Dieseling

[dremu]

Okay, I'm stumped. Which doesn't take much, but I'm learning

Truck is my '74 K5, with a fairly new (20k maybe) GM crate 350, and a '75 hot-air style intake mainfold. Carb is a new-with-the-crate Edelbrock Q-jet replacement, 1903 I think.

Distributor is a reman; cap, rotor, wires, plugs are all new.

I've played with the timing until I'm blue in the t!ts and the damn thing still diesels when I turn it off. It's worse when the engine gets warm, i.e. it's really bad in hot weather.

After reading the diagnostic section in the factory service manual, I replaced the EGR valve, which seems to have helped. At the least, she starts much better.

However, she still diesels, and I hate turning her off in gear every time.

One oddity is that the points have been replaced with a Pertronix electronic ignition thing ... don't ask, not my idea. I could drop an HEI distributor on there, but I don't have one handy (well, except the one that's in my '77, and it's running so I'm loathe to mess with it )

Should I go and play with the timing again, now that the EGR is presumably good ... or is there something else that causes dieseling?

-- A

 

[dirtwarrior17]

All i can think of is timing or too low octane... what compression does it have?

does it ping?

 

[R72K5]

it is timing too advanced or idle too fast

real easy fix

do it before you damage engine,,,

if enigne starts right up without any hesitation and runs fine then just turn idle down

make sure choke fast idle cam falls all way donw and isnt keeping idle faster

 

[dremu]

All i can think of is timing or too low octane... what compression does it have?

does it ping?

Hmm. Donno about compression; the crate is the GM stock-replacement, not a high-output type, so I'll assume it's okay on 87. I spose I could try a tank of 91, but danm, at three bucks a gallon that's $100!

-- A

 

[dremu]

it is timing too advanced or idle too fast

real easy fix

do it before you damage engine,,,

if enigne starts right up without any hesitation and runs fine then just turn idle down

make sure choke fast idle cam falls all way donw and isnt keeping idle faster

Yeah, I know (engine damage) ... 'swhy I turn it off in gear. Which works fine, but it's just wrong

I'll take a gander at the idle setup, and recheck the timing while I'm in there.

Thanks all!

-- A

 

[big jimmy 91]

It sounds like you have a carbon build up causing hot spots that ignite the fuel after you shut it down.

You can run carb cleaner through it(spray in while running type) or pour water in it while reving it up not allowing it to stall.

Either method should help out the problem

 

[diesel4me]

idle speed!

My 86 305 is very fussy about idle speed with the Edelbrock "performer" on it--if its even 100 rpm's too fast,it will diesel every time I go to shut it off hot,unless I leave it in drive or reverse!-

-I tried retarding and advancing the timing,moving the vacuum advance hose from ported to manifold vacuum(and it runs better on manifold vacuum --first small block I've had that did!)..but lowering the idle speed is what stopped the dieseling for me..Its possible something is getting hot enough in the combustion chamber to ignite the fuel and keep it running after the key is off--but that would lead to detonation or pinging under load too..if its not pinging,its probably not the cause...

You might have to find a anti-dieseling solenoid and bracket for your carb(also called an idle stop solenoid)--GM used them in the 70's to eliminate this problem,it was quite common--it is a solenoid that has a plunger that pops out and sets the idle speed when the key is on--when you shut the key off,it retracts,and makes the throttle butterflies shut completely closed so it cant get any air,and it makes sure it shuts off...

I think they have the brackets and solenoids available from Edelbrock,or you could get one off a junkyard carb--IF simply turning the idle speed down does not cure it,thats what you'll have to do...or leave it in gear when you shut it off!--dieseling can cause timing chains to jump,and do other nasty things if allowed to continue...

 

[TorkDSR]

shut it down still in gear... it dies right away. or you could fix it... but i just shut it down in gear