running without a key

[Rust Bucket]

i ran my car ('72 240z..im in the middle of restoring it) for a bit today, but when i turned it off, the engine kept running for about 20 more seconds.

i did everything like i normally do, turn the key all the way to off/lock, turned off the fuel pump, and closed the choke, but it's never done this before.

can anyone tell me what's wrong?

[JimmyZ]

If you had a non-stock carb with a bad fuel solenoid... I'm sure that's not the case since you prob have SU's.

When you say "running" do you mean running or just motor turning as in back firing, dieseling etc.. Was it running as if the key was in it?

[Rust Bucket]

i have the origional (i think) SUs in there.

i had it running as in the key was in the ignition, motor idling at 900 rpm.

[JimmyZ]

I'd have to look at the electical diagram but it might be a relay stciking. One sec.

[Rust Bucket]

ok..but if that's the case, i probably won't know what to do to fix it.

[JimmyZ]

I'm digging for my FSM and can't find it. The fact that it's running normally after ign removal suggests the ignition is somehow staying on. I can't imagine the switch causing this but maybe there are some frayed wires near the backside of the switch??? (unlikely)

Run a voltmeter to the coil "+" terminal. (If you can get it to repeat the running after key off bit.) You want to test if there is voltage after switching the key off.

If there is voltage try attaching the voltmeter to the ballast resistor. At least this will tell you if it's the run or start voltage that is keeping it running. (Run =6V Start is the full 12) Simply hooking up to the coil would tell if it's the 6V or 12V but perhaps you miswired the ballast/coil. I did this when putting mine together.

It is slightly possible that someone wired something into your harness that is creating this. (doubtful)

The relay can be heard clicking when you cycle the ignition switch. It is a rectangular metal box the size of your thumb near where the passenger's right knee would be.

My Haynes manual's elec diagram is kind of fuzzy but the FSM has a better one and describes/has a picutre of the relay.

[Rust Bucket]

well, if this helps at all,

it only ran normally for about 3 or 4 seconds and then it started sputtering and it almost sounded like it had just run itself dry.

[JimmyZ]

but when i turned it off, the engine kept running for about 20 more seconds.

3-4 seconds is a bit different:) Sounds like it's "dieseling"

Here's a link to an excellent thread on the relay I was thinking of. Escanlon studied it well. (Nice writeup by Escy)

http://www.classiczcars.com/forums/showthread.php?t=21059

If it is dieseling then here's a snipit from an online article

This condition can occur for a multitude of reasons:

Built-up carbon in the ignition chamber can glow red after the engine is off, providing a mechanism for sparking unburnt fuel. Such a thing can happen when the engine runs very rich, depositing unspent fuel and particles on the pistons and valves. Similarly, non-smooth metal regions within the piston chamber can cause this same problem, since they can glow red. It has also been suggested that an improperly rated sparkplug can retain heat and cause the same problem.

A carburetor that does not close entirely can contribute to running once the engine is off, since the extra fuel and oxygen mixture can combust easily in the warm piston chamber. Similarly, hot vaporized oil gases from the engine crankcase can provide ample fuel for dieseling.

Incorrect timing.

An engine that runs too hot or too lean may produce an environment conducive to allowing unspent fuel to combust.

An idle speed that is too fast can leave the engine with too much angular momentum upon shutdown, raising the chances that the engine can turnover and combust more fuel and lock itself into a cycle of continuous running.

Check your timing and idle speed first.

Jim

[Rust Bucket]

thanks.

this actually really does help a lot.

[Exdamyankee]

Pull your plugs first and tell us what they look like. If they're black & sooty you're running too rich and that will contribute to a run-on problem.

If you do have a carbon buildup problem you might try a can of Seafoam ($6 at Discount A.P. or A-Zone). Cleans the crap out of the combustion chambers pretty well. Worked really well for me.

If you decide to do this follow the directions carefully or risk hydro-locking the motor (stuff goes in through the intake).