My 389 Pontiac has started to diesel after shut off. I don't think it is idling too fast. Noticed it earlier this summer and put a new carb (Edelbrock 650) before taking it to MOKAN. Still doing it. Motor has about 120K on it, could the timing chain be getting stretched and throwing the timing off? Doesn't seem to be running hot and starts great. I didn't change the carb to fix the problem, the old one was junk. What do I check?
try looking for a vacum leak, collasped vacum hose, blown carb base gasket, bad or sticking pcv valve something like that. also make sure your choke and fast idle cam are working properly. timing will not cause dieseling
I considered the carbon build up but I figured flogging it four hours both ways to the drags should have blown the cobbwebs out pretty well. Looks like a difference of opinion on the timeing. Guess I'll check it to make sure.
i did not mean to sound like a smart ass, i was writing my post when you were posting yours, sorry.....
I say it's junk sell the car to me Ceap an I'll be there early in the morning to pick it up.... I think it's timing. Or bad gas
usually timing and or a little too high an idle would cause it not a collapsed hose or pcv or whatever
GV, I guess you are talking about your Grand Prix. Is that 120K without ever having a new timing chain? I have owned a boat load of 389 and other Pontiac V-8 cars, and never had one go over 60K without a chain change (except for my '55's). Even if it had one at 60K or so, it's probably due again, and that is the first thing I would try. Don
My '64 Motors Repair manual on Pontiacs; "When there is a complaint of carburetor troubled and the car is equiped with a PCV Unit it is advisible first to check the operation of this unit before servicing the carburetor." If it has a PCV valve it could be stuck open making a big vacuum leak, or plugged up causing you to need the carb idle speed too high to compensate for the lack of the regulated vacuum leak a PCV is designed to allow. Either way, it could be the problem. Or it still could be carbon causing glowplug like hot spots. In the '70's GM put electric solenoids on the fast idle that completely closed the throttle when the ignition was turned off. Did your (earlier) car have something like that at one time?
Why do you say it won't cause dieseling? If the timing cycle is out of phase it will definately try and detonate.
First thing is to try better grade of gas first then try timing change. I have a 454/427hp in my Lincoln and it does that when I run low octane gas.
i have never quite figured out how timing can cause dieseling, if the power is cut to the distributer the plugs wont be fireing, it has to be a hot spot inside the combustion chamber lighting things off. what is the cause?, is it only carbon build up?
timing that is set too slow, or retarded will require a larger throttle opening to maintain proper idle speed, the large throttle opening causes the dieseling, its just a vacum leak. if you set the timing correctly and do not adjust the idle it will still diesel, or after run.
Even when things are adjusted correctly a abnormally high idle will also cause dieseling when you shut off.
Idle speed too high, carbon buildup are the most common. If the bottom end & rings are in good shape about 10 full throttle starts up to 70-80 MPH (just stick it in "Super" & punch it) will clean the crap out of the chambers, sometimes fouling a plug or 2 from flying chunks of carbon. Then set the idle @ 450-475 in "Drive" with all electrical turned on & park brake set. Give the gas pedal a quick tap; if it kills the engine, slowly raise idle speed til it stays running. If your engine is AFB equipped, use the large air bleed screw to adjust speed, not the mechanical linkage. If 2 bbl, check the front carb mount studs - heat riser gasses will erode them & base gasket loses seal. Timing chain ? Pontiacs with timing chain problems just jump & quit ...
If your positive the timing is right and you cant find a vacume leak,then i would have to say its carbon build up on piston tops .I have heard of people pouring ice water down the carb while the motor is hot and running to make the carbon turn loose.
so i dont see how the throttle plates in a carb being open a little or a lot can make a engine diesel, if you roll you engine over on the starter with the ingition turned off and you foot holding the throttle open it wont start will it? if you have an engine that does not diesel, and you turn the idle screw in till its running at 2000rpm's and then kill the ignition will it diesel?
I've done the water trick before, and it does work, IF cabon buildup is the problem. Most likely it is the problem. You might try a little vodka or scotch with the water, your engine might like it! Bernie
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