Dieseling Problem - The 1947 - Present Chevrolet & GMC Truck Message Board Network

BigRed · 09-09-2003, 11:15 AM

I'm having a problem with my truck dieseling after I shut it off. I seems to do it more when it is hot. I also noticed that the idle is kinda high after I put it into park and about to shut it down. It's not high when I first start it up. If the AC is on when I shut it down, the dieseling doesn't happen. So, it seems like the dieseling always happens when the idle is higher.

What can I check out to fix this problem? If I remember right, overheating is one reason for dieseling, however I'm only running at about 190 F.

By the way, my truck is a 1986 Suburban, 350, quadrajet, stock HEI, 700r4.

Thanks!

PHOENIX · 09-09-2003, 01:06 PM

My truck was dieseling from high idle too.
Didnt do it everytime but would do it from time to time.
I would leave the electric fan on when I turn it off and it helped.
I lower my idle speed and it did it a time or two after that, and hasnt for months now.

Check timing, tune carb, set idle to correct speed and see how it acts.

When I was having my time to time problem I posted something and talked with some local friends. From what I have been told (by a few people) if your idle is high for awhile it will cause build up in the engine. The build up heats while the engine is running and causes dieseling when you turn it off. I do not know if this is true, but it sure acted that way when I changed my idle speed. It did it a time or two then hasnt since.

Hope that helps somewhat, better then nothing I guess.

BigRed · 09-09-2003, 01:08 PM

Thanks Eddie. I'll check everything you mentioned. I haven't touched a Quadrajet, so this should be fun. I'm sure the idle screw is easy to find though.

Yeah, mine only happens every once in awhile but was doing it alot yesterday. Never happens in the morning though.

86SILVERADO · 09-09-2003, 07:10 PM

Scott, You should be able to remedy the problem by retarding the timing or switching to a higher octane gas, given no other problems such as vacuum leaks or EGR problems.

*1FaastC10* · 09-09-2003, 08:21 PM

Scott, you'll have to slightly modify your stock QJet if it hasnt already been modified. i was just out working on mine, and the idle mixture screws are sealed from teh factory with a small plastic plug. pop the plugs off, and you should be able to tune it just like an old non-smog QJet.

Eddie, it's funny that you leave your fan on, and the dieseling problem goes away. when i had my TA, i had to shut the fan off, or i had a dieseling problem. i guess the fan still spinning was actually generating power, and backfeeding it through the system.

**swervin ervin** · 09-10-2003, 12:23 AM

What about the idle compensator? This is the little deal at the throttle that raises the idle when the AC is on. If this thing sticks open, your rpm will be high and cause it too diesel. Reason it don't do it with the AC on is the rpm is lower. Same thing with the fan on, it reduces the rpm.

My vote is check the idle compensator and make sure the wire is plugged onto it. It's located on the front drivers side of the carb.

BigRed · 09-10-2003, 10:17 PM

Thanks guys for the info. Mike, I had a little time tonight and checked that idle compensator. The wire going to it was not connected. I then connected it but nothing happened. Truck didn't run any different. Is this lead supposed to have 12v to it? I'll check that tomorrow. I may see if I can find that tomorrow at the auto parts store and just swap it out. It's worth a shot.

**swervin ervin** · 09-10-2003, 10:34 PM

Scott,

it is supposed to only move when you turn on the AC. It raises the rpm so the engine will idle smoothly with it on. When you cut the AC off, it's supposed to retract, back to the normal idle speed. That's all it does. I suppose if it were to stay out after the AC is off, this would cause a higher than normal idle rpm.

Things that can cause run on, is too low of octane fuel, too high of timing, too high of idle rpm, too high of engine coolant temp, hot spots in the combustion chamber, just to mention a few. I have had it caused by every one of these and not all at the same time. You will need to elimate each one to figure out what it is.

Depending on how many miles are on the engine, might try running a can of top engine cleaner through it. This will help clean any deposits out.

BigRed · 09-10-2003, 10:51 PM

Ok, thanks Mike for the suggestions. Guess I better get busy.

