MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Ignition Off - Engine keeps running rough

When I drove my TD today, a warm day, when I turn the ignition off, the engine continued to try to run, running rough, coughing sort of.... this is a symtom of what??? Thanks for any help and suggestions.
B Moreland

Running on is the result of a hot spot in the cylinders that will ignite fuel that is being drawn in after the ignition is turned off. The hot spot can be carbon build up in the cylingers, a rough spot in the cylinder, on the piston top of in the combustion chamber of the head. In our cars, that don't have anyking of anti run-on valve to shut off the fuel from enteringthe carburetor from the float bowl, everything has to be set right to avoid run-on, especially on hot days. One of hte most common causes is ignition that is excessively retarded, requiring that the idle settings need to be incerased to make the car idle properly. Essentially, this means that at idle, the throttle disk in the carburetor is open further than it should be and it can draw in fuel after the ignition is shut down. Unfortunately, the same thing that contributes to run-on, also causes pinging and the most common attempt to reducing pinging is to reard the ignition, which slows the idle, which requires increasing the idle setting, which causes thre run-on - a viciou circle. Sometimes ust letting the car idle for a minute or so after driving will minimize the chance of run-on. If one is cordinated sufficiently, they can leave the car in gear with the clutch depressed and the brakes applied when they turn the ignition off, then let the clutch when the engine winds down t a stop and before it makes the fist kick of run-on, let the clutch out. this will stop the run-on withut the car jerking (in practice, I manage to hit everything just right withthat operation about one time out of fifty). Cheers - Dave
David DuBois

David, thanks for your advice... will see what happens. It was awfully hot today when we took our drive thru the country.... as you probably guessed, this is my first TD.... and learning as I go.
B Moreland

This is a symptom of using fuel that has a too low octane rating as well as the timing that david mentioned. This is all related as is the compression ratio as our heads have mostly been shaved to raise compression.

Just try a tankfull of higher octane fuel and see if this makes the difference. It certainly does in my car and also other cars in our club.

Cheers,

Paul.
Paul van Gool

Dave,

Sorry to disagree, but there are other reasons one of which, Paul van Gool has alluded to.

In my case, I have a a gorgeous cylinder head, Not a hot spot in sight - all cc-d, clean and well-polished, but I still had run-on.

That is, 'till I installed my pertronix. Its now a thing of the past and I continue to use regular gas. Before, I can only assume I continued to get spark as the engine rotated to a stop.



Gord Clark
Rockburn, Qué.
Gordon A. Clark

I've suffered only one such situation since the purchase of my TF some 4 years ago. Cleaning the engine bay one summer day somehow I managed to open the block drain cock , and having seen nothing come from it, believing it was closed, I drove into town (about 6 kms). On the way I saw the temperature guage rise quite a bit above normal, just as I was pulling into the gas station. I started to hear rapping noises as I pulled beside the pumps and turned the engine off, only to experience the same rough run-on situation you describe, and I stalled the engine out. To my horror there was no coolant in the engine. After a fuel up and a safe half hour cool down, I closed the drain cock properly, filled the little gem with water and now 3 years later have never had a problem with the engine or a repeat of the rough run-on. Could you have the same high temperature problem traceable back to a coolant blockage perhaps?
Gene
Gene Burgess

My TD displays run-on unless I shut off the ignition with the engine idling at low RPM (~500 RPM).

Depressing the clutch lowers the RPM by putting additional drag on the engine.

Another way to briefly lower the engine RPM is to "blip" the accelerator. The RPMs initially increase, but then quickly drop down. I've also heard it said that the brief enriching of the fuel mixture cools the cylinder, lessening the effect of hot spots.

Larry
Larry Shoer

Thanks for the suggestion, Larry. I never have tried this. I suppose the inertia of the flywheel slowing down may have a little bit to do with it too? (If you time it right, turn off the switch as the engine is slowing.)

If it doesn't work, I'll revert to my clutch out thing.
Tom

The exact sequence I follow is to "blip" the gas then, a few seconds later when the RPMs drop, press in the clutch and, a moment later, turn off the ignition key. It's what I'll call a "graceful and planned engine stall!"

Larry
Larry Shoer

Gordon - You are correct, I didn't cover anywhere near all the possible causes of run-on and in a perfectly clean cylinder/combustion chamber, the ground or center electrode of the spark plug could well be the hot spot. I would discount the idea of a spark being generated after the ignition is shut off, unless there is a wiring problem with the car. Run-on is normally caused by spontaneous ignition of any fuel getting into the cylinder (thus the retarded ignition timing/ increased throttle opening being one of the causes). This is the reason that run-on is also called dieseling (but I think I/m preaching to the choir here). Cheers - Dave
David DuBois

Might be worth mentioning that 'pre-ignition, run-on, dieseling' can also do a fair bit of damage. When happening, the engine no longer has a firing order and you can have two cylinders firing at the same time... not a good thing for rods/bearings/etc.
Better to correct the problem!
gblawson - TD#27667

I'm old enough to remember this was a common problem with about everything in the 70s. Sometimes a car would "diesel" for several seconds- hard to stop with an automatic transmission. A carburetor still supplies mixture as long as the engine is spinning, and any of the hot spots mentioned above, too fast idle, and other problems can keep it sputtering along. Guess we get spoiled with our electronic fuel injected modern cars. George
George Butz

This thread was discussed between 17/06/2007 and 19/06/2007

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.