i have a 94 z28 auto trans 66000 miles

When the vehicle idles when hot or cold the revs climb from 650 rpm to 2500 rpm and then settles at 650rpm but climbs to 1500 rpm almost as if you were playing with the gas pedal. With the autoxray scanner no error codes appear all sensors operate within the prescribed parameters .

Stepper motor increases between 12 steps and 65 / 70steps

Is this car haunted

 

May want to check and or clean the TPS (throttle Positions sensor) located on the TB. Mine had the same problem and after I cleaned it it works so much better

 

Also a common symptom of a failing opti. I'd check the idle air control actuator first to make sure it's not gummed up.

 

I took a TPS off another lt1 and she did the same eratic idling.
it is an electrical part to what extent is it cleanable and with what can one clean it?

 

I have removed the stepper motor (iac) cleaned the plunger and the spring. I am a bit hesitant to go crazy and spray carburator cleaner in to it as I dont know if the electrics can get damaged.
After cleaning the IAC the scanner indicated a reading of 0 steps and climbed as the rpm climbed . What puzzles me is the learned position is 23 steps but this car idles at 0 steps to 10 and wolud idle there for a while and sudenly climbs to 70 steps as the rpm increases.

What commands the stepper motor to climb with no load added the the engine as I have noticed that the steps increase as load on the engine is increased eg. aircon

 

Ooooooooooo. . . . . Shoebox!!!!!!

 

i have replaced the iac and thought that the gremlins
would leave .

but when she reached operating temp the iac showed les than 10 step and occasionally climbed to 30 + steps when the gremlins decided to rev the car

any idea as to where one can start lookin for the vacuum leaks

 

did vacuum check on the intake manifold and all pipes leading to it .
the various pipes showed no sign of leakage.
when i checked the vacuum on the intake manifold while idling she registered 20 inches which i believe to be the norm but she would suddenly jump to 23 inches and the revs would climb to above 1000 rpm and the iac would be on 7 steps and climb to 30 + steps.
the rpm would settle back at 650 rpm the vacuum to 20 and the iac to 7steps. this would be very intermittent.

does this call for an intake gasket replacement.??

 

Did you actually use a vacuum tester (ex. Mityvac) to check all the hoses? If you think it might be an intake gasket leak, just for grins you could see if any all the intake bolts were tight or at least spray some tb cleaner around to see if you can pinpoint any leaks. Your IAC counts being that low makes me think it is getting some air from somewhere else.

 

hi shoebox
used mity vac on all hoses and sprayed wd40 around intake ports but cant see if if any leaks round the intake ports
have not checked bolts yet.
do u think this could be the problem

thanx for the reply

 

It's possible the intake could still be a problem if it was leaking at the runners on the lifter valley side. Injector o-rings could be another potential spot. I would only pull the intake if I was confident I had ruled out anything else.

 

hi shoebox

tightened 8 bolts either side of the injector ports and this solved the reving high and low problem. thankx

i now find driving at a steady 75mph or 90 mph the vehicle surges . could this still be air related.

at idle there is also an intermitent miss could this be air as well or more likely opti. the coil was replaced recently.

if i do rip the manifold off what would you suggest i replace while the manifold is off.

once again thanx you have been most helpfull and my z28 is smiling .