IAC maybe?

[scotts62]

I have a '92 350/4l60 Xcab and it seems to have a higher idle after its warmed up and driving, it seems to come off high idle a bit as it sits and warms up but when i go and drive it and stop at a light or something its idling higher but as im sitting there it slowly starts to drop and idle where its suppose to, im just wondering if its the IAC or something else. Any ideas?

[ChevyTech]

Try stopping vacuum on the hose going to the brake booster.

If the booster gets a torn diaphragm in it, excessive vacuum will be flowing, and it creates a vacuum leak.

We did a lot of diagnostics on one of the boards recently and the problem was the booster.

Check for other vacuum leaks too.

An engine coolant sensor failing or with a bad connection can make the ECM think the engine is cold and the ECM will speed up the RPM.

The IAC could be getting bad.

Sometimes when the distributor gets worn, the shaft wobbles around, and the ECM gets a poor RPM signal for controlling the idle speed.

How’s that for a few ideas!

[scotts62]

Originally Posted by ChevyTech

Try stopping vacuum on the hose going to the brake booster.

If the booster gets a torn diaphragm in it, excessive vacuum will be flowing, and it creates a vacuum leak.

We did a lot of diagnostics on one of the boards recently and the problem was the booster.

Check for other vacuum leaks too.

An engine coolant sensor failing or with a bad connection can make the ECM think the engine is cold and the ECM will speed up the RPM.

The IAC could be getting bad.

Sometimes when the distributor gets worn, the shaft wobbles around, and the ECM gets a poor RPM signal for controlling the idle speed.

How’s that for a few ideas!

You always seem to always have a few ideas that help!

I think i can rule out the distributor because it is new (dont mean anything though) the coolant sensor is new also but you know how the new parts go sometimes..

I will try the booster line and see.

Funny thing was is this morning after i posted the question and went to leave for the day it warmed up fine and drove good and even at stop lights all was good so it got my hopes up but then coming home it was doing it again

[Boog]

If the spring is broken in the booster it can puncture the diaphram and plug the hole sometimes as well as it moves back and forth so it leaks sometimes and not sometimes.

[bwood]

Could it be a cracked line between the EGR Solenoid and EGR valve creating a vacuum leak? I was thinking the EGR didn't begin to operate unit the truck warms up.

[scotts62]

Havent had a chance to check anything out, caught the funk and have been down the last day or so but will try all your ideas asap. thanks guys

[Mechanic77]

Just another suggestion.... the throttle body base gaskets on those have a tendency to get sucked in, causing a vacuum leak and a high idle. The gaskets cheap, and it only takes 15 min. to change it I've seen probably a dozen or so in my 10+ years of wrenching.

[scotts62]

Well right after i asked the question i got Pneumonia and that kept me down and indoors for a while but im back now. I pulled the booster line and held my thumb over it and it didnt change RPM at all so i think that rules it out, right?

I have a spare newly used cool temp sender that im going to put in and see what happens.

[ChevyTech]

Yes; that rules out the booster if it still runs to fast with your thumb plugging the hose.

When checking for leaks make sure you check the base gasket under the TBI. As Mechanic77 stated it is a common problem.

You said it eventually slows down, so the IAC could be getting bad a sticking.

Usually the IAC does not go bad electrically, but it is common for them to stick and stop moving.

The IAC is a stepper motor and uses short pulses of electricity on the wires to move its position.

If the IAC sticks as does not move as it should for each pulse, the IAC count will get off because the computer does not know the actual position of the IAC, but the ECM tracks the commands it has sent to the IAC and assumes it has moved every time it has been sent a pulse.

To clean the IAC, you should remove it, and use a wet rag to wipe the tip of the plunger, while holding it with the plunger pointing DOWN. Be careful, the IAC will break easily. It is aluminum. Clean the passages in the TBI unit while you have the IAC removed.

I do not suggest unscrewing the pintle (plunger). If you don’t get in back in far enough, the IAC can get damaged if the “pintle” (plunger) bottoms out in the TBI, when you screw the IAC back in.

You can look down from the top and see the IAC plunger if it is in the normal idle position.

Glad you are feeling better.

