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Jaguar Repair/1984 XJ6 not starting


 I drive my Vanden plas Monday to Friday and lately it's been hard to start on Monday morning.  Now it won't start, the engine turns but doesn't fire.  I pulled 2 plugs and cranked the engine and there is a spark but it seems weak.  I have 40 lbs/square inch of fuel pressure at the fuel rail while cranking.  I noticed some coolant has leaked on my distributor so I pulled the cap, didn't see any water but sprayed it with wd40 anyway.  I also tried some starting fluid in the air cleaner but it has no effect.   I pulled the fuel injection temp sensor connector and cleaned it and put it back in case it was not connecting well.  Occasionally it sounds like one cylinder fires but mostly the engine just turns over and over.  I even replaced the coil but that didn't help either.   I haven't checked compression but I don't think it could suddenly lose that on all the cylinders ?  What's next to check ?  I suspect the ignition...



ANSWER: Hi Steve,

When you have a no start you can not skip things and leave stones unturned. There is a saying among mechanics and that is the hardest car in the world to fix is one that don't start on occasions and the easiest one in the world to fix is one that don't start.

There is a procedure that must be followed and nothing skipped. There are only 3 items that make a gasoline engine run and they are Compression, Fire and Fuel with conditions on each. They need to be tested in order too.

First --- Compression. This must not be skipped even if it was tested yesterday. All plugs out and the (+# wires removed from the coil. Hold the throttle wide open and spin the engine over at least 5 or 6 revolutions. On a XJ-6 you should see 150 PSI or more on every cylinder and very little difference between cylinders. Don't skip this for any reason or rational.

If that is good and only if that is good can you proceed to the Second.

Second --- Fire. #Ignition# With all the plugs still out place each plug #cleaned or new# on each plug wire and lay the plugs on the head so that the case of each plug contacts the head and put the #+# coil wires back on the coil. Place your thumb over the front plug hole #Jaguar calls it #6) and have someone spin the engine over while you watch the front plug on it's wire. As the engine spins the compression will blow your thumb off of the plug hole and make a "Pop" sound. The plug will spark with a "click" sound and it should seem that the "Pop" is causing the "Click". If it is "Pop-Click" or "Click-Pop" then the timing is probably off far enough to cause it to not start. The spark must be colored blue (not in bright sun light) and must be thick as a pensile lead. If the spark is colored Orange or Yellow and thin like a hair the spark is weak and probably will not fire under pressure of compression.

If it looks good test all 6 plugs and holes the same way. Then and only then if they all look good can you go to Fuel last.

Third --- Fuel, You already did the fuel test by spraying starter fluid into the intake. If you didn't get it to fire off on starter fluid even for a second or two, then you for sure have a "Fire" or "Compression" problem because starting fluid will ignite with even a faint spark.

The only exception to that is a 100% stopped up exhaust system. (melted CAT) this is easy to test, just remove either of the large hex caps on the exhaust manifold and force a piece of hose into the hole and put a low pressure gauge (a vacuum gauge with a section for testing fuel pump pressure works well) With the plugs back in and the (+) wires off of the coil spin the starter for about 20 to 30 seconds and you should see almost no pressure build up. If you see pressure remove both caps and your hose and put the (+) wires back on the coil and use starter fluid and try to start the engine and if it fires off, shut it off right away as fire will be coming out of the cap holes.

In one or more of those tests you WILL find the problem or at least the section of the problem.

Let me know,

---------- FOLLOW-UP ----------


  Thanks so much for the detailed diagnostics, I did the compression test but I don't know how accurate my 30 year old tester is !  I did 3 tests per cylinder for 6 compression strokes each time.  Results per cylinder number are:

1 = 150 psi, 2 = 145 - 148 3 = 130 - 140 4 = 140 - 143 5 = 140 - 145 6 = 132 - 140  

 So if the tester is good they are mostly low.  I did notice the reading dropped gradually as I was looking at it for some cylinders so my tester may have a problem or a leak.  I've had cars before that has similar readings and ran fine so I wasn't sure this was the whole problem and went on to the ignition test with the plugs out and my finger over the hole.  The timing of the pop & click was the same so timing appears ok, but the spark was thin and yellow so it appears to be a weak spark.  I think this is the main problem the car won't start - what do you think ?

