S4 sporadically runs rough
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Joined: Jan 2009
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From: CT
S4 sporadically runs rough
My '88 S4 just started running rough...but not all the time. Here's what's going on...
A couple weeks ago I took the car for a ride and it ran fine. I went home, parked it for an hour or two, started it up again and while idling, the engine was reving up and down on its own... the tach was constantly bouncing from 700-1000, up and down every second or two.
I just shut the car off and didn't do anything with it because I was leaving on a vacation the next day.
I got back home this week and started the car on Wed, a rainy, misty, cool (50 degrees) foggy day. The car started but ran like crap from the start. Little power, I felt like it was going to stall all the time (but it didn't) and this time there was no engine reving.
Yesterday, I start the car again (beautiful, dry, 80 degree day). The car started and ran fine. No issues at all. I went for a 20 minute ride...no issues.
About an hour or 2 later, I started the car up again and it did the reving thing again...the tach was constantly bouncing from 700-1000, up and down every second or two... just like it did 2 weeks ago.
I'm not really mechanically inclined, but I try.
If anyone has any ideas on where I can start diagnosing this problem, please let me know. I'm all ears.
-John
A couple weeks ago I took the car for a ride and it ran fine. I went home, parked it for an hour or two, started it up again and while idling, the engine was reving up and down on its own... the tach was constantly bouncing from 700-1000, up and down every second or two.
I just shut the car off and didn't do anything with it because I was leaving on a vacation the next day.
I got back home this week and started the car on Wed, a rainy, misty, cool (50 degrees) foggy day. The car started but ran like crap from the start. Little power, I felt like it was going to stall all the time (but it didn't) and this time there was no engine reving.
Yesterday, I start the car again (beautiful, dry, 80 degree day). The car started and ran fine. No issues at all. I went for a 20 minute ride...no issues.
About an hour or 2 later, I started the car up again and it did the reving thing again...the tach was constantly bouncing from 700-1000, up and down every second or two... just like it did 2 weeks ago.
I'm not really mechanically inclined, but I try.
If anyone has any ideas on where I can start diagnosing this problem, please let me know. I'm all ears.-John
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Joined: Feb 2002
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From: Cambridge England
LH ECU and ISV are OK, otherwise the rpm wouldn't rev up and down.
Try a battery ground disconnect for a few seconds. In the trunk of the car, just under the bottom edge of the tool tray, big flat strap. Unbolt, wait a few secs, then rebolt.
Other possibilities are igntion circuits, as the car seems to alternate between perfectly OK and bad.
Check when weather is damp or very humid, cold start, in the dark, look for fireworks under the hood.
Check king lead connections at the coil end, they can corrode.
Try a battery ground disconnect for a few seconds. In the trunk of the car, just under the bottom edge of the tool tray, big flat strap. Unbolt, wait a few secs, then rebolt.
Other possibilities are igntion circuits, as the car seems to alternate between perfectly OK and bad.
Check when weather is damp or very humid, cold start, in the dark, look for fireworks under the hood.
Check king lead connections at the coil end, they can corrode.
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From: CT
Hi John. Thanks for the reply. I did disconnect the battery cable. In fact, I took it out completely and cleaned it thoroughly. I also removed the engine ground (underneath the engine on the passenger side) and cleaned that as well.
I'll do the firework test you mention (that sounds scary).
If you don't mind, can you provide more info on the "king lead connections?" I'm not sure what that is or where/how to check it. THanks.
-John
I'll do the firework test you mention (that sounds scary).
If you don't mind, can you provide more info on the "king lead connections?" I'm not sure what that is or where/how to check it. THanks.
-John
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Joined: May 2003
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From: Frankenmuth, Michigan
^^^^
What John said.
I have a rough running issue which ended up being a corroded coil and wire.
Also, check connectors like the crank position sensor (CPS), temp2, WOT/Idle switch (TPS).
This CPS wire lead cause a no start issue for me.
Also check throttle cable adjustment. You problem is intermittent, so it's unlikely. But you TPS may not be switching to idle.
What John said.
I have a rough running issue which ended up being a corroded coil and wire.
Also, check connectors like the crank position sensor (CPS), temp2, WOT/Idle switch (TPS).
This CPS wire lead cause a no start issue for me.
Also check throttle cable adjustment. You problem is intermittent, so it's unlikely. But you TPS may not be switching to idle.