[wrangler94]

Sorry to revive a long dead thread, but this upcoming weekend not only am I replacing the door hing pins for the second time this year, because the Dorman ones fell apart and the door is worse now than before I fixed it, but I'm trying to diagnose and fix a high idle, bad gas mileage, and sometimes stumbling and hesitation issue in my 92 3500 with a 5.7. I get, and can find an AC delco IAC valve if that turns out to be it, and I'll check the brake booster too, but the coolant sensor in the intake, is it the same one as the one for the cluster? just a second one? auto parts store only shows one for the truck. guage doesn't work either, and gonna check for correct thermostat as well this weekend just because.

[ChevyTech]

Originally Posted by wrangler94

... but the coolant sensor in the intake, is it the same one as the one for the cluster? just a second one? auto parts store only shows one for the truck. guage doesn't work either,...

The coolant sensor in the cylinder head for the gauge is different then the coolant sensor by the thermostat that the computer monitors.
The sensor by the thermostat has two terminals. The sensor in the head has one terminal and a prong in the center so if installed by the thermostat the wiring will not plug onto it.

If you want to test the sensor that the computer monitors here is information on that.

To test the coolant sensor, unplug the wire connector and test the resistance across the two terminals of the SENSOR with an ohm meter.

If you see any coolant leaking out the terminals replace it.

If the terminals in the connector are corroded replace the “pigtail”.

On the TBI trucks the sensor screws into the intake manifold, on the front passenger side, by the thermostat housing.
On the Vortec big block it's on the front of the intake ahead of the thermostat housing.

Coolant sensor approximate resistance specifications:


177 ohms @ 212 deg. F. or 100 deg. C.
241 ohms @ 194 deg. F. or 90 deg. C.
332 ohms @ 176 deg. F. or 80 deg. C.
467 ohms @ 158 deg. F. or 70 deg. C.
667 ohms @ 140 deg. F. or 60 deg. C.
973 ohms @ 122 deg. F. or 50 deg. C.
1188 ohms @ 113 deg. F. or 45 deg. C.
1459 ohms @ 104 deg. F. or 40 deg. C.
1802 ohms @ 95 deg. F. or 35 deg. C.
2238 ohms @ 86 deg. F. or 30 deg. C.
2796 ohms @ 77 deg. F. or 25 deg. C.
3520 ohms @ 68 deg. F. or 20 deg. C.
4450 ohms @ 59 deg. F. or 15 deg. C.
5670 ohms @ 50 deg. F. or 10 deg. C.
7280 ohms @ 41 deg. F. or 5 deg. C.
9420 ohms @ 32 deg. F. or 0 deg. C.
12300 ohms @ 23 deg. F. or -5 deg. C.
16180 ohms @ 14 deg. F. or -10 deg. C.
21450 ohms @ 5 deg. F. or -15 deg. C.
28680 ohms @ -4 deg. F. or -20 deg. C.
52700 ohms @ -22 deg. F. or -30 deg. C.
100700 ohms @ -40 deg. F. or - 40 deg. C.

[wrangler94]

awesome, thank you. I will check that this weekend.

[wrangler94]

While fixing the power steering hose yesterday, and replacing the o rings on the other hose that didn't blow out, I also replaced the pigtail on the intake coolant sensor since it was melted looking. Cranked the truck and used a thermometer to check and confirm actual temperatures, and found that the thermostat was stuck open. Replaced that and now it runs at proper temperature and idles much better. There is still a very intermittent miss but I can hear a sound when it misses so I'm 99% sure that's a plug wire. I will replace those this weekend. Thank you again for all the help.

[ChevyTech]

Originally Posted by wrangler94

While fixing the power steering hose yesterday, and replacing the o rings on the other hose that didn't blow out, I also replaced the pigtail on the intake coolant sensor since it was melted looking. Cranked the truck and used a thermometer to check and confirm actual temperatures, and found that the thermostat was stuck open. Replaced that and now it runs at proper temperature and idles much better. There is still a very intermittent miss but I can hear a sound when it misses so I'm 99% sure that's a plug wire. I will replace those this weekend. Thank you again for all the help.

Good progress!
If plug wires are not the problem you may consider carbon build up because it was running rich. If they run cold the system is running rich.

These TBI trucks can get very heavy carbon deposits on the valves when they run cold.

I have used SEA FOAM several times with great success. I have also used GMs TOP ENGINE CLEANER at the dealership many times with good results. The GM Top Engine Cleaner smells just like SEA FOAM to me and I have wondered if they are the exact same product. I have never had any problems from using these products, but if an engine has very heavy carbon, one treatment may not get it all out. My biggest concern is that it is not poured in so fast that it liquid locks the engine.