Thanks again,


ANSWER: Compression is probably ok. When we got readings like that we would do a "Wet" test. (squirt about 4 or 5 shots of engine oil into the plug hole and run a new compression test. The "Wet" test as we called it will always show higher but it should not be more then 10% to 15% higher or it is a problem.

A large increase in pressure of a "Wet" test is clearly a ring problem. A small increase is normal. No increase in the "Wet" test is an indication of a gasket leakage or more likely a valve leakage. An intake valve leakage will cause a "No Start".

A yellow or orange hair thin spark is clearly a weak spark and will not even spark at all under the pressure of compression. The color can not be confirmed in bright sun light but in a garage it must be blue and thick.

The Series III Jag has suffered from a odd problem from day one. Symptoms are that the engine spins over fast but no start and all tests seem ok. Here is the test for this odd problem. Take a known good battery and use it on the car as though you have a dead battery even though your battery spins the engine quit fast. Add this "Jump Start" from the added battery as though yours is dead. If the engine fires off and starts, you need a new "Deep charge" battery or a diesel battery.

My son worked for Jaguar through the span of the Ser III and we always had a free supply of batteries for all of our other cars because when a battery would not keep a ser III car starting that same battery would work fine in other cars for years.

One customer had a battery go bad and he purchased a low grade battery and it wouldn't start the ser III Jag even though it would spin the starter motor over fast.

Run the "Jump Start" test and if that don't make it start let me know and I will write up the test procedure for the Ignition system. Your starting fluid test was the test for the fuel system.


---------- FOLLOW-UP ----------


  I did a wet compression test, readings went up by  7 to 15 %.

  Then I jumped the car with another battery - and what do you know the car started immediately !!!  As soon as you mentioned the battery I was suspicious, I've been using cheap 'blem' (blemished) batteries from a local battery store for years for all my cars and up till now never had a problem.  My sister has a modern Jaguar X type and had to get a battery and was told it had to be a deep charge or deep cycle, but I can understand that with all it's modern conveniences.  I think she paid over $200 for one.

I've had my battery on a charger in between all my tests but I guess it needed a good boost to energize the ignition system properly.  After I disconnected it from the other battery the car still started but I don't think I should trust it.  

I was wondering if the problem is due to the XJ6 using a 12 volt coil and if the battery can't maintain that during cranking it won't start ?  My TR7 has a 6 volt coil with a ballast, does that mean it always would get 6 volts even if the battery was down to only 10 volts ?  Could I change to a 6 volt coil or would that cause other problems ?   Otherwise I guess it's time to get a better battery !

Thanks again for all your help Howard !


Hi Steve,

I never tried to install a 6v coil and all the bypass wiring and resistor necessary to use a 6 v coil. But I believe it is a combination of the draw of current that the starter, the ECU, The resistor pack for the injectors, the cold start injector and the ignition module, not just the coil.

I have successfully used a GM diesel battery in the series III cars several times in the past so check on the price of them before trying to add the bypass wiring necessary to make a 6v coil operate. Keep in in mind it will be easy to miss the resistance by a small amount and thus fry the ignition module at $300.00+ as all of the current going through the coil goes through that module mounted on the front of the intake manifold. Not to even mention what can happen at the ECU in the trunk as it must receive an ignition pulse signal from the ignition system via one single wire on top of the intake manifold.

As a mechanic in several Jag dealerships over the years I suffered from electrics as did most mechanics back then so I took a course from RCA in electronics and even with that it would scare me to try to get it right on the amount of current going through everything at the same time and not fry something else. You are not just dealing with voltage, you are now dealing with amperage and amps are 100% dependent on load. When you change the load (coil) you also change the charge rate of the alternator which also raises the voltage too. Too big of a monkey puzzle for me. That's why I never tried it.


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Howard M. Fitzcharles III


Jaguar from the XK 120 to XJ-6 ser. 3 1987


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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Import Car magazine

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