09-09-2003, 11:15 AM	#1
BigRed Registered User Join Date: Jun 2002 Location: DFW, TX Posts: 1,680	Dieseling Problem I'm having a problem with my truck dieseling after I shut it off. I seems to do it more when it is hot. I also noticed that the idle is kinda high after I put it into park and about to shut it down. It's not high when I first start it up. If the AC is on when I shut it down, the dieseling doesn't happen. So, it seems like the dieseling always happens when the idle is higher. What can I check out to fix this problem? If I remember right, overheating is one reason for dieseling, however I'm only running at about 190 F. By the way, my truck is a 1986 Suburban, 350, quadrajet, stock HEI, 700r4. Thanks! __________________ Scott Shelbourne

09-09-2003, 01:06 PM	#2
PHOENIX Registered User Join Date: Oct 2002 Location: Phoenix, Arizona Posts: 4,703	My truck was dieseling from high idle too. Didnt do it everytime but would do it from time to time. I would leave the electric fan on when I turn it off and it helped. I lower my idle speed and it did it a time or two after that, and hasnt for months now. Check timing, tune carb, set idle to correct speed and see how it acts. When I was having my time to time problem I posted something and talked with some local friends. From what I have been told (by a few people) if your idle is high for awhile it will cause build up in the engine. The build up heats while the engine is running and causes dieseling when you turn it off. I do not know if this is true, but it sure acted that way when I changed my idle speed. It did it a time or two then hasnt since. Hope that helps somewhat, better then nothing I guess. __________________ * AVOID: LOPER'S PERMORANCE / LOPERSPEED.COM OF PHOENIX, AZ & COTTMAN TRANSMISSION *

09-09-2003, 01:08 PM	#3
BigRed Registered User Join Date: Jun 2002 Location: DFW, TX Posts: 1,680	Thanks Eddie. I'll check everything you mentioned. I haven't touched a Quadrajet, so this should be fun. I'm sure the idle screw is easy to find though. Yeah, mine only happens every once in awhile but was doing it alot yesterday. Never happens in the morning though. __________________ Scott Shelbourne

09-09-2003, 07:10 PM	#4
86SILVERADO Registered User Join Date: Dec 2002 Location: Hills of Western Mass Posts: 626	Scott, You should be able to remedy the problem by retarding the timing or switching to a higher octane gas, given no other problems such as vacuum leaks or EGR problems. __________________ 86 Silverado C10 back to 305 power! 67 C30 Dually Dump, with 350 transplant, Rockhauler 05 Duramax 3500 "If at first you don't succeed, try a bigger hammer" Member of the 1-Ton club!

09-10-2003, 12:23 AM	#6
swervin ervin You get what you pay for Join Date: Dec 1999 Location: Cherryville, NC Posts: 4,798	What about the idle compensator? This is the little deal at the throttle that raises the idle when the AC is on. If this thing sticks open, your rpm will be high and cause it too diesel. Reason it don't do it with the AC on is the rpm is lower. Same thing with the fan on, it reduces the rpm. My vote is check the idle compensator and make sure the wire is plugged onto it. It's located on the front drivers side of the carb. __________________ Mike 1985 Chevy C-10

09-09-2003, 08:21 PM	#5
1FaastC10 Account Suspended Join Date: Oct 2002 Location: Elkhart, IN Posts: 6,399	Scott, you'll have to slightly modify your stock QJet if it hasnt already been modified. i was just out working on mine, and the idle mixture screws are sealed from teh factory with a small plastic plug. pop the plugs off, and you should be able to tune it just like an old non-smog QJet. Eddie, it's funny that you leave your fan on, and the dieseling problem goes away. when i had my TA, i had to shut the fan off, or i had a dieseling problem. i guess the fan still spinning was actually generating power, and backfeeding it through the system.

09-10-2003, 10:17 PM	#7
BigRed Registered User Join Date: Jun 2002 Location: DFW, TX Posts: 1,680	Thanks guys for the info. Mike, I had a little time tonight and checked that idle compensator. The wire going to it was not connected. I then connected it but nothing happened. Truck didn't run any different. Is this lead supposed to have 12v to it? I'll check that tomorrow. I may see if I can find that tomorrow at the auto parts store and just swap it out. It's worth a shot. __________________ Scott Shelbourne

09-10-2003, 10:34 PM	#8
swervin ervin You get what you pay for Join Date: Dec 1999 Location: Cherryville, NC Posts: 4,798	Scott, it is supposed to only move when you turn on the AC. It raises the rpm so the engine will idle smoothly with it on. When you cut the AC off, it's supposed to retract, back to the normal idle speed. That's all it does. I suppose if it were to stay out after the AC is off, this would cause a higher than normal idle rpm. Things that can cause run on, is too low of octane fuel, too high of timing, too high of idle rpm, too high of engine coolant temp, hot spots in the combustion chamber, just to mention a few. I have had it caused by every one of these and not all at the same time. You will need to elimate each one to figure out what it is. Depending on how many miles are on the engine, might try running a can of top engine cleaner through it. This will help clean any deposits out. __________________ Mike 1985 Chevy C-10

09-10-2003, 10:51 PM	#9
BigRed Registered User Join Date: Jun 2002 Location: DFW, TX Posts: 1,680	Ok, thanks Mike for the suggestions. Guess I better get busy. __________________ Scott Shelbourne



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