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High Idle Stays Around 2500.

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BumpOnHood

Probationary Member
8
0
Jan 29, 2012
Melrose Park, Illinois
Hello, I just recently bought a 95 gst and i had to get it towed to my house because the intake piping was not assembled. The first couple of things that we did to it were minor things we replaced the spark plugs and changed the oil. We then got the stock intake piping and hooked it up, and started the car. from a cold start it goes to 2000 and after five seconds or so it drops close to 1000 and stays there for 15 seconds then it just climbs quickly back up to 2500. The idle is stays around 2500-2700 even when its at running temp. We got rid of all the boost leaks, and for the most part its driveable but it moves by itself throught 1st gear, it's a manual. Once i take the car out of gear it stays at 2500-2700. Weve replaced the throttle body gaskets, and cleaned the MAF sensor. I also removed the throttle cable to make sure that the throttle cable was'nt the problem and it still idled at 2500-2700. We have run out of other ideas for the idle problem and i wanted to ask, before i take it to a shop. Thanks.
 
Usually high idle is an indication of a boost leak. If you are sure that you have no boost leak then check to make sure that the throttle body cable isn't to tight or the stop screw isn't holding the throttle plate open to far. There is a possibility that the cold start valve on the bottom of throttle body is not functioning correctly, its the valve that has water lines going to it. It stays open till the water warms up and then it closes dropping the engine rpm to normal idle. Also check to make sure that the BISS screw is not turned out to far, though its not likely that the BISS screw will cause such high idle.
 
Okay, I'll check the throttle screw stopper. I've check the throttle cable and its fine, I've also deleted the FIAV . and the biss screw is fine :( but ill check the screw that stops the throttle right now.
 
Okay i check the stopper screw and it wasn't that.

Nope i do not hear any sounds coming from intake manifold :(

I have not tested the ISC i will do i that as soon as i can thanks.

i just want to put out a bit more info. When i start the car it stays at a decent 1k idle but after like 15 seconds it shoots up.
 
If the ISC turns out to be good check your EGR valve, if you still have it of course. Take the vacuum line off of it and see if that changes anything, if it doesn't I would take it off and check the valve itself to make sure its not stuck open.
 
2500 is too high. You're probably thinking, "No kidding Sherlock" but think about it; if the ECU knew the throttle was closed, and that the RPM was that high, it would cut fuel, and it would be surging. There's a huge clue for you right there. Check idle switch, and my favorite, tach signal to the ECU.

Read this: http://www.dsm.org/how-tos/tttt/tttt3.html it should help you get inside the ECUs head, so to speak, about how it manages idle.
 
Okay thanks a lot guys, i'll let you guys know if i was a able to fix the problem soon.:hellyeah:

Well so far i replaced the ISC motor and it still does the same thing. :banghead:
 
Then you might have a problem with what Colt4g63 pointed out. Check your TPS to make sure that its not advanced to much. Loosen up the screws and turn it counter clockwise all the way, this should put your TPS at 0% when throttle is closed and engage closed throttle switch. Also make sure that your TPS has 4 prongs and not 3, 1g cars use 3 prong TPS because the closed throttle switch is part of the stopper screw, maybe the previous owner put a wrong TPS on it.
 
I would think if you changed the ISC, and its still not even surging and just stays at high rpm, it might be time to check wires/pull the ecu and check for burns/seafood smell. When a cap or driver burns it usually has an obvious old seafood type smell.
 
Yeah there's obviously more going on there. The ECU would cut fuel if it were something as simple as a vacuum leak. It would never just rev that high and stay there.

CHECK THE TPS FOR IDLE SWITCH OPERATION, CHECK THE ECU. THIS IS YOUR FREE PROFESSIONAL HELP HERE.
 
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