Engine "run on" aka dieseling

[acaciolo]

I've had a great couple of months with my new delorean! It has been running like a champ since I had DPI's exhaust installed and some general maintenance items (hoses, belts, plugs, wires, fuel lines, valve adjustment, brakes, shocks, tires, springs, etc.)

Tested with a dwell meter and it is dead on. Idle speed is a tiny bit high. Dave M's RPM relay works great. Amazing how fast it starts.

The only minor annoyance (which hasn't been looked at by an expert yet) is that I have a little engine run on after I shut it off. not quite a backfire but a little stumbling. It usually happens more when it is hot.

timing issue? leaky injector? neither has been checked yet...but i am seeing if I can get some pointers at to where to start.

thanks!

tony c
3886

[D Knight]

Not a good thing for pistons or aluminum heads. Your combustion chamber temps are too high. Maybe an intake vacuum leak. Sounds like it's running too lean at idle. That would also contribute to your high idle. You should get it checked with an air/fuel ratio gauge and smoke test for leaks. If this goes on, you will damage the pistons and combustion chambers in the heads


-D Knight-

[acaciolo]

thanks for the tip! I won't be driving it until after I get it in to be checked out. We just replaced some vacuum hoses, so there might very well be a leak!

tony

[David T]

I've had a great couple of months with my new delorean! It has been running like a champ since I had DPI's exhaust installed and some general maintenance items (hoses, belts, plugs, wires, fuel lines, valve adjustment, brakes, shocks, tires, springs, etc.)

Tested with a dwell meter and it is dead on. Idle speed is a tiny bit high. Dave M's RPM relay works great. Amazing how fast it starts.

The only minor annoyance (which hasn't been looked at by an expert yet) is that I have a little engine run on after I shut it off. not quite a backfire but a little stumbling. It usually happens more when it is hot.

timing issue? leaky injector? neither has been checked yet...but i am seeing if I can get some pointers at to where to start.

thanks!

tony c
3886

Engine run-on is impossible IF the fuel pump is hooked up and running correctly. Just as soon as you turn off the ignition switch the fuel pump should stop running and the motor can't run if the fuel pump isn't running.

[acaciolo]

it literally is for about 1 second or less. before I even pull the key out it stops. it is just like a quick little engine stumble.

[David T]

it literally is for about 1 second or less. before I even pull the key out it stops. it is just like a quick little engine stumble.

That is not engine run-on or "dieseling".

[DMCMW Dave]

That is not engine run-on or "dieseling".

Agreed. It's probably just extended spin-down from idling too fast.

[acaciolo]

thanks dave and dave. thanks for correcting my terminology and letting me know what it probably is.

dave s, see you at dcs.

tony c.

[82DMC12]

Bringing this post back because I actually experienced this yesterday. Actually I first experienced it about two years ago after doing a valley dive and putting in NGK plugs. Sometimes on shutdown I would pull into the garage, put the shifter in neutral, set the parking brake, then turn the key off, and the engine would mostly stop but then stumbled four or five times before actually stopping.

I thought I had it figured out after switching to Bosch Copper plugs, doing a smoke test and fixing a couple of very minor leaks, and above all, getting the throttle set correctly.

Now this weekend I replaced the wires going to my idle speed microswitch which required unhooking the throttle linkage and removing the W pipe. I put it back together and re-adjusted the throttle (first setting the spool 1/8" off the stop and then shortening the linkage just enough so it's about 1/8" short of snapping onto the spool). I didn't do anything with the curb idle stop but all looked good inside the throttle body anyway. Once it was back together I went for a nice long drive including interstate driving. Engine coolant temp was normal. When I pulled into the garage after a few seconds I shut the engine off and for the first time in two years it did the diesel-ing /after-run thing for a second. I started the car up again, let it idle for 30 seconds, then shut it off and it didn't do it again.

So, not quite sure what causes this but I have a spare set of 022 injectors (unused) I am considering installing just to eliminate the possibility that one of my injectors is dripping into the cylinder on shut-down. I do have the McKeen RPM relay which runs the fuel pump for a second or two on shut down to boost the rest pressure. Maybe a leaky injector along with a pump that doesn't shut off immediately with key-off could cause this?

I had my plugs out for inspection early this summer and they looked practically brand new with zero deposits or signs of excess heat.

[Ron]

You could have a fuel leak, but something has to be igniting it... Sharp edge (plugs, valve), carbon build up, etc.

[82DMC12]

You could have a fuel leak, but something has to be igniting it... Sharp edge (plugs, valve), carbon build up, etc.

Hmm. How would one begin to diagnose this? If all the plugs look good, it seems difficult to figure out even which cylinder could be to blame. Here is what my plugs looked like in June this year:

69890

[Ron]

Hmm. How would one begin to diagnose this? If all the plugs look good, it seems difficult to figure out even which cylinder could be to blame. Here is what my plugs looked like in June this year:
I would check the current condition of the plugs then confirm all basic settings are correct (especially idle speed and timing). If no luck, since it happened after a long run, I would check the engine temp (not with the dash gauge;-) after another. Look for carbon deposits in the cylinders if you can get your hands on a scope.
...You may just need to run some Seafoam or similar through it (in gas tank and directly into the intake).

