Engine keeps running when cold

[Peer81]

Hello everybody,

Question about something I didn't have before. I changed the 160F thermostat back to the original 195F thermostat to get the ECM (for my E4ME carb) into closed loop. After that engine is running great except for one thing.

When the engine is hot there is no problem, I turn off the key and the engine dies right away. But when the engine is cold (coolant temp is around 200F and oil is warming up) and then I turn off the key the engine keeps huffing and puffing for about 1 sec, then rev's up to 500 - 600 rpm (WITH THE KEY TURNED OFF) and then the engine dies.

Thing about this problem is that I read it could be the exhaust valve being to hot (running lean) and that would ignite the mixture, but with the engine still warming up and on the choke I don't think this is it. A good friend of mine said I should check the spark advance and change it from the 10 degrees (its on right now) so something more or less untill the engine stops right away. But I think the engine should stall right away no matter what the advance is (between factory 6 to say 14 degrees). Second option, with the higher thermostat the valve would expand more and not close perfectly. But with the engine warming up I don't know how this could work.

Thing is the choke is set like Lars describes and when fully hot there is no problem at all. Who has an idea?

Greetings Peter

 

[Peer81]

Nobody? :ugh

Greetings Peter

 

[n8dogg]

If I remember right doesn't the 81 have a shutdown relay on the carb that shuts down the fuel or air flow when the key is turned off? My 79 was doing this intill I took it out on some back hiways and ran it hard enough to get the carbon burned out of the cylinders was so bad that I had to shut it down in drive then go to park

 

[bb2kksy]

cold running engine

My 79 used to do that, then I started using mid blend fuel and it stopped.

 

[JohnZ]

"Running-on" as you describe is generally caused by the primary throttle plates being too far open at idle; when you turn the key off, the engine still gets air (and fuel) through the gap between the throttle plates and the throttle bores. I'm not familiar with late C3 Q-Jet setups, but some C3's had an ignition-activated solenoid that established the idle speed, and when you turn the key off, the solenoid retracts to fully close the throttle plates.

It's not the timing - when the key is off, there's no spark.

 

[bb2kksy]

I have an early 79 with a Mexican replacement engine. The only electrical connections on my q-jet is for the A/C when on ( which does not currently work) Wher I burn regulay gas, do not run it long enough to heat up, shut it off it will attempt puy-put. When I shifted to the mid-blend ( 89 octaine ) it stopped doing that. The onlt real electronic items in/on this vette is the ignition mod and the radio. Just mt two cents.

 

[Peer81]

Thanks everybody,

I'm not willing to change to a higher fuel octane. We're now at 1.50 euro's for 1 liter (euro95).
It could be the throttle plates, I couldn't get her under 800rpm so the plates don't seal perfectly to the bores. But how is this possible when there isn't no 12v on the ignition?
The late Q-jets don't have a idle solenoid.

Greetings Peter

 

[Rex S]

Pete, could also be a vacuum leak just about anywhere. Try shutting off the engine while still in gear (with your foot on the brakes-just for the ambulance chasers) as the extra load will not allow the engine to run-on.