Jaguar Repair: jag XJ-6, vacuum test, manifold vacuum

Question
So I did another compression test this morning when the engine was cold(75deg F).  Starting with #1(back) 140, 140, 130, 135, 125, 115.  Oil pressure is good but none of the cylinders got up to the 150-170psi that you spoke of.  The so called "rebuilder" seems like a hack to me so how deep into the engine he went with his rebuild is questionable at best.  



Additional info...

75deg F

TTS          221 Ohms

Gauge Temp Sensor         7,620 Ohms

Computer water Temp Sensor 2069 Ohms

195deg F

TTS          223 Ohms

Gauge Temp Sensor          309 Ohms

Computer water temp sensor  230 Ohms



Smoke test ok

Fresh Gas

New Plugs and wires

No injectors leaking

New Injection Harness

Ignition Timing ok

Aux Air Valve Working

Good Spark

Anything Below 11-1200RPM the engine dies

Coil wiring seems a bit odd,  2 wires to the (+) side and 4 to the (-) side??

Still bad Multiple Random Cylinder Misfires



Anything I left out just ask

Thank You for your time and help, it is much appreciated

Answer
The ECU temp sensor reading is important and seems to be close to correct and with new wires and even a new injection harness and a random miss and the compression showing very low on #6 and the others not that great. I would run a vacuum test to see if you are loosing manifold vacuum at low RPM. Also put about 4 or 5 squirts of engine oil in each plug hole and run another compression test (throttle open) to see if the loss of compression is due to rings. If you don't get a large boost in compression with the "Wet" test then it is valves so you should check valve clearance.

If it was not a Jag mechanic he most likely didn't have an assortment of valve pads to do the valve adjustment correctly. A manifold vacuum test and the ring "Wet" compression test may lead you to the problem as the intake manifold on a Series III jag was not well designed to start with and I have noted that even a slight leaking intake valve will blow into the intake and cause a loss of vacuum for other cylinders and that will cause a "no run" at low RPM. Instead of the vacuum going up at low RPM it goes down with a intake valve leaking. At low RPM the manifold gets pressurized with burned gases from a leaking intake valve.  

Also, you need to look at the color codes of the 4 wires on the negative side of the coil as I question that. I don't remember any series III cars having more then 3 wires there. (ECU, Ignition system and tach) They all must be white wires with colored tracers on negative side. If someone has installed an aftermarket ignition system they may have added a wire for some reason. ???

If you can set the throttle at an RPM that you have the most miss-fire you may be able to do a cylinder kill test to see which cylinder it is on. But don't pull the plug wire to do the test. Because the spark may jump down the coil tower to the primary of the ignition system and wipe out the module mounted on the front of the intake manifold. If you don't have scope you can put pins through each plug cover and just go down the line touching the head of each pin with a ground wire to kill that cylinder and watch for the RPM change. You will have to rev the engine between each kill to clean the plug as the injection will continue.

I would also test your compression gauge on a known good engine to be sure it is not a faulty gauge. Also make sure the throttle is open when running the compression tests.

Let me know,

Howard