Why "dieseling" after ignition off?

What causes my engine to run a few seconds more after I shut the ignition i.e. “dieseling”.

I dont think it overheats and there is no carbon on the spark plugs. And believe engine is running too lean.

I had that happen before it turned out the timing was too advanced.
I also had it happen on an old mini and the previous owner told me it was because I was running unleaded in leaded heads.

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All Etypes have hardened valve seats, they work fine on uldeaded. Running lean is a common problem on E10, which is stochiometric at 14.1, as opposed to gas which is 14.7. So a richer needle may help. Other causes:

Air leak at the throttle spindles

Retarded timing

Hot range spark plugs

Carbon build up

Also adjust the idle speed as low and you can



I meant to adjust the idle speed as low as you can


This dieseling happened to me this summer in my 1969 E-Type FHC a few times on hot days so I changed gas brands and that fixed it. My E-Type needs premium fuel (93 Octane) and has occassionally dieseled if I used a lower octane. I recommend changing fuel brands and always use 93 octane and seeing if that helps.
BTW, in addition to the causes already suggested, another possible cause could be that your engine is running at a higher than spec compression ratio if the head and/or block have been skimmed too much. This can be checked if you suspect this may be a possibility.


Has anyone noticed that, when dieseling, the engine is actually running backwards?

Yep… no real harm, because it doesn’t last long.
The Jeep did it couple of times, this summer, but lowering the idle cured it.

Most of us are old enough to remember the 1970s… Dieseling was a real problem then. One of the tricks with an automatic transmission car was the shut the car off with the car in drive and then shift it into park. The extra load of the transmission eliminated the dieseling.

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With the S1 cars this problem can be caused by the cooling fan causing power to reverse leak back into the ignition system. Simple test is to disconnect the fan, run the car and see if the post-ignition (the correct name by the way) occurs when you switch off. Had it on my car and solved it by adding a diode in the fan circuit. More common cause is carbon buildup in the combustion chamber.


If you compare the s3 wiring diagram to earlier ones, you’ll see Jaguar deliberately disconnect the radiator fans by earthing both sides when power is disconnected.

Perhaps if you wire your fan relay the same way, the run-on problem will go away.

kind regards

First of all, there’s a big difference between dieseling and fan-induced run on. Dieseling tends to be irregular, because it’s caused by one or two hot spots. So all six pistons aren’t engaged. If the fan is causing run on, the engine just runs on as normal.

A diode can fix fan induced run on at the expense of somewhat reduced CFM output. The better way is to use a relay and supply power to the contact and winding from opposite sides of the ignition switch. Jaguar’s trick of grounding the fan output doesn’t do the trick, but looks good on paper. What it actually does is cause back EMF to brake the fan when it isn’t powered, increasing natural airflow.

If the fan run on problem isn’t too bad, you can “fix” it by just stepping on the brake as you turn the key off. The draw of brake lights may bleed off just enough power to stop the engine. Won’t work with LED bulbs, though.


The other issue can be that one or more butterfly’s are not seating properly ….after it happens ……check that they are properly closed by pushing on the connecting rod

I had that problem on my Series 2, including the running backwards thing at the end. Mine was definitely running lean, as it needed to be partially choked, even when running at normal temperature. This was after rebuilding the carbs, installing new throttle shafts and seals, and verifying there were no other air or vacuum leaks. This problem has existed on my car since I bought it in the 1980’s. I previously “bandaided” it by raising the float level, which is a trial and error process. I finally replaced the fixed jets with adjustable jets from Joe Curto, and enrichened the mixture. This seems to have solved the problem.

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Yes, exacerbated by higher engine temps and EGR mandated by US pollution regulations. some Jaguars were fitted with “anti run-on” solenoid valves which applied manifold vacuum to the intake, including the float chambers. That stopped fuel flow and hence dieseling.

With ignition switched on and the engine running (with oil pressure) the run-on valve was off. With ignition switched off, 12V appeared on one side of the solenoid and (provided their was oil pressure) the other side of the solenoid was grounded through a special “reverse” OP switch. When the engine finally stopped (no fuel into the carbies) the OP switch went open circuit and the solenoid valve shut down…and would remain shut down until dieseling returned after the next run.


The problem is that the non ethanol gas is always 90 octane so if that’s what you want to burn you might have to put up with run on

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My first car was a '58 MK1 that the original owner had rebuilt with .040" over 12:1 Mahle racing pistons, for some unknown reason. He told me that the way to stop the engine without dieseling was to floor the accelerator and turn the key off about a tenth of a second later, when it hit 1500 RPMs, and keep it floored until the engine stopped. The inrush of cold air would quench any hot spots. Try it, see if it works for you.

Would full choke also kill the engine?
Something is too hot and sustains ignition. Wouldn’t an engine ping as well if it was that bad? I know dieseling is a problem and have had it in one or two cars on occasion. But it indicates that something isn’t perfect.

Hey George….have you worked through all of this advice…do let us know

Appreciate all the advice but still have the problem. I hate to give up and just adopt the “prevent engine turning when you turn off ignition”.solution.?
Separately, could low octane gas really cause this problem? Low octane causes pre ignition: is that connected with dieseling?