I'm trying to test the FPR on a 1996 XJ6.

I pulled the vacuum line and it is not leaking. I hooked up a vacuum pump and when I applied vacuum it did absolutely nothing. Usually when I did it on other cars if I applied vacuum manually the car would run different or even die, but this one does nothing. Has anyone ever seen one stick?

How much power are these cars supposed to have? I know it's 245HP, but should they be able to light up the tires from the stand still? This one doesnt.

 

Whenever I want to check fuel pressure I install a mechanical gauge in the supply line.

I made an adapter when the XJ40 (AJ6) was introduced in the 1980s and it works fine on the AJ16 engine as well.

The vacuum application will only change the pressure several pounds per square inch. (something like 36 PSI to 40 or 43 PSI depending on the absolute pressure)

Are you watching the pressure or just listening for a change in the engine?????
Listening can be subjective. I can tune (ancient) S.U. carbs by listening but it is a LEARNED technique.

Modern engines can adapt to different fuel pressure via O2 sensors. A few PSI can be compensated (to a degree) with Pulse Width adjustment by the ECM over time.

Gone are the days of moving the distributor back and forth to get the engine to run right or 'get more power' and a few PSI might not cause a significant reaction.

Running too rich or too lean can cause a lack of power but there are other things as well.

Can you monitor the STFT when you apply vacuum to the FPR????

More diagnosis???

bob gauff

 

I had a scanner hooked up and was monitoring all 4 o2 sensors voltages. There was no visible change, unless the ecu adapted quickly enough.

 

Try to monitor STFT while you change air pressure (vacuum) to the FPR.
The Short Term Fuel Trim should react to the fuel pressure change.

The upstream O2 sensors should notice the change.

bob gauff

 

Hi adam699,

The ability to light up the rear tires is perhaps less a factor of pure horsepower and more a factor of torque and overall gearing. Normally-aspirated Jag straight-six engines aren't torque-monsters to begin with, and the transmissions/differentials are geared to intentionally minimize rear wheel spin off the line.

Cheers,

Don