I slowly pour in a half can in the hot engine at fast idle then stall the engine and let it sit ten minutes. Then I slowly pour in the rest of the can at a fast idle, and then go drive it.

[wrangler94]

Originally Posted by ChevyTech

Good progress!
If plug wires are not the problem you may consider carbon build up because it was running rich. If they run cold the system is running rich.

These TBI trucks can get very heavy carbon deposits on the valves when they run cold.

I have used SEA FOAM several times with great success. I have also used GMs TOP ENGINE CLEANER at the dealership many times with good results. The GM Top Engine Cleaner smells just like SEA FOAM to me and I have wondered if they are the exact same product. I have never had any problems from using these products, but if an engine has very heavy carbon, one treatment may not get it all out. My biggest concern is that it is not poured in so fast that it liquid locks the engine.

I slowly pour in a half can in the hot engine at fast idle then stall the engine and let it sit ten minutes. Then I slowly pour in the rest of the can at a fast idle, and then go drive it.

Changed the plugs, wires, distributor cap, rotor, and fixed a couple grounds and some other wires that looked like they'd seen better days as well. Also broke out the timing light and followed the proper procedure of unplugging the tan/black wire to check the timing. it was off a good bit, so reset it to 0 and plugged the wire back in and revved it and all and rechecked.
The plugs looked surprisingly good, driving it a week either cleaned the carbon and rich looking plugs up, or by some miracle with it running as crappy as it was 2 weeks ago the plugs stayed ok.
It runs sooo much better now, no stumbling like when I back off the gas heading to a red light and it turns green and get back on throttle, etc. it's like a new truck again now. Thank you again for the tips and help.

[ChevyTech]

Originally Posted by wrangler94

Changed the plugs, wires, distributor cap, rotor, and fixed a couple grounds and some other wires that looked like they'd seen better days as well. Also broke out the timing light and followed the proper procedure of unplugging the tan/black wire to check the timing. it was off a good bit, so reset it to 0 and plugged the wire back in and revved it and all and rechecked.
The plugs looked surprisingly good, driving it a week either cleaned the carbon and rich looking plugs up, or by some miracle with it running as crappy as it was 2 weeks ago the plugs stayed ok.
It runs sooo much better now, no stumbling like when I back off the gas heading to a red light and it turns green and get back on throttle, etc. it's like a new truck again now. Thank you again for the tips and help.

You're welcome!
Glad to hear it is running well.
The plugs clean up quickly if it is driven at high enough RPMs when fully warmed up.

[wrangler94]

Originally Posted by ChevyTech

You're welcome!
Glad to hear it is running well.
The plugs clean up quickly if it is driven at high enough RPMs when fully warmed up.

OK all the work I've done lately has made the truck run fantastic most of the time, thank you again ChevyTech. Maybe I should start a new thread but i'm getting replies in this one. One issue the truck has sporadically had, when it sat alot it was more common, since Christmas when it became my daily driver again I almost thought it had gone away, but this morning it did it again.
Driving to work I hit a pothole and the truck went from running at the RPMs needed to keep 35 mph, to idle or less, no throttle response at all, but oil pressure was up, engine was running enough (pretty sure) then it died and check engine light flashed and i was coasting.. shifted to neutral and recranked it and it ran kinda rough but ran for a few more miles. Then same thing again, cut off at 60 mph, but not like flat out died, just no throttle response. Engine died finally and recranked it again and been ok since,

I'm thinking ICM maybe? that's one of the few parts I haven't changed yet. Maybe all the extra heat from having a working thermostat and truck running at 195 degrees is making the sporadic problem more common?
It is my only transportation now days, and starting a new job next week, letting the problem get bad enough to finally fully fail so i can easily find it is a last resort lol.
Thoughts?

[Just call me Sean]

Usually it's the factory connector hose that turns to mush. All the replacement pumps I've bought came with a decent hose to reconnect the pump.

[wrangler94]

Posting a little more info to possibly help in diagnosing, as well as explanation of history a bit

It did it again this morning. For the last couple years it's only been sporadic, but the engine was running ridiculously cold for the last couple years too. I used to maintain the truck myself but I left the company for 5 years and over that time is when all the issues started.

