dieseling (sp?) - The 1947 - Present Chevrolet & GMC Truck Message Board Network

PureChevy · 08-19-2003, 11:42 AM

Every now and then my truck trys to keep running after i turn it off. I think this is called dieseling. I am not sure. I was wandering what causes this and how can i fix it.
thanks

Will

VTKidder · 08-19-2003, 12:13 PM

Dieseling is caused by gas still making it's way into the engine, and igniting even when the ignition is off. I've heard that in a hot engine, carbon still left in the combustion chamber can be hot enough to act as an ignitor, and ignite the fuel still being dumped in. Other places that can be a snag and remain hot enough to ignite fuel are sharp edges of the combustion chamber or head ports caused by head rework. A small burr with very little volume will heat up very quickly compared to the rest of the metal in the head, and could be red-hot during operation, but still ignite fuel when ignition is killed. Other causes I've heard of are the incorrect heat range plugs not sufficiently burning the fuel-air mixture and a lean fuel-air mixture which will burn at a higher temp than a rich mixture. Timing is generally not the culprit here; timing that is off will lead to knocking under load, not dieseling. The first places I would try are the plugs and the fuel-air mixture. Any head rework done before? Just some places to check.

tom hand · 08-19-2003, 12:14 PM

Lots of thens can cause this but the biggest offender is to much idle speed. Try reducing that first and see if it helps

PureChevy · 08-19-2003, 12:19 PM

my idle speed is right at 600 rpms. is that too high
It does seem like it happens when i have stopped abruptly and turned my truck off, like when pulling in my drive when i have been in a hurry or something.

tom hand · 08-19-2003, 12:23 PM

600 is correct but sometimes 590 will cure the problem...it only takes a screwdriver and five minutes to find out...

PureChevy · 08-19-2003, 12:25 PM

ok
thanks

*67Fleetsidedream* · 08-19-2003, 01:17 PM

Crappy gas LOW OCTANE fuel will cause it to diesel.
High compression ratio will cause it to diesel.
Carbon buildup on the pistons and heads will cause it to diesel.
The timing being off will cause it to diesel.

PureChevy · 08-19-2003, 01:21 PM

i run 87 octane. It is a mild 350 rebuilt about 40,000 miles ago.

Longhorn Man · 08-19-2003, 04:09 PM

VTKidder has a very good handle on the situation here (not to downplay anyone else's responce)
The first thing i would do is gas up with some 89 octain. This will likely solve your problem. If id does not, and your timing isn't advanced very much, then I would spray some water into the carb.
Yeah, you read right. Set the idle at about 1200 - 1500 RPM (with the engine already at normal temp and air filter removed) and then take a spray bottle and set it to stream. Speay the water directly into the carb's primarys. Don't spray it onto the walls, but rather aim it at the opening of the throttle blades. This stream of cold water will help break up deposits of carbon on the combustion chaimber.
As long as you are not dumping the water in there, it will do no harm to your engine. Old timers in California are familiar with this trick, it used to be a good way to get a car that just failed the smog test to pass on the second time.
Don't be allarmed when you see steam coming out your pipes, and if you are lucky, you may see chunks of carbon fly out too.
When you are done. reduce the idle back to your previous speed, and then shut it off. Go for a test drive and haull ass back home and kill it as soon as you put it in park. See it it worked.

PureChevy · 08-19-2003, 04:12 PM

I will try that this weekend as soon as i get a chance. Should i run 89 octane normally or just every once in a while? I know i have to run 93 in my camaro, but it is high performance.

Longhorn Man · 08-19-2003, 04:21 PM

If the 89 solves the problem, then you should run it all the time.
Octain is basicly resistance to ignition. If it is igniting without a controlled spark (your spark plug) then you need a grade that will have more resistance to ignition.

