QuestionThis car is fitted with a lumenition ignition system, and has been quite reliable up until now. The engine starts and runs up to normal temperature ok, but if caught in traffic with the engine at idle for a while, it can begin to run rough, and stop completely after a few minutes. (I should point out that the engine in no way over-heats and the temperature gauge reads 'normal' or just below at all times). Restarting can only be acomplished by allowing the engine to cool down sufficiently, but it will sometimes restart and run rough while still quite hot. I have eliminated the coil, as I have replaced this when the engine was hot, but it made no difference to the rough running. Thinking that the problem was a component getting too hot, I sprayed freezer spray on the ballast resistor, amplifier (and old coil) when hot, but again this made no difference. Any ideas?
AnswerHi Jim,
I never found it to be of any use at all guessing what is wrong when an engine stops running.
All engines require only 3 items to run and when an engine quits running at least one of the 3 is missing. It is extremely rare for more than one of the 3 to go away at the same time. So to narrow down what is causing the problem all you need to do is to test to see which is missing at the time of failure.
The three items, Compression, Fire and Fuel have conditions on each.
Compression will usually not go and come back so you should concentrate on the other two.
Fire is easy to test, by connecting a timing light to a plug wire and run the timing light out from under the hood (bonnet) and tape it to a wiper arm and if it is a bright sunny day tape a piece of cardboard over the light end of the timing light so you can monitor the flash of the timing light while driving. If your timing light has a trigger to operate it you will need to tape the trigger down so it flashes all the time.
When an engine cuts off it takes a little tile for the engine to actually stop turning so you need to watch closely when the engine dies to see if the flashing stops before the engine stops rotating or if the flash continues on as the engine coasts to a stop.
Your Freon test of the electrics is effective some times but is not a 100 test. Watching the end result (flash of a timing light) is a 100 test.
For a test of fuel supply first you will need to put a "T" in the fuel line and run a long hose out from under the hood (bonnet) and put a fuel pressure gauge under a wiper arm to monitor the fuel pressure at the exact time of failure.
A series II in the UK is four Stromberg carburetors correct?. Usually any failure in fuel supply will deplete the fuel in one or more carburetors first so when the engine dies it will not die abruptly but drop several cylinders first then die.
From what you tell me I will bet that you will see the timing light go out before the engine stops rotating.
I don't have much data on the UK version of the XJ-12 so you will need to look to see if the added electronic LED ignition system uses a slave coil. (a second coil like some later models used)
Howard
