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Old 10-19-2018, 07:16 PM   #1
drzoidberg
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Default Sudden bad idle, fixed itself eventually

As I was pulling into my driveway after a 2 mile drive home tonight, my 98 SC2 automatic with 110k on it (and yes, it does have the brass ECT sensor) suddenly started bucking pretty badly. Idle was swinging back and forth about +/- 50 to 100 RPM at about a 1Hz rate. Car was bucking in drive or in park. Seemed to smooth out some if I gave it gas in park but when I let it get below maybe 1200 RPM it would start bucking again immediately. Letting the RPMs fall quickly didn't kill the car.

Car would restart after just a couple cranks as usual, but hesitated a bit and didn't sound great on startup, and immediately started bucking again. Pulled the electrical connector to EGR, restarted car, same symptoms. Reconnected EGR connector and this time I started car with foot on the gas slightly to check IAC. This time, car started fast/smoothly, RPMs jumped up to 3k immediately (with foot on gas), when I let off car was idling normally at 800 RPM hot idle. I hadn't moved the car at all during those 2-3 restarts. Seemed like maybe a sudden vacuum leak, but can't find any visible issues with the vacuum hoses I can see. There is a hissing that I can't quite locate somewhere in the vicinity of the brake booster, EGR valve, and intake, but it seems loudest with my ear near the booster. Couldn't locate it using a piece of hose on ear either. Brake booster itself seems to be holding vacuum and is good for 3-4 pedal depressions before becoming hard. Is it normal to have a noticeable hissing noise in the vicinity of the brake booster? Unfortunately I wasn't able to probe around with propane since it started acting OK again.

For a minute or two the booster-vicinity hissing was slowly increasing, then decreasing to zero hissing over the course of several seconds while idling with a steady engine speed and no bucking/surging. Then, after the next restart, it was/is just at a constant amplitude hiss at idle. Very strange.

Tried about everything I can think of to get the car to act up again. Let it get good and hot, took for a warm test drive up to 2 miles. Restarted about 5-6 more times and no issues each time. I do notice a bit of a "whooshing" noise inside the car at certain speeds and often while coasting down to a stop, but not sure if that is related or not. I think it's been around at some level for a while, but may be a bit more noticeable, or it could be my imagination.

Is it possible the EGR was sticking, and that by removing and reattaching the connector I jarred it loose again, or does it sound more like maybe a vacuum leak or other issue? Thanks in advance for any suggestions, these intermittent problems are real pains...

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Old 10-20-2018, 10:35 AM   #2
drzoidberg
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Default Re: Sudden bad idle, fixed itself eventually

Couple additional details I left out before that might be relevant...

When the engine was surging I was able to view some of the live data. Seemed like the ignition advance was around -12, and the LTFT was about -9, which I recall is fairly typical for this car at least over the last several years. Nothing else jumped out at me as being unusual but I wasn't able to snapshot the data. Not sure if the retarded timing is a problem itself, or if that's just a symptom. I replaced the CKP sensor a few years / 10-15k miles back with an autozone Duralast unit because the car had been suddenly dying when hot (thanks Fdryer for your help with that one!). Not sure if this temporary rapid surging could be that failing again in a different way?

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