Cherokee XJ 1998 fuel line

[Fourpot]

Hi Folks,

 

My 4.0 (actually 4.6) petrol 1998 XJ is suffering from fading fuel pressure. If left overnight it drops to zero and when the ignition is turned on, comes up to about 20-25 psi. It takes a few seconds of cranking to start, then runs fine with pressure at the desired 49 psi. Turn the engine off and the pressure drops away again to below 30 psi within 15 to 30 seconds. It should take more than 5 minutes?  This is with a fuel pressure tester connected to the fuel rail Schrader valve.

If left for just a few hours or so, it starts straight away, but then sometimes revs high for a few seconds before settling down. Then everything runs perfectly.

So I guess the problem is somewhere between the fuel pressure regulator and the injectors. The whole fuel pump assembly was replaced along with serviced and tested injectors, when I put the new engine in, in 2021, but the piping and fuel rail are all original. (I want to replace the whole fuel line anyway from regulator to fuel rail, with something flexible and ethanol proof.)

As all the fuel lines are metal I can't clamp any section off to eliminate the pressure drop to a particular area. Short of replacing absolutely everything one thing at a time, does anyone have any ideas as to how I may narrow it down a bit?

 

[V]

On a steel fuel tank XJ fuel bleed down can be caused by a leak on the short length of submersible hose on the pump assembly. On a plastic tank XJ I think bleed down could be a leak at the sprung fuel tube or the fuel pressure regulator filter at the top of the fuel pump sender assembly. I think the later XJ fuel pressure regulator has combined with a filter and a check valve, if so a faulty check valve could enable the fuel to drain back on the pressure side.

 

I don't think the pump has a problem as you wouldn't get 49psi.

 

I think the most likely problem is a leaking or stuck fuel injector nozzle. This would explain the pressure drop in 30 seconds. I bad leak is likely to throw a DTC for rich running. The spark plug may also be carbon fouled. It is possible to feel the vibration of a fuel injector operating when the engine is running. Touch the body of the fuel injector with a small screwdriver while keeping your fingertip on the shaft of the screwdriver. You should be able to feel a faint clicking. If you can feel it on five of the injectors the other one could be stuck open. I guess a less detectable click could be a leaking but otherwise working nozzle but I guess the spark plugs would be a better indicator.

 

It could also be a fuel injector O ring leak but you would smell or see fuel leaking.

 

[Fourpot]

Thanks Vince,

 

All the plugs are good and recent MOT showed a nice lean run (The issue was there when the MOT was done, but because I drove it there, the pressure was up and stayed up during the test enough to not produce the symptoms). No codes up. No fuel smell. I'll try the vibration thing but I had all the injectors out the other day, all o-rings are new and I re-seated them all with a little Vaseline.

I've dropped the fuel tank now (plastic one) to see if there's anything obvious, no visible clues. I'm going to replace the cheaper bits one by one until it's fixed, starting with the upper fuel pipe.  If I can find something to fit, I'll stick the pressure tester into the end of the fuel pipe where it joins the rail. May show if the leak is up stream or down stream from there

[Caroline12]

I don’t know petrol fuel systems, sorry if I get something wrong. I know diesel. 

first I would look at any non-return valves in the system, leaking valves drop pressure.

is their one line from fuel pump to injectors, like a diesel common rail system, any leak off lines off the injectors ? Or fuel lines to each injector. 

when you had the injectors out did you pressure test them, injectors leak/seal tests. There are 4 tests you can do on injector testing for pressure. A leaking injector will constant drip leak fuel, dropping pressure in the rest of the whole fuel line. This would produce higher exhaust temps of that cylinder and poor combustion and a sooty/carbon cylinder space. A borescope down the spark plug will tell you if sooty or carbon.

 

I would check your fuel filter as well. If blocked with crap, pressure will drop. 
 

you can do a injector leak off test with bottles and plastic hoses when running, if 1 injectors leaks off more than the rest, that’s the injector with issues. 

