Boost sometimes at -7.5 PSI at idle, engine surging - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 08-04-2005, 11:21 AM   #1
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Default Boost sometimes at -7.5 PSI at idle, engine surging

Occasionally over the last several months, my '92 sse supercharged has had a problem in which I'd stop at a light or at my destination and the boost is at about -7.5 PSI rather than near -10 PSI and the engine surges up and down a little. It only happens sometimes, and so far it has happened only during my 20 mile morning drives to work but not on the way home. It makes no sense to me. I first suspected that maybe the throttle linkage was sticking, but that'* not the case. I also suspected that maybe the fuel sock or fuel pump or baffles may be causing the problem if something was floating around in the tank and getting occasionally sucked and stuck to the pump, but that'* only a wild guess.

I think the engine may have a vacuum leak somewhere, but I've been unable to find it; sometimes the idle is smooth, and sometimes it'* rough. If it were a constant vacuum leak, I'd expect it to always have a rough idle when the engine is warm, but that'* not the case. I'm also wondering if the rough idle could be caused by the fuel vapor canister solenoid, the EGR, or the supercharger boost solenoid.

Any ideas?
Thanks!
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Old 08-04-2005, 11:22 AM   #2
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Only time I notice this is when the a/c is on and cycling.

EDIT: Let me expand. I don't consider it a problem In my case the A/C is clearly working the motor and therefore vacuum is less than if the motor was at rest.

Do you have the A/C on since the weather has been hot nearly everywhere?
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Old 08-04-2005, 11:23 AM   #3
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Same problem...
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Old 08-04-2005, 11:29 AM   #4
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Jwikoff99, does yours have the same problem?

Yes, the boost is at -7.5 PSI just as it would be if the AC were on...but the AC isn't on...and when this happens, the engine surges a bit up and down but the RPMs don't change very much at all, the gage reads around 750 RPMs and only varies about 50 RPMs or so.
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Old 08-04-2005, 11:33 AM   #5
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Quote:
Originally Posted by billha
Jwikoff99, does yours have the same problem?

Yes, the boost is at -7.5 PSI just as it would be if the AC were on...but the AC isn't on...and when this happens, the engine surges a bit up and down but the RPMs don't change very much at all, the gage reads around 750 RPMs and only varies about 50 RPMs or so.
Mine acts very similar.
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Old 08-04-2005, 11:45 AM   #6
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I've experienced this with 2 different cars, 2 different causes. 91 GP had a clogged catalytic converter, 99 GP had a bad MAF sensor. A vacuum hose that a quick change oil place knocked loose caused a similar problem. All with the surging problem. Can't report on the boost as they were N/A
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Old 08-04-2005, 11:53 AM   #7
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I had the catalytic converter replaced a few months ago, just because I though it might have been causing the problem when it got hot. Before replacing it, the muffler shop owner said a plugged cat wouldn't cause this problem, and it turns out that he was right.

I've swapped the MAF with another one, and it made no difference.

All vacuum hoses look to be intact and connected properly.
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Old 08-04-2005, 12:21 PM   #8
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Hmmm I'd be looking toward items that control idle.
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Old 08-05-2005, 11:54 PM   #9
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Quote:
the muffler shop owner said a plugged cat wouldn't cause this problem, and it turns out that he was right.
Ah...well, it must've been the new bolts on the clamps then.
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Old 08-23-2005, 11:10 AM   #10
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The boost was acting up again this morning, and this time I pulled over, shifted to park, and opened the hood to look around. Unfortunately, it was still dark outside, so I wasn't able to see much, but the arm on the boost bypass valve was in the up position. I manually moved the valve actuator arm down and up again with my hand, to see if it was sticking, and it didn't seem to be, but the boost was still stuck at -7.5 PSI and the idle was surging, still in Park. I also pressed on the electrical connector for the boost control solenoid to see if maybe there was a bad connection, but it made no difference.

What could cause boost to be at -7.5 PSI when the car is in Park? The boost is supposed to be completely bypassed and at -10 PSI, right? I'm thinking there must be an intermittent problem with the boost bypass valve, its vacuum canister, or the boost control solenoid. And why does it only happen in the morning?
Do any of you have any experience with this?
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