2001 Pontiac Grand Prix Poor/Looping Idle

Tiny
MAGUS732
  • MEMBER
  • 2001 PONTIAC GRAND PRIX
  • 3.8L
  • 6 CYL
  • FWD
  • AUTOMATIC
  • 139,000 MILES
After getting the intake manifold gasket replaced on my 2001 Grand Prix a few months ago (and shoftly after that, a new catalytic converter), it started doing this; it will dip to under 1000 RPM, as normal for idle, but then it loops back and forth between roughly 600 and 900. As far as I can tell, the engine behaves normally at cruising speeds, both on side streets and the freeway. Last Monday, it started showing the oil pressure light, indicating the pressure's "dangerously low", but I'm taking it to a nearby mechanic tomorrow to have them diagnose THAT issue. I was told after the manifold gasket replacement that it's running rich, which doesn't explain why, after running rich all winter before carbonizing the catalytic converter (forcing me to replace it), it's suddenly causing this problem at idle. The trouble codes I'm getting are as follows: P0171 (System Too Lean; Bank 1) (this one was actually displayed TWICE when I scanned the system); and P0300 (Random Misfire Detected).
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No
Sunday, June 15th, 2014 AT 4:28 PM

7 Replies

Tiny
RANDICO
  • MEMBER
A lean code condition can set a random misfire and a random misfire can set a lean code condition, You already replaced the catalytic convertor for being carbonized? Rich running condition shortens the life a catalytic convertor very fast and so do misfires. When you previously replaced the intake gasket did the engine overheat at any point. It is to you advantage you have the engine check for any low compression.
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Sunday, June 15th, 2014 AT 6:52 PM
Tiny
HMAC300
  • EXPERT
Try cleaning throttle plate on both sides and iac hole with choke cleaner then check fuel pressure with a gauge autoparts rent them. Disregard last reply not an expert. Fo 52-59 for vin K and 48-55 psi for vin 1
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Monday, June 16th, 2014 AT 6:36 AM
Tiny
MAGUS732
  • MEMBER
To Randico: No, the engine has never (to my knowledge) overheated, at least not to the point of failure; 3.8s put out a lot of heat, and when I come to a stop on hot days, the temp goes up, but not to the point of hitting the redline (that issue I CAN fix myself; I need to check/clean the radiator fins after I get it back, anyway). I took it in for the aforementioned oil pressure problem this morning, so I can't do any further diagnostics until I get it back, but it did actually take all winter to heavily clog the catalytic converter; the reason I know that is because around the end of November 2013, I noticed it sometimes acted like there was a little bit of drag on the engine (slightly higher revs to achieve decent acceleration), so in March of this year, when it became bad enough to make it impossible to drive, I took it in, and they told me the cat was gone; I needed to get it replaced. Turns out, the cause, or so I was told at the time, was a vacuum leak around the intake manifold gasket, causing the air/fuel ratio to skew toward the rich side to compensate; they replaced the faulty gasket before I had the cat replacement done. When I went to the place for the cat replacement, they told me it was 50% blocked with carbon buildup (the tech who replaced it showed it to me, and said he'd actually seen them worse), and told me I needed to get it in for a tuneup as soon as possible. The AAA Car Care Center I called to schedule the tuneup told me to give it a week, and the rich mixture should even itself out, since the PCM needs time to readjust. That was 3 months ago, and, obviously, it's still doing the same thing it was back in March.
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Monday, June 16th, 2014 AT 9:40 AM
Tiny
MAGUS732
  • MEMBER
To Hmac300: I have only a limited amount of automotive expierence outside of lightbulb changeouts, oil and coolant checks, and A/C recharging, mostly brakes and door panels (with my father), but I do have a Haynes manual, so I have detailed instructions on how to take the engine apart, but not the experience to safely do so, or put it back together correctly (without help, anyway). I may have the place checking on it now do that.
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Monday, June 16th, 2014 AT 9:45 AM
Tiny
RANDICO
  • MEMBER
Rich mixture do not even out eventually, The PCM can only adjust so much, have the sparkplug check if you need it replaced that would be a start. Having the tune up would be a start if the sparkplugs are worn. Its the oil pressure to low that is the concern, that should be addressed first. The reason I do not mention checking fuel pressure is that the vehicle run good at cruising and highway speed, So you would spin your tail in just getting it check. Having a lean condition lets rule out a cylinder head problem, This is not a fool proof method but it gives you a idea of where to go, When the engine is at operating condition feel the upper radiator hose if it is hard as a rock lean toward a mechanical condition causing the lean condition.
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Monday, June 16th, 2014 AT 11:01 AM
Tiny
MAGUS732
  • MEMBER
Turns out a broken vacuum hose was leaking, causing an imbalance in the air/fuel mixture. AAA replaced the hose, and now it runs perfectly (as far as I can tell).
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Sunday, June 22nd, 2014 AT 2:31 PM
Tiny
RANDICO
  • MEMBER
Great, unmetered vacuum leak as long as it is hopefully resolved, and good luck, I really believe it is truly is resolved vacuum leak can do exactly what is describe. Best regard to you Magus 732
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Sunday, June 22nd, 2014 AT 2:43 PM

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