98 Idle surge / rough idle
Thread Starter
Heel & Toe
Joined: Aug 2020
Posts: 21
Likes: 0
98 Idle surge / rough idle
Hi again,
I recently picked up a very clean 98 c5 A4 with 68k on it. Since ownership, I’ve had this consistent issue with my idle. Basically when i start the car, it revs to 2k for a few seconds before returning to normal (about 500-800). It then idles very choppy. When i shift into drive, the idle smooths out and it runs fine. Shift back into park and the idle will surge to about 1k then drop and almost stall before returning to the choppy idle. It recently had a CEL on for an o2 sensor which i replaced; this did not fix the problem. Car is stock from what i can see except a magnflow cat back/ and k&n intake. No other codes are coming up. Any ideas what to check first?
I recently picked up a very clean 98 c5 A4 with 68k on it. Since ownership, I’ve had this consistent issue with my idle. Basically when i start the car, it revs to 2k for a few seconds before returning to normal (about 500-800). It then idles very choppy. When i shift into drive, the idle smooths out and it runs fine. Shift back into park and the idle will surge to about 1k then drop and almost stall before returning to the choppy idle. It recently had a CEL on for an o2 sensor which i replaced; this did not fix the problem. Car is stock from what i can see except a magnflow cat back/ and k&n intake. No other codes are coming up. Any ideas what to check first?
Safety Car
Joined: Mar 2015
Posts: 3,638
Likes: 245
Hi again,
I recently picked up a very clean 98 c5 A4 with 68k on it. Since ownership, I’ve had this consistent issue with my idle. Basically when i start the car, it revs to 2k for a few seconds before returning to normal (about 500-800). It then idles very choppy. When i shift into drive, the idle smooths out and it runs fine. Shift back into park and the idle will surge to about 1k then drop and almost stall before returning to the choppy idle. It recently had a CEL on for an o2 sensor which i replaced; this did not fix the problem. Car is stock from what i can see except a magnflow cat back/ and k&n intake. No other codes are coming up. Any ideas what to check first?
I recently picked up a very clean 98 c5 A4 with 68k on it. Since ownership, I’ve had this consistent issue with my idle. Basically when i start the car, it revs to 2k for a few seconds before returning to normal (about 500-800). It then idles very choppy. When i shift into drive, the idle smooths out and it runs fine. Shift back into park and the idle will surge to about 1k then drop and almost stall before returning to the choppy idle. It recently had a CEL on for an o2 sensor which i replaced; this did not fix the problem. Car is stock from what i can see except a magnflow cat back/ and k&n intake. No other codes are coming up. Any ideas what to check first?
I'd do a basic tune up
Last edited by My Vette Life; Oct 18, 2020 at 12:26 AM.
Thread Starter
Heel & Toe
Joined: Aug 2020
Posts: 21
Likes: 0
I think changing the plugs out is a good idea. I just pulled all the codes the other day with a scanner. The only one was for the o2 sensor which I had replaced.
Safety Car
Joined: Mar 2015
Posts: 3,638
Likes: 245
yes, I'd go for a basic tune up just for a base point.
Last edited by My Vette Life; Oct 18, 2020 at 12:42 PM.
Drifting
Joined: May 2007
Posts: 1,547
Likes: 210
From: Huntersville, NC
could be several possibilities. With your scanner and engine cold, see if IAT and ECT temps are the same. Then look at fuel trims. Regarding theO2 sensor, a lot of times aftermarket do not perform. Which O2 did you change? Also look at MAP and MAF numbers. you should compare MAP with barometric pressure with the engine cold and not running, then see if MAP changes as you press on the ACCel pedal with the engine running. Justin Miller, Youtube has a great procedure for checking MAF to see if it is giving linear output WRT RPM. All you need is a scan tool and EXCEL on your computer.
Melting Slicks
Joined: Mar 2018
Posts: 2,502
Likes: 187
From: California
I would get the original stock file and insert that ....one that untouched .....sounds like your in gear / out of gear parameters are not set right / fueling is all messed up...and your throttle crack at the least ...try a fresh file ....can’t help ya as I have a manual car I’m sure other guys here or hpt website is full of them ...if problem keeps persistent change plugs/ wires / maf / iat sensors might be doing it possible a injector . Doubt it as it seems more a file issue
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Have you checked ALL possibilities that you have a vacuum leak? The early LS-1 cars are notorious for rotted / cracked /damaged PCV lines! Especially where the lines converge into the Y fitting behind the manifold. They crack and leak and cause all sorts of idle havoc.
Check ALL of your vacuum lines.
Plug the vacuum line for the PCV and do an idle relearn and see if the issue goes away or change.
You can also spray solvent (brake parts cleaner) on suspected vacuum leak areas and see if the idle changes. If it does, you can pin point the source of the leak. Spray around the manifold to head connection area gaskets, TB, PCV fittings and lines, Also check the air bridge connection between the MAF and TB.
If it were me, I would remove the filter assy and air bridge and clean the throttle body (TB) and the throttle blade and give the MAF a good spray cleaning. Pretty easy and relatively inexpensive. If you have never done it, it most likely needs it anyway. Then make sure all those connections are redone air tight. .
BC
Check ALL of your vacuum lines.
Plug the vacuum line for the PCV and do an idle relearn and see if the issue goes away or change.
You can also spray solvent (brake parts cleaner) on suspected vacuum leak areas and see if the idle changes. If it does, you can pin point the source of the leak. Spray around the manifold to head connection area gaskets, TB, PCV fittings and lines, Also check the air bridge connection between the MAF and TB.
If it were me, I would remove the filter assy and air bridge and clean the throttle body (TB) and the throttle blade and give the MAF a good spray cleaning. Pretty easy and relatively inexpensive. If you have never done it, it most likely needs it anyway. Then make sure all those connections are redone air tight. .
BC
Last edited by Bill Curlee; Oct 19, 2020 at 12:03 PM.
Melting Slicks
Joined: Dec 2004
Posts: 2,847
Likes: 1,144
From: Sleepy Hollow- NJ
Have you checked ALL possibilities that you have a vacuum leak? The early LS-1 cars are notorious for rotted / cracked /damaged PCV lines! Especially where the lines converge into the Y fitting behind the manifold. They crack and leak and cause all sorts of idle havoc.
Check ALL of your vacuum lines.
Plug the vacuum line for the PCV and do an idle relearn and see if the issue goes away or change.
You can also spray solvent (brake parts cleaner) on suspected vacuum leak areas and see if the idle changes. If it does, you can pin point the source of the leak. Spray around the manifold to head connection area gaskets, TB, PCV fittings and lines, Also check the air bridge connection between the MAF and TB.
If it were me, I would remove the filter assy and air bridge and clean the throttle body (TB) and the throttle blade and give the MAF a good spray cleaning. Pretty easy and relatively inexpensive. If you have never done it, it most likely needs it anyway. Then make sure all those connections are redone air tight. .
BC
Check ALL of your vacuum lines.
Plug the vacuum line for the PCV and do an idle relearn and see if the issue goes away or change.
You can also spray solvent (brake parts cleaner) on suspected vacuum leak areas and see if the idle changes. If it does, you can pin point the source of the leak. Spray around the manifold to head connection area gaskets, TB, PCV fittings and lines, Also check the air bridge connection between the MAF and TB.
If it were me, I would remove the filter assy and air bridge and clean the throttle body (TB) and the throttle blade and give the MAF a good spray cleaning. Pretty easy and relatively inexpensive. If you have never done it, it most likely needs it anyway. Then make sure all those connections are redone air tight. .
BC
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