I recently started a thread about low fuel pressure on my 98 XJ-8. It was running 24psi on the fuel rail. I have noticed that my short term fuel trims have decreased quite a bit recently compared to the positive 19.5 that it showed on both banks before. They now run near zero on idle and even in the negative numbers at cruise. The LTFT's are both 14.8, but will decrease in cruise as well. I checked the fuel rail pressure again today and it read 34psi . This would confirm that I am getting more fuel, but still not 41psi. Now....this is what I discovered: After I shut down the engine, the fuel pressure will creep up to 41psi and stay there. Does anyone know why? Where is this pressure coming from ? I thought the fuel pump cut off with the ignition switch. Is it possible for the diaphram in the regulator to exert pressure back in to the system enough to cause this rise ? Is this a sign of a bad fuel pump or bad regulator ?
Thanks in advance for any and all ideas.

Are you testing fuel pressure with vacuum to the regulator, or removed and capped? That pressure spec is likely for no vacuum, which implies Wide Open Throttle or a high Throttle percentage.

When you shut the engine off, manifold vacuum goes to zero, so fuel pressure climbs slightly, usually around 5 psi. This is normal.

I checked fuel pressure both with and without vacuum connected to the regulator. Slight increase from 34 to 35 with vacuum removed. I ran engine at idle and at 2500rpm with no change in pressure. I'm thinking a bad fuel pump or at least an obstruction or clogged sock in the gas tank. I replaced fuel filter a couple weeks ago.

You should have seen more of an increase with vacuum removed... There isn't any evidence of liquid fuel in the vacuum line, or vacuum connection, to the regulator is there?

The fuel pump may be going out, since your readings seem pretty sporadic... The pump itself has never been replaced, has it? I've seen a post on this forum from someone who had a fuel issue and found a kinked hard-line from whoever had already replaced the pump prior and tried to short-cut it by bending the fuel lines, likely so the tech wouldn't have to drop the tank entirely...

Is the pump louder than normal or whining?

I have owned this car three years. As far as I know, it still has the original fuel pump. I have looked at the fuel lines under the tank and they look ok. I will disconnect the vacuum hose on the regulator again and try to determine if the diaphragm is damaged. I have seen no evidence of fuel in the vacuum hose. What do you suggest I try ? No unusual noise coming from fuel pump. I would never have known I had a problem if the check engine light had not lit. I can reset the computer and drive about 150 miles before check engine light comes on again. I changed the fuel filter a couple weeks ago, but no change to fuel pressure as far as I can tell. Thanks for you help.

CEL? There are no codes associated with fuel pressure. What code(s) are coming up?

I still get codes P0171 and P0174 about every 100-150 miles after I reset the computer. For more info, here is a link to my previous thread: https://www.jaguarforums.com/forum/x...essure-107717/

Haven't read you other post but P0171 and P0174 say that you have a leak in the air intake system between the MAFS and the intake valve. Accordion bellows in the filter to TB duct is famous for splitting over the exhaust manifold heat.

Have tried a test with a vacuum pump to ascertain diaphragm integrity?

Yes, but those are mixture related and probably derived from the O2 sensor readings.

I would suspect in the present case that they can also be thrown without a leak when fuel pressure is low because it will cause a lean mixture as well.

Those codes, Bank 1 Lean and Bank 2 Lean, could be caused by low fuel pressure, but an air leak is much more commonly associated with them, as test point mentioned.

I would check the inlet behind the MAF sensor as he/she suggested, but the Intake Plenum Gaskets are a common failure on this car. There are 8 small gaskets, all individual and of the same size, made of rubber that seal the Intake Plenum Runners. Any air leak around those gaskets will show up as a lean code and depending on how bad the leak is, various things may happen.

Small leaks tend to show up as a slightly high or hanging idle, a stumble when you crack the throttle, and lean codes. The computer then takes over and begins adding fuel to the trims until it's seeing the O2 millivolt range it's looking for. Larger leaks will result in a surging idle, stalling, and stumbling, as well as higher positive trim numbers, in the ballpark of 10+ %...

If your car was in my shop, the first thing I would do is pull a large-diameter vacuum line, like the one running to the brake booster, and plug a Smoke Tester into it, then stuff a towel or plug into the throttle body with the air intake removed, and slowly fill the manifold with smoke. If there are any plenum leaks, you will see the smoke seeping out around them. The "backwoods" way of doing it is taking a can of Throttle Body Cleaner and with the engine idling, spray it around each runner individually and see if the idle changes. Do it one at a time, and don't drench the thing if the engine is hot because the vapors are flammable.

My original thread :https://www.jaguarforums.com/forum/x...essure-107717/ No one responed to my post.

This explains what I have already checked. I am pretty certain the problem is low fuel pressure, and I was hoping to be able to verify my fuel pressure regulator was either working as normal or not. 34 psi on the fuel rail is 17 percent lower than the 41 psi it is supposed to be. Initially I only had 24 psi, so I am going to pull out the tank and replace the pump and screen. If this doesn't cure my problem, then the reulator will have to come off next. Thanks for all your suggestions. I will report back when I swap out the fuel pump.

If you plug the return line from the regulator, can you maintain a higher pressure?

If so, that would point at the regulator. If not, then the fuel pump.

Hi Plums !
I have considered removing the pressure regulator and covering that port just to see what kind of pressure is getting to the fuel rail. Might do that before removing the tank.
Thanks to all !

Update on my problem: Since I last posted, my car has been running a little better, not cured by any means, but with the cold weather upon us, I kept on driving . Last week I went to start the car and could not get it to fire. There was no fuel pressure on the fuel rail. After several attempts it did start, ran very rough for a couple of minutes and this was like the kick in the pants. I drove to the parts store and purchased a disconnect for the fuel return line coming off the fuel pressure regulator. I attached my fuel pressure gage to the fuel rail and a clear vinyl hose to the fuel return line going to a gas can. As I suspected, I was only getting about 30psi fuel pressure to the rail and not return fuel going back to the fuel tank. So, I have eliminated the fuel pressure regulator as the reason for low fuel pressure. I will now attempt to remove the fuel tank and replace the fuel pump and screen.....

Great thread so far and don't want to steal it but where is the fuel pressure regulator located?

The regulator is onthe fuel rail. Identify it by the fuel return line and vacuum line connecting to it.

These symptoms sound a lot like what happened when my fuel pump inlet screen plugged, except mine got worse faster. The screen would apparently plug as the engine ran, then the crud would fall off the screen when shut down, then replug when it ran. New screen, flush tank (with water hose!, same old pump. 50,000 miles later I am pretty sure of the diagnosis.

I finally got around to replacing my fuel pump, and now my fuel pressure is where it should be about 40 psi. The fuel pressure increases about 3 psi when I rev the engine. I can say this was not a fun repair, but I took my time and did no damage to the fuel lines, vent hose or wiring. I think I will start a new thread explaining what I learned while replacing my fuel pump. Thanks to all who helped me so far....I am very lucky my old fuel pump did't completely fail prior to my fix.