Idles high after going from park to gear and back.
 

Hello,
Its been awhile since I've been on here. I figured the experts here are my last hope to solve this issue.

So I have a 1989 c3500 7.4L with a 3sp th400. It has 116k on it. It is set up as a wedge ramp single car hauler I use for personal vehicles. I am having a very unusual problem. It starts in park and idles perfect at 750rpm. If I go into gear it idles down to around 625rpm and then slowly starts rise up to about 750-800rpms. when I shift back to park it will flash up to 1700rpms then slowly over a minute settle at 1200-1300rpms. If I shut it off and restart it. It idles perfect at 750 until shifted into gear again.
I have done these and replaced the following:
1. New IAC and have done the relearn many times while diagnosing and disconnecting the batt.
2. New TPS set to .62
3. rebuilt TBI
4. New base gaskets. (both of them) The BB has the base coolant chamber to help warm up the TBI in cold weather
5. I have done the base idle set procedure. I have set it at 500rpm and it still ends up at 1100-1200 I have also set it to 625rpm and it still settles at 1100-1200 RPMS.
6. Replaced the egr control module. The egr valve tests fine.
7. new fuel filter
8. Cap rotor plugs and ignition module. Wires are MSD less than a year old
9. Capped off IAC and it Idles fine at set base idle. So there is no vacuum leak.
10. new pcv
11. New ecu
I am down to thinking It has to be something in the Prom? Telling it that it should be at that Idle. I don't want to just throw more parts at it at this point. It runs great down the road but at a light it starts to get warmed up because you're fighting that high idle with the brakes. Also it revs up to 1700 when you stop and park and its a bit embarrassing at a car show. Everything I have listed that I have replaced was a neglected part from the previous owner. I've had the truck about 5 mos. So I didn't mind replacing those things and each one made the truck run smoother but never fixed the idle issue. I've several friends that are gm mechanics and they just scratch their heads like its possessed. lol
I have no problem spending the money to make it right just don't need to waste money on nonsense. I have checked everywhere I can think of for vacuum leaks and there is none.
So I'm down to PROM. If the PROM carries the idle info, could it have got corrupted?
Borrowed a scan tool and you see the IAC counts changing and hear the idle change accordingly.
Also the p/n switch shows its working in the scanner.

Any info would be greatly appreciated.
Thanx in advance for any help given.
CJ

Re: Idles high after going from park to gear and back.
 

A PROM issue is likely. Did you check for codes? It doesn't matter if the CEL is on or off. If you get a 51 code, then PROM is suspect. Google your PROM code off the PCM label and see if you can find a replacement...good luck because it took me 4 months to find one.

But try wiggling the PCM connector and see if you have a loose connection. Read my story below about this.

_________________________________
Here's my experience with OBS PROMS:

I worked on a 1995 C1500 and the issue with the truck dealt with idle rpm with and without a load, so very similar to yours. I worked on the truck prior to the rpm issues and I know timing and idle was dead on and all components were new or fairly new.

At idle with no load, rock solid at 750 rpm for about 10-20 seconds. Then it climbed up to about 2200 rpm all by itself. I put it in gear, didn't matter which one, the rpm went down to about 1200 and then stabilized at about 1500. I noticed the CEL was flickering between bright and dim. This was a late model 1995, so it had the 1996-2000 instrument cluster. I placed it back in park and let it idle with no load.

The flickering is what concerned me, so I went to the PCM behind the glove box. I was wiggling and pressing the red connector into the PCM and I noticed the rpm dropped to 750 and the CEL went off. Wiggled it ever so slightly and the rpm jumped up but no CEL. I was able to change the rpm by wiggling the red connector. The blue connector did nothing.

I jumped the OBD 1 connector and there was a 51 code set, which is a PROM error. I figured the loose connection and the voltage fluctuation caused the PROM to freak out and be intermittent. I mean, when I hold the red connector in the right spot, the truck runs great under no load and a load. However, computer chips are sensitive to voltage changes so loose connections are never good for electronics.

I replaced the PCM, which you already know doesn't come with a PROM. I searched high and low for a PROM because they are no longer made and you have to find the exact PROM that's listed on the PCM sticker. Mine was a BJYB and I was lucky to find one. Replaced the PROM and the problem was fixed.

