No fuel pressure at rail

I have a '93 XJ 4.0 that's been taking longer to start for about the past two weeks. Did a tune up including fuel filter, have good voltage and cranks strong. Heres my question...Before turning the key I have zero pressure. Turn the key and have 5 lbs, start it and I have 35 lbs, turn it off and I have zero. No apparent fuel leaks anywhere so I thought maybe a leaking injector so I replaced them. Runs better but still the same pressure results Next thought would be weak pump or leak in the in tank fuel hose maybe? Am I on the right track? With such low pressure before starting, why would pressure be fine after? After it starts it runs great... WTH?

There is a fuel pump suction check valve to provide quick priming of the pump for immediate fuel pressure. Since you have replaced the fuel injectors; you have ruled out that they are leaking by when the fuel pump is off.... It sounds as though the check valve is leaking back to the tank which bleeds the fuel pressure off the fuel rail.
You can try turning the ignition switch to ON, but don't crank the engine. Wait 5 seconds and allow the fuel rail to pressurize; the engine should start immediately once you crank the engine over.

I had the same problem on a '90 XJ. The issue turned out to be the feeder hose from the fuel pump to the pickup inside the gas tank. It is a relatively short piece of rubber but it had slipped back from the pickup tube that exits the tank. It was so short that it couldn't completely pop out so when the pump was running it would eventually build up pressure and run but (I suspect it was spraying gas inside the tank) the second I turned it off, pressure would drop to zero. A bit of a PITA to fix (I had to drop the tank). Hope yours is just the check valve.

It is easy to mount an inexpensive external check valve on the '93 if that turns out to be your problem. I did this on my '93 some years ago. My problem turned out to be an intermittent failure to prime but the valve is still there.

Thank you for your help. Sounds like the problem. I read in another thread that some have used a BMW in-line check valve but it doesn't seem to be available any longer. Can I use any ol check valve as long as it's the correct line size? And would I mount it before or after the filter? Also it looks as if I can replace the pump without dropping the tank on my '93. Can anyone confirm this? Is the check valve part of the pump or a separate part of the assembly? Thank you again...I really want to get this fixed!

I got mine off ebay, probably here but I can't swear to that: http://cgi.ebay.com/ebaymotors/One-W...Q5fAccessories

I did the install in May 2009 and there were several threads about this problem in the first several months of that year which you may have already seen. Would have to look but I believe it mounts before the filter where there is room to cut the flex line from the tank.

Edit: I see the link isn't working -- worked earlier. Will check it out and re-post.

Seems kinda funky to me to have to put in an aftermarket check valve on the fuel line. I'd almost bet money on the problem being the little short hose from the pump to the pickup frame inside the tank, it's pretty common. I've seen this multiple times where the rig was hard to start but stayed running once it was going, because the hose was just cracked enough to let the fuel in the line bleed back into the tank over time, but not bad enough to cause it to completely lose pressure while the pump was running.

I guess it's worth it to try if the check valve you get is cheap enough and easy to install, but even if it fixed the problem I'd still be wary that I was just bandaiding a problem with that hose inside the tank. Good luck and let us know what the results are.

I had that problem with the fuel pump hose when I first bought the XJ and I paid a mechanic to replace the fuel pump but I guess the PO's had used a non fuel rated hose and yes it was leaking back into the tank. Luckily I found one of those very rare honest mechanics and he replaced that hose about 3.5yrs ago. I can't be 100% positive but I don't think that's the issue. As soon as it starts pressure is 35-36 lbs compared to when the hose was leaking the fuel pressure was only about 23-24 lbs. Also when I do the poor mans prime it fires right over. So my $'s on the check valve. I haven't gotten any answers about where in the assembly it's located so I'll have to do some research. I'll update when I find out. Thanks again!

I believe the Fuel pressure regulator is mounted at the end of the fuel rail. A new one would cost about $40 to $50 bucks ... check at Rockauto.com for part price and picture.

The check valve is part of the pump assembly. If the tank sender is on the front of the tank, then you shouldn't need to drop the tank.

four_shot said:

The check valve is part of the pump assembly. If the tank sender is on the front of the tank, then you shouldn't need to drop the tank.

Click to expand...

Sweet! Thank you for your response! Rocks...

Here is the fuel pressure leak down test procedure... it indicates that either the check valve or the pressure regulator can be the problem.

Fuel Pressure Leak-Down Test
1) Remove cap from pressure fitting on fuel rail. See Fig. 3.
Connect a 0-100 psi (0-6.9 kg/cm​

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regulator. Replace fuel pressure regulator.