Jaguar Repair: 84 XJ6 loss of power, continued, fuel pressure gauge, air flow meter

Question
Howard,

I have performed the next round of tests and found 30 PSI fuel pressure at idle, under throttle, and with vacuum removed. The fuel pressure never changes at all. Vacuum pressure was 15 in HG at start up, settling in to around 18-19 as the car warmed up. Now, to establish failure mode, I drove my neighbor hood street, then ventured out on to the highway to the gas station. After adding 5 gallons of 87 octane, I restarted and pulled on to the highway to find I already had less power. After a quarter mile, I was creeping, and I barely made it the next quarter mile back to my garage. This is my failure mode, and upon parking under the lights, I found fuel pressure to still be at 30 PSI, vacuum at 19 in HG, but when throttled to 2000 RPM, vacuum drops to zero, RPM drops to 1000 and there is a backfire at the air filter housing. Please tell me what is next - it seems failure occurs after warm up, of course, but what of the fuel pressure?

Thanks,
Michael

Answer
Hi Michael,
When the vacuum goes to zero that is a indication of a stopped up exhaust system. Pull one of the hex plugs in the exhaust manifold and rig a temp fitting with a hose to your vacuum gauge (if it has one of those sections for low pressure to check carb fuel pump pressures.

You don't have to run it long, just rev it up several times to a high RPM to get a reading. Any pressure reading of higher than 1.5 PSI is a stopped up exhaust. Don't run it long as the exhaust manifold will quickly get hot and burn what ever hoses you force into the fitting.

If it is stopped up you will probably quickly see 6 or 7 PSI on a few rev-ups.

Your fuel pressure is low considering that you have seen the vacuum go to zero which should have boosted the fuel pressure to about 42 PSI. With the fuel pressure gauge on in a "T" turn the key on and hand open the air flow meter flap and watch the pressure and if you still have just 30 PSI pinch off the fuel return line coming from the regulator to see if you can boost the pressure. If you can't you have a Pump, lines or filter problem. If you can boost the pressure, you have a bad regulator.

Howard