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.

[MadRatt]

Originally Posted by Just call me Sean

My truck had a grounding issue that altered the oxygen sensor readings. It was supposed to fluctuate back and forth between 0.1 and 0.9 volts but it was going into the negative. The computer thought it was running lean and dumped a bunch of fuel into the engine. It probably affected other systems, but that's how I found it. I had to ground the computer case to the battery, plus the headlights and blower motor, to make it work right.

Thanx for responding.

I have cleaned all the ecu grounds. The o2 is new and seems to be reading right. I might have to redo all the engine to chassis and to body grounds as well. It's acting so irregular that it could very well be grounding issue. Although the ecm is reading correct and steady voltage.

[ChevyTech]

Originally Posted by MadRatt

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.

That is designed in. It is like a dash-pot used on some carburetors. It is to slow the deceleration of RPMs to prevent backfiring in the exhaust.

Does your truck have a tilt column that has gotten loose?

[MadRatt]

Originally Posted by ChevyTech

That is designed in. It is like a dash-pot used on some carburetors. It is to slow the deceleration of RPMs to prevent backfiring in the exhaust.

Does your truck have a tilt column that has gotten loose?

Hey Chevytech.
What should my Map Kpa be at idling in gear? I am looking over my logs and when I idle in park my Map Kpa is 35(seems about right). But when I go to gear the rpms drop to 475 and it goes to 58 and as the idle comes up to achieve 750rpm the map finally reads 68 at 750rpm idling in gear. IACs are at 145 at this point. When I shift back to park the RPMs jump to 1500-1700 and the map drops back to 35 and the IAC doesn't drop. Does this seem normal for Idle in gear? It just seems to me something is loading the engine more than normal when in gear causing it to need to add more fuel to achieve its 750rpm idle in gear. As if I'm power braking it. Still unable to get winaldl to work with my connector or I would send you a log.

[ChevyTech]

35 Kpa would be about 10.3 inches of mercury which is low
58 Kpa would be 17.12 inches which is good
68 Kpa would be 20 inches

I guess it is possible that the torque converter is not free enough causing a drag. The only other thing turning is the transmission pump when it is standing still while in drive.

Something is wrong when the idle is up at 1500-1700 and the IAC count is not dropping.

I would switch between data readings, and writing down the readings, and looking at what all changed when it is running at the high idle speed verses at normal idle. Look at what the ECM data shows for RPM, coolant temp, system voltage, as well as the MAP and IAC count. Make sure the run time keeps going and is not reseting. Make sure it shows that it is being shift into gear and back to park.

If the ECM and PROM are new and good something has be be causing bad data.

WinALDL makes that easier to look at all the readings at once but if you write all the readings down you can get close to the same thing, and start comparing sets of readings. The bad part is it does not sound like what you are using gives as many data readings.

[MadRatt]

Originally Posted by ChevyTech

35 Kpa would be about 10.3 inches of mercury which is low
58 Kpa would be 17.12 inches which is good
68 Kpa would be 20 inches

I guess it is possible that the torque converter is not free enough causing a drag. The only other thing turning is the transmission pump when it is standing still while in drive.

Something is wrong when the idle is up at 1500-1700 and the IAC count is not dropping.

I would switch between data readings, and writing down the readings, and looking at what all changed when it is running at the high idle speed verses at normal idle. Look at what the ECM data shows for RPM, coolant temp, system voltage, as well as the MAP and IAC count. Make sure the run time keeps going and is not reseting. Make sure it shows that it is being shift into gear and back to park.

If the ECM and PROM are new and good something has be be causing bad data.

WinALDL makes that easier to look at all the readings at once but if you write all the readings down you can get close to the same thing, and start comparing sets of readings. The bad part is it does not sound like what you are using gives as many data readings.

I'm also in Florida so my Map Kpa with the key on engine off is 101

Got this Direct hit file from a Automotive Diagnostics Specialist:
He is coming this week to run some tests to confirm but without me telling him what I thought it was this was the 3rd piece of test info he sent was this. Maybe I'm on to something.
See what you think.

[ChevyTech]

It sure is possible.
It sounded like there is an unusual load on it to you.
The low vacuum at idle in gear fits with an unusual load
My thought of transmission torque converter or transmission pump as a cause of extra load.

I think you are onto something. My guess is that your hard work and not giving up has brought you to the answer.

[MadRatt]

Originally Posted by ChevyTech

It sure is possible.
It sounded like there is an unusual load on it to you.
The low vacuum at idle in gear fits with an unusual load
My thought of transmission torque converter or transmission pump as a cause of extra load.

I think you are onto something. My guess is that your hard work and not giving up has brought you to the answer.

UPDATE: RESOLVED!! Just wanted to give an update as to what we found.
I took the truck to a trans specialist that did some testing and concluded that my converter had no flash stall. So I decided to replace the converter with a HD unit and have them do a inspection of the transmission. He stated that he wasn't sure how the truck actually drove as good as it did based on what he found(he actually test drove it himself). The converter was so loose and actually was able to be spun in the wrong direction without effort(only supposed to spin 1 way). Definitely junk. On inspection of the transmissions clutches he found the second gear packs were completely burned up and the steels were fried. The rest were also over heated just not as bad as the seconds. He thinks maybe something to do with converter just couldn't be sure. So he put all new HD clutch's and steels in it and reinstalled the trans and would ya believe it. It drove home effortlessly. I had to reset all the base Idle stuff in gear and it idles in gear now at 750rpm with 18 counts on the iac and back to park it idles at 850rpm at 13 counts on the iac. I think i'm going to drive it a bit and then reset the base 1 more time and we are good. Couldn't believe that this was the problem but there you have it. $1500 later for all the trans stuff and all the $ for all the other stuff I did. At least I know now that EVERYTHING IS NEW. Should be good for awhile now.

Thank you for your help and everyone else who chimed in with their ideas. It's much appreciated.

[Palf70Step]

Glad to hear you got the situation rectified.

[ChevyTech]

Originally Posted by MadRatt

UPDATE: RESOLVED!! Just wanted to give an update as to what we found.
I took the truck to a trans specialist that did some testing and concluded that my converter had no flash stall. So I decided to replace the converter with a HD unit and have them do a inspection of the transmission. He stated that he wasn't sure how the truck actually drove as good as it did based on what he found(he actually test drove it himself). The converter was so loose and actually was able to be spun in the wrong direction without effort(only supposed to spin 1 way). Definitely junk. On inspection of the transmissions clutches he found the second gear packs were completely burned up and the steels were fried. The rest were also over heated just not as bad as the seconds. He thinks maybe something to do with converter just couldn't be sure. So he put all new HD clutch's and steels in it and reinstalled the trans and would ya believe it. It drove home effortlessly. I had to reset all the base Idle stuff in gear and it idles in gear now at 750rpm with 18 counts on the iac and back to park it idles at 850rpm at 13 counts on the iac. I think i'm going to drive it a bit and then reset the base 1 more time and we are good. Couldn't believe that this was the problem but there you have it. $1500 later for all the trans stuff and all the $ for all the other stuff I did. At least I know now that EVERYTHING IS NEW. Should be good for awhile now.

Thank you for your help and everyone else who chimed in with their ideas. It's much appreciated.

Glad to hear it is fixed, and thank you for the update. I wish I could have been more helpful at getting to the cause more quickly. I will remember this thread for a long time.

It is good to see the IAC counts down where they are.