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86 high idle

F

fatboyreyn

Guest
My 56 coupe idles around 900 or 1000 rpm, even after engine is warm. I know idle is controlled by ECM, but is there anything to do to lower it? No engine codes, but I just found out it does have a hyperchip.
 

jsinga

Well-known member
Joined
Nov 23, 2001
Messages
280
Location
Savannah, GA, USA
Corvette
87 Silver Coupe Way over 200k miles
clean the throttle body and IAC pentel. You may have a TPS adjustment problem or a minimum air screw out of adjustment.
A manual will tell you how to set the adjustments or I think you can do a search and find info.
reach up and push the throttle closed and see if it will slow down. If it does I would look at the IAC. It looks like a solinoid. If the end is crudded up air can bypass and speed up the idle. Or it could just be toast. It's a stepper motor the ECM commands in and out.

JS
 
F

fatboyreyn

Guest
clean the throttle body and IAC pentel. You may have a TPS adjustment problem or a minimum air screw out of adjustment.
A manual will tell you how to set the adjustments or I think you can do a search and find info.
reach up and push the throttle closed and see if it will slow down. If it does I would look at the IAC. It looks like a solinoid. If the end is crudded up air can bypass and speed up the idle. Or it could just be toast. It's a stepper motor the ECM commands in and out.

JS
I cleaned TB, adjusted TPS by voltage, according to manual. It seems to idle fine at startup, but gets increasingly faster as motor gets hot. I'm thinging coolant temp signal to ECM? what do you think? Also no codes.
 

gmjunkie

Well-known member
Joined
Nov 11, 2004
Messages
4,547
Location
Old Fort NC/Vero Beach FL
Corvette
03 Z-51,78 L82 Coupe
I cleaned TB, adjusted TPS by voltage, according to manual. It seems to idle fine at startup, but gets increasingly faster as motor gets hot. I'm thinging coolant temp signal to ECM? what do you think? Also no codes.
Thats very possible!!!! $12-$18 and you will know for sure!!:upthumbs
 

jan-erik

Member
Joined
Mar 23, 2004
Messages
12
Location
Norway
Corvette
1985,4+3,Z-51
86 high idle.

HI
I have an 85, but basically they are the same.
This may sound as a vacuum leakage some place. Check all the hoses and black thin brittle plastic tubes also they underneath the plenum. Check also the PCV valve on the valve cover.
When that is done spray some startgas or propane over the areas where the runners enter the intake and also over the plenum. If you experience an rpm increase there is a vcuum leakage in the area. Vacuum leakage always increase the rpm on idle or it can give a hunting idle
Hope this helps
Jan-Erik
 
F

fatboyreyn

Guest
HI
I have an 85, but basically they are the same.
This may sound as a vacuum leakage some place. Check all the hoses and black thin brittle plastic tubes also they underneath the plenum. Check also the PCV valve on the valve cover.
When that is done spray some startgas or propane over the areas where the runners enter the intake and also over the plenum. If you experience an rpm increase there is a vcuum leakage in the area. Vacuum leakage always increase the rpm on idle or it can give a hunting idle
Hope this helps
Jan-Erik

I agree a vaccum leak effects idle, but idle is perfect untill engine reaches operating temp. I am going to take off intake and check the vaccum lines.
thanks
 

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