Engine keeps chugging on after turning ignition off

bonzai272 · February 7, 2007

Hey guys. I just picked up a 1979 GMC C10 pickup. Its got a straight 6 in it, column shift 3 speed. When turning the key to turn the engine off, it will continue to chug and stumble along. It will sit there and chug itself on and on unless you turn the key back to the on position it will start to idle normally again. The only way to stop it is to stall it out. The guy at the smog station said the timing may need be retarded 2-3 degrees. He said it is set at 10 degrees now which is where it should be but that the law in CA will allow it to be retared or advanced up to 3 degrees each way and that I should retard it 2-3. Does this sound correct? if so, how is this done? Thanks for everything!

C & A s Dad · February 8, 2007

It could possible be a timing issue but before I retarded the timing, which well reduce your power slightly, I would suggest you complete a Seafoam treatment. Your condition is commonly called "dieseling" meaning the carbon in the combustion chambers is thick enough that it is actually keeping the engine running after the ignition is off. This is because the carbon becomes very hot, glowing red in fact and will continue to ignite the incoming fuel air mixture.

On this engine, I would pour a bottle of Seafoam in the tank and run it for a couple of hundred miles. Then, with the engine warm and at high idle, slowly pour about 1/2 of a can of SF through the throat of the carb. It will stumble and smoke something terrible but keep the idle high enough that it does not go dead. After about 1/2 to 2/3 of the can has been poured in it, dump the rest in at a faster rate a let off the gas so it kills the engine. Let it set for about 10 minutes and crank it back up. It will be hard to start and once it does, it will make you think the entire neighborhood is on fire with all the smoke. Drive that puppy up the road a couple of miles and you will see a tremendous difference in the way it runs.

The instructions to do this are also on the can.

bonzai272 · February 8, 2007

It could possible be a timing issue but before I retarded the timing, which well reduce your power slightly, I would suggest you complete a Seafoam treatment. Your condition is commonly called "dieseling" meaning the carbon in the combustion chambers is thick enough that it is actually keeping the engine running after the ignition is off. This is because the carbon becomes very hot, glowing red in fact and will continue to ignite the incoming fuel air mixture.

On this engine, I would pour a bottle of Seafoam in the tank and run it for a couple of hundred miles. Then, with the engine warm and at high idle, slowly pour about 1/2 of a can of SF through the throat of the carb. It will stumble and smoke something terrible but keep the idle high enough that it does not go dead. After about 1/2 to 2/3 of the can has been poured in it, dump the rest in at a faster rate a let off the gas so it kills the engine. Let it set for about 10 minutes and crank it back up. It will be hard to start and once it does, it will make you think the entire neighborhood is on fire with all the smoke. Drive that puppy up the road a couple of miles and you will see a tremendous difference in the way it runs.

The instructions to do this are also on the can.

Awesome! I have never even heard of Seafoam. So this is supposed to clear out the carbon build up I assume? Thanks again for you generous help. I will try that stuff this weekend and let you know how it worked! thanks again, I really appreciate it.

bonzai272 · February 9, 2007

Well I went to get it smogged (before SeaFoam treatment) and I passed the smog test except there was one vacuum line from the vapor canister to the EGR valve that was not hooked up so the smog tech made me hook it up and run the smog test again. I still passed the second time but the NO2 levels shot waaaay up. I asked how to bring those numbers down and he suggested cleaning or replacing the EGR valve.

He also suggested that a fauly or dirty EGR valve might be cause of some or all of the dieseling issue. Id rather just pull off and try to clean the EGR valve with carb cleaner instead of replacing it. I will do so tomorrow....hopefully that will help.

I also will do the SeaFoam treatment tomorrow after cleaning the EGR valve. I dumped a pint into my fuel tank last night so the first step is done.

