Question/suggestion about stopping a dieseling engine.

[Guest bastaad525]

Well I'm still having problems with dieseling. I've tried and checked everything I know to be a cause or remedy but have had no luck. Checked for vacuum leaks, adjusted timing both ways, tried different spark plugs, had the carbs fine tuned on a exhaust gas sniffer machine... you name it. Anyways, that's not why I'm posting this. Up til today, I've been using the tranny to stop the engine from running on, by putting it in 5th gear and slowly letting out the clutch to stall the motor after turning the key off. This still seems to be pretty rough on the drivetrain, and nine out of 10 times the car shuts off with a very loud clunk. Anyways, today I had this idea, to instead try to shut off the car by opening the choke fully. Sure enough this stalled the engine near instantly. What I'm wondering is, am I doing any harm to anything by suddenly enrichening the mixture by so much, so quickly? Even though the motor shuts off right away? Would this method be preferable to using the tranny to stall it, or should I keep doing it that way? Advice please

[Guest zfan]

My Toyota did this and low and behold the bugger needed a timing chain/belt. I hope this is not the case.

 

Mike

[Guest greimann]

By pulling the choke you are effectively having the motor shut down the sway it is supposed to: removing the spark and air. All the choke does is close the flapper door above the carburetor to enrichen the mixture by removing air. It does not add any fuel. Your dieseling is most likely caused by having your idle circuit open too far. This can be caused by cranking up the idle screw because you have capped off the PCV, You haven't capped off the PCV have you?

 

Otherwise, combustion deposits are also a contributor to dieseling. Run a tank with some STP valve cleaner.

[Guest bastaad525]

my chain is still good, it was replaced about 20k miles ago with a factory Nissan chain. I haven't capped off the PCV valve. About the choke richening up mixture, I have to clarify. I'm running SU carbs on an L28, and by opening the choke it is most certainly richening the mixture, by lower the nozzles and leaving more space open between the nozzle and needles. Also it cracks the butterflies open a bit more when opened all the way.

[Guest bastaad525]

Anyone else? Am I harming my engine by stalling it out with the choke or what?

[DavD3]

I have the same problem as you have.I run L28 with E88 and ZTherapy SUs,engine is dieseling almost every time as i turn it off.I figure out that the reason for this is excessive heat under the hood,because i tried to shut off engine with hood open and it was normal,no dieseling whatsoever.I switch to 280z ventilated hood,little bit better,but still have a problem.Next step is putting electric fan.SummitRacing has one for about $80 MrGasket,CFM close to original.I should mention that i have now an extra one pusher type from 200sx,but this one and original didn't help.So next step if electric fan will not help,i'll go to Stant 160 degree thermost or i might do it at the same time.I also thought of a header blanket ,as long as my header doesn't have jet hot protective coating.Not sure about exhaust jacket or header blanket though,but MSA catalog say that header wrap decreses temperature under the hood up to 70 degrees.The only thing my 240z is not my daily driver and right now i don't sufficient funds to go all the steps,have to feed my new business,so if you find something else let me know how to cure this problem.

[Mike C]

Lower your idle speed and lean out the idle mixture. This should help significantly. Also, a switch to colder plugs can make a world of difference.

[Pop N Wood]

Higher octane gas will almost certainly cure the problem. Exspensive solution though.

 

Also buy a can of starter fuild and check the SU throttle shaft bushings for air leaks. Spray the starter fluid around the throttle shafts and various manifold gaskets while the engine is idling. If you see a change in idle speed, you have a vacuum leak that is contributing to your problem. I had this very problem in my 70 240. Once the seals are gone, you have to enrichen you idle mixture to compensate. This will contribute to dieseling.