John81287
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When shutting truck off...
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mtnmankev
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Low compression ?
firebane
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Usually referred to as dieseling.
lusktommy
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wow ive heard this happening before but never did get an answer on why it did.. i would like to think high compression?? or maybe ignition not shutting off completely? ive never really had to diagnose this problem on gasoline engine before. now diesel different story
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Dieseling, afterrunning, scares many drivers.
"Dieseling" in a spark ignition engine is a term used to describe a run-on condition which occurs after the ignition is turned off. It can be caused by a number of things, but your initial timing setting is not one of them. "Dieseling" occurs when the ignition is off, and therefore no spark is present.
For "Dieseling" to occur, you need the presence of air and fuel, as well as a source of ignition. In carbureted engines, an air/fuel mixture can continue to enter the engine during the coast down of the engine after the ignition is turned off. The engine continues to pull air thru the carburetor, and the carburetor contines to mix fuel with it. This can be aggravated by a high idle setting; more air/fuel is delivered. The source of ignition can be a "hot spot" in the combustion chamber, which remains hot enough after the ignition is turned off to initiate combustion. This can be a carbon deposit, sharp edge, or even the spark plug electrode itself.
Modern fuel injected engines are not plagued with "Dieseling", as the fuel AND ignition are turned off when the engine is stopped.
Some early emissions controlled carbureted engines exhibited "Dieseling", even when new. To control this condition, these engines used a solenoid operated idle throttle position control which permitted the throttle to be held in a more open position to provide sufficient idle speed, but closed completely when the ignition was turned off to prevent "Dieseling".
You need to focus on the causes: Too much air/fuel after ignition shut-off, the "hot spot" (source of ignition", or both.
Also when the cylinder creates its own combution heat...without the need for a spark. It's a combination of timing, running hotter than engine design intended (for emissions reduction) and higher compression. A normal internal combustion engine doesn't have enough compression to ignite the fuel charge only with compression heating effect. But, throw the other conditions into the mix, and it can (and does) happen with the ICE. GM used a lot of 'tricks' to prevent this, such as:shutting the air supply off completely, turning on the A/C compressor when the engine is cut off so that it will load it enough to stop, etc.
If you have a hi-compression engine with the emissions stuff still intact, you will have to get 'creative', too. Back in the day, most of us just put the car in gear when we turned the engine off (released clutch a bit after engine "off"). That would 'kill' the engine with no problem.
"Dieseling" in a spark ignition engine is a term used to describe a run-on condition which occurs after the ignition is turned off. It can be caused by a number of things, but your initial timing setting is not one of them. "Dieseling" occurs when the ignition is off, and therefore no spark is present.
For "Dieseling" to occur, you need the presence of air and fuel, as well as a source of ignition. In carbureted engines, an air/fuel mixture can continue to enter the engine during the coast down of the engine after the ignition is turned off. The engine continues to pull air thru the carburetor, and the carburetor contines to mix fuel with it. This can be aggravated by a high idle setting; more air/fuel is delivered. The source of ignition can be a "hot spot" in the combustion chamber, which remains hot enough after the ignition is turned off to initiate combustion. This can be a carbon deposit, sharp edge, or even the spark plug electrode itself.
Modern fuel injected engines are not plagued with "Dieseling", as the fuel AND ignition are turned off when the engine is stopped.
Some early emissions controlled carbureted engines exhibited "Dieseling", even when new. To control this condition, these engines used a solenoid operated idle throttle position control which permitted the throttle to be held in a more open position to provide sufficient idle speed, but closed completely when the ignition was turned off to prevent "Dieseling".
You need to focus on the causes: Too much air/fuel after ignition shut-off, the "hot spot" (source of ignition", or both.
Also when the cylinder creates its own combution heat...without the need for a spark. It's a combination of timing, running hotter than engine design intended (for emissions reduction) and higher compression. A normal internal combustion engine doesn't have enough compression to ignite the fuel charge only with compression heating effect. But, throw the other conditions into the mix, and it can (and does) happen with the ICE. GM used a lot of 'tricks' to prevent this, such as:shutting the air supply off completely, turning on the A/C compressor when the engine is cut off so that it will load it enough to stop, etc.
If you have a hi-compression engine with the emissions stuff still intact, you will have to get 'creative', too. Back in the day, most of us just put the car in gear when we turned the engine off (released clutch a bit after engine "off"). That would 'kill' the engine with no problem.
chengny
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The two most common causes I see are:
Excessively high idle speed (generally due to overly-advanced timing - but sometimes it's just that the idle stop screw is set too far in). Oh yeah, forgot one - it can also be due to an intake manifold vacuum leak(s)
Incomplete closure of the throttle blades (weak return springs).
But it really doesn't matter how - the air and gas are drawn into the cylinders. Just those two components alone, without some type of ignition source, cannot cause combustion (and the dieseling effect).
The root cause (as per Mr. RC10S) is the presence of some type of unintended ignition source. The ignition system presumably shuts down the instant the ignition key is moved to OFF. So the heat required for combustion comes from something other than the arc across the spark plugs. Generally that is from hot spots that develop on built up carbon deposits.
