Idles high after going from park to gear and back.

[MadRatt]

Originally Posted by ChevyTech

Yes the ECU aka ECM used in these trucks is very limited in setting codes, and these early years are slow systems.

I don't have a way to look at those files. People in the past have found a way to convert the data to excel files that I could look at. There were sites they could post the data at and post a link.

As you work at this, keep in mind that some modes of looking at data cause the system to alter the desired RPM. This altered RPM is not as drastic as your truck is.

If the ECM data has the RPM higher then the desired RPM, and the TPS data shows the throttle is closed, then the IAC should be closing to bring the idle down, which the readings from an earlier post showed it was not doing that. Double and triple check that the desired RPM, actual RPM, and the TPS reading. Take a look at the temp reading several time to make sure it is not jumping around. I have run across a few of these early ECMs that the coolant temp data was not near correct at times.

There is also a deceleration feature that when the MAP vacuum is high the IAC will not close as fast as it would otherwise to help prevent back firing in the exhaust. It just slows the rate of change.

Even if there is a mechanical drag the system still should not have a high IAC count with a rpm above the desired RPM with the throttle position closed. The system should be trying to slow the RPM with the IAC.

If all data looks believable at the same time the problem occurs, and the RPM is staying above the desired RPM with the throttle showing it is closed, I can't think of any thing else but the problem is within the ECM aka ECU.

Originally Posted by ChevyTech

The more it think about this the more I am suspicious of the ignition switch breaking connection.

See if you can find a data reading for percent throttle open.

Just so I'm being clear I'm not adjusting the idle with the iac connected. I was just doing that to see what responses I would get from the iac. Not having any ignition issues that I notice.

Also thought this was weird but why when I rev the rpms up why does the iac counts go up? I thought the iac was only for idle. The higher I rev the engine the more open the iac goes. Shouldn't it be closing the more the butterflies open? I'm going to actually drive it tomorrow and see if it's also doing it while driving.