As I said, it was pretty common at one time, but a new distributor was put in because of the slop in the shaft, and that made it very sporadic again. I've been daily driving since basically new years day this year because the company shut down and the truck became mine. In that time, first 3 months of this year, it's only cut off like that maybe twice the whole time.
Now that the bad thermostat was replaced and the engine is running at 195 degrees again (2 weeks) it's done it now 3 times in the last 2 days, so I'm thinking wahtever it is is heat related. No pothole this morning but shut off again.

[ChevyTech]

If you think it is heat related then the module in the distributor could be the problem. Many parts store will test the module for free if you bring them the module.

Make sure you put heat sink compound under the module when you install it.

The replacement distributor could have had a low quality module in it depending on what distributor was bought.

I keep a spare module in my truck.

[Just call me Sean]

I set my timing once and the bypass wasn't making contact after I plugged it back in which gave me the code 42. It was pretty crusty inside and I had to clean it and reset the computer to end the code.

[wrangler94]

Originally Posted by Just call me Sean

I set my timing once and the bypass wasn't making contact after I plugged it back in which gave me the code 42. It was pretty crusty inside and I had to clean it and reset the computer to end the code.

Yes as mentioned in the post above, that was possibly the problem, or the bent pin at the computer, but in any case, so far, the truck has thrown no codes, but it does cycle from closed to open loop a lot. I've got a suspicion that the O2 sensor is reading well enough to not throw a code, but not reading as it should be, so that will be replaced this weekend. one of the two codes it was throwing before was for the O2 sensor.
Now that the immediate problems seem to be handled, I will be going through and replacing basically every sensor on the truck as funds allow with all AC Delco stuff or Delphi if I can't find AC Delco, just to help the computer as much as possible and hopefully improve gas mileage. I went so far from seeming like gallons per mile to a couple weeks ago 9.3 mpg and last week 10.56 mpg.. not sure what to expect from a crew cab long bed with a 350 in it, but maybe i can get it to 13 or so at least lol.

[ChevyTech]

Glad you are making progress.

I would not replace any sensor just to replace parts as you have funds to do so.

The quality of replacement parts is poor. I would prefer a used good original parts over much of what is being produced today.

When you buy a new oxygen sensor I suggest staying away from BOSCH brand.

[wrangler94]

Originally Posted by ChevyTech

Glad you are making progress.

I would not replace any sensor just to replace parts as you have funds to do so.

The quality of replacement parts is poor. I would prefer a used good original parts over much of what is being produced today.

When you buy a new oxygen sensor I suggest staying away from BOSCH brand.

Just adding another update. Truck has not shut off at random now for 2, maybe 3 weeks now. Thank you again for all the help with that.
I did replace the O2 sensor saturday and it stays in closed loop as it should now, mostly at least. Not sure how much it should cycle to open, but it's almost always closed now.
When replacing the fuel filter i found hacked together wiring for the transmission plug for the 4L80E and wasn't sure what to think, until I did some digging and the plug has been updated, but it looks like a monkey with a butterknife did the upgrade. At this point I'm happy and not gonna chase it so hard, but the scanner does show that the torque convertor never locks up.
Obviously the bad wiring job is a prime suspect, but research has shown that a bad brake light switch can cause it too, and when the scan tool is plugged in i throw a code (not on scanner, in flashing brake light warning) of stop light switch. Scanner shows that brakes are off or on as i press the pedal, and brake lights work, but is it possible that the switch is somehow not giving the transmission what it needs? The computer seems to get signal, but that code does keep coming up, even with resetting the computer. Again, only when the scan tool is hooked up. I unplug scaner and brake warning light goes off again.

[ChevyTech]

You're welcome! Glad you are making even more progress.

The brake switch has multiple circuits through it and the brake lights can work while other circuits going through it don't work. The brake switch is a weak point on these trucks. The brake switch could cause the lock-up converter to stop locking up. The brake switch can cause anti-lock problems while the brake lights still work.

Does your 3500 series truck have rear disc brakes?

There are different brake system and I am not sure what your trucks has as for brake issue while using the scan tool. Some trucks have 4WAL some have RWAL and if your truck could have ZPRAWL if it has rear disc brakes.

By the time your 1992 was built some of the RWAL and ZPRWAL codes were soft codes which need a scan tool to read. The hard codes can be read without a scan tool.

The anti-lock issue can turn into a long thread just by itself. If you want to go into that you should start a new thread for it, and if I don't reply just send me a private message to get my attention. I have posted on the RWAL system several times over the years and a search should be able to find these. The ZPRWAL has some different procedures, like how to clear codes.