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1990 Grand Prix Rough Idle - Runs great
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p11g20t
Novice
Sep 3, 2012, 10:02 PM
Post #1 of 4
(2383 views)
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1990 Grand Prix Rough Idle - Runs great
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1990 Grand Prix, 3.1L
Just bought it to flip, heres the issue:
First, it always fires up easily.
When its cold it idles fine. When its in park or neutral it typically idles fine.
The second you start driving, it runs great. When i let off the gas and the rpms drop, the surging starts. when you come to a stop, foot on the brake, in gear, it surges up and down - and sometimes will die. If i flip on the headlights or fans (which are wired to a switch), it bogs the motor down even more and will sometimes die from that.
the IAC has been replaced, i pulled the EGR, it looks clean enough to not be the problem.
unplugging the iac, MAP sensor, EGR, or TPS doesnt make any noticeable difference for the better or worse. I have tried the steps to "relearn" the idle.
i have done a decent search for vacuum leaks, however i have to wait til tomorrow to pick up some starting fluid to check more thorough.
if i jump pins A and B on the diag port while running, it smooths out some, but not completely. its throwing no codes currently.
when i first fire it up and its already warm, if i put it in gear it doesnt always surge, but it does idle somewhat rough still. if i flip on the fans and lights it will either die or bog and start surging.
i do not know this history of this problem, this is my first day with the car. he said it starting running a bit worse after he was told to replace the intake manifold gasket, and he did.
he said they had to remove pushrods in the process, which makes me wonder (although reading around i believe that was correct). it has non adjustable roller rockers so im not too worried about what he did there.
again it drives really well the second you start to accelerate.
ideas?
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p11g20t
Novice
Sep 3, 2012, 10:53 PM
Post #2 of 4
(2361 views)
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Re: 1990 Grand Prix Rough Idle - Runs great
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hey i guess ill answer my own thread!
i tried figuring this out for like 9 hours today, and finally posted this thread. there are literally TONS of threads out there with this same problem, and hardly ANY resolved. a few, but they varied.
heres what happened. i went out after it cooled off some, and it wouldnt run. it would start then die. i decided this was a good time to recheck everything, because if it started, that was a good sign.
in the end, i unplugged the TPS again, and it fired up. i drove it around the block and it gave me no problems at all.
i unplugged it before and i thought it hadnt changed anything but i guess i didnt test it enough.
tomorrow when i come back to work i will test it out again and make sure its all good, but it drove great for a bit here a minute ago!
new tps here i come!
unfortunately i decided to try the high beams, and i got one click out of it, turned them on. now they wont go off unless i pull the fuse lol. even with the car off. AWESOME! guess ill replace that switch too.. *rolls eyes*
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p11g20t
Novice
Oct 4, 2012, 7:08 PM
Post #4 of 4
(2178 views)
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Re: 1990 Grand Prix Rough Idle - Runs great
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that would be the previous owner that wired things up like that. dont look at me!
anyways, unplugging the TPS helps quite a bit, but doesnt solve it. putting a new TPS in doesnt change anything. so i have that unplugged, and thats how its been. its been doing a lot better, but recently has started dying at a stop, or when you put it in reverse.
ive unplugged almost everything one at a time trying to make a difference, and frankly nothing seems to make it better OR worse.
ive done a thorough starting fluid check for vacuum leaks, because i have no vac gauge setup. found nothing.
im doubtful but could it be either a weak alty or fuel pump? it runs great at anything other than idle. and under electronic load at idle it gets worse.
my friend is actually the one driving the car recently, but if i recall, there is a CEL, a coolant level light, and a check gauges light on. All of which are meaningless. no codes. i think its a soldering problem, because i have read around on this model and when a coolant level sensor goes bad, you can unplug it and the light will go out (not stay on). but when i unplug it, it stays on, which i have read=bad soldering. i figured they were all a case of that as my coolant level is fine, gauges are all fine, and there are no codes.
could this be related to a deeper electrical problem?
thanks guys
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