Vintage Cars: Engine Over Running, coolant level, vacuum tubes

Question
I have a leyland princess car (o series engine).sometimes, when i turn off the car the engine continues to run for a moment then gives a little splutter before stopping. what is causing this?

Answer
You don't give a date, which is important. There are 1950s Princesses (Austin limousines), and 1980s too (more likely, since you describe it as Leyland). I am not familiar with the latter, but let's try...

Anyway, run-on, aka dieseling, is usually caused by carbon buildup in the cylinder head. But that's assuming you are using premium gasoline (petrol). If you are not, simply upgrade the gas and it should disappear.

(i) First, check (actually, change is better) the spark plugs. If they look awful (either all pale flaky, or crudded with soot), change them, and get the car checked for mixture: if injectors, have them cleaned; if carbs, a proper tuneup. (ii) It may also be a result of overheating. Does the car have a temp gauge; and is it in the OK range? If it isn't, check the coolant level. If low, look for leaks, and also check the tube from the radiator to the reservoir for cracks. (iii) Go for a good drive and then take off the radiator cap (be VERY careful: goggles, thick gloves, and stand back; loosen slowly) and shove a thermometer in. It should be around 160-190F. If it's high, replace the thermostat.

If nothing untoward so far, and if you are already on premium, the head should probably come off to clean out the deposits (and always a good idea to check the valves, springs, guides at the same time). Any competent machine shop can manage the head/valve job once it's off, though if it's a 1980s car taking the head off and replacing it may take some work (sensor connections, vacuum tubes, etc.).

The problem is NOT anything to do with ignition timing, because the engine is still running when there IS no ignition! On the other hand, it also will commonly occur at the same time as pinking (tinkling on hard acceleration, esp. at low revs). That can be cured a bit by retarding the ignition. Dieseling cannot.

Best of luck--

Jo

PS If you have the head job done, you can minimize the future risk with a good tuneup to get the air/fuel mixture right.