1990 Suburban high idle/whistle from IAC

[SoCalSlaughter]

I've had this problem with my Suburban for as long as I can remember. First off, when the truck is started cold, it runs a bit rough, once it warms up it runs much better. It will idle fine, has power, no engine light. But after about 10 minutes of driving, I notice on acceleration, after I let go of the gas, I can hear a whistle. When I come to a stop light/sign, the trucks idle is high, about 1,200rpm. If I put the truck into park it will idle at 1600/1800 rpms. Once the truck has been turned off and started back up the idle will be fine until I start driving again. I've compression tested all 8 cylinders, which where all between 147 to 151. Ive replaced the throttle body gasket, replaced the IAC motor and have tried switching it out from a friends 1990 Suburban that was known working, replaced O2 sensor, coolant temp sensor, throttle body was rebuilt with new injectors, all normal maintenance has been done. Ive replaced the positive and negative batter cables, knock sensor has been replaced, the ECM has also been swapped out to rule out a faulty ECM. The wiring harness has been pulled out of the truck and checked for bear marks and/or bear wires. The only code the truck has gotten recently was a code 42 (ignition control module). That code hasn't been set in about 2 months. All vacuum lines have been replaced. The truck runs awesome when its not in this condition. I am now at a loss of what to do or check for. It seems I've gone through the engine bay 30 times. The truck was also able to pass Calif smog running like this. But I think it may be affecting my mpg's. The truck is a 1990 Suburban V2500 5.7 TBI 4X4 TH400. Thanks guys!!!

[67 Burb]

Have you checked the EGR valve?

[SoCalSlaughter]

I've used a vacuum pump to confirm the EGR valve is holding vacuum which it does. I've replaced the EGR base gasket. Could the EGR not be closing all the way cause a vacuum leak? Also an update, yesterday I was driving the truck around, got off of the free way and just before I came to the stop light, the truck surged and the engine light came on for the O2 sesnor saying "Lean". The engine idle was at 1200, and after sitting at the light for a minute the whistle went away, the rpm dropped and the light turned off. The truck just went back to normal running condition. I'm lost. haha

[67 Burb]

Originally Posted by SoCalSlaughter

I've used a vacuum pump to confirm the EGR valve is holding vacuum which it does. I've replaced the EGR base gasket. Could the EGR not be closing all the way cause a vacuum leak? Also an update, yesterday I was driving the truck around, got off of the free way and just before I came to the stop light, the truck surged and the engine light came on for the O2 sesnor saying "Lean". The engine idle was at 1200, and after sitting at the light for a minute the whistle went away, the rpm dropped and the light turned off. The truck just went back to normal running condition. I'm lost. haha

If the EGR valve is not closing when it is supposed to, problems will occur.

Wikipedia states: "EGR is typically not employed at high loads because it would reduce peak power output. This is because it reduces the intake charge density. EGR is also omitted at idle (low-speed, zero load) because it would cause unstable combustion, resulting in rough idle.

I have had problems with the EGR valve in my 95 Chev, and it nearly drove me nuts because I initially only tested it like you did; checking that it held vacuum. Which it did. I finally took the EGR valve off and stuck a piece of clean heater hose on the valve (see arrow in picture) and blew into it. Then I knew that the valve wasn't closing.

[VetteVet]

Here's some threads that may help.

..........http://www.google.com/custom?domains...D%3A1%3B&hl=en

[SoCalSlaughter]

Hmm well the EGR looks factory. Haha I'll have to take it off and check to see if its sealing properly. Thanks for the info! I'll post back with the results!!

[68c10airstream]

Hi, worked as a gm dealer mechanic during the 90's. Egr open at idle (should be closed) will cause a stall or low rough idle. If your egr valve is original look at the part number. Most likely a 8 digit number followed by a letter "P" (for positive pressure) or a "N" (for negative pressure). Negative pressure can be tested easily with the engine shut off. Pull the vacuum hose off the egr valve, hook up hose to the egr and suck on the hose and watch the stem as it should rise up and hold a steady position and not leak down. This would be a good egr valve. Now start up the engine and suck on the hose and the engine should stall out if the passage ways are not all plugged up with carbon. Now to the positive egr test. Hooking up a hose with the engine shut off will NOT raise up the stem. This is normal. Start the engine, hold the rpms up to maybe 1500 rpms and now suck on the vacuum hose and the stem should rise up. This is because the positive pressure in the exhaust is closing a valve inside the egr valve to seal it up and allow it to open up. I used to call the positive egr valves as "self regulating" because egr action and quantity was based on exhaust back pressure. This info is from recall, i didn't google it but i'm pretty confident it's correct. I've worked on gm's since the 70's and remember the anemic 80's feedback carb v6's with plugged up cats would just off idle open up the positive pressure egr valves all the way up. Talk about soggy acceleration. My quick inspection was to look at the egr stem. If it was a fat stem it was positive press. egr. I would remove the o2 sensor, thread in a fitting with a hose and read the pressure inside the exhaust BEFORE the cat. Anything above 1.0-1.5 psi at 2500 rpm's was bad news. Now i check for plugged up exhaust with this test; record the inches of vacuum at idle, rev and hold to 2000-2500 rpms, and with correct timing, etc vacuum needs to be at least a little above the idle value or greater. Now onto trouble code 42. If you clear all codes, and it returns, pm me and i will tell you the test to check for a poor ground at the distributor neck. Never put a part in a truck to fix trouble code 42, just clean certain parts. God i'm getting old remembering all of this. Good luck, Brian Fuller

