Jaguar Repair: XJ6 1980 miss firing once engind in at running temp., gasoline engine, coil wire

Question
Howdy, My 1980 XJ6 runs well till he warms up.  One so normal operating temperature is achieved by the car begins to misfire any eventuality becomes difficult to start.
Is it possible that once the ignition coil warms up from the engine heat that it will faulter?  
As soon as the car cools down in runs perfectly again.  How do I have check if the caller is faulty?  

Kind regards
Michael Chorney

Answer
Hi Michael,

Yes, a coil can do that, but you have picked one of a hundred things that can do that.

I learned while working in Jaguar dealerships that most of the time you can not diagnose a problem by symptoms. Even though the symptoms can lead you into an area of a problem, they are for the most part useless.

The only sure way to correct a problem is by testing. There are only three things that are necessary for a gasoline engine to run and they are, Compression, Fire and fuel with conditions on each. Also the tests MUST be done in a specific order. Compression first, Fire second and fuel always last.

Your symptom does tend to put it into either Fire or Fuel as Compression does not usually come and go and come back. So you can put Compression aside for now.

Fire (Spark) is easy to test for. First take a timing light and put it on the coil wire and tape the trigger down so the light flashes all the time. Then start the engine and watch the light. Because it is on the coil wire the flash will look almost as a steady light. Warm the engine up and when it starts to falter ans misfire you can see even the smallest flicker in the steady light. This tests the coil, and all of the primary circuit of the ignition system. The only thing it does not check is the cap, rotor, plug wires and plugs, but they are less likely to change from good to bad and back to good as per engine heat.

If the light is steady without flickering then you most likely do not have an ignition problem.

Fuel is easy to test too. Just take a spray can of starting fluid and keep it handy and warm the engine up and when it starts to falter, spray starting fluid into the intake to see if you can keep it running on spray starting fluid. If the spray helps it to run smoother then it is a Fuel problem.

With this method you isolate the problem to a section not an item so once you have identified the section (Fire or Fuel) then you start testing that section.

Let me know what the results are in these tests and I will give you the next set of tests.

I found to never try to guess what might be wrong as there are too many possible parts that can fail. By first testing for the section then test in that section you will always find the problem 100% of the time. I get people who start guessing and start replacing parts and they start to run out of money before finding the real problem. Working it dealerships I had to find the exact problem before replacing any parts.

Howard