Intermittent IAC issue 89 V1500

[jhe4500]

Hey guys, been chasing gremlins on my 89 Suburban since I replaced a blown motor. Replaced with lower mileage '87 350 from a 1-ton GMC. Kept all my accessories/intake manifold/TBI unit. The truck has been going through IACVs like they're going out of style. I've tried the idle relearn/reset several times without any luck. Recently, I've discovered that when I have a high idle issue, I can eliminate it by running the truck up to around 65 on the speedo (75ish ground speed, on 35s). Inevitably it always comes back in one way or another. Sometimes it starts fines and won't idle down, other times it struggles to start and idle without throttle input for 60-90 seconds. We've replaced the IACV, TPS, CTS, and I rebuilt the TBI. CTS and TPS have new pigtails, IACV does not. Only code is for the deleted EGR. The truck runs great otherwise. We're planning a trip to Oregon in June and we're camping from OH to OR and back. This is the only real bug I'm having currently and would LOVE some insight. My dad and grandpa always ran Chevy's in the towing business and dad has tons of experience with TBI, but I've got him stumped. Thanks in advance!

 

[Bextreme04]

Hey guys, been chasing gremlins on my 89 Suburban since I replaced a blown motor. Replaced with lower mileage '87 350 from a 1-ton GMC. Kept all my accessories/intake manifold/TBI unit. The truck has been going through IACVs like they're going out of style. I've tried the idle relearn/reset several times without any luck. Recently, I've discovered that when I have a high idle issue, I can eliminate it by running the truck up to around 65 on the speedo (75ish ground speed, on 35s). Inevitably it always comes back in one way or another. Sometimes it starts fines and won't idle down, other times it struggles to start and idle without throttle input for 60-90 seconds. We've replaced the IACV, TPS, CTS, and I rebuilt the TBI. CTS and TPS have new pigtails, IACV does not. Only code is for the deleted EGR. The truck runs great otherwise. We're planning a trip to Oregon in June and we're camping from OH to OR and back. This is the only real bug I'm having currently and would LOVE some insight. My dad and grandpa always ran Chevy's in the towing business and dad has tons of experience with TBI, but I've got him stumped. Thanks in advance!

It sounds like a vacuum leak. Or maybe a bad ground from when you swapped the engine over? Did you get all the grounds put back on in the right spots?

 

[jhe4500]

It sounds like a vacuum leak. Or maybe a bad ground from when you swapped the engine over? Did you get all the grounds put back on in the right spots?

I have t smoke tested yet, but carb cleaner hasn't revealed any vacuum leaks as of yet. I made new grounds after the swap because I was charging issues after the swap. New ground from battery to frame, frame to front of block, passenger cylinder head to firewall, and new hot from alternator to battery. Also had to replace a fusible link that got pinched between starter and firewall junction

 

[YakkoWarner]

How high is high? I'm asking because I was pretty convinced I also had a high idle based on how rough the switch from neutral to drive or drive to reverse was. With a data logging tool I confirmed it was actually pretty much dead on perfect at 750 which is what the computer is apparently programmed to keep it at.

Any kind of vacuum anomoly could cause it - leaking brake booster, bad vac line, even the HVAC controls are vacuum operated and a leak there could cause it. I had to replace my throttle body with a rebuilt unit because of worn shaft bushings but you've already rebuilt yours. I broke down and bought the datalogging setup because I have things that don't follow logic on mine, and definately gave me insight into what was going on but no silver bullet solution. Having the data logging will let you set/calibrate the TPS, monitor the IAC step counts, watch for changes on the MAP sensor, what your coolant temp sensor is actually telling the computer, etc. If the computer is getting incorrect readings it will do incorrect things, but even with all that its not assured to pinpont a problem.

 

[jhe4500]

How high is high? I'm asking because I was pretty convinced I also had a high idle based on how rough the switch from neutral to drive or drive to reverse was. With a data logging tool I confirmed it was actually pretty much dead on perfect at 750 which is what the computer is apparently programmed to keep it at.

Any kind of vacuum anomoly could cause it - leaking brake booster, bad vac line, even the HVAC controls are vacuum operated and a leak there could cause it. I had to replace my throttle body with a rebuilt unit because of worn shaft bushings but you've already rebuilt yours. I broke down and bought the datalogging setup because I have things that don't follow logic on mine, and definately gave me insight into what was going on but no silver bullet solution. Having the data logging will let you set/calibrate the TPS, monitor the IAC step counts, watch for changes on the MAP sensor, what your coolant temp sensor is actually telling the computer, etc. If the computer is getting incorrect readings it will do incorrect things, but even with all that its not assured to pinpont a problem.

Normally around 1100 rpm. I bought the aldl cable and downloaded the software for it, but I haven't had any luck getting all the information I've been after. I have a file of the FSM and tested the MAP, set my TPS voltage, all that jazz. I haven't had a chance to check any of the HVAC, but I've gone as far as disconnecting all the vacuum lines from the TBI and plugged the ports - no change.

 

[jhe4500]

This weekends plan is to test the IACV using the diagnostic tree in the FSM. I'll be checking the resistance across the coils and all that fun stuff

 

[gmbellew]

It won’t affect idle, but if you are still running a 4L60, you need to adjust the DRAC to get the Speedo right for TCC lockup and fuel management at highway speeds. The ECM uses the speed for some things.