PROM issues are not rare in the TBI trucks. Actually, it was a common error back in the day and I remember you could buy blank PROMS in 5-packs. The dealerships had the programmers to complete the fix. However, as these trucks became older, experiencing PROM issues is frustrating because replacement PROMs are so difficult to find.

Re: Idles high after going from park to gear and back.
 

Thanx for the response. I am down to thinking that is my problem. Today I'm going to replace the temp sender and egr valve just because it is the only 2 things left that I haven't done. If that doesn't fix it. A jet performance chip is the next step. You are right about finding a stock chip. And I'm not willing to gamble on a used one. So I guess if I must it will be the Jet 6 pack that has 6 tunes for towing and performance as well as an economy tune at the push of a button. Just hope I'm not wasting my money and it still has the problem after all that. Just wish it was easier to diagnose the issue more accurately but it is what it is.

Re: Idles high after going from park to gear and back.
 

Is the IAC new?

The IAC is a stepper motor and if it does not move for the pulses sent to it the system knows the idle is not right but it does not know if the IAC actually moved. They can get sticky. The IAC gets short pulses of electricity to move.

If any carburetor cleaner gets into the IAC it can damage it.

Another IAC problem that happens is the terminals in the IAC connector can loosen up so they don't have tension for a tight fit.

Looking at the ECM data would be what I would do. You could watch the desired RPM, the IAC count and the actual RPM reading. Another thing to look at would be the Park/ Neutral switch as you shift.

Re: Idles high after going from park to gear and back.
 

Confirm that the JET is not a piggyback chip. In the 90s, that’s how the JET and hypertech chips were designed. Back then you needed to use both the factory PROM and the JET chip.

Re: Idles high after going from park to gear and back.
 

Thanx for your response. All those answers are in my opening post.

Re: Idles high after going from park to gear and back.
 

Does it have hydra-boost or vacuum assist power brakes?
A torn diaphragm in the vacuum brake booster can move and leak at varying amounts each time the brake pedal is pushed so when you apply the brakes to put it in gear the leak is affected.

Did you take note of the IAC count at hot idle in Park? If so what was it.

Did you ever see the IAC count go to zero?

Re: Idles high after going from park to gear and back.
 

Hydra boost. IAC a little high at 750rpms the counts are 35. Starts at 0 and works its way up. Noticed today on the scanner that when in gear the idle settles at 750 and the iac is at 145 counts. Back to park and the counts go down to around 128 counts and its idling at 1700rpms. Cycle the ignition and idle is back to 750 and 35 counts on iac.

What do you think I should set the base idle to? I've been getting many different answers. Some say 450 others say 125 below target others say 550. (can't find any info probably cause the don't want ya messing with it). not sure what the best base is to achieve a 30 iac count.(this is based off of gm specs for the 454) any suggestions?

Re: Idles high after going from park to gear and back.
 

For the base idle, aka minimum idle speed, which later was commonly called the minimum air rate, I would use 125 RPM below the target. For your big block that will be higher then 450. I don't have specifications for you truck but I would guess it would be around 600 RPM. When you get everything else taken care of I would used the IAC count to adjust the stop screw.

When you restart the truck and get a low count, that is because the ECM clears the count every time you turn it off. When you restart the engine, the ECM tracks every command it has sent to the IAC and that establishes the current IAC count.

Because the IAC count hit zero it looks like the stop screw is adjusted to slow. But the fact that it climbs so much makes me think the IAC is not moving with each pulse sent to the IAC. It is common for the connector that plugs onto the IAC to get terminals that are loose so they make and break connection as the engine shakes. The pulses alternate between the two windings in the IAC so missed pulses are very problematic. I like using extra terminals to test the fit of both sides of the connection to the IAC to make sure there is a tight fit at all the terminals.

If you are sure the IAC is good and you find the connector terminals are in good condition I would be suspicious of the ECM being the problem. Having a 128 IAC count and an idle at 1700 RPM without see the IAC count going down right away, and then cycling the ignition bringing the RPM down makes me think the ECM is the problem. NOTE: when the scan tool is connected to the vehicle it can alter the desired RPM as well as other things. Good scan tools have different modes you can put them in depending on what you are doing.

