Hi I have a 72 csl with original d jet system that occasionally runs on when the ignition is killed.
It has had new injectors put in it and been dyno tuned. I was having trouble with the auxhilary air valve but now that seems to be fixed and the cooling system is all in order.
Any ideas or thoughts would be most appreciated.
“Run-on” on a d-jet is unusual. Run-on is often described as dieseling. It typically occurs when there is enough heat or compression remaining in one or more combustion chambers to ignite residual fuel. This situation is more common for carburetor-equipped vehicles and is the reason for solenoid controlled idle jets in later model carburetors. The Zeniths found on CS models were so equipped and - when the ignition was switched off, the fuel feed would be cut off too. Therefore, no run-on. In the fuel injected model, the system is under pressure, BUT with the ignition key in the off position, your system should shut off fuel to the engine.
There are several generic considerations to control run-on.
First, consider the source of the fuel to the engine when the injection system is shut down with the turn of the key. If your injectors are leaking, something is wrong. What was wrong with the old injectors?
You mentioned dyno-tuning the d-jet. What fuel pressure are you running at the fuel rail? Has someone tweaked the pressure regulator so that it is substantially higher than the recommended 30psi? Even slightly higher than recommended pressures should not normally cause the injectors to leak.
What about the cold start injector at the top of the manifold? Could it be dribbling or leaking fuel? (The fact that your auxiliary air valve is not fully closing suggests you are possibly jury rigging something to obtain an acceptable idle. ??The ecu potentiometer setting? The co adjustment at the throttle body? Timing??)
Next, consider the source of the ignition after you turn off the ignition key. As noted by others, advanced can result in higher combustion chamber temperatures. (Same with leaner mixtures, as may occur if parts of the d-jet system has been modified e.g., a modified ecu or psu.)
Spark plugs that are too hot for the application can become incandescent. Have you tried a colder range plug? What is the overall state of the combustion chambers? Carbon-laden combustion chambers have a tendency to become aglow and also foster run-on.
What is your compression ratio? Have you any compression test readings? Engines with shovel loads of carbon deposits can raise the compression ratio, and, in turn, promote dieseling - not to mention pinging and preignition. This problem can be compounded by using low octane fuel.
What is your idle speed? Too high and the engine can seem like a perpetual motion machine until it runs out of go juice.
In sum, make sure everything is mechanically sound. Try upping the octane and see if things don't improve. If not, colder plugs, check fuel pressure, lower idle, retard timing or blow the carbon out with combustion chamber cleaner and/or a good long hard run.
hth :wink:
The following are NOT endorsements.
