Well, I'm posting for several reasons.

First of which is to formally introduce myself. I've been a member for almost 2 years now and I just realized that I have yet to do just that. My name is Uriah. (It's Hebrew if you are wondering.) I have owned my Grand Prix white 1990 964 Targa Tiptronic for 3 years this March and she has been a joy to drive. I still can't stop staring at her curvy butt.

Now that I've done that I have to say that I'm a bit frustrated at the moment. Recently, I have been having idling problems. While I was else where on vacation I had my girlfriend take her in to get an oil change. That is when the problems started. I came back and it was idleing rough, searching I think is the term, and about 200 rpms lower than normal. I am a fairly experienced DIYer so I took a look at it myself starting with the oil system. The dipstick was not on tight which I thought would cure my problems when seated correctly. It was also overfull and the oil had gotten into the intake system.

Now I set about to correct the overfill. I took out some of the oil with a long suction thingy ( sorry cant for the life of me remember what it is called) and then took off the intake manifolds, ISV, and airflow sensor to clean them. It was quite messy.

I had thought that when I put everything back together the idle would be stabilized. Instead she wouldnt stay running unless I kept my foot on the pedal. Immediately after I let off she dies. I was thinking that maybe it was a leak at the hoses connected to the ISV so I tightened those. Still no luck. I also tried replacing the DME relay to no effect. And a battery disconnect...

I then started poking around and found that the distributor caps were worn much differently than each other. #1 or bottom dist. was much more worn than #2 or top dist. #2 looked practically brand new while #1 had a lot of sooty deposits.

I was wondering what could be causing this and also what your thoughts are on the idle issue.

Thanks in advance

P.S. I would like to post some pictures of the dist. caps but im not sure how to do that from a desktop folder. If someone could explain it to me I will clarify what I mean by sooty.

 

Oh I should probably mention that I have searched the archives for an answer but there are just too many things that can affect the idle for me (a lowly DIYer) to narrow it down.

 

How did you clean the ISV? Did you just spray it out with some carb cleaner or try to insert something in it to clean it out. I remember a tread where someone did that and scratched the inside of the ISV and it quit working.

It sounds like the overfill was the cause and probably something with the intake is the problem (vacuum leak, ISV, etc). I wouldn't think it would be in the Ignition system.

 

To post pictures click the Manage Attachments button at the bottom of the start thread or reply screen.

This will open a pop up box (make sure pop up blocker is off) where you can browse and then upload photos from your desktop.

Cannot help you with the idle problem.

 

I used some carb cleaner and sprayed the heck out of it. I didnt stick anything in there but I did allow the cleaner to go to the bottom of the ISV and swished it around down there. What I am assuming is oil came out with the cleaner when it was tipped over again. I'm not sure if it can rust in there. I would hope not, that would mean that I have a leak somewhere in those lines that is allowing moisture in.

I agree. I've since checked the vacuum lines and couldn't find any cracks or any visible cause for a vacuum leak.

 

Hi, Uriah.
New caps and rotors are never a bad recourse for a first time troubleshooting mission. I don't know that that is your problem (and through the blur I don't see anything obviously awful with your caps...) but a caps & rotors change never hurt anyone. It's a simple DIY and parts will come to less than $150, I believe. Break open that piggy bank.

I, too, had a rough idle problem for the first year I owned my car.
Sometimes I would roll to a stoplight, the idle would be rough, I'd tap the accelerator, and it would steady out to a nice, smooth idle. It finally began to become more frequent, and while chasing the fix to something else --naturally! -- I ran across what the problem was to this...

It traced to the throttle cable being adjusted too tight. (Released, it wasn't allowing the throttle body workings to engage the Idle Microswitch. -- DOH!)

You adjust the throttle cable via it's tension adjustment under the car, near the front of the transmission housing. (Do a search, around here, and you'll find threads & pictures on Idle microswitches and throttle cable adjustment.)

Normally, you can check whether your Idle and your Wide Open Throttle microswitches are working by listening for them.
(Open the hood, and with the engine off work the throttle body manually. At both ends of the it's motion you should hear those two microswitches alternately go, 'click'. I wouldn't be surprised if you don't hear a 'click' -- as you should -- when your throttle body is slowly released to its throttle-off position.)

It's great you're trying to tackle this yourself. Sounds like you've got the right step-by-step temperament to squash this problem, but good!
Hang in there, and keep posting your progress.

And, Brother! -- step back a foot with your camera from the subject. Your pictures remind me of being in the garage without my glasses!

 

In Tiptronic model is only throttle position potentiometer, no idle and WOT microswiches. Maybe can´t TPS (dirty or broken) send idle position to DME.

 

Great catch, rollingjack!
(Sorry, Uriah.)

HEY, RUBE!
You Tiptronic-guys: Your brother, Uriah, above, is in a spot of idling trouble.
Some help, please.

 

Ok, so I've checked the idle microswitches and I didn't hear a click at WOT. The off-throttle position is a very audible click. I'm not sure this is the problem though. I have checked the throttle cable before and don't remember ever having heard a click at WOT.

I wasn't actually refering to a problem with the caps but rather the distinct difference in deposits inside the caps. You can see that the top picture from dist #2 (top dist) has deposits like its been used for 10000 miles while the #1 cap in the bottom picture looks practically brand new. Very shiny in other words. I dont think the problem is with the caps nor that it is related to my idle problems but I am wondering what could cause such an obvious difference in wear???? Or is this normal?? It's in my curious nature to question this.

I did do the caps and rotors myself about 3-4k miles ago.

 

Ha ha. Well there goes that idea... THANKS LONNIE!!

What exactly is this TPS and where can I find it?

 

Could you have a bad coil to upper distributor and be running on six spark plugs?

 

I had thought of that but im not sure how to test it right now without starting the car.

 

TPS is Throttle Position sensor, mounted by throttle housing, connected with throttle flap axle. Maybe in right side of throttle housing. OK I´m not 100% sure, my car is with manual transmission.

 

Isn't the bottom dizzy that one that runs off the belt of the top dizzy? If the bottom one isn't worn as much, then I would check the belt first thing - or just click the starter with the cap off to make sure the rotor is spinning.

 

All that tracking on the 2nd cap tells me that the rotor is no good .
That the spark was jumping from the post in the middle of the cap to the posts around the cap .
If you look you will see tracks running from the center outward .

I have in my collection a rotor that I pulled out of a rich guys gelanderwagen ,
that has a deep crater , deep enough to stick your pinky finger into .
The spark had been eating away at it for a long time .