08-19-2003, 11:42 AM	#1
PureChevy Registered User Join Date: Jun 2003 Location: Hickory, North Carolina Posts: 340	dieseling (sp?) Every now and then my truck trys to keep running after i turn it off. I think this is called dieseling. I am not sure. I was wandering what causes this and how can i fix it. thanks Will __________________ 1972 k10 4x4 LWB (Sold in 2005)

08-19-2003, 12:14 PM	#3
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	Lots of thens can cause this but the biggest offender is to much idle speed. Try reducing that first and see if it helps __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

08-19-2003, 12:19 PM	#4
PureChevy Registered User Join Date: Jun 2003 Location: Hickory, North Carolina Posts: 340	my idle speed is right at 600 rpms. is that too high It does seem like it happens when i have stopped abruptly and turned my truck off, like when pulling in my drive when i have been in a hurry or something. __________________ 1972 k10 4x4 LWB (Sold in 2005)

08-19-2003, 12:23 PM	#5
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	600 is correct but sometimes 590 will cure the problem...it only takes a screwdriver and five minutes to find out... __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

08-19-2003, 12:25 PM	#6
PureChevy Registered User Join Date: Jun 2003 Location: Hickory, North Carolina Posts: 340	ok thanks __________________ 1972 k10 4x4 LWB (Sold in 2005)

08-19-2003, 12:13 PM	#2
VTKidder Registered User Join Date: May 2003 Location: Vermont Posts: 996	Dieseling is caused by gas still making it's way into the engine, and igniting even when the ignition is off. I've heard that in a hot engine, carbon still left in the combustion chamber can be hot enough to act as an ignitor, and ignite the fuel still being dumped in. Other places that can be a snag and remain hot enough to ignite fuel are sharp edges of the combustion chamber or head ports caused by head rework. A small burr with very little volume will heat up very quickly compared to the rest of the metal in the head, and could be red-hot during operation, but still ignite fuel when ignition is killed. Other causes I've heard of are the incorrect heat range plugs not sufficiently burning the fuel-air mixture and a lean fuel-air mixture which will burn at a higher temp than a rich mixture. Timing is generally not the culprit here; timing that is off will lead to knocking under load, not dieseling. The first places I would try are the plugs and the fuel-air mixture. Any head rework done before? Just some places to check.

08-19-2003, 01:17 PM	#7
67Fleetsidedream Wiseass secured himself a BAN Join Date: Nov 2000 Location: MOON!!! Posts: 2,282	Crappy gas LOW OCTANE fuel will cause it to diesel. High compression ratio will cause it to diesel. Carbon buildup on the pistons and heads will cause it to diesel. The timing being off will cause it to diesel.

08-19-2003, 01:21 PM	#8
PureChevy Registered User Join Date: Jun 2003 Location: Hickory, North Carolina Posts: 340	i run 87 octane. It is a mild 350 rebuilt about 40,000 miles ago. __________________ 1972 k10 4x4 LWB (Sold in 2005)

08-19-2003, 04:09 PM	#9
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	VTKidder has a very good handle on the situation here (not to downplay anyone else's responce) The first thing i would do is gas up with some 89 octain. This will likely solve your problem. If id does not, and your timing isn't advanced very much, then I would spray some water into the carb. Yeah, you read right. Set the idle at about 1200 - 1500 RPM (with the engine already at normal temp and air filter removed) and then take a spray bottle and set it to stream. Speay the water directly into the carb's primarys. Don't spray it onto the walls, but rather aim it at the opening of the throttle blades. This stream of cold water will help break up deposits of carbon on the combustion chaimber. As long as you are not dumping the water in there, it will do no harm to your engine. Old timers in California are familiar with this trick, it used to be a good way to get a car that just failed the smog test to pass on the second time. Don't be allarmed when you see steam coming out your pipes, and if you are lucky, you may see chunks of carbon fly out too. When you are done. reduce the idle back to your previous speed, and then shut it off. Go for a test drive and haull ass back home and kill it as soon as you put it in park. See it it worked.

08-19-2003, 04:12 PM	#10
PureChevy Registered User Join Date: Jun 2003 Location: Hickory, North Carolina Posts: 340	I will try that this weekend as soon as i get a chance. Should i run 89 octane normally or just every once in a while? I know i have to run 93 in my camaro, but it is high performance. __________________ 1972 k10 4x4 LWB (Sold in 2005)

08-19-2003, 04:21 PM	#11
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	If the 89 solves the problem, then you should run it all the time. Octain is basicly resistance to ignition. If it is igniting without a controlled spark (your spark plug) then you need a grade that will have more resistance to ignition.

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