this vroom, vroom, vroom thing until you give it some gas and THEN perfect idle... until the battery meter drops and the oil pressure sensor wiggles all over the place, give it a touch of gas, good idle again. Took it on a short interstate run, it was smooth and powerful but when we coasted a back road there went the battery meter again and the vroom, vroom, vroom. It seems basically drivable again but something is still screwy. I found this cryptic line in the Chilton's: "Bypass Signal- above 400 rpm, the ECM applies 5 volts to this circuit to switch spark timing control from the C3I module to the ECM" How do I investigate this circuit? And could this be a ground issue? I read that there are grounds somewhere on the edge of the floorboards and we've never checked them, are there others??

[Twystedhemp2]

Well, spoke too soon. Went out and started it again and it stalled

[MattStrike]

OK, time for the fun side of troubleshooting...

You have:
-new CPS
-new battery
-new MAF
-new EGR

It sounds like it's not able to maintain idle, which is causing the PCM to cut off the alternator. That's just a side effect of the issue.

Plugs, wires, coils, ICM, and fuel injectors need to be tested in that order at this point. Also test the wire harness that goes between the crank, cam, and ICM if you haven't already. Intermittent connections (the terminals inside the connectors tend to 'un-sping' over time) might be a contributor.

[96 SSEi]

f its only at idle, then I would next inspect/ clean the iac

[1oldman]

OK, time for the fun side of troubleshooting...

You have:
-new CPS
-new battery
-new MAF
-new EGR

It sounds like it's not able to maintain idle, which is causing the PCM to cut off the alternator. That's just a side effect of the issue.

Plugs, wires, coils, ICM, and fuel injectors need to be tested in that order at this point. Also test the wire harness that goes between the crank, cam, and ICM if you haven't already. Intermittent connections (the terminals inside the connectors tend to 'un-sping' over time) might be a contributor.

Matt, just a thought, that green '92 you have did the same thing several years back. I had to put on a TB from the JY to get it to stop after replacing IAC, TPS, MAF, and I don't remember what else. I did a lot of chasing and replacing to get it to stop. - BC

[Twystedhemp2]

The IAC was one of the first things we replaced. Had another diagnostic done and it's still showing the MAF circuit. The belt was a bit worn so we went ahead and replaced it and the tensioner just in case. No change. So the testing you're recommending, we need a multimeter to do these right? The mechanic is totally stumped, his only suggestion is try another MAF even though this one is new or replace the PCM. I've read there are sometimes issues with the wiring to the MAF, what is the most effective test?

[Twystedhemp2]

Oh! Here's a mind boggler for you. The oil filler cap...It is sucking in air there. That's not normal at all is it? Also on a cold crank the vacuum hose on the back of the engine is blowing out, very dramatically.

[ron350]

With vacuum at the oil filler cap the first thing to check is the PCV valve o-ring to make sure it is in place and not split.

[MattStrike]

Testing the coils:
http://www.pontiacbonnevilleclub.com/fo ... c1683.html

Alternate method is to pop all the plug wires loose so that with the engine running you can slowly lift one away from a coil. You're looking for a strong blue spark. Yellow spark or no spark is a weak/bad coil.

Plugs and wires are a visual inspection, any mechanic should be able to do that.

testing the wire harness between the ICM and the crank and cam sensors needs a multimeter with a diode tester or can be done with a test light if you remove the harness.


Air flow through the open oil cap is normal - if you're cruising down the road at light throttle. The PCV valve is supposed to regulate how much flow there is through the PCV system (runs from the Throttle body into the crank case, then up into the PCV valve and into the vacuum side of the intake system). I know the series 2 engines had an issue with the PCV valve o-rings, so I would start with that first before moving on to check anything else. It shouldn't be sucking a significant amount of air at idle.

Which vac hose is blowing out? I can't think of any good reason why that would be the case.

[Twystedhemp2]

The hose that keeps flying out goes from the back of the engine to the brake booster and has a few other connections i believe one is cruise control. The PCV valve is new and the o ring is intact. Could it be the wrong one? The auto part store gave me the wrong belt the other day so now I am suspicious of half the parts I've put on.

[ron350]

Please post a picture of this possessed motor.

[96 SSEi]

if its not the pcv, it may be the fresh air intake hole within the throttle body is plugged.

[95naSTA]

If your booster hose coming out of the back of the upper intake keeps flying out, it's not in there correctly. That would cause a vac leak.

[Twystedhemp2]

if its not the pcv, it may be the fresh air intake hole within the throttle body is plugged.

We took apart the throttle body and thoroughly cleaned it.

[Twystedhemp2]

If your booster hose coming out of the back of the upper intake keeps flying out, it's not in there correctly. That would cause a vac leak.

Is it supposed to have an O ring or something? It shook loose while on a rough road years ago and we only had it come loose a few times since, but the car ran fine.