oldjoe91s15

my 91 2.8 TBI 5 speed manual, has a extremly high idle at cold start up, in the 2,000 rpm range. once up to normal operating temp it idles down to 500 rpm. Once i have made it to my destination and let the truck set for 1 to 8 hours right back to the high idle it goes. I have checked vacume lines but all appear good.
It suggestions on what elese I should be a looking for, and the procedure to rule out the things I will be checking will be appreciated.
My Haynes and Chiltons manual was really no help.
Any and All welcome !

__________________
JOE M.
IOWA
1991 EXT S15 2.8 L 5 SPEED MANUAL
MILAGE AS OF 10-24-07 102 K

blazes9395

this can be a few things. check and make sure you have no vaccuum leaks at the intake or the throttle body gasket. If you snap the throttle at the throttle body is it smooth? Is the throttle body itself clean and allowing the throttle plates to close completely? Do you have any SES codes popping up or in memery?

Oh and welcome to AF!

oldjoe91s15

I have finnally had time to check that i had no leaks at the intake, by spraying carb cleaner around the base of the intake, and around all bolts.

same thing with the throttle body, no leaks.

when i snap the throttle plates open and close smoothly and tight, it is a clean as a mirror inside the bores.

oreilly auto found no codes whatsoever.
no ses codes.
No what should i be looking at ?
thx for the help thus far.

__________________
JOE M.
IOWA
1991 EXT S15 2.8 L 5 SPEED MANUAL
MILAGE AS OF 10-24-07 102 K

oldjoe91s15

Any other ideals, still looking for the cause.
tia

__________________
JOE M.
IOWA
1991 EXT S15 2.8 L 5 SPEED MANUAL
MILAGE AS OF 10-24-07 102 K

blazes9395

Alright, you found no leaks and all looks good with the throttle body.
Try this;

1) Ground the ECM at the diagnostic port(like your checking for codes).
2) Turn the ignition on and leave it, again just like when you are reading it
for codes.
3) While the ECM is displaying the codes , wait 30 seconds (leave ignition
on and let it keep showing codes) and then get under the hood,
disconnect the IAC valve and leave it that way for the moment.
4) Get back in the truck, turn the ignition off and disconnect the ground
from the diagonstic port.
5) Then go ahead and start it. You may have to use the pedal to start it,
but it should start.

What your doing here is disabling the IAC valve and not allowing the ECM to control idle. Its at this point you'll find out whether you have an engine problem, or its electricl/computer related problem. Try this and post back.

Also do this when your engine is cold, and when your done the test, just plug in the IAC valve again and your ECM will regain power to control idle.