Jaguar Repair: after it runs for awhile it wont take fuel, low pressure gauge, vacuum gauge

Question
QUESTION: HOWARD.
MY 4.2LTR JAG AFTER RUNN A LITTLE WHILE THEN I GIVE IT THE GAS
IT JUST WILL NOT TAKE THE FUEL AS IF SOMETHING IS HOLDING IT BACK.
WHILE GIVING IT THE FUEL IT BACKFIRES.I HAVE CHANGE COIL, BALLAST
RISISTOR AND FUEL FILTER  but to no prevail.sounds like it after a
while begains to idle rough. WHEN I PUT IT IN GEAR OR TURN ON THE
AC IT WILL CUT OFF AS IF IT IS STRUGGLING FOR FUEL BUT IT IS GETTING ENOUGH FUEL I THINK.
WHAT DO YOU SUGGEST.
REGARDS,
RICHARD

ANSWER: Hi Richard,

Yes, it could be fuel related and there is a simple test for that.
Just spray a little starting fluid into the intake horn as you try to rev the engine. If you can rev the engine with the added starter fluid it is for sure fuel related and you need to start testing the injection system.  The first test of the injection system is system pressure at the fuel rail. (32 PSI with the engine running and about 42 PSI without the engine running.) However, if you still can't rev the engine high with the starer fluid you need to run a different test.

Remove one of the two plugs on the exhaust manifold and put a low pressure gauge in the hole. I use a vacuum gauge that has a section for testing fuel pressure on carburetor fuel pumps. I add a piece of rubber hose to the end of the vacuum hose so as to fit the hole snugly. You can't leave it in there long as the manifold will soon get hot and melt the hose but you don't need to for this test.

With the gauge connected to the exhaust and the engine cold all is start the engine and watch the gauge as you SLOWLY try to raise the RPM. As the RPM slowly raises you should not see more than 1.5 to 3 PSI. If it raises up 4,5,6,7 PSI and then the engine fails to go any faster. You most likely have a burned up CAT or a stopped up muffler.

If that is the case the next sure test is to loosen the first CAT from the two exhaust manifolds so that you have about a 1/4 to 1/2 inch gap in the exhaust and start the engine and quickly rev up the engine. If it revs up ok then is it a sure thing.

Let me know and I will give you the next set of tests.

Howard


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

QUESTION: HOWARD
I DID A FPT. ON THE JAG AND IT READ BETWEEN 23/24 PSI.
I THINK THAT IT MIGHT BE THE FUEL PUMP.I BELEIVE THAT YOU
SAID THAT THE PSI SHOULD READ ABOUT 32PSI? IF THAT BE THE
THE CASE I THINK IT IS THE PUMP......... I WILL LET YOU KNOW.

REGARDS,
RICHARD

Answer
It could be the pump or a partially clogged fuel filter or a faulty pressure regulator. To test the regulator just watch the fuel pressure gauge while you pinch off the return line from the regulator. If you can raise the pressure, the regulator is faulty. Look in the tank filler neck while the pump is running and see if any fuel is returning to which ever tank is selected. If no fuel is being returned to the tank and you only have 24 PSI look at the fuel filter or look under the car for any crushed fuel lines. Then test the pump itself.

Howard