IAC maybe?

[Camaro_nut2001]

Originally Posted by ChevyTech

Immediately after you shut off the engine look at the TBI unit and see if fuel is leaking.

A damaged injector o-ring can allow fuel to dip in the throttle bore. A gasket leak can on the upper TBI also cause this problem.

Another problem is that the TBI unit is thin by the injector o-rings, and if lube is not used on the o-rings, sometimes the TBI aluminum cracks in the area of the o-ring.

Another possibility is the coolant temp sensor connector is in poor condition and got bumped when you were working under the hood. Any resistance or connection problem will make it read colder then it really is. Colder means more gas gets injected.

Thank you ChevyTech. I did pull the breather & watched it after turnin the key off & sure enough, the top o-ring on the left side injector was leaking. I pulled the top back off, injector out, found an o-ring in my parts bin that was almost the correct match, enough so that it worked lol, lubed it & the bottom o-ring up with a little wheel bearing grease & problem solved. Thanks for your very quick reply to help me.

[ChevyTech]

You’re welcome, and thank you for letting us know what fixed it.

[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?

[ChevyTech]

The module in the distributor is a common failure part but that does not fit the hitting a pothole scenario. These do get problems with the connection where the wiring plugs onto the module on the outside of the distributor which does fit with hitting bumps.

First thing that comes to my mind is the jolt affecting the fuel pump. It is common for the hose between the fuel pump sender and the fuel pump to get mushy and leak. The jolt could be affecting that hose. These hoses seem to fail from a vehicle that sits a long time without being driven.

The ignition switch on the lower area of the steering column can get worn and lose connection from hitting bumps.

It would be a good idea to check for trouble codes which can be done without a scan tool or code reader. Here is a link on checking codes.
https://www.chevytalk.org/fusionbb/s...php?tid/273660

[wrangler94]

Originally Posted by ChevyTech

The module in the distributor is a common failure part but that does not fit the hitting a pothole scenario. These do get problems with the connection where the wiring plugs onto the module on the outside of the distributor which does fit with hitting bumps.

First thing that comes to my mind is the jolt affecting the fuel pump. It is common for the hose between the fuel pump sender and the fuel pump to get mushy and leak. The jolt could be affecting that hose. These hoses seem to fail from a vehicle that sits a long time without being driven.

The ignition switch on the lower area of the steering column can get worn and lose connection from hitting bumps.

It would be a good idea to check for trouble codes which can be done without a scan tool or code reader. Here is a link on checking codes.
https://www.chevytalk.org/fusionbb/s...php?tid/273660

yes from reading other threads you've helped in on these issues that thought has also crossed my mind about the fuel hose deal.. and fuel filter hsn't been replaced likely since last time I personally did it at least a decade ago lol.. (was a company truck but I always felt sentimental to it, since i helped start the company and sold my car to buy it)
As to that fuel hose breaking in the tank, does it matter that it had a full tank, as in minutes before just filled it to the brim to test to see if I now get miles per gallon instead of gallons per mile?

[ChevyTech]

Originally Posted by wrangler94

yes from reading other threads you've helped in on these issues that thought has also crossed my mind about the fuel hose deal.. and fuel filter hsn't been replaced likely since last time I personally did it at least a decade ago lol.. (was a company truck but I always felt sentimental to it, since i helped start the company and sold my car to buy it)
As to that fuel hose breaking in the tank, does it matter that it had a full tank, as in minutes before just filled it to the brim to test to see if I now get miles per gallon instead of gallons per mile?

In past discussions on fuel hose failure some people think the hose fails faster when there is very little fuel in the tank so it is not submersed.

Where I live there is high ethanol content in the fuel. I have seen the fuel damage small engine rubber components so fast I could watch the damage happening.

If your truck had never been exposed to ethanol and had a low quality piece of hose installed during a previous repair and the hose was deteriorating then you put an high ethanol blend of fuel in the tank it may have done damage that quickly.

[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.

[wrangler94]

Originally Posted by ChevyTech

The module in the distributor is a common failure part but that does not fit the hitting a pothole scenario. These do get problems with the connection where the wiring plugs onto the module on the outside of the distributor which does fit with hitting bumps.


It would be a good idea to check for trouble codes which can be done without a scan tool or code reader. Here is a link on checking codes.
https://www.chevytalk.org/fusionbb/s...php?tid/273660

Didn't want to leave this hanging and unanswered, but just started a new job so haven't had the time or funds til today to mess with the truck any more. I borrowed an old OBD1 scan tool and drove around with the truck, O2 sensor error (code 13) and code 42, ignition control module failure of some sort. Clearing the code it comes right back too, so definitely going to change that. Thank you again for all the help.

[ChevyTech]

Originally Posted by wrangler94

Didn't want to leave this hanging and unanswered, but just started a new job so haven't had the time or funds til today to mess with the truck any more. I borrowed an old OBD1 scan tool and drove around with the truck, O2 sensor error (code 13) and code 42, ignition control module failure of some sort. Clearing the code it comes right back too, so definitely going to change that. Thank you again for all the help.

Thanks for the update.
Note: Code 42 will set if the set timing connector is left unplugged.

[wrangler94]

Originally Posted by ChevyTech

Thanks for the update.
Note: Code 42 will set if the set timing connector is left unplugged.

That's plugged back in, and with it plugged in the timing does move around according to the timing light, So I doubt it is an electrical problem there.
Also just to isolate another possible issue, I unplugged fuel pump relay and truck still runs, but cranks way slower, so I know that the redundant systems of fuel pump relay and oil pressure sensor aren't the problem, unless somehow both fail at the same time when driving.
I'm thinking that the ICM in the truck is a cheap knockoff, AC Delco D1943a is going back in it. When i googled the part number (DKD1980C-21905) the results always sent me to the D1943A, but I could never identify whoever made the one that's in the truck now. Thermal paste is ordered as well. You said earlier put it under the module? like before i bolt it down? and should i also use it as dielectric grease basically?

[ChevyTech]

Originally Posted by wrangler94

.... Thermal paste is ordered as well. You said earlier put it under the module? like before i bolt it down? and should i also use it as dielectric grease basically?

The thermal paste is used under the module before you bolt it down. Make sure the spot where the module mounts is cleaned off first. Do not put the thermal paste anywhere else.

[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 ChevyTech

Thanks for the update.
Note: Code 42 will set if the set timing connector is left unplugged.

Writing again to update you because you helped so much, and to hopefully help others in the future. Today I replaced the ICM, and put dielectric grease on every connector under the hood just because i had a full tube. Made sure to put heat sink paste under the ICM, there was a tiny bit under the last one, but not alot.

Since we were under the hood, we busted out the timing light again, and it was at base time. we messed with the timing connector, and restarted it, and it was letting the computer change timing again. We replaced that connector and Cleared the codes, and all seemed good with the scan tool, went on a test drive and luckily Russell was riding with me and watching the scanner the whole time. It all of a sudden threw the 42 code and cut off again. Then the 42 code wouldn't clear again, just like before.

We got it home, and had found a diagnosis chart to basically test everything but the computer itself, and had about decided the computer was bad. We unplugged the computer, (last changed in 2009) and found a slight bent pin that happened to be one of the ones linked to the ICM. We straightened that out, plugged it back in, and after driving a couple hours now, it goes into closed loop and hasn't cut off (yet) so I may be done chasing that ghost.
it doesn't stay in closed loop, it goes back open sometimes, but seems like the more i drive it the more it stays in closed loop now. I'm not sure how often, if at all, it should be cycling.
Thank you again for all your help, and as i said I hope this input helps someone else one day.

[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.

[Just call me Sean]

I think I got 13 once with my extended cab long bed 350.