When you get this sorted out, I would want the IAC count no higher then 20, Idling in park, with a warm engine, and all accessories off. You never want the count to reach zero.

Re: Idles high after going from park to gear and back.
 

The more I re-read your first post, which is well written and covers the topic well (I don't know how I missed so much when I made my first reply) the more curious I am becoming. You even replaced the ECM already. If you can use WinALDL to capture all the data idling slow and idling fast and post the data, and post it, I would surely look it over.

I went looking for specifications for your truck. I don't have a 1989 manual but I do have a copy of the 1988 fuel & emissions manual.

Specifications for a 1988 7.4L with an automatic trans:
The minimum idle speed (stop Screw) is 700 + or – 25 rpm IN DRIVE with a warm engine and all accessories off
The controlled idle speed is 750 in drive – warm engine & all accessories off

The IAC count spec is 5 to 30

Re: Idles high after going from park to gear and back.
 

Did you look and see if the coolant temperature had changed to showing a cold reading when the idle goes higher?

Yes the PROM does hold the desired RPM within it. Could it be the problem? It could.

This one is going to be twisting in my brain all day.

Re: Idles high after going from park to gear and back.
 

I changed the temp sender yesterday just because its one of the last things I hadn't changed. The temp doesn't change from park to gear and or back. Everything looks as it should. I'm going to reset the base idle again and see where my counts end up. I also found a diagnostic tree from a service manual that shows how to test the wiring going to the IAC. All wires tested fine. Also Tested resistance on the old IAC and that was fine. The new IAC never helped this issue but I am going to pull that out and double check its resistance as well. The more I go the more I think the prom is the issue.

I have read several posts on here from awhile back that say these year trucks were prone to PROM issues. Someone even said in the early 2000's he used to go to the junk yards and grab several PROM's (as they were many of these trucks in there back then) and would have to go thru a couple till he found one that didn't have some running issue with it. So it wasn't unusual at the time.

My brains been twisting for months on this as well. Any other good info you can think of would be of great help. Oh and I don't have an ALDL cable as of yet to get better data. Ordered one last night. should be here next week.

Re: Idles high after going from park to gear and back.
 

If you don't mind can I get the info on that cable please? I am wanting one too.

Re: Idles high after going from park to gear and back.
 

I will be watching this thread for any new posts or WinALDL data.

If I come up with a new theory I will post it.

Re: Idles high after going from park to gear and back.
 

I ordered it off ebay. Heres the link. He carries both 12 and 16 pin. This is the 12 pin.

https://www.ebay.com/itm/11486945734...d512%7Ciid%3A1

Re: Idles high after going from park to gear and back.
 

Thanks. And you're going to use it with WinAldl? Let us know how that works out for ya.

Re: Idles high after going from park to gear and back.
 

Going to try and use it with ALDLAndroid. I've read about a couple rv guys using it with their android based tablets not only as a scanner but as a guage pod while driving for a tach, speed and other actual info their rv doesn't have on the dash. I have an old android phone that I think will work for that. Stick it to the windshield and have a tach( my truck didn't come with a tach.)and real time guage data. I'll let ya know how it works.

Re: Idles high after going from park to gear and back.
 

The next time you to look at the data, there is another thing I want you to check which is the system voltage.

There is a correction done which includes the desired RPM if there is a system voltage problem. The service manuals just states the system functions will be modified. The training manuals show some of the changes are the desired RPM, and fuel pulse.

I have never been able to find how much the RPM changes for low voltage reading. My guess is it is a small correction, but I thought I would mention it.

Re: Idles high after going from park to gear and back.
 

For what it's worth, I had a random creeping idle a month or so back. After reading data and watching IAC counts just creep up, changing the IAC, and checking everything else, it turned out to be a very slight leak in a vacuum line from the MAP to the TB. Turns out the line had stiffened up and sometimes would allow a little air in. Swapped it and no more issue.

Re: Idles high after going from park to gear and back.
 

