Engine spasms to a stop after turning the car off?!

[Guest livedrive]

Hey guys, I have a weird problem that just cropped up in the last week. When I put my car into park after running for a while and then turn the key off the engine spasms to a stop. It's like it's still getting spark after the car is off, and so it just chokes from lack of fuel/air I guess. I really have no idea, but it really resembles dumping the clutch on a manual shift, which is what makes me think that...

 

What is really weird is that is I pull my car in, leave it in drive rather than park on neutral, and then turn the car off it doesn't do it. I'm at a loss. Does anyone have an ideas as to what could be causing this?

 

Thanks.

 

BTW: If it helps at all I have a Chevy 383 in a '73 240Z.

[DaleMX]

Could be dieseling from carbon buildup on the heads and valves. I assume it knocks also when doing this.

 

If your not running a good name brand gasoline then you should be. This can also be caused by oil consumpsion caused by bad valve seals, or rings, or coming into the intake via the PCV system.

 

Get some GM topend cleaner and run it through (can't remember the name, but it sounds like soap). It will smoke like hell, but it will clean it up good.

 

Dale

[Guest 305240]

While twisting wrenches at dealerships before the fuel injection era, many a car come in doing that. We called it deciling. What was happening was the butterflys in the carb wasn't closing completely. It was a simple matter of adjusting the carb. When it "ran on" after shutting off, it was the open carb letting fuel into hot carboned cylinders. Stopping the engine in gear shut the engine off. I used to clean the carbon out with a pop bottle of water. Just make sure you have the engine reved up a bit and don't pour too much down the carb at a time as to choke it out. Just another old school trick.

[Pop N Wood]

I agree with the guys above. A bit of carbon build up on an already high compression engine causes an even higher compression ratio. This makes the engine run just like a diesel, the gas self ignites. Shutting the engine off while in gear puts a little extra load on the thing and makes it stop quicker.

 

My Z use to do that unless I bought premium gas and used octane boosters. Finaly realized the carbs throttle shafts leaked air. Putting on a set of webers allowed it to run fine with regular.

 

So do like the guys said above. Get out on the freeway and open it up to "blow out the carbon". But also check for vacuum leaks.

[Guest 305240]

Hey Livedrive! I've got a suggestion for ya that we can't turn down. How bout you get a keg or so , a bunch of steaks, and a bunch of us will come over and blow out the carbon for ya? All you have to do is make sure the beer is cold and the steaks arn't burnt! I talked to a few buddies and they want to come along too. How many more want in on this?

[Guest livedrive]

Hey guys, thanks a bunch for the info... One piece of info that Cozy Z Cole posted for me was this link: http://www.misterfixit.com/deton.htm

 

I'm going to get some of that top end cleaner you suggested, but I did note one thing from this site and that is that I recently had my plugs replaced... Do you think that could be my problem, or would you still think the most likely cause is the carbon build up?

[Guest BillV]

I have had plugs cause that, but they were old, not new. Check plugs, timing, use good gas. I have heard, but don't know if it's true or not, that when an engine diesels , it is running backwards. I don't think so, but have heard this more than once. Anyone else heard that?

[Capt_Zorro]

Bill,

It is possible for it to run backwards as the cylinder approaching tdc is fired by compression instead of spark, also possible for two cylinders to fire at the same time. Needless to say it can be hard on an engine for it to diesel on.

We used to just back the timing down a couple degrees to compensate for the carbon buildup, lower octane of the gas and it would eliminate the problem.

 

Adios Amigos,

[SpeedRacer]

It sure sounds like the engine is dieseling (also called run-on). Since it didn't do it before you changed your plugs it might be that the new sparkplugs are to hot. Since a 383 isn't a stock engine it may be a bit difficult to find the right heat range plug. I'd try using a set of plugs that are a couple of heat ranges colder and see if that helps.