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Username Post: 95 350 high idle check engine light        (Topic#95451)
Posts: 76

Loc: Twin Cities, MN USA
Reg: 01-11-03
03-24-05 08:25 PM - Post#657820    

I've got a 95 Chev 350 TBI. My check engine light comes on when sitting at stoplights. I was told my a mechanic that checked it with the code reader that it was a low idle sensor, something I could change myself. I blew it off as it wasn't affecting the performance any at the time. (a few months ago)

Now, just recently, my truck idles high (1300 RPM) while in park or neutral, when the truck is fully warm. My check engine light is still coming on too. I'm thinking it's that sensor really going south now.

Any idea on where this sensor is and how difficult it would be to change?


'02 Chev K1500 Z71 5.3L 50k miles
'97 First C5 Corvette ever produced 31k miles
'93 GMC K1500 Z71 5.7L 241K miles

Super Senior Member
Posts: 6501

Reg: 07-03-01
Re: 95 350 high idle check engine light
03-24-05 09:40 PM - Post#657821    
    In response to kenwood

Possibly the Idle Air Control (IAC) stuck open. Located on the throttle body, right side (passenger side) with a 2 or 3 wire plug. Has approx. 1" hex to unscrew it from the body.

I usually remove it, start the engine, plug the inner port with my finger. If it idles down, you know the IAC wasn't closing.

Doesn't necessarily mean the IAC is faulty, but it's a good bet. Could also be the wiring, a temp sensor, etc. causing the IAC to stay open.

Vacuum leaks can also give you a high idle.

Posts: 103

Loc: South Carolina
Reg: 03-17-05
Re: 95 350 high idle check engine light
03-25-05 04:24 AM - Post#657822    
    In response to kenwood


... that it was a low idle sensor, something I could change myself.

I have a 94 Suburban that used to be a stock TBI system. I did not have a "low idle sensor", so I would not expect that the 95 TBI system does either. High and low idle would be sensed within the PCM, using inputs from other sensors (i.e. MAP, TPS and RPM).

First thing to check for is that the throttle blades are fully closing to the set screw. The shaft may be binding and a good dose of carb cleaner will help free it up.

I inadvertently set off a "high idle" code when I shimmed the idle set screw. I hated the 450 RPM idle and wanted to raise it. Since the idle set screw is set with a plug from the factory, I installed a sheet metal shim between the screw tip and the throttle return surface. This raised the idle about 100 RPM, but set off the code. My point here is that the sheet metal was maybe 1/32" thick. If your throttle shaft is dirty and binding even a little, it will not take much to set off the code.

If that is not it, then you need to move on to the IAC and possibly TPS. But the TPS would throw its own code, so I would be more inclined to suspect the IAC. A new IAC for that system is not very expensive (less that $50). After a while, carbon builds up on the pintle head and spring. It very well could be it is not closing when commanded. After all, the IAC is nothing more than a controlled vacuum leak.

Check for binding, the IAC and if that does not solve the problem, you may have another vacuum leak elsewhere. Be sure to check all your hoses. The hose between the rear of the throttle body and the MAP sensor has a section of hard plastic tube. Mine cracked and you could not readily see it.

Good luck troubleshooting this problem, Ed.
My 1981 Camaro Z28 TPI Conversion - 1994 Suburban

Posts: 76

Loc: Twin Cities, MN USA
Reg: 01-11-03
Re: 95 350 high idle check engine light
04-04-05 04:45 PM - Post#657823    
    In response to kenwood

After a week or two of changing various things, including the idle control motor, it came down to a faulty throttle body / base plate gasket.

A buddy of mine who is a mechanic sprayed carb cleaner along side of where the throttle body meets the intake. As he sprayed, idle went down.

Spent a few hours of head scratching and almost $100 to fix a $12 seal.

Back to normal on idle - no codes. Thx for input guys.

'02 Chev K1500 Z71 5.3L 50k miles
'97 First C5 Corvette ever produced 31k miles
'93 GMC K1500 Z71 5.7L 241K miles

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