'95 LT1 Corvette - High Idle
Thread Starter
Intermediate
Joined: Jun 2010
Posts: 47
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From: Yorktown VA
'95 LT1 Corvette - High Idle
Odd week with this car. Started off week with car running really bad. Problem went away on its on - believe it was bad gas (probably water in fuel) that was resolved when new gas mixed in.
Shortly after, I find my idle now is running about 1600-1700 RPM - for no noticeable reason, as I didn't do anything to the car. Drove car for an hour this morning, and start to finish, same high idle. As an old 'carb' guy, I would have hunted for things like vacuum leaks, throttle plate blockage, etc - and just cranked down my idle screw until I found the problem. With this engine, don't know what the likely cause would be or any way to manually bring the idle down until I find problem. Nothing special about car - LT1, 6spd, with 190K miles but has been very reliable for a high mileage engine since I bought it a year ago.
Any ideas??
Shortly after, I find my idle now is running about 1600-1700 RPM - for no noticeable reason, as I didn't do anything to the car. Drove car for an hour this morning, and start to finish, same high idle. As an old 'carb' guy, I would have hunted for things like vacuum leaks, throttle plate blockage, etc - and just cranked down my idle screw until I found the problem. With this engine, don't know what the likely cause would be or any way to manually bring the idle down until I find problem. Nothing special about car - LT1, 6spd, with 190K miles but has been very reliable for a high mileage engine since I bought it a year ago.
Any ideas??
Le Mans Master
Joined: Jan 2006
Posts: 5,621
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From: Orlando FL
It's almost a sure bet the idle speed control motor (IAC, idle air control) is probably bad. You can take it out and clean it, make sure the passages (plunger and seat) are clean. If idle changes or if it works better, it's probably defective.
These things go bad quite frequently on all cars and found from personal experience if it seems to be bad or is definitely bad, just replace it. Usually there are no tests that will show any signs of a problem with it.
It's located toward the front right side of the plenum near the TPS (throttle position sensor (throttle plate)).
These things go bad quite frequently on all cars and found from personal experience if it seems to be bad or is definitely bad, just replace it. Usually there are no tests that will show any signs of a problem with it.
It's located toward the front right side of the plenum near the TPS (throttle position sensor (throttle plate)).
Thread Starter
Intermediate
Joined: Jun 2010
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From: Yorktown VA
PColt94 - Thanks for the info. Found link on forum with info on how to remove/install. Think it is just "idle air control valve" from a parts standpoint that I need. Can I use Berryman's to clean??
Melting Slicks
Joined: Mar 2008
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From: Delta B.C. Canada
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After you've carefully cleaned the AIC, make sure you follow the proper reset proceedure. Search for threads on that.
Also, try to clean the orifice in the throttle body as well.
Also, try to clean the orifice in the throttle body as well.
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
It is really a stepper motor moving in increments as commanded by the PCM (with feedback) to screw the plunger in or out toward the seat to control amount of air flow.
Thread Starter
Intermediate
Joined: Jun 2010
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From: Yorktown VA
Not having much luck here - finding it let alone fixing -and my FSM1 is not much help.
Am on the right side of the engine, on the side of the throttle body. Electrical connection removed, but what I see is an orange rubber gasket about an 1/8 in thick, about 1 in in diameter (?). Remove it and I see some type of flat rod that is obviously actuated by the electrical connection I just removed. Can't figure out how to remove it so better re-think.
Then I notice that directly under it is another electrical connection, very similar in looks. Then I'm wondering - do I even have the right device, as my FSM doesn't show anything like this.
Thought I would look at my 89 C4 for help - but it is totally different - except that it has the same two type of devices, one under the other - but the upper one is different in shape than the one on my 95 that I just took off.
Have not found a good post with pics so if anyone has a link or can help out, it would be appreciated. Idle will still stay at 1500-1700 usually, but of course today, it starts out at about 1200 - not great but better.
Am on the right side of the engine, on the side of the throttle body. Electrical connection removed, but what I see is an orange rubber gasket about an 1/8 in thick, about 1 in in diameter (?). Remove it and I see some type of flat rod that is obviously actuated by the electrical connection I just removed. Can't figure out how to remove it so better re-think.
Then I notice that directly under it is another electrical connection, very similar in looks. Then I'm wondering - do I even have the right device, as my FSM doesn't show anything like this.
Thought I would look at my 89 C4 for help - but it is totally different - except that it has the same two type of devices, one under the other - but the upper one is different in shape than the one on my 95 that I just took off.
Have not found a good post with pics so if anyone has a link or can help out, it would be appreciated. Idle will still stay at 1500-1700 usually, but of course today, it starts out at about 1200 - not great but better.
Thread Starter
Intermediate
Joined: Jun 2010
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From: Yorktown VA
Well, thoroughly cleaned throttle body, IAC and all associated parts, replaced gaskets, etc, and re-installed. Problem still exists - might even be idling a bit higher. Assume that the next step is to buy a new IAC and hope that was the problem. Any other suggestions? TPS not usually part of the problem? 
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Check for vacuum leak. Remove the fuel rail covers. While it is at idle disco each vacuum line one at a time and plug the hole with your finger/thumb. Do the PCV U shaped hose both ends at once.
Make sure the throttle is closing. If you disconnect the throttle cable, can you close the throttle more? Does the butterfly valve move smoothly? Are it's axel's holes nice and round or oval?
Obviously the engine is getting the air from somewhere. You'll have to find out where.
Make sure the throttle is closing. If you disconnect the throttle cable, can you close the throttle more? Does the butterfly valve move smoothly? Are it's axel's holes nice and round or oval?
Obviously the engine is getting the air from somewhere. You'll have to find out where.
Thread Starter
Intermediate
Joined: Jun 2010
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From: Yorktown VA
Replaced IAC and TPS. Idle is down from a weekend high of almost 2000 now to about 1200-1300 - still pretty high. TPS (from Chevrolet) had instructions on how to set (Tech 1, etc), none of which I had so just installed and hoped for best. Will keep looking for possible vacuum leaks but am looking for other suggestions. Do I need to follow some procedure to set the TPS when new? Should I invest in some type of tech tool/scanner to better test out these type of fixes (and future tuning, etc)? If so, what are your suggestions?
Drifting
Joined: Sep 2003
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From: 1994 LT1 Coupe 6-speed with FX3 & 2000 LS1 Vert 6-Speed with F45 Hunterdon County, NJ
FSM Book 2 - 6E3-C2
See FSM book 2, section 6e3-C2-16 (+/-).
Likely the procedure for the 95 is the same as for my 94 -- which is:
5. Reset IAC valve pintle position (as follows):
a. depress accelearator pedal slightly.
b. start and release accelerator pedal, run engine for five seconds.
c. turn ignition 'OFF' for ten seconds.
d. Restart engine and check for proper idle operation.
Thread Starter
Intermediate
Joined: Jun 2010
Posts: 47
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From: Yorktown VA
I have the FSM - but only Vol 1, so I miss alot of these type tips. Have driven the car for a day, and idle is still around 1500. Will run through the listed procedure, just in case. Question about vacuum leaks - if I had a leak that was causing the high idle, would I not notice any performance peculiarlities (which I haven't)?
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