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Engine run on?


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Currently im having a problem with my engine, its has preignition/dieseling/Run on.

Basically the engine still turns a few turns after i turn off my key, after i switch off i hear a few turns then it slowly grumbles to a somewhat sucking sound, Cant explain this sound but i guess u guys know what i mean.


the mods/details of the engine are -

stock fuel injected l24, with a single carb manifold and a 720 carb.

matchbox dizzy

msd blaster coil 2

ngk plug wires, with ngk plugs (less than 300km since install.)

90 amp alt from sr20


the engine is mated with a 4 speed automatic in a c32 laurel.


i am using-

95 octane

My engine runs a 1/4 temp, i still have the stock shroud and clutch fan.

idle is set at 850 at hot idle.

my choke isnt functioning so i just step on the gas a few times and the idle smooths out, I live in a Caribbean country to the temp is always around 26-35 degrees Celsius.


i came across a post with a similar problem and im seeing the idle cut solenoid is maybe the problem, my carb has a wire behind it. to a unit similar to a electronic choke but i never bothered with it , it has a bullet connector if that helps.









i managed to get a pic online of the carb setup.



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The red wire in that picture is connected to your fuel cut solenoid. If it isn't working, it will allow dieseling after you turn it off. If your choke isn't working either, check your fuses and relays. The fuel cut and the choke are on the same circuit, so a blown fuse or bad relay would allow for this to happen. Just to clarify - you have the choke and fuel cut wired up still, right? Don't get rid of those wires. Also, make sure your choke is fully open when the vehicle is warmed up. If the choke is not opening up all the way, it will run really rich and that's another reason why it would diesel. Timing could be an issue that causes dieseling too, but I'd focus on the other stuff first.

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If idling at 850 then it isn't the idle cut. The idle cut (when off) shuts off fuel to the idle circuit. Won't idle below 1,200


That's known as dieseling or run on. I have no idea what Datsun or engine you have or the year, or if it has the stock carb. Include all this information in a separate thread. No the resistor or any part of the ignition can cause this as the ignition is turned off and the coil is not firing. .
Dieseling in a gas engine is the firing of the cylinders without the ignition on, caused by the heat of compression igniting the fuel and air. Causes can be...
1/ Too low an octane gas. The higher the octane the harder the gas is to ignite.
2/ Compression too high. Compression concentrates the heat in the air. A diesel engine can produce temperatures above 1,200 F. A gas engine needs less than half that amount to self ignite.
3/ Spark plug is too high a heat range. Tip becomes red hot and with the compression the gas auto ignites.
4/ Combustion chamber has excess carbon deposits that heat up glowing hot. Over rich carb, choke on, idle mix set too rich and excessive idling.

5/ Idle set too high. Allows too much air and gas mixture into the combustion chamber when shutting off.
6/ Engine running too hot. Over heating.

Check the choke is fully off when engine is warmed up.

stock fuel injected l24, with a single carb manifold and a 720 carb.

Obviously you've mixed and matched some parts. The carb pictured is from an L20B (they were on the '80 720) but maybe this carb is too small, wrong jets, flooding?

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I cant find the fuel cut or the choke relay anywhere, thats my problem.


i noticed that the wire to the carb was connected to the coil, i believe on the negative side. I was doing some body work and removed a few of the wires, this was one of them. Never put it back. I used my powerprobe 3 and sent a signal to it and the idle dropped but the choke didnt move, ( i was trying to figure out weather the choke was functioning or not)


I currently have a weber 32/36 with necessary parts needed for install, The weber doesnt have the choke and the idle cut either, sooo i guess this problem would continue with the weber huh?

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the mods/details of the engine are -

stock fuel injected l24, with a single carb manifold and a 720 carb.

matchbox dizzy

msd blaster coil 2

ngk plug wires, with ngk plugs (less than 300km since install.)

90 amp alt from sr20




This is the choke...




The idle cut solenoid is right beside it lower right. Hexagon shape with a Red wire. (with black covering) Find a 12 volt source that turns on and off with the ignition.... like the + side of the coil. Choke heater takes 8 min to warm up and open. Idle cut makes a clicking sound when key turned on and off.






Choke ON



Choke OFF...


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So i did the following last weekend.


i ran a ground to my vehicle body( didnt have one,just engine and alt)

i also ran a ground to my dizzy housing plus the stock ground on the vaccum advance.

removed my stock resistor.


I noticed smoother idling, Before i used to get a miss

no breaking up around 3500rpm, 14.4v hot idle voltage.


and the Run on problem didnt show up since. So i guess thats solved.


oh and i also installed a starter relay.

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