Hey Chevy Tech,
Sorry it has been a minute since I posted an update. Got my bluetooth cable and it works great with ALDLDroid. But it only does basic live read data logs. Which I could send you just don't know if you'll be able to see them if you're not running ALDLDroid. I am trying to figure out how to use it with Tunerpro. Will you be able to look at a tunerPro data file? You asked about system voltage and It seems good. I have also put new injectors and a Jet 6pack prom upgrade in since last posting to no avail. I'm at my witts end here. Not sure where else or what else to try. I am unable to set my base idle in drive to 750rpms and 5-30 counts on the IAC with out needing 2 feet on the brake peddle and then back to park it soars to over 2000rpms even if I cycle the key. I'm starting to think that there is something overloading the engine to make it idle down so far at first shift to drive. Maybe converters to tight? It seems to drive as it should once your rolling. No lugging or over reving or staggered shifting so the converter seems fine but maybe at idle its wrong. With the Idle set in PARK to be 750rpms and 20 IAC counts. IF I unplug the IAC while its shut (0). When I shift to drive it luggs down to 450-500 rpms. Now that didn't seem to bad but when It's supposed to be 750RPMS and the IAC counts under 30 there is no way the IAC is going to only open 10 counts or less to achieve the proper in gear idle. So is my rpm drop from park to gear only supposed to drop maybe 100rpms? if so what's causing the load? And that still doesn't explain why when I go back to park it doesn't correct for the high RPMs. These ecu's are dumber than I'm used to. In my opinion there should be some type of code set with any of these things happening.
Thanx in advance for any Input you may have.

Re: Idles high after going from park to gear and back.
 

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.

Re: Idles high after going from park to gear and back.
 

New ecu and prom. No change. I'm really at my witts end here. I cant see what the ecu is commanding for this but it is supposed to be 750 according to a gm service manual from 89. I can manually manipulate the idle speed in park and make the iac counts go down then up. But if i make them go up first then they won't lower. As if there's more fuel and it's seeing proper air fuel and ignoring the rpm. Not sure what would make that happen. It's very frustrating at this point. What should the map be reading for a big block at idle in park vs. In gear? Is the vacuum for the map ported?

Re: Idles high after going from park to gear and back.
 

On this system the ECM looks at the TPS voltage when the key is turn on and remembers that voltage a zero percent throttle open. If you move the throttle stop screw so the TPS voltage goes higher then the ECM logic says the throttle is open so do not move the IAC. When you back the stop screw out after a engine start the voltage goes lower so the throttle is not detected as being open so the system will run the IAC. Every time you adjust the throttle stop screw the engine has to be turned off and restarted for the system re-determine the throttle position.

If you have a habit of holding down the throttle when you get in to start the truck and the ECM does not see a TPS reading with the gas pedal not being pushed it can cause all kinds of problems including the IAC issues.

The MAP is the same way in that the system looks at the reading when the key is turned on before the engine is cranked and uses that reading for the Barometric pressure reading.

Ignition switch failure is common on these trucks and I wonder if it is breaking connection as you turn the key to start it and losing the base reading from when the key is first turned on. Have you noticed any issues with the ignition switch?

The MAP reading is manifold vacuum, not ported vacuum.

The normal MAP reading varies depending on variables like altitude, engine condition, and weather so there is not a tight specification. The rule of thumb is 1 to 2 volts at idle. It should go above 4 volts with the key on and engine off.

There should be a desired RPM somewhere in the data.

Re: Idles high after going from park to gear and back.
 

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.

Re: Idles high after going from park to gear and back.
 

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.

Re: Idles high after going from park to gear and back.
 

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.

Re: Idles high after going from park to gear and back.
 

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.

Re: Idles high after going from park to gear and back.
 

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?

Re: Idles high after going from park to gear and back.
 

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.

Re: Idles high after going from park to gear and back.
 

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.

Re: Idles high after going from park to gear and back.
 

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.

Re: Idles high after going from park to gear and back.
 

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.

Re: Idles high after going from park to gear and back.
 

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.

Re: Idles high after going from park to gear and back.
 

Glad to hear you got the situation rectified.

Re: Idles high after going from park to gear and back.
 

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.