Jaguar Repair: 87 xj6

Question
I have an 87 xj6, Melloy Jaguar has been working on it for weeks, finally got it where it will start without dying, but occasinally will act up, usually will start after 4 or 5 tries.  Igntion ampfiler, distributon, re wired, last time they set the timing
and it performs as it should at this time. Thanks for your help, Ron

Answer
Hi Ron,

I would not want to second guess the Jag dealership. Sometimes a problem is not present when testing. Also most dealerships have a scope and if the scope sees a problem in those parts that were replaced the mechanic would automatically replace them even if they were not the cause of the original problem.

Normally the Series III XJ-6 is a quick starting car. You need to note and probably document on a note pad when it is difficult to start or restart. For example, does it happen mostly just after driving it? Like, Went into a store and came out and it was difficult to restart. Or, Went out in the morning and it was difficult to start cold.

The reason you need to know this is because there are different parts and systems involved under those two situations.

The only true method to find the problem other then a shotgun approach of tossing parts at it, is to test it at the time of failure. The easiest car in the world for a mechanic to fix is one that don't run. The hardest car in the world for a mechanic to fix is one that don't run occasionally. The intermittent fault is the most difficult problem to fix.

But there is hope. There are only three items that make a engine run. Compression, Fire and Fuel. So when an engine does not start it is for sure one or more of the three is missing. Compression does not come and go so you can skip it in your case. Now it is just fuel or fire. The "End Result" of fire is spark at the plug so to visually see the spark without removing a spark plug at the time of failure. (it is no use checking it when it is running right) So a simple 90% method is to put a timing light on a plug wire and tape the trigger down and put the light under a wiper arm so you can see the flash of the timing light when sitting in the car and trying to start it.

So how do you know if there is fuel at the time of failure? It would be difficult to tell if there is fuel to the combustion chamber at the time of failure (no start) but you can start by testing fuel pressure to the fuel rail at the time of a no start by putting a "T" in the fuel line right at the point were the fuel line connects to the fuel rail. (It does require a gauge that can read from 32 PSI up to 45 PSI) It is true that a fuel injection test gauge is expensive but a industrial pressure gauge (0 to 100 PSI) is very inexpensive and works well. (Just be sure to put hose clamps on each joint.) a gauge and a long injection hose available at any auto parts store and rout the gauge out from under the hood and put it under the other wiper arm.

You should see 45 PSI when starting. If you see the timing light flash and you see 45 PSI or close to it and no start. Go check the power to the cold start valve (cold start injector) while in the "Start position". Then test the ground side to see that it is grounded when cold. (it is grounded by the "Thermal Time Switch" (largest switch in the coolant rail over the intake manifold) usually the front one.

The cold start injector works without the computer and is powered by the ignition switch and grounded by the thermal time switch by the coolant temp. If however you try several times to start it and it don't start the thermal time switch will disconnect the cold start injector so you can't flood the engine.

Here is a shortcut many mechanics use. When it does not start, have a spray can of starting fluid available at any auto parts store, and have someone try to start it while you give the air intake next to the radiator on the right side, about three shots of spray. If it don't start you have an ignition problem. If it does start when it wouldn't before, then you have a fuel related problem.

Testing is the only method to diagnose a problem. Diagnosing by symptoms is just a guessing game.

Howard