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General IDLE - ECU possible culprit

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barza21

15+ Year Contributor
106
31
Oct 11, 2006
Fairbanks, Alaska
Hello,

The car in question is a 1991 Eagle talon TSI AWD (6bolt). I just rebuilt this engine about 100 miles ago. Here is the issue that I am having: The car won't idle correctly. It surges. I have done some research and have gotten some interesting results back from the testing that I completed. I have narrowed it down to being an electrical problem (or so I think). I have taken care of the boost leaks. The TPS is adjusted properly and so is the idle switch. The TB is properly grounded. I did a basic test of the IAC. I took it out of the IM and had somebody turn the ignition on. When I did this the IAC simply rattled. It didn't move smoothly how it's supposed to. I bought a new one, connected it to the harness and the same results occurred. I swapped the original ECU with a spare one that I had, and it still does the same thing. I checked the wiring from the ECU to the IAC connector, they are ok (good continuity). I checked the power wires to the IAC connector, they are ok(good continuity). I checked for voltage at the IAC connector (pins 2 & 5) , 12v were present there). What are the odds of another bad IAC? The resistance of the coils are way within specs (39.2-39.6 ohms on all coils). I have opened the ECU and inspected for burned ISC drivers and I cannot see any sign of a burn or damage that has occurred. Are there any other suggestions as to why the IAC is not testing properly when I hold it in my hand and it connected to the harness? When the car is on and idling, when I disconnect the idle switch, it does nothing for the idle. It doesn't stabilize it. It keeps surging. I checked the switch and it checked ok. Any other suggestions as to what to check? What am I missing?

Thank you for your time.

Oscar
 
Thank you for the replies. There are no boost leaks. The last one was the tb shaft seals and I took care of that.
Thank you fro the link. I will take a look at it.

In reference to the ISC, I did swap a new one in and got the same results. From what I have read the new ISCs have a higher resistance (39 ohms).

So, after posting this I went outside and fired her up. The idle hunted at the beginning, it warmed up, then it came down and stayed at about 1700rpm, drove it around, after it fully warmed up the idle would settle at about 1200rpm (it would hunt for a bit and then it would settle). When I go home and parked, the rpm would not drop below 2000rpm. I started shaking all the wires and sensors that are related to the idle. I finally got it to come down when I moved the throttle lever (towards the close position). Which brings me to the following question: what is the ECUs priority when it comes to idle. Does it listen to the TPS and then the idle switch or is it the other way around?

Thanks for the help.


Oscar
 
The idle switch should be the last thing pushed..... and without digging into the assembly code for the ecu it'd be hard to say which is considered first.

Is your ignition timing advance set properly? Is the TPS set to 14% (or whatever it is) at rest? Is there a little tiny bit of slack in the throttle cable?

I had a throttle that would stick open slightly because i had the cable a bit too tight. When the car was cold it would idle ok, but once the engine bay heated up it caused the throttle to do something else and it's stick a higher idle than normal. Giving some slack to the throttle cable fixed the issue.
 
I agree with checking the throttle cable and making sure it isn't too tight at the intake manifold. Have you considered leaks at the FIAV, too, or do you think your boost leak test would have caught that?
 
I know this is bringing a thread back from the dead, but I thought I should share with the community what fixed my issue (Just for future reference). I ended bypassing the fast idle air valve in the throttle body. I did the free mod for the FIAV. Knocked out the freeze plug, screwed in the valve, put some rtv around the plug and punched it back in. After doing that, the car now idles good. I adjusted the idle per vfaq.com.

I hope that this helps somebody in their future endeavors.

Oscar
 
Nice, sounds like a worn out bi-metal spring in the FIAV was the issue then. This is a common one and why a lot of people end up blocking theirs off. I still wish there were some easy way to screw in that part that turns under the freeze plug. Sometimes if it's junked up w/ intake crap it is very difficult if not impossible to turn.
 
I did. There was a couple of things wrong. The first one was that the idle switch wire connector was hanging on by a couple of strands of wire. That combined with some corrosion could've been causing part of the problem. The other part was that the throttle cable was not adjusted correctly. Now, here's what screwed me. When the car was turned off and cold, I would adjust the cable. I would check the tension and it felt like it had a bit of slack. For some reason when in would go out for a drive and let the engine warm up, the cable tension increased. I discovered this on one of my test drives. I pulled off to the side of the road, and for giggles I decided to check the throttle cable to make sure it was ok. It wasn't. It was tight as hell and holding the throttle partially open. No wonder the steper motor couldn't ever find a happy spot. After I fixed that it got better, but the final fix was the FIAV.
 
...Now, here's what screwed me. When the car was turned off and cold, I would adjust the cable. I would check the tension and it felt like it had a bit of slack. For some reason when in would go out for a drive and let the engine warm up, the cable tension increased...

I discovered the same thing - opposite of what I thought would happen. I would expect the heat to loosen things up and not tighten things up. I would definitely recommend to everyone that the throttle cable be adjusted on a warm/hot engine, not cold. For reference, the vehicle has the non-cruise control cable - not sure if that make any difference.
 
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