Idles high after going from park to gear and back.

[MadRatt]

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

[EVRLET]

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.

[MadRatt]

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.

[ChevyTech]

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.

[EVRLET]

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.

[MadRatt]

Originally Posted by ChevyTech

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.

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

[ChevyTech]

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?

[sntrym95]

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.

[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.

[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.

[Palf70Step]

Glad to hear you got the situation rectified.