For the IAC, it is a tricky one that you can’t readily check with a voltmeter. And even with the software, all you can see is the IAC position being commanded. With a tech2 I think you can command the IAC and do some more tests, but those are hard to come by.

Are there ever any intermittent codes? Could be the plug wires laying on a sensor wire causing electromagnetic interference? The ECM grounds at the back of the engine and by the coolant temperature sensor. I’ve had issues with the ICM harness not being protected by loom and causing weird issues. It is up against the firewall and can easily rub through and can cause issues even if it doesn’t touch anything. Check the TPS, as it is part of “idle” vs “off idle” engine management. Check the CTS as it affects idle down based on engine temp. ICM harness feeds the ECM RPM data.

I’d wager the IAC isn’t the issue, but that the ECM is seeing something that is causing it to open the IAC. And the fact that it is intermittent means it will be harder to track down. If you can get the software running to see the ECM parameters, it will probably be a huge help

 

[jhe4500]

It won’t affect idle, but if you are still running a 4L60, you need to adjust the DRAC to get the Speedo right for TCC lockup and fuel management at highway speeds. The ECM uses the speed for some things.

For the IAC, it is a tricky one that you can’t readily check with a voltmeter. And even with the software, all you can see is the IAC position being commanded. With a tech2 I think you can command the IAC and do some more tests, but those are hard to come by.

Are there ever any intermittent codes? Could be the plug wires laying on a sensor wire causing electromagnetic interference? The ECM grounds at the back of the engine and by the coolant temperature sensor. I’ve had issues with the ICM harness not being protected by loom and causing weird issues. It is up against the firewall and can easily rub through and can cause issues even if it doesn’t touch anything.

The truck still has a 700R4. Where is the ECM ground at the back of the engine? I cleaned up the ground at the water neck already. No other codes since I sorted out my low voltage issue (faulty duralast reman alternator). I thought about the plug wires too, already re-routed and tied up from the rest of the harness as best as possible

 

[gmbellew]

The truck still has a 700R4. Where is the ECM ground at the back of the engine? I cleaned up the ground at the water neck already. No other codes since I sorted out my low voltage issue (faulty duralast reman alternator). I thought about the plug wires too, already re-routed and tied up from the rest of the harness as best as possible

I was adding more info to my post when you quoted.

The other ECM ground is on the passenger side back of the engine. You can check the harness at the ECM to make sure the grounds are good on the ground circuits.

 

[jhe4500]

Awesome, I'll make sure to check. I've got an old OTC scan tool with the correct adapter for the truck, I'll hook it up and see what I can get from a live data stream while I'm at it.

 

[Bextreme04]

CTS could definitely cause high idle. It will also run pig rich if it thinks it is colder out than it is.

 

[jhe4500]

CTS could definitely cause high idle. It will also run pig rich if it thinks it is colder out than it is.

Thank god for warranty replacement. We have a commercial account with AZ and AAP. However, I've been tossing all those parts in favor of ACDelco when I've been doing my replacements.

 

[YakkoWarner]

CTS could definitely cause high idle. It will also run pig rich if it thinks it is colder out than it is.

That was part of how I limped mine off a highway shoulder into a parking area 3 weeks ago - plugged in a spare CTS and just left it hanging loose, it should have run stupid-rich but instead got just enough fuel to idle (but not really drive) and was able to get off the state highway and avoid a police tow/impoundment/citation/court costs/county storage fees/etc...

 

[Bextreme04]

Thank god for warranty replacement. We have a commercial account with AZ and AAP. However, I've been tossing all those parts in favor of ACDelco when I've been doing my replacements.

You can chase your tail forever if you just throw parts at it. Test and verify, then replace parts.#1 cause of people to junk their TBI system is inability to fix it because they have replaced every sensor and it still doesn't work right. Get the manual and run through the troubleshooting procedure.

That was part of how I limped mine off a highway shoulder into a parking area 3 weeks ago - plugged in a spare CTS and just left it hanging loose, it should have run stupid-rich but instead got just enough fuel to idle (but not really drive) and was able to get off the state highway and avoid a police tow/impoundment/citation/court costs/county storage fees/etc...

They tend to fail full cold(-40) and that is what makes them run stupid rich. If you plugged a good one in, it would have brought it back up to whatever abient is, which is much better than -40(hopefully)

 

[YakkoWarner]

You can chase your tail forever if you just throw parts at it. Test and verify, then replace parts.#1 cause of people to junk their TBI system is inability to fix it because they have replaced every sensor and it still doesn't work right. Get the manual and run through the troubleshooting procedure.

They tend to fail full cold(-40) and that is what makes them run stupid rich. If you plugged a good one in, it would have brought it back up to whatever abient is, which is much better than -40(hopefully)

Yes the spare is good one. The installed one does read correctly, plugging in the spare was a pure desperation play to force the ECU to give me as much fuel as it could, which was at least enough to idle up the shoulder. Once that spare one warmed up to the full under-hood ambient temp it leaned out and died again but got close enough to push into a parking spot.