1984 c4 all stock..Car runs very well..About two weeks ago car started dieseling for about 3-4 sec after shut down..Complete tune up..rebuilt throttle bodies..reset iac,s..tps voltage .53..tried seafoam and injector cleaner..only run premium gas..There are no vacuum leakes either..Even switched from shell to exxon to mobil..It does it hot or cold..I am completely stumped..It actually does it worse when cold..Only does it sometimes when hot..Temp steady at 190..Idle steady at 600rpms..Throttle blades do close completely..Im at a total loss here..

Thanx for any help

 

Seems like the only thing it could be are leaking injectors. Pull the air cleaner and start it up, have someone else shut the key off and watch the injectors. No fuel should come out of them once the key is off.

 

Thats it..when i shut down the one injector does spit out a shot of fuel..Now i assume its probably the o ring not the injector itself? By the way thanx for solving this for me..

 

Also put a volt meter on the power wire to the distributer and make sure it immediately goes to zero. It seems strange to me that it dieseles when cold because you need heat and fuel for a car to diesel. Maybe the power to the distributor is not turning off.

 

I will check that too
thanx...what would cause power to still b in distrjibutor? Ignition switch?

 

That is possible, but let's first see if you still have power at the distributor when you key off.

 

I will check this saturday and post the results..thanx guys

 

I wil check that also..If that happens to be it then alternator needs replacement or is there something else causing it?

 

ok. I will check that tomorrow and get back to u..Thanx

 

Ok....neither injector is leaking and voltage goes right to zero when shut off plus no difference when alternator is disconnected. ..ive gone over everything i can think of today..Driving Me Nutz!!

 

Holy crap...I JUST FOUND THE PROBLEM...VACUUM FITTING ON TOP PLATE NEXT TO WHERE BRAKE BOOSTER LINE CONNECTS WAS CRACKED...PUT BRASS FITTING IN ITS PLACE AND PROBLEM SOLVED. .THANK U ALL SO MUCH FOR YOUR IDEAS...!!!!

 

I was wrong...thiught i found problem..Shut down correctly a few times..went to store and it diesels again..Back to looking for any suggestions

 

Is your base timing set correctly at 6 degrees? Seems very odd that it diesels with no spark and no fuel. It must be residual fuel in the intake and a hot spot in a few cylinders causing the issue.

 

I have a C3 that likes to diesel but have never heard of a C4 doing it. On the C3 it only happens when the engine and the ambient temperatures are HOT and you can look down the carburetor as see the fuel being pulled down into the engine and helping it diesel. I am running 12.25-1 Compression in my L-71 427 engine and it never dieseled until after I put a carburetor that was not set up properly and was running a bit rich. Fortunately I have a 4 speed so I can release the clutch and force it to stop, I just don't like doing that.

The suggestion to check your timing might be a worthwhile one. I have seen more timing cause/aggravate this sort of thing.

On my C3 it turned out that there was a lot of carbon on my cylinder head and when the carbon gets hot and glows it will cause the car to diesel if there is fuel present. I checked inside the cylinder using a small bore-scope and saw the carbon deposits on the cylinder head and the intake valves. I am going to try to remove the carbon without removing the cylinder heads. I am going to try the BG 44K fuel injection cleaner first and then go to the more drastic fix. BG makes a newer product designed to clean the combustion chamber and it looks very promising. It is called "BG Platinum Air Intake, Valve and Combustion Chamber Cleaner" and claims to help eliminate deposits inside the engine.

There are naysayers who call BG products "Snake Oil" and ridicule the users, I only recommend products from companies that I have experience with and TRUST! I have used BG 44K for a few decades and it has always helped my fuel systems. If they did not work I would not suggest them to you. I first learned of BG from a mechanic friend and Mechanics all over the country use this stuff every day. BG is a quality company who makes quality products, No, I do not work for them...

Does your car have any aftermarket ignition or charging equipment?

I wish you a very Happy New Year driving your Corvette!

 

Hey..my base timing is at 6deg..And i agree how strange this is..If i go outsude and start car for first time right now.. (30 degrees outside) and then sbut it down right now it will run on worse now then at any other time today..when its warm its not as bad..Im gonna put new ac delco plugs in it tomorrow just for the hell of it..ngk V plugs in it now.. (under 6 months old).. I spent over 5 hours yesterday pulling everything apart .cleaning and checking and resetting everything possible.. And the car is completely stock except for poly bushings and muffler eliminators..

 

bjankuski is right; shouldn't diesel when cold. Shouldn't diesel at all, being EFI.

I'd love to see a video of this. I'm not convinced it's actual dieseling that we're talking about here.



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I was thinking that also..Maybe its not "dieseling"...But it does run 2 to 3 sec after key is off..sputters til it completely stopped..Like it takes a big breath before it stops...I will take a video of it tomorrow cause im not going to be home today. Not sure if this helps but car doesnt start by just turning key..I do have to put foot on gas pedal when cranking..If i only turn they key it will try to start but it will usually just stall out.It does have a new updated fuel pump..

 

have to be timing

 

Its not timing..set at 6 degrees...

 

Why would timing make it run on after you cut the spark? With the ignition off there is no timing.