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Erratic Idle after 100K Service

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Old 07-05-2007, 02:34 PM   #1
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Default Erratic Idle after 100K Service

I've searched the site and I seen lots of posts on major idle issues, but not exactly this problem. I just did a 100K mi maintenance on my stock A4 '02SS (except for lid). Changed items included: Delco Iridium Plugs, Delco Plug Wires, 180 Deg Thermostat, Coolant flush & fill w/Dexcool, Delco Accessory Drive Belts, Seafoam in the top end and the gas tank. I also cleaned the TB. I have driven about 400 miles after this maintenance work and ever since then, when I stop at a light, the idle indicated by the tach is around 550, (same as before) but after 20-30 seconds, there is a slight stumble, the idle drops by roughly 50 rpm according to the stock tach, then it picks up and smooths out again. This cycle repeats after another 20-30 seconds. Any suggestions? Vacuum Leak, something stuck in the TB? When its not stumbling, the engine is running nice and smooth. BTW I haven't changed the fuel filter yet - its still the original.

One more thing - I did drop a plug trying to get the threads started in lovely #8 cylinder and of course it landed wrong and the gap was messed up. I re-gapped it and installed the re-gapped plug in #1 for easy monitoring.
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Old 07-06-2007, 02:13 AM   #2
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well if they disconected the battery the car has to relearn its idle.
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Old 07-06-2007, 07:28 AM   #3
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Thanks for the reply.

Yes I disconnected the battery while doing the maintenance. I wanted to flush the overflow tank, which necessitated battery removal.

How long does it take for the system to adapt the idle?

I've driven over 400 miles since I did the tuneup and my daily commute is the usual suburban crawl with lots of idling, but I have had her out on the highway for about 250 of those miles. Sometimes I think that the problem might be getting better but I can't believe that it takes a couple of weeks (or over 400 miles) for the idle to stabilize. I am more inclined to believe that weather changes are making more of a difference. Cool and rainy seems to have less fluctuation than hot. (with our without the A/C)
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Old 07-06-2007, 10:08 AM   #4
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It does not take 400 miles worth of driving to relearn idle.

I bet one of the plugs are cracked or something. Did you mess with the idle screw on the TB?
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Old 07-06-2007, 10:29 AM   #5
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I didn't mess with the TB other than to clean it. However, I did drop a plug and of course it landed on the ground electrode had to re-gap it. If it's cracked would it show up in a scan (of course I don't have an OBDII scanner - good excuse to buy one) or would I need a scope to see it?
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Old 07-06-2007, 10:48 AM   #6
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If it was cracked it "should" show a misfire and throw a SES light but then again when I had my 1996 Camaro I had a cracked plug and no SES light ever came on. I had to pull the plugs and see what was up. I would just buy 1 new plug. Its only a few bucks and if thats not the problem, at least you ruled something out.
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Old 07-06-2007, 11:01 AM   #7
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True - and I was thinking that I might need to replace the plug when I dropped the #8 plug, so I moved the re-gapped plug to #1 just in case. I have no No MIL (Malfunction Indicator Light - check engine) so I assume that I have no codes.

However, I definitely agree that a cracked plug doesn't always set a code - last time I replaced plugs on my '94 LT1, the plug by the alternator, #4? was also a royal PITA and sure enough I had cracked it. No signs other than a slightly rough idle too. But, I didn't think that just dropping a plug would crack it, but what the heck its only like $9.00 for a single plug! (if I buy one locally)
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Old 07-06-2007, 11:18 AM   #8
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Damn dude, what kind of plugs you use for $9.00 a piece!?!?!?!?!
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Old 07-06-2007, 11:53 AM   #9
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Delco Iridium - they are over $9 from the local dealer, but Summit has them for $6 each or a World Parts search will turn them up for $5.63. (which is what I paid when I ordered 8 plugs, the belts, and plug wires)

These plugs are the recommended stock replacement and I really don't mind paying that much because I don't want copper plugs like the NGK TR55's where I need to change them every 20K mi on my DD.
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Old 07-06-2007, 11:53 AM
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