tricky question for the experts (I'm stumped) - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 05-19-2005, 01:03 PM   #1
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Default tricky question for the experts (I'm stumped)

Ok, heres the picture. 1994 Bonneville, 73K miles, exceptionally good shape (it was garage kept before I got it). When I'm driving down the highway, at 50-60 mph steady speed, I'll feel a strange lurch from the car. It feels just like a quick transmission shift, (lag in power then return). The transmission stays in 4th with TC lockup, RPMS do not dip according to the tach and there is no service engine light. Happens 3-4 times on a 14 mile highway run, seems random, starts in the middle of the run. Will occaisonally do it when first starting from a stop. Idle is perfect and I don't think it'* ever done it while accelerating hard. Doesn't stall either.
The car has new plugs (1K on AC Delco plugs), brand new NAPA wires and coils, good battery, good alternator. I recently cleaned all under hood grounds and cables as I was stranded one night about a month ago due to a bad grounding strip under the drivers rug (fixed and double grounded now).
Unfortunately I can't pull any soft codes as I have the stupid hybrid OBD1/2 connector. I've cleaned the MAF, checked the TPS with an ohm meter (looks fine, changes value smoothly) and have a new EGR and idle air solenoid.
So I'm curious about the new (6 months old) EGR valve. I'm wondering if it is getting stuck. It'* a Wells brand from Autozone and I know they have a spotty reputation. Would it hurt to unplug the EGR and drive it home as a test? I'm also going to try going home in 3 rd gear to see if that makes a difference.
Anyone have any other ideas? I take very good care of my cars (I hate the slightlest malfunction) and I'm afraid this might be a tranny issue. (It was serviced and flushed (last summer) or a really weird electrical issue.
Thanks for all your help,
Rob
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Old 05-19-2005, 02:03 PM   #2
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It really doesn't sound like a tranny issue. Unplugging the vacuum to the EGR won't hurt it but make sure to put a small screw in the end of the line to prevent a vacuum leak. It sure sounds like a electrical problem more than anything. Or just an odd occurance. Using a scantool on it would really help.
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Old 05-19-2005, 02:11 PM   #3
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The EGR on the 88-current 3800'* use no vacume... Its all electronicly controled...

You can unplug the EGR'* connector as a test... The SES light will set sooner or later... But the car will run fine... Do not do this but for a brief amount of time, as in your test...

It almost sounds like you have another issue somewhere... But we'll have to look at it one by one... Good Luck

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Old 05-19-2005, 02:34 PM   #4
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I'm going to yank the EGR plug for the ride home. I'll start off with no EGR and use 4th/OD and see if it happens. If it does I'll kick it down to 3 (non-od) and see what happens. Any other tests or symptoms I should look out for? The most annoying thing about this whole problem is it'* randomness. Just about the time that you start to relax and say "maybe it was just a freak thing" it happens again...
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Old 05-19-2005, 02:38 PM   #5
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Its most likely ignition related... But thats hard to say at the moment... Possibly an unhappy coil or shady ICM... Could be a few things...

The interesting thing about the 94-95 bonnies is that the PCM will show live missfire data if you have a scantool...

See if that EGR makes any diffrence... The PCM does monitor the EGR... But not sure of what it takes for it to throw a code... Say it was open when the PCM expected that it was closed...

Let us know what you find..

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Old 05-19-2005, 02:48 PM   #6
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I put on 3 new napa coils (to replace the factory ones) and new wires last night, no change.
I'm also suspicious of the ICM but want to rule out as many other items as possible before shelling out $164.
I had a ground failure last month that stranded me and I'm wondering if the ICM was damaged by the bad grounding. It certainly seems like the motor is losing spark for a fraction of a second but I would expect the ICM to also fail at idle (which never misses). I'm ruling out the crank sensor and main ECM as there is no twitching of the tach needle when it happens.. But the ICM could be a culprit. I'll post later tonight and let you know what I find.
The computer on this car bums me out, I have both OBD 1 and OBD 2 scan tools and can't read the stupid codes. I diagnosed a lot of minor issues on my 92 Achieva by watching the real time data and would love to know what changes when the car burps. I hate guessing and shotgunning parts..
Thanks for your help.

Rob
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Old 05-19-2005, 02:57 PM   #7
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I understand how you feel...

I hate doing that as well... Helps to have a friend that works on cars and sometimes has an ICM or ECM sitting in the house waiting to be transplsnted...lol

The 94-95 models are all OBD1 compliant and use the same protocall, but they use the OBD2 connector and codes... If you have a tool capeable of reading a 94-95 GM it should read the 94-95 Bonneville as long as you have the OBD 2 cable...

I actually swapped to the Type 2 ignition on my 91( same as your 94 ) the ICM I used came from a 98 bonneville... I had it on my car for 3 months and started having issues with it... It only got worse... I replaced the ICM and never had another problem...

But I do agree, troubleshoot it before spending that kind money on a new ICM...

I even have an ICM the we need to install in my Neighbors 88 Grand Prix... That joker went out... lol.. Gotta love these cars and their electronic controls...

Let us know what you find... Its a learning experience for all of us..

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Old 05-19-2005, 05:19 PM   #8
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Exact same thing happened to me as well. On my car it was more consistant though, OD/TCC lockup, as soon as I was underload and losing speed (going up medium hills) I would get the lurch. Eventually, it got to the point where I was going up hills and it would just shutter and it seemed like the car SHOULD downshift but never did. So I dropped it back into 3rd and limped it home.

I had my dad drive the car, and we replicated the problem. He said it felt like a miss. So he started doing some high RPM accelerations and you could feel it every once in awhile at high RPMs.

I had new plugs and wires in the garage....just hadn't gotten around to changing them yet. When I was changing them, I checked each one. All of them were gapped WAY larger than they were supposed to be. What really caught my eye was the electrode on the number one cylinder plug. It was rounded like all the others, but had dimples like a golf ball. I assume that is from the misses.

As expected, changing the plugs and wires fixed the problem. I know you said that you changed the plugs wires, and coils.

I'm not real good with this whole engine thing, and that is why I read more than I post on here.... But is it possible to get a "bad plug." Or a wire with too much resistance?

I hope your problem is as cheap to fix as mine.
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Old 05-19-2005, 05:31 PM   #9
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I had the same problem as described by Logan. New plugs & wires last weekend did the trick. Plugs looked good except for the electrode on the plug from #5 cylinder.

In your case, it could be EGR. You might want to take another look at the plugs just to make sure.
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Old 05-19-2005, 11:14 PM   #10
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Well my test ride was inconclusive. I do have some interesting observations.. First, a 12 mile, mostly highway ride with the egr valve unplugged did not set the "Service Engine" light. That surprised me. Second, the car runs a lot better with EGR than without. The motor is smoother, revs better and seems more peppy.
As for my stumbling, on the way home (without EGR) it stumbled once, stronger than before and then stopped doing it for the rest of the ride. Reconnecting the EGR did not produce any stumbling. So the EGR test was inconclusive. I'm taking out my double barrell parts shotgun and I'm changing the ICM and the plugs again. These will go along with the new coils and wires and hopefully the problem will go away.
I have no clue what to shotgun after the ICM and another set of plugs.. Fuel pressure regulator maybe?
Comments? Criticisms?

Thanks!
Rob
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