79bonanza
1/2 ton status
i bought a 250 inline six 1978 chevy 4x4, and everytime i shut it off it diesels for a second, any idea what causes it.
CK5
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what causes a engine to "diesel"
- Thread starter 79bonanza
- Start date
GMC1977
1/2 ton status
idle speed set too high....
crashandburn
1/2 ton status
X2. Crappy fuel can cause it too. It is caused by the heat in the engine (some of it from compression) causing the fuel to ignite even though there is no spark. Just like how a diesel runs, hence the expression. I had an old car do it so bad once that I gave it some gas and it actually sped up. A cold motor will not diesel. Lower your idle and it should go away. Does your truck ping going up hills? If so forget the idle, and switch to better gas.
Can Can
Pusher Man
Too much fuel, too much timing, too high of an idle.........
Take it from there and get back to us.
Take it from there and get back to us.
79bonanza
1/2 ton status
no pinging but it does idle warm at like 1100 rpm, i though it was high but i never owned a inline motor so i figured it was normal
DEMON44
Low-Tech Redneck
also if theres enough carbon build up in the hole and its running a little lean or something that carbon gets hot enough to run the charge a few cycles after keying off.
1978 is getting pretty close to the "diesel years". For a while back in the 70's just about every engine out there would diesel when you shut it off because of various smog regulations relating to timing, and other things.
Since the car companies had to live with government engineering instead of automotive engineering, they put in antidiesel gadgets.
Most of them were carbs that would close completely when you took your foot off so as to choke the engine down and not let it have fuel or air.
The idle was controlled by an antidiesel solenoid. When you turned on the key and stepped on the gas, the solenoid would extend to hold the throttle open the correct amount for an idle.
When you turned off the key, it retracted, and the throttle closed completely.
You set the idle on those, by cranking the engine, and adjusting the length of the plunger.
But, be careful. Some of them were a two position plunger. It extended longer when the engine was cold for a fast idle.
Some 4 bangers, used the air conditioner compressor. When you turned off the key, it would turn the compressor back on for a few seconds to stall the engine.
Life was fun in the 70's.
Since the car companies had to live with government engineering instead of automotive engineering, they put in antidiesel gadgets.
Most of them were carbs that would close completely when you took your foot off so as to choke the engine down and not let it have fuel or air.
The idle was controlled by an antidiesel solenoid. When you turned on the key and stepped on the gas, the solenoid would extend to hold the throttle open the correct amount for an idle.
When you turned off the key, it retracted, and the throttle closed completely.
You set the idle on those, by cranking the engine, and adjusting the length of the plunger.
But, be careful. Some of them were a two position plunger. It extended longer when the engine was cold for a fast idle.
Some 4 bangers, used the air conditioner compressor. When you turned off the key, it would turn the compressor back on for a few seconds to stall the engine.
Life was fun in the 70's.
Fordum nailed it. If you can't seem to fix whatever is making it diesel then shut the engine off in gear then put it in park and it won't do it.
Man, speaking of dieseling, this last summer we went houseboating in the British Columbia Interior and our houseboat was fitted with a little 3.0L Mercruiser 4 banger that would diesel like crazy! It was actually amazing how long that little thing would keep firing after shutting the key off, I timed it for 2 minutes once!
I had brought a timing light with me to use on our personal boat since it had been giving us grief earlier in the year (3.0L Mercuriser, and you guessed it, dieseled!). I found the timing was stupid advanced so I backed it off and set the idle to a more reasonable speed, and while it no longer dieseled, it also wouldn't start anymore either... I played with the mixture for a long time and couldn't get the thing to run unless the timing and idle were both big time advanced.
So, I set it back to where I found it and let it diesel it's little heart out after every run, haha Same problem with our boat BTW, could not get that little engine to run without idling high or dieseling on shutdown. Must be a 3.0L Mercruiser thing.
Either way, I never even thought about shutting the engines down in gear -- Definitely gonna have to try that next go around.
I had brought a timing light with me to use on our personal boat since it had been giving us grief earlier in the year (3.0L Mercuriser, and you guessed it, dieseled!). I found the timing was stupid advanced so I backed it off and set the idle to a more reasonable speed, and while it no longer dieseled, it also wouldn't start anymore either... I played with the mixture for a long time and couldn't get the thing to run unless the timing and idle were both big time advanced.
So, I set it back to where I found it and let it diesel it's little heart out after every run, haha Same problem with our boat BTW, could not get that little engine to run without idling high or dieseling on shutdown. Must be a 3.0L Mercruiser thing.
Either way, I never even thought about shutting the engines down in gear -- Definitely gonna have to try that next go around.
It's either bad gas or bad timing. Forum nailed it x's 2. Does it start up fast when you turn the key? Or does it whirl, whirl put, put...crank? Does it smoke when you first start it? What color?
79bonanza
1/2 ton status
well it doesnt smoke and it doesnt start up real quick you know just kinda average for a carbed engine and idles around 1000 to 900 rpm, once it fires it runs pretty good once the choke pulls off, i put some new gas in it and it didnt make much of a diffrence i think ill check the timing just for the heck of it i guess but it seems right has good power all the way to about 4200 rpm doesnt miss or studer or anything. idk ill have to check over somethings i guess, im gettin ready to rebuild my carb maybe it will help some
This is your problem. When you shut the engine off it is pulling fuel from the metering circuit instead of being in the idle circuit. This is why alot of carbed engines used an anti dieseling solenoid on the carb, it allows the idle to be set to the proper spec but once you turn the ignition off the solenoid relaxes and fully closes the throttle blades so NO fuel continues to drip and make the engine diesel.
Scott got it here, when your idle is high it will diesel.
Now one more thing that can be improved that will cause dieseling is too much carbon deposit, if the psitons have too much deposits they wil change the compressions ratio, I don't remember if it's what causes the high idle to begin with or that is sepreate but I know lots of the four bangers I had in the 70-80s had that problem, and we just went with higher octane rating which helped. Higher octane will prevent detonation which is actually what keeps the engine running after you turned the ignition off.
As for timing, those years they had different timing from what we are used to seing, because of all the smog stuff that was imposed on them.
First time I was adjusting my 86 olds with a CCC carb and ESC dist, and set it at 8 BTDC it ran like crap, then I checked and it was supposed to be 20 BTDC.
And another I can't remeber what year was At TDC
79bonanza
1/2 ton status
what should i idle at, i thought standards were suppose to idle higher that automatic, should it be like 600 or 700 rpm not in gear idleing
Idle is usually between 550-600 rpm
79bonanza
1/2 ton status
ahhh ok ill reset my idle well and prolly my timing and see if that helps
Yeah, you could go as high as 700 but 900-1000 is too much.
79bonanza
1/2 ton status
ok, idk where i seen it but some engine is suppose to idle at like 900-1000 rpm idk tho i think it was a newer truck tho
Kenny78
1/2 ton status
I know with autos you set the idle in gear so that it wont lug at stoplights. I would think that with manuals you could set it at the low 550-600 not in gear.
