Jaguar Repair: 1984 XJ6 fuel issue, saturation point, pressure gauge

Question
Howard,
I just performed the test you suggested. I remember this one now from before. With the key on, I did get just over 40 PSI at the fuel rail, and the pressure climbed quickly towards 50 PSI when I pinched the return. However, immediately after this, it took several, several start attempts before the engine would crank, and within 5 minutes, the pressure had dropped to around 25 PSI, idle had slowed, and a pull on the throttle would decrease pressure to as low as 10 PSI.

I began to wonder today if a fuel filter has a saturation point that is reached. That is to say, it is not completely clogged, but begins to choke down over time. I can only get the car to start after several tries, every time - even if it has just been shut off. The pump can be heard running, and remember last night, I put the pressure gauge just on the pump as a dead - end and got 60 PSI +.

I look forward to reading your thoughts.

Michael

Answer
Either the pump is failing after running a short time or your filter is partially stopped up thus restricting volume of fuel but not static pressure.

A good test I use for this is to put the "T" fitting in the fuel line just at the fuel rail and run a hose out from under the hood and put the gauge under a wiper arm so you can see fuel pressure as you drive the car. Then either find a long hill to climb or take it out on a long straight road where you can accelerate hard up to a good cruise speed so that you use a lot of fuel and watch the gauge during the run. (be sure to put clamps on all hose ends)

When static testing a certain amount of fuel is pumped up the the rail and the regulator maintains 32 PSI by dumping most of the fuel back to the tank. But when the car is running under load a much larger volume of fuel is needed to maintain that same 32 PSI of pressure and if a filter is partially stopped up, the volume can not make it up to the rail.

You can sometimes test a pump by just doing the static test but leave it running for 10 or 20 minutes. A pump that heats up and fails will usually start to falter in that time of just pumping fuel up front and back to the tank.

Howard