Engine sputter after turning off ignition

[904Zster]

Hey guys, just wondering if anyone had this experience. After I turn the keys to the off position, I'd expect the engine to stop. But I am experiencing something that I don't expect. My engine will sputter on for a while. Sometimes just a few seconds, sometimes it'll sputter, crank, then final come to a stop.

I have a 240Z with new pertronix electronic ignition.

Any ideas?

[superfunk]

I've had that happen on older cars before. I believe term for it is Dieseling. Not too sure what causes it, but its pretty much just the momentum of your engine keeping it going.

I'm sure someone here knows exactly what causes it and how to stop it.

[e_racer1999]

IIRC it's caused by to rich of a mixture. there is unburnt gas in the cylinder and it ignites with the heat of the CC. or is it that it's too lean and the hot piston ignites the fuel? i don't remember. but you can probably consider it a fuel mixture problem. if you're running SUs, test and see if you're running a proper ratio.

now that i think about it, i think that when i advanced my timing it went away.

[The Wingnut]

...it's also a result of too much advance or not a high enough grade of fuel. Try to find a happy medium.

[904Zster]

I was always under the assumption that since my 240 is carbureted, that I can put in regular 89 octane(which I did this morning!). So, I guess I was wrong. What is the recommended octane for the 240s?

[904Zster]

Well, now my friend who use to have a 260 told me that regular gas for 240s are fine. So, it may not be the gas. Carbon buildup? hot spot on the valve? My mechanic told me that this is common on these old Z's.

So I guess I'll just have to deal with it and see if it goes away...

[DeesZ]

From my experience, "run-on" is most often caused by a timing issue. Fuel could also be involved. Make sure that you have good fresh fuel. Mine (straight stock) runs flawlessly on 87 octane as it is supposed to. Valve clearance adjustment or excessive carbon in the combustion chamber could be at issue too. Try the process of elimination doing the easiest first - timing adjustment. Work your way up to the harde$t task, that of removing the head to get the carbon out - also, some folks like to run Sea Foam through the engine to loosen up carbon - many swear by this solution and you will find some threads on it here in the archives.

Please let us know what you find.

[Z train]

1-Set the timing

2-set the idle

3-set the mixture

[904Zster]

Wow. SeaFoam?! never heard of the stuff. As for all your recommendations, especially the one from DeesZ, I'm gonna assume that since I just got the car back from my Z guy, that timing is probably not the issue. The engine is running great. It purrs down the road when I'm driving it.

Plus, I'm not a mechanic, and not really qualified to open up anything in the engine, without messing something up. So for me, it seems like SeaFoam is the best first line of attack. From all the stuff I just dug up it looks pretty straight forward, and not really all that technical. The only thing that I might have to do is a simple oil change afterwards to get all the gunk out.

Anyways, thanks to all who responded and gave me tons of great solutions.

[FREE-Z]

i had this and all it took was to back off the idle screw about a 1/16th turn. also to make sure the rpms were around 1000 or just below before turning off the key. very simple for mine but all the other stuff is a possibility. please dont take my word for fact here, i am a rookie. :stupid: but this was something even i was able to slove. be easy on the carb screws. it freeks me out to mess with them. good luck!

[DeesZ]

Really good point on the idle setting!

Also, in your shoes as things seem to be fresh back from the 'Z-guy', I'd take it back to him and let him have a chance to give you what you paid for, a reasonably good running car. If he knows his stuff, this should be a quick fix since he's aware of what he's done up to this point, and something has changed recently from when it ran well. Perhaps the bolts were not seated tightly enough on the distributor allowing it to shift slightly (which would alter your timing), or something simple like that? Personally I think that Sea Foam may get some crap out of the engine, but I still leaning to the more mechanical adjustments as being the root. In his post Z train kinda summed it up pretty well.

[FREE-Z]

just wanted to be more clear on my deisling experience. when i'm out running around and come to a stop the idle is at about 1100 to 1300 rpms. i tap the gas kinda hard and let off....the rpms drop back to about 800 to 900 rpms. then off goes the key. this seems to clear the lines and the carbs of extra fuel. before my 1/16th turn adjustment it ran a bit higher. dropping to 1100 to 1000 rpms. (it may have been even 1/32 turn. keep it small and wait to see if it helps, or try the gas tap trick first) this seems like a good first step as it will be much more simlpe then valves and timing. like yours my engine purrs like a cougar when at running temp so i doubt it is timong or the like. no offence z train. im a rookie here:stupid: hot spots in the fireing chamber would be true diesling and much more serious. this sounds like your just sucking gas. i also run as high octane quality fuel as i can. dont skimp on gas. modern fuel has more ethenol then 1972 gas. especialy the cheep crap. get high octane w/techron or even add "stabil" fuel stabilizer to keep it all water free and clean. i add "stabil" for the winter and my spring fire up seems better then in the fall! on the fuel note , at 20 cents more per gallon, at ten gallon fill up, thats 2 dollars more per fill up. seems like a get about 5 to 6 dollars more out of it in milage. same goes for my 2000 ford. that pays itself off, especaly when considering wear and tear, spark plugs and filters and such. i hope this is a help and not a hinderince. good luck! i hope all it takes is a tap on the gas and or a slight turn of a screw for you! drive safe out there!:laugh: