Know this is easy-Why does a gas engine diesel when u shut it off?

[Tims450SL]

I know how elementary this must be, but it could be the root of my terrible gas mileage...all seems ok slight miss during speed, but then on occassion it like diesels when I shut it down. What is that about?? :0) Thanks all!

[Roncallo]

Originally Posted by Tims450SL

I know how elementary this must be, but it could be the root of my terrible gas mileage...all seems ok slight miss during speed, but then on occassion it like diesels when I shut it down. What is that about?? :0) Thanks all!

You have leaky injectors and it may be the cause of your terrible gas milage. Try a bottle of Cheveron Techron before doing anything.

John Roncallo

[Carson357]

i had that problem, mine was i was using very low octane gas in a high compression engine, my car would literally run in reverse when i would shut it off, once i began using better gas with higher ocate problem was solved.

[ScottinSoCal]

Originally Posted by Tims450SL

I know how elementary this must be, but it could be the root of my terrible gas mileage...all seems ok slight miss during speed, but then on occassion it like diesels when I shut it down. What is that about?? :0) Thanks all!

Diesel fuel doesn't require a spark to explode, compression and heat cause the explosion. Gasoline shouldn't explode until it gets a spark. The octane rating on the pump is a measure of how resistant the gas is to exploding with just heat and pressure. Higher octane=more resistance to spontaneous explosion. A gas engine will diesel after you shut it off if there's enough heat and pressure to overcome the gas's resistance and it explodes spontaneously anyway.

Ron is right, in that you have to keep a supply of fuel going to the engine for this to happen, and with one of these cars that can only happen if your fuel pump is still energized (bad wiring) or you've got a leaky injector. However, having fuel available isn't enough - there's still that heat and pressure causing the gas to explode without a spark. In older engines, carbon deposits can build up in the cylinder, leaving less room in the cylinder and increasing the compression of the engine. Using a higher octane gas would overcome this, if the deposits aren't continuing to build up. If they are, eventually even the highest octane gas can get enough pressure to explode spontaneously. The real solution is to clean up the deposits in the cylinders.

The good news is, you've got good compression in your engine. The bad news is, you've got too much of a good thing.

[ScottinSoCal]

One more thing:

Originally Posted by Tims450SL

...all seems ok slight miss during speed

This may not be a miss. Combined with the dieseling you've got, this is probably pre-ignition (knock and ping), which is the gas exploding before it's supposed to. It's the same problem as the dieseling, but it's happening when your engine is running. This damages your engine. I'd suggest using a higher octane gas until you get the other issues taken care of.

[Tims450SL]

i did it- and it doesnt deisel anymore thanks!

[tk0084]

Can you tell exactly what you did please? Did you change octane, replace injectors, fuel pump wiring????

Thanks

[Tims450SL]

i used supreme, and a bottle of the chevron tecalyne (sp) and I now have noticed it it running better too. I put mid grade in it this weekend (since our prices are sky rocketing) and it is doing the diesel and wasting gas. I guess it is about Super and drive very lightly.