L28 runon!

[zeiss150]

Hey Z people... Why oh why, is my car running on!!!!! My valves are perfect(10 I 12 E) my timing is about 10 degrees advanced. I use 91 octain (I dont think that makes a differance). My compression ratio sis 9.2 :1. I have a performance cam 460 lift 270 duration with beef'd up springs. So if my valves are set right and my timing is good what elese could be causeing the runon?! Could it be something in the dizzy? could it be an exaust manifold leak? Or maybe a hidden vacum leak? Any Ideas would be great, I'm pretty tired of using second gear to stop my car.

Rock on Z people

Matt-

[MikeW]

It seems to me like this would be an electrical problem. When you turn off the ignition the spark plugs are supposed to quit sparking, right? Isn't that what really stops the engine - not cutting off the fuel flow. I'm no expert on the ignition system, especially for a fuel-injected 280Z, but that's where I'd start looking.

[zeiss150]

OOOOPs! I forgot to mention that my L28 is an SU powered hot rod. No fule injection for me (untill I drop a turbo in). My SU's also have SM needels. Your probably right MikeW It could be and electrical problem, on the other hand it could be that the compression champer glows red hot and with the higher compression that could be enough to ignite the air/fule mixture. By the way It only does it when the engine warms up.

Matt-

[MikeW]

Will it continue running indefinitely? I'd carefully pull off the coil wire while it's running. I'm guessing that will stop the engine. If not then your hypothesis may be correct. Since you don't have fuel injection fuel will continue to flow and if it's continuing to run then that might indicate the equivalent of pre-ignition. If that's the case then a higher octane might make the difference.

[zeiss150]

It wont keep running indefinatly but for about 5 seconds or so. Ill try the coil trick, carfully! Im already using 91 octain which is the highest we have here in Southern California.

Matt-

[MikeW]

Since it only runs for a few seconds then I'd say it is what you suspected. Your compression ratio is obviously higher than normal so I would expect that you'd notice some pre-detonation while driving as well. I wonder if one of those octane boosters would help?

[v12horse]

Does the runon occur even more when it is hot out or does it not matter? I had runon before and it was because of an exhaust leak. During the heat of the day it would runon a lot more than in the mornings.

Ben

[2ManyZs]

Before you go too far, the obvious thing no one has mentioned is the idle speed. If it is 850+ combined with the advanced timing, that could be the largest contributor to the problem.

Make sure your idle is under 800, and perhaps try to retard the timing back one or two degrees, if for no other reason than to rule out the timing as being the culprit.

[zeiss150]

good good ... more tips! Keep them comming!

[Gav240z]

You beat me to it 2 many I was gonna say the same thing. An advanced ignition timing will cause the motor to run hotter. Combined with the higher idle when you switch her off it's very hot inside the chamber and the higher the idle the more likely it is to keep on ticking over when you get pre-ignition.

It could also be carbon build up on the plugs or inside the motor in general. However this is less likely.

Don't worry I know how annoying it is to have to stop it by using second gear ... :cheeky:

[tehghey]

Sounds like it's "deiseling" - it combustion without spark like a diesel engine. I had the same problem on my 210. It has an anti-dieseling solenoid that cuts off the gas to the idle circuit after the car is shut off. My solenoid wasn't receiving power and so it wasn't working, and when it was very hot it would run on for a few seconds after shutting off the car. But I guess the Z engine doesn't have something like that so I'm not sure how you would fix it. I think it's bad for your engine though. Hopefully one of the suggestions from the other guys will help, but I would lean more toward adjusting the idle than messing with the distributor or other parts of the ignition system. GOod luck