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Car keeps running after key is shut off.

Z-Priv
Z-Priv
in Help Me !!

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Posted
comment_1748

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.


comment_1751

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-

  • Author
comment_1781
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..

comment_1782

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.

comment_1884

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.

comment_1885
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]

comment_1886
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).

  • Author
comment_1887
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..

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