Do you think checking/cleaning the EGR valve could be a solution?

redvett · February 11, 2007

Lots of engines in that era had idle solenoids if it does it may be adjusted wrong or not working. The solenoid opened the throttle to idle speed and when the ign. was shut off the solenoid closed the throttle blade to stop the engine.

bonzai272 · February 12, 2007

Well I dumped a pint of SF into the fuel tank and I put almost another full can into the carb until it stalled, waited 10 minutes and fired it back up, smoked a bit at first but nothing too crazy, smoke went away after a few minutes. I put the remaining SF of the second can into the crankcase per directions on the can. It still chugs and diesels on after turning the ignition on...although it does seem to run alot better, Im still concerned about the running on. It doesnt run on then stop after a few seconds.....itll keep running and running and struggling unless stalled out or turning the key back to the on position. Im confused!

C & A s Dad · February 13, 2007

Matt, if you put the SF in the crankcase as you stated, I would suggest changing the oil pretty soon. SF will clean out a lot of stuff and it will end up in your oil.

As far as the "running on", I am going to have to defer you to a real Tech. I would suggest you take a look for a Idle Solenoid as Redvett has suggested.

bonzai272 · February 13, 2007

Awesome...thanks for the help guys. I will change the oil this week and check witha tech or into that idle solenoid. Ill keep updated.

frdfandc · February 13, 2007

Dieseling is caused by fuel still getting into the combustion chambers which would point to one of a couple of things.

1. Timing way out of adjusment - possible stretched timing chain

2. Carbuerator needs to be either tuned, overhauled or replaced.

3. Too high idle speed - should be around 750 rpms.

4. Or built up carbon in the combustion chambers. - which you have attened to already.

mudrunner80 · April 7, 2007

It could possible be a timing issue but before I retarded the timing, which well reduce your power slightly, I would suggest you complete a Seafoam treatment. Your condition is commonly called "dieseling" meaning the carbon in the combustion chambers is thick enough that it is actually keeping the engine running after the ignition is off. This is because the carbon becomes very hot, glowing red in fact and will continue to ignite the incoming fuel air mixture.

On this engine, I would pour a bottle of Seafoam in the tank and run it for a couple of hundred miles. Then, with the engine warm and at high idle, slowly pour about 1/2 of a can of SF through the throat of the carb. It will stumble and smoke something terrible but keep the idle high enough that it does not go dead. After about 1/2 to 2/3 of the can has been poured in it, dump the rest in at a faster rate a let off the gas so it kills the engine. Let it set for about 10 minutes and crank it back up. It will be hard to start and once it does, it will make you think the entire neighborhood is on fire with all the smoke. Drive that puppy up the road a couple of miles and you will see a tremendous difference in the way it runs.

The instructions to do this are also on the can.

I've never heard of sea foam either.WOW I really need to get out more lol. my 79 does the same thing but assumed it was a timing problem. concidering its only a plow truck for my private drive way I was never too concerned. Its currently only run about 2 hours a year...I put more time in changing the oil then I spend actually driving the truck. Hopefully this year I'm able to find time to place that engine (350) into my 80 GMC. As of right now. its a bit of a mystery engine. starts today, won't start tomorrow.wait a day and it fires up. I'm guessing its a timing problem but I admit it...I'm a wimp to the cold I'm in no great rush lol

Back to the sae foam stuff...is that the actual name? and is it located by the other engine additives?

C & A s Dad · April 10, 2007

It is in a white 16 0z can with writing all over it. The part # is SF 16 and yes it is called Sea Foam. It is also avail in an aerosol called Deep Creep. We sell enough of it in a year to float the Midway.

Draenor · April 10, 2007

1. Timing way out of adjusment - possible stretched timing chain

Cadha13 · April 13, 2007

The GM L6's had more durable timing gears, not chains. I drove a 1978 GMC C15 250 L6 for years as my daily driver. The dieseling will disappear after setting the base idle (solenoid off and turn the screw in the back) to 425 rpm, Then set the curb idle (solenoid on, turning the whole solenoid) to 750 RPM with A/C on if equipped @ 10 degrees base timing for manual trans with the purge hose plugged and vacuum advance disconnected. With all the hoses connected the curb idle should be as high as 1000 RPM for the L6. The fast idle on the high step @ 2200 RPM Then the engine will be easier to drive. Unusual, but it works with and engine with over 180K miles and get good gas mileage. This is the specifications for a 1978 GMC C15 with light duty emission for federal (49 State) tuning. My 1978 shop manual has all of the specifications for all of the engines in all classes.