[SoCalSlaughter]

I removed the EGR, and confirmed it is sealing, so no air is getting past the air way when its closed. I've used a hand held vacuum pump and applied vacuum to the EGR and it did not leak down, even after 3 minutes. Also, the cat was recently replaced last year (for smog) I removed the cat about a month ago to make sure it hasnt fallen apart inside. When the truck is up to temp in park, from the intake manifold its pulling 20lbs of vacuum, and it holds steady. Doesnt jump around or anything. When I give the truck gas the vacuum drops to about 11-14 lbs of vacuum. And the timing is set to 2 degrees advance. Also, I was driving the truck Thrusday, went to the store, and when I was leaving the store parking lot, the check engine light came on while I was at a stop light, then when I started driving, the light turned off. Used the old paper clip to check the code and it was a code 13 (O2 Lean). And now the code 42, haha I haven't gotten the code in about about 2 months. I've replaced the distributor, took the hold down clamp and took it to a wire wheel, I've added a ground from the intake manifold to the frame, also from the block to the firewall (the body). The truck defiantly isn't running right, its not getting the mpg's it used to get. I'd also like to note I have an Edle Brock intake manifold. But these problems existed before the intake was installed. I've also removed the intake manifold to check the gaskets.

[SoCalSlaughter]

Latest update. Tonight I was in a drive thru, the truck seems to be running well. I had pulled to the window, turned my headlights off and put the truck in park, it was idling around 850rpm. Was sitting there for about 5 minutes, then I heard a whistle and the rpm kicked up to 1500rpm. I turned the truck off, waiting a minute and started it again and all was good. Drove out of the parking lot, and about a minute of driving down the road, the engine light came on. Got home and checked the code, and it was code 21 (TPS signal high) Ive never had this code before and the TPS sensor is about a year old. Started the truck up again and drove around my block, light was off and truck drove normally. Haha I dont know whats going on!

[SoCalSlaughter]

4 years later the truck still has the issue. I drive it maybe 2000 miles a year. Around town driving its fine, after getting off hwy, and I put it in park, it idles at 1200. If I turn the truck off and turn it back on it idles around 800 like normal. In the past 4 years Ive replaced all the sensors, fuel pump, rebuilt throttle body. Always passes smog no problem, no engine light.

[rpmerf]

My thoughts:
Determine what conditions cause the IACV to open. Not what causes your issue. In the ECU's programming, what conditions make the ECU tell the IACV to open? Certainly cold idle (Coolant temperature sensor). RPM too low (HEI module)? Does it open a bit while the vehicle is in motion (Vehicle speed sensor)? I wonder if there is a write up or logic chart somewhere.

Can you disconnect the wires from the IACV and verify this condition does not happen. Working on 80's Dodges, I would pull a vacuum line. This causes the RPM to go up and the IACV to fully close. Unplug the IACV. Reconnect the vacuum line. You may need to play with the base idle to get it to idle at a good RPM.

It might be useful if you could get a scan tool to monitor the sensors while the engine is running to compare good condition vs bad condition. Also this is what the ECU sees, not what you are seeing from a multimeter. Useful in the event those values are different for some reason.

Seeing some interesting stuff here:

http://www.gearhead-efi.com/Fuel-Inj...pdated-12-4-00

Now that the engine is idling, sooner or later the driver will hit the
gas. Now the throttle follower logic kicks in. As the tps% increase, so
does the IAC steps. Again, this action is lag filtered. As the throttle
is then closed, the IAC retracts, lag filtered.

I'm wondering if there is something going on like the TPS sticking slightly causing the ECU to think you are applying throttle, which opens the IACV. Once you restart, it resets the minimum value and recognizes idle again.

There are additional factors that affect the IAC position. If the battery
voltage dips, the IAC can increase the idle. The A/C turning on, or
decel fuel cutoff (DFCO), can affect the current IAC position.

Battery voltage - bad grounds was mentioned earlier in the thread. I try to use 4 gauge wire and go battery -> body -> frame -> engine block. Maybe not in that order, but hit all those locations with good solid connections.

[SoCalSlaughter]

The idle screw in the throttle body has a metal cap over it, so its not user adjustable. Also, voltage. I'm running a 140amp alternator. I've checked grounds. Any grounds reading over .5 ohm I replaced the wire or fixed. The TPS was recently replaced with an AcDelco unit, no codes present and throttle linkage is free and has full travel. I was getting a code 13 for a O2 sensor. But the ground was reading 1.7ohm. I replaced ground and it hasnt been back on.