[82DMC12]

I would check the current condition of the plugs then confirm all basic settings are correct (especially idle speed and timing). If no luck, since it happened after a long run, I would check the engine temp (not with the dash gauge;-) after another. Look for carbon deposits in the cylinders if you can get your hands on a scope.
...You may just need to run some Seafoam or similar through it (in gas tank and directly into the intake).

Will do. What do you suggest for checking engine temperature other than the dash gauge? I do have an IR thermometer gun. Is it reasonable to add a second wire to the engine temp sender so it can be read in parallel to the dash gauge with a multimeter?

[Ron]

Will do. What do you suggest for checking engine temperature other than the dash gauge? I do have an IR thermometer gun. Is it reasonable to add a second wire to the engine temp sender so it can be read in parallel to the dash gauge with a multimeter?I'm thinking reading it in parallel would throw both off..
But an IR gun would be good. Read all around the thermostat housing and the ends of the heads.

...repeat the long drive, make sure the idle is ok before you shut it off -- It could be the linkage was hung up.
If, while you were doing the wiring etc, you got the vacuum line going to the solenoid valve on the wrong port, it may not vent and keep the timing high until you shut the engine off, iirc.

[82DMC12]

I will definitely give everything another look. I pulled my plugs out in June because I noticed with the IR gun that my passenger side valve cover and passenger side exhaust was about 15F hotter than the driver side. Like 182F vs 198F on the valve covers and 205F vs 225F on exhaust collector. I mentioned this to a vendor who said it's common and just how k jet is (not very precise). So I pulled the plugs and saw what I posted earlier, they all look the same and clean.

Seems that is sort of common for V-type engines, may have more to do with coolant flow than anything else. But, would love to know if that's a PRV thing too.

I'll check them again and maybe run a can of seafoam before swapping injectors.

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[Bitsyncmaster]

I've not heard of dieseling problems in cars since carburetors have been eliminated. So my guess is most of that was caused by improper mixture. Is your lambda system working and adjusted with the dwell meter?

[82DMC12]

I've not heard of dieseling problems in cars since carburetors have been eliminated. So my guess is most of that was caused by improper mixture. Is your lambda system working and adjusted with the dwell meter?

I would say yes, I have a dwell meter and I check it periodically to make sure it is still within spec. However as I mentioned I was playing with the throttle a little bit and it's possible that one particular time I shut down something was wacky. I think I'm going to check my plugs tonight, and if they look good, I will run a bottle of Techron. After I burn a tank of gas I'll pull the injectors out and check spray pattern and volume again. Any question about them and I will just swap them out with a brand new set of injectors I have as spare.

Who knows, maybe it will never do it again, but it is concerning. I've had the intake manifold off a few times and I always peak down into the intake valves - they don't seem that dirty but maybe there is something inside that is heating up and igniting if I have a leaky injector.

[David T]

My guesses are you do not have the throttle plates adjusted correctly. They should be shut at idle. If they are open at all you would get enough air to keep it running but as I said in a previous post, the fuel pump would have to be running too unless you are just running on what pressure is left. Make sure the fuel pump stops when you turn off the key and make sure you have no power to the coil with the key off. Recheck you have the correct heat range spark plug. Test the fuel injectors for opening pressure, a leaky one may be the cause. The motor should not be running at all with the key off.

[82DMC12]

Well I pulled the plugs out and they look absolutely perfect. Does anyone disagree? They are gapped to .024 to .025 in.

I will check the closed throttle body and maybe put my original RPM relay back in as an experiment and drive to work tomorrow.

If I get the dieseling thing again I am going to put my unused spare set of injectors in. https://uploads.tapatalk-cdn.com/20230801/3c951c6aa2e3406a3a8f841679675052.jpghttps://uploads.tapatalk-cdn.com/20230801/b6b22f4a94b5dc0bde6aff6348181c14.jpghttps://uploads.tapatalk-cdn.com/20230801/0154fec48d53c15cfc53e642bddfb2c0.jpg

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[82DMC12]

Just thought I'd give a little update.

I haven't changed anything with the throttle since the dieseling event a few days ago. However I did add a bottle of Techron to 3/4 of a tank of gas and checked the plugs as in the previous post. Plugs looks great but I haven't pulled the injectors out. I was wondering about the fuel pump priming on Key On as well as when shutting down when using the McKeen RPM relay. I thought I'd try running with the old black 1981 RPM relay and see if I got the dieseling. I went for a spirited drive yesterday taking the same route as before including 75 mph interstate driving. On shut down, there was no dieseling or drama. In fact, the car is starting faster now too, firing on the second engine revolution both hot and cold. Before, sometimes it would take up to ten revolutions when cold for the engine to fire, and it usually ran a bit rough for the first few seconds.

Anyway, it seems to be better now so I will keep running with the original relay for a while and see how it goes. I will still pull the injectors out eventually and check for leakage but perhaps the Techron will clear that up.