Stange Shuddering of motor when i turn car off

dvldoc

Registered User
The car is now shuddering after i turn it of after driving it, I only does it after you drive it about a mile. Car is running fine, tach is steady at 600rpms nothing else changed on the car except new gauge cluster.

The whole motor vibrates for about 4 seconds and then stops, almost like a old carb vehicle running still after you turn off the ignition.

Any suggestions?
 
almost like a old carb vehicle running still after you turn off the ignition

Does the motor actually keep turning (running) after you turn the key off? Or are you saying that it shakes LIKE when a motor is dieseling (running without ignition)?

A carb'd motor had the ability to diesel after the ignition was turned off because the fuel flow wasn't disabled. Carb'd vehicles with a fuel-shutoff solenoid on the carb couldn't diesel/run on.

On a FI vehicle, the computer controls both fuel and spark. When key is turned off, both the fuel and ign are removed. The only way the motor could keep running is if there was fuel still getting to the engine. I would assume either leaking fuel injectors or a problem with the evap system.

Did this problem show up at exactly the same time you changed the cluster?

Vince
 
I had this problem with my 89 n/a T-bird, but don't remember how I resolved it. Is it possible that a faulty ignition switch could cause this?
 
Typical cause

...is carbon buildup in the cylinders. Carb or fuel injected motors will both exhibit the same symptom of 'run-on' if enough carbon builds up on the cylinder head. Once it gets sufficiently dense, parts of it will act like a sparkplug and cause fuel that is still in the intake to be ignited.

You can try various aftermarket cylinder/head/intake cleaning chemicals and maybe get lucky, or....you may have to resort to pulling the heads and having things cleaned up.

In any case, if this is your problem, you'll also want to determine why things are running rich enough to cause carbon buildup to occur. Start by running the codes, checking your air filter and considering a general tune-up.

Ken
 
KMT: You're bang on about the carbon acting like a sparkplug. My point is that in a MPFI (multipoint fuel injected) engine, there shouldn't be enough fuel in the intake for this to happen. On a TBI maybe, but not MPFI which is what all SCs are.

The computer injects the fuel charge for each cylinder while the intake valve is open. The purpose of the cam sensor is to determine when to inject fuel. In theory there should be no "latent" fuel in the intake of an MPFI engine when it's turned off. In practice, there likely is a little bit, but nowhere near enough to make an engine diesel for 4 seconds!

If dvldoc's engine is indeed dieseling for 4 seconds, then it has to be getting fuel from somewhere... Leaking injectors would be my bet.

Vince
 
Well found out what it was, fan was dying and that noise was water boiling back into the radiator overflow on shut off. Trust me my engine internals are dang near spotless after 2 years of alcohol injection.

Found out it was the fan when the sucker started to overheat just as I got home and the fan was damm near on fire and it melted the plug harness. The noise is now gone, but i had to pull the T stat to drive the car, Good think i only live a mile from work.
 
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