This is not to say that your pistons/valves are loaded with carbon - they are probably just as clean as everyone else's.
But everyone else might not have the idle set too high. Or too much throttle plate clearance. Or too much advance.
To achieve the diesel effect requires both an ignition source and a fuel/oxygen supply. Apparently, you have both those elements available in the combustion spaces - after the truck is shut down.
Excessively high idle speed (generally due to overly-advanced timing - but sometimes it's just that the idle stop screw is set too far in). Oh yeah, forgot one - it can also be due to an intake manifold vacuum leak(s)
Incomplete closure of the throttle blades (weak return springs).
But it really doesn't matter how - the air and gas are drawn into the cylinders. Just those two components alone, without some type of ignition source, cannot cause combustion (and the dieseling effect).
The root cause (as per Mr. RC10S) is the presence of some type of unintended ignition source. The ignition system presumably shuts down the instant the ignition key is moved to OFF. So the heat required for combustion comes from something other than the arc across the spark plugs. Generally that is from hot spots that develop on built up carbon deposits.
This is not to say that your pistons/valves are loaded with carbon - they are probably just as clean as everyone else's.
But everyone else might not have the idle set too high. Or too much throttle plate clearance. Or too much advance.
To achieve the diesel effect requires both an ignition source and a fuel/oxygen supply. Apparently, you have both those elements available in the combustion spaces - after the truck is shut down.
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rich weyand
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What is your base timing set to? Where is your vac advance connected, to ported vacuum or manifold vacuum?
John81287
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Thanks for the responses guys! I'm gonna go with the idle being too high probably. Just got a new carb a not ago and haven't monkeyed with the adjustments on it yet. I know I know I'm being lazy! @rich weyand. I don't remember where my base timing is set to, I do know vac advance is ported though. Ran like a top with the old carb just was a pain to start. I will look into this more tomorrow! Thanks again guys!
rich weyand
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Dieseling can also be caused by timing set too far retarded. The charge is still burning on its way out the exhaust valve, heating the valves and heads, and a higher idle throttle setting is required to keep idle rpms up.
Base timing set in the 4-6 degree range is too late. Better is in the 12-14 range, and you can go higher. I am running base timing at 17* BTDC with no knocking problem at all.
You should also connect your vacuum advance to manifold vacuum. You are idling way retarded on ported vacuum. Ported vacuum was introduced in 1968 to accommodate AIR pumps. Manifold vacuum has been used on engines without AIR pumps since 1930. If you have no AIR pump, you should run on manifold vacuum. Running on ported vacuum heats the heads and exhaust valves, which could be your ignition source for dieseling.
The combination of low base timing and ported vacuum advance requires a higher idle throttle setting to maintain idle rpms at spec levels, which means the throttle plates are open further when shutting off the engine. That's where your dieseling is probably coming from.
Also, what plugs are you running? I think a 1987 305 calls for R43TS. If you are running hotter-than-recommended plugs (like the R45TS used in 350s), the plugs can run hot enough to be your ignition source for dieseling.
Base timing set in the 4-6 degree range is too late. Better is in the 12-14 range, and you can go higher. I am running base timing at 17* BTDC with no knocking problem at all.
You should also connect your vacuum advance to manifold vacuum. You are idling way retarded on ported vacuum. Ported vacuum was introduced in 1968 to accommodate AIR pumps. Manifold vacuum has been used on engines without AIR pumps since 1930. If you have no AIR pump, you should run on manifold vacuum. Running on ported vacuum heats the heads and exhaust valves, which could be your ignition source for dieseling.
The combination of low base timing and ported vacuum advance requires a higher idle throttle setting to maintain idle rpms at spec levels, which means the throttle plates are open further when shutting off the engine. That's where your dieseling is probably coming from.
Also, what plugs are you running? I think a 1987 305 calls for R43TS. If you are running hotter-than-recommended plugs (like the R45TS used in 350s), the plugs can run hot enough to be your ignition source for dieseling.
350runner
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You should take the time to set-up the carb. You'll need a cheap vac gauge to set the air fuel screws. It's pretty simple and you'll like the results. Then set the timing after...
Edhard_Stark
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I'm currently dealing with the same issue, and it's more apparent after driving for a decent amount of time. When I put it in park I hear the engine(no tach) rise about 400rpms. Then I shut it down and it diesels for about 4-6 seconds. I turned down the idle air screws and I have yet to try it again since yesterday. I'll tinker with it again later today.
rich weyand
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Yeah, your idle is set way too high. Change the vac advance to manifold, then dial the idle down with the idle screw, not the idle air (mixture) screws.
That combination should solve the problem. It certainly won't hurt!
Edhard_Stark
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Haha yup I was going to do that today too, the previous owner had it tee'd with the trans to the carb. The idle screw is set, I just need to double check the vac advance after I separate the vacuum lines. The truck was running rich sitting, so that's why I tinkered with the idle fuel screws.
