Car keeps running after key is shut off.

[Z-Priv]

Hey all,
I know this is probably a dumb question but i cant remeber what causes this. I have a 71' 240z which run great except when you turn the car off it sputters and revs and doesn't fully shutoff for about 5 or so seconds. I know this is a comman occurance with cars i'm just not sure exactly what causes it. :confused: I searched the forums already and didnt find anything, if there is already a thread about this that someone knows about please let me know. Thanks in advance for any help.

[SoCalZ]

Z-Priv check the timing. See this under: the weather...
Author: Matt 78Z http://www.zcar.com/forums/read.php?f=1&i=229083&t=229083

:)

[Dale B.]

My first Z did something like that. I had a bone stock '73 and sometimes when I would shut off the engine it would sputter then free spin backwards! If you had it in 1st gear and let the clutch out while this was happening it would lurch backwards! Very unsettling! I was led to believe that this could be caused by carbon deposits on the tops of the pistons igniting unburnt fuel after shutdown. Anyone else experience this? That particular Z had Over 200k miles on it and had never been apart. My current '73 has only 30k miles on it and it stops running the second the key is off.

-Dale-

[Z-Priv]

Originally posted by SoCalZ
Z-Priv check the timing. See this under: the weather...
Author: Matt 78Z http://www.zcar.com/forums/read.php?f=1&i=229083&t=229083

:)

Thanks for the link SoCal, i was thinking maybe timing also.

[Z-Priv]

now after reading those posts im not too sure now. seem it could be a few things.. gave me a couple of things to look into though.

[SoCalZ]

My 73' does it with regular gas so I run mid grade now. Do you a standard ignition in it or upgraded to electronic?

[Z-Priv]

Originally posted by SoCalZ
My 73' does it with regular gas so I run mid grade now. Do you a standard ignition in it or upgraded to electronic?

It's totally stock. am going to put a new ignition in though. I got a lot of idea from the zcar forums i can try, including better gas and some decarbon.. timing and the carbs were both dialed in two weeks ago, might get a valve adjustment also.. thanks for your help..

[PetesPonies]

you have what is called preignition. This is where the fuel charge is burned before a sprak occurs. In your situation, there is no sprak at all because the ignition is turned off. There are many reasons for this. As fuel degraded, this became a problem with car companies. They did several things to combat it. One was to have a solenoid control the throttle valve on the carb. When the key was turned off, the throttle closed competely which did not allow air into the engine, thus nothing to burn. This is not the case with these engines however. As carbon builds up in the combustion chamber, the carbon holds heat. this heat is enough to fire the mixture. Sometimes a little water poured into a running engine will help get rid of the carbon. You can also run cooler spral plugs which will disapate hear more and not stay as hot, which can burn the mixture too. Higher octane fuel doesn't burn as easy as lower octane. This is exactly what you pay for., having the fuel more difficukt to burn. This in turn stop fuel from burning when it is not spposed to, either as preignition or along with a spark which is detonation. High comprerssion engines will detonate easier because of the added heat from compression. So high octanee fuel resists this. You may try some higher actane. A vacuum leak of any king will contributr to this. as will improper timing. Try some of these things and let me know what you find out.

[2ManyZs]

Something else crossed my mind, besides timing etc. being the culprit it could possibly be that your ignition switch is bad. My first Z wouldn't shut off without wiggling the key in the switch. Just another idea, in case the other things don't help.

[Mike]

Originally posted by Z-Priv
Hey all,
I know this is probably a dumb question but i cant remeber what causes this. I have a 71' 240z which run great except when you turn the car off it sputters and revs and doesn't fully shutoff for about 5 or so seconds. I know this is a comman occurance with cars i'm just not sure exactly what causes it. :confused: I searched the forums already and didnt find anything, if there is already a thread about this that someone knows about please let me know. Thanks in advance for any help.

Does your car 'diesel' or continue to run very poorly? You did mention that it sputters and chokes, but, I wasn't really sure.

Anyway, if it continues to chug and lurch like it wants to continue to run.... you might have a problem with the idle (or even the carb adjustments).

Sometimes people tend to adjust the idle screw because their car doesn't idle smoothly. So, what do they do? They just increase the idle speed. This isn't good because it can cause the 'dielseling' symptom.

I would start with a good timing adjustment, a good carb adjustment, and also check to make sure there is enough OIL in the carb pistons.

You might also want to change the spark plugs and check the engine compression while you're in there.

[m]

[Mike]

Originally posted by Z-Priv
It's totally stock. am going to put a new ignition in though. I got a lot of idea from the zcar forums i can try, including better gas and some decarbon.. timing and the carbs were both dialed in two weeks ago, might get a valve adjustment also.. thanks for your help..

If someone adjusted your carbs, they might have done a few things to 'cheat' in order to get it running smoothly. My previous message indicated that some people modify the idle screw in order to get it to idle smoothly.

A good idle (for a car with halfway decent SU's) is about 800 rpm. If you're idling over 1,000 or so... you might get that effect.

I used to be good enough at adjusting my SU's so that the idle screw wasn't even touching the carb. They would idle perfectly at 800rpm w/out any help of the idle screw.

A good way to check this is to loosen your idle screw completely (when the engine is running).

[Z-Priv]

Originally posted by Mike


Does your car 'diesel' or continue to run very poorly? You did mention that it sputters and chokes, but, I wasn't really sure.

Anyway, if it continues to chug and lurch like it wants to continue to run.... you might have a problem with the idle (or even the carb adjustments).

Sometimes people tend to adjust the idle screw because their car doesn't idle smoothly. So, what do they do? They just increase the idle speed. This isn't good because it can cause the 'dielseling' symptom.

I would start with a good timing adjustment, a good carb adjustment, and also check to make sure there is enough OIL in the carb pistons.

You might also want to change the spark plugs and check the engine compression while you're in there.

[m]

had everything you suggested checked and adjusted two weeks ago and it still did it..
2manyZ's you where right it was the ignition switch. Fixed it this afternoon. Thanks for the suggestion..

[Mike]

Originally posted by Z-Priv
had everything you suggested checked and adjusted two weeks ago and it still did it..
2manyZ's you where right it was the ignition switch. Fixed it this afternoon. Thanks for the suggestion..

Can you explain the fix? I don't see it. You replaced the ignition switch and it fixed your dieseling?

Or, was your car continuing to run at full idle speed after you turned off the key?

[2ManyZs]

Glad I could help. Everyone else was giving you the obvious things that cause it, I just thought of the one thing people don't think about until it's the obvious one.

My first Z was a 73 I bought around 81 or 82. It wouldn't run when it when the engine was warm so the girl traded it in. I bought it and tinkered with it for a couple weeks before someone told me to check the water contol valve going to the carbs. I blocked that and it ran great. It also had a ignition switch that I could start the car and then take the key out while it was running. After a while it wouldn't shut off without stalling it in gear, it was shorting out in the switch. Sometimes it ran smoothly, other times it ran rough like it was "dieseling". Since these cars are now 30 years old it is going to be more common since this happpened to me when my car was less than 10.

The greatest thing about this forum is the variety of answers you can get to a question, the answer is bound to be there somewhere.

[My260Rocks]

Yea, im confused how an ignition switch would fix the problem. My car liked to diesel until I dropped its idle abt 250rpm and that was the end of it. If you turn off the key and the thing sputters it should be a predetination(sp?) problem. If it were an ignition problem then the dern thing wouldnt spark OR you'd turn off the key and the dern thing would run until you unpluged your distributor or coil! But hey, these cars to the craziest things.
:confused:

[2ManyZs]

In my case and evidently in his the switch contact were worn, the barrel of the switch gets worn out, and what happened is it wasn't cutting off the ignition currant. It was as if the key was still in the run position. When you are dealing with cars as old as these weird things are bound to happen, at least they are still simple enough for most anyone to fix, sometimes it just takes a little time to find the cause.