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Strange idling issue 98 Cavalier
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loligator
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Jul 1, 2010, 7:42 PM
Post #1 of 7
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Strange idling issue 98 Cavalier
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Vehicle info: 98 Chevrolet Cavalier Z24 with 2.4L 4 cyl. The vehicle is distributorless.
So here's the issue and all the history behind it. I've owned the car for 5 years. I can't remember exactly when this started happening but maybe a year after I got it Essentially, I'll be driving along and the RPMs will suddenly move rapidly between 1 and 2k. Up and down, up and down, up and down. As suddenly as it comes on, it's gone. It lasts for maybe a minute then goes away. I'd guess it happens maybe once every 6 months. Originally to resolve the issue, I changed the plugs, air filter, fuel filter, put Lucas fuel treatment through the system a few times, and have even taken it to the mechanic and he couldn't find anything wrong with it. I always change the oil and filter every 3,000 miles.
Last week, the check engine light came on and my OBD II scanner reported a bad throttle position sensor. The car wouldn't shift out of first (automatic) and ran very rough. I swapped the sensor out and the check engine light went off and the shifting/rough running issue cleared up. However, the strange idling issue mentioned above now happens about every other time I drive the vehicle. It's lasting longer also. It tends to start after I'm driving for a few minutes and hit the brakes. Normally, the RPMs decrease smoothly until I stop and the car it's idling around 600 to 700. What's occurring s the RPMs will start to decrease as expected when the brakes are applied, however, the RPMs fluctuate rapidly from about 1k to 2k, as described above. Once I hit the gas again, it seems to straighten itself out until the next time I brake.
I've removed the IAC valve and cleaned it with WD40 as I've read elsewhere but it hasn't seemed to help the issue. The onboard computer isn't picking up any codes and I'm at a loss at what this could be.
Any help would be greatly appreciated
(This post was edited by loligator on Jul 1, 2010, 9:24 PM)
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Sidom
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/ Moderator
Jul 1, 2010, 9:20 PM
Post #2 of 7
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Re: Strange idling issue 98 Cavalier
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Ok, you're right, this is a strange problem & it sounds like it's been going on for a while....
Probably the 1st things I would check would be the basics. Load test the battery to be certain it's good and make sure all connections are clean & tight (you'd be surprised at some of the weird electrical problems a simple defective battery can cause). You mentioned this has been an ongoing problem. If you've changed batteries in this time period and the problem remains the same, more than likely it's not the source of the problem but needs to be verified as good anyway....
I didn't see any TSBs pertaining to this exact problem but did notice a couple of recalls relating to the ign switch. The next thing I would probably do is make sure all the recalls were taken care of and the PCM had/has the most current flash.
After that it's going to come down to checking the duty cycle for the IAC to see if in fact for some reason the PCM is trying to control the idle or there is an intermittent power or ground issue or if you just have a defective IAC.
From what you've posted I doubt the IAC is bad, that would be too easy.....lol
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loligator
New User
Jul 1, 2010, 9:27 PM
Post #3 of 7
(2689 views)
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Re: Strange idling issue 98 Cavalier
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Any idea if a bad IAC valve would make the check engine light go on?
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Sidom
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/ Moderator
Jul 1, 2010, 9:41 PM
Post #4 of 7
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Re: Strange idling issue 98 Cavalier
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Yes it can and I believe you would get a P0505 or P0507..... can't remember the exact code right now.......sux having a bad memory.....
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loligator
New User
Jul 2, 2010, 6:54 AM
Post #5 of 7
(2676 views)
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Re: Strange idling issue 98 Cavalier
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Thanks for the quick reply. It's interesting you point out that code, the P0507. Although the check engine light is no longer coming on, I'll still pick up a P0507 code occasionally. Any idea if I can test the valve? I'd just hate to put out $78 and have the issue be somewhere else 
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Sidom
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/ Moderator
Jul 2, 2010, 8:17 PM
Post #6 of 7
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Re: Strange idling issue 98 Cavalier
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Without the proper equipment, there is no way to really test the valve. If you had a hard fail with no idle control all the time you could do some checks with a volt meter, but it sound like for the most of the time you have idle control. So to try to find the problem you would need a bi-directional scanner & scope (around 8 to 10k worth of tools) PLUS you would need to be checking it while the problem is happening...(I really hate intermittents sometimes.. ).
What I can tell you, while it's not a 100% guarantee, with that code, the PCM is expecting the IAC to do something and the IAC isn't doing what the PCM thinks it should be doing. So it sounds like the PCM is trying to control the IAC but the IAC isn't behaving. If this was a case of the PCM controlling the IAC the incorrect way for the conditions, thus causing the idle problem. Chances are pretty good you would see a code for the circuit that is giving the PCM the bad advise.........
Also, this is a 2 trip failure code, meaning it has to fail on 2 consecutive trips to turn the light on. The 1st fail sets the code up as "pending" and the 2nd fail turns the light on......If it doesn't fail on the 2nd trip, the cycle starts all over......
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loligator
New User
Jul 6, 2010, 7:05 AM
Post #7 of 7
(2655 views)
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Re: Strange idling issue 98 Cavalier
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Okay, I'll keep all of that in mind. Of course it's been running without issue for the past few days... I know as soon as I start driving it regularly again it will probably break down. If I find a solution to the problem I'll be sure to post it here.
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