[Fourpot]

13 hours ago, Caroline12 said:

I don’t know petrol fuel systems, sorry if I get something wrong. I know diesel. 

first I would look at any non-return valves in the system, leaking valves drop pressure.

is their one line from fuel pump to injectors, like a diesel common rail system, any leak off lines off the injectors ? Or fuel lines to each injector. 

when you had the injectors out did you pressure test them, injectors leak/seal tests. There are 4 tests you can do on injector testing for pressure. A leaking injector will constant drip leak fuel, dropping pressure in the rest of the whole fuel line. This would produce higher exhaust temps of that cylinder and poor combustion and a sooty/carbon cylinder space. A borescope down the spark plug will tell you if sooty or carbon.

 

I would check your fuel filter as well. If blocked with crap, pressure will drop. 
 

you can do a injector leak off test with bottles and plastic hoses when running, if 1 injectors leaks off more than the rest, that’s the injector with issues. 

Thanks mate. It's a very simple system. Fuel pump in the tank (annoyingly goes in the top, so you have to drop the tank top get to it). The filter and pressure regulator are built into the pump, but both can be removed from it when it's out. Then a straight line of pipe all the way to the fuel rail, each injector comes off that (so just like common rail diesel). No valves except in the regulator, and no leak off lines. Plugs are all good. 

I've taken the regulator out now. I couldn't actually remeber if I'd replaced the fuel pump or any of that module, assuming I had because it looked clean and shiny in my posts at the time I took the tank off to clean it. But now I've puylled the regualtor out, it most definitely is not recent, it's a Chrysler part and looks old in the bits that don't see daylight and the o-rings are rock hard, whereas the filter is clean and the injectors are recently serviced and tested. A new regulator is not expensive, so I'll start with a new one to see how that goes.

[Fourpot]

I'd bought a new pressure regulator on Ebay for about £40. when it arrived it looked odd, and when I checked it properly, saw that it wasn't straight and on offering it up, it wouldn't seat properly. So it got sent back. 

Bought another from a reputable Jeep parts place in Hull. More expensive but fitted perfectly (and came with new spongy pad and clip).

Everything back on. Ignition on and off a few times to get the system primed and pressure to about 30psi. Ignition off, key out. Pressure sat at 30psi until I got bored (about 15 minutes). So switched back on and it started up instantly. Ran engine for a few minutes, pressure steady at 49psi, switched off, pressure went very slowly down to 40psi and then stayed above 40psi until I got bored again. That'll do nicely.

[Fourpot]

I put the wide 'fender flares' (wheel arches to us over this side of the pond) on ages ago. They come with self-tapping screws which have held the arches on reasonable well, but every time you remove them, as I have done several times now, the holes get bigger and the screws don't grip. So I bought some 'nutserts'. The are bascially nuts that are 'rivetted' into the steel panel. Dead easy with a decent tool. Practiced with a bit of off-cut steel the same thickness-ish. Used M6 nutserts and M6 japanned bolts and washers (the tool comes with a range of sized fittings). Now very secure and can be taken off and on without any degradation.

[V]

Mine were nutserted more than 20 years ago. I cannot easily take them off now. I assume a small amount of corrosion around the nutsert has lost some grip so the nutsert spins in the panel when I try to remove the screw. I should have splashed a weld on them when they were fresh or used epoxy around the hole before fixing the nutsert. My flare screws are all stainless steel, but the nutserts are steel.

[Caroline12]

9 hours ago, Fourpot said:

I put the wide 'fender flares' (wheel arches to us over this side of the pond) on ages ago. They come with self-tapping screws which have held the arches on reasonable well, but every time you remove them, as I have done several times now, the holes get bigger and the screws don't grip. So I bought some 'nutserts'. The are bascially nuts that are 'rivetted' into the steel panel. Dead easy with a decent tool. Practiced with a bit of off-cut steel the same thickness-ish. Used M6 nutserts and M6 japanned bolts and washers (the tool comes with a range of sized fittings). Now very secure and can be taken off and on without any degradation.

Rivnuts, I’ve got the same Sealey Rivnut tool, once I worked out how to use it, it’s fine. 
I used it to do carbon steel rivnuts for my rock rails as the aluminium ones supplied were rubbish. I’ve also used them to attach my wheel arch inserts, it works well, just not on plastic very well. 
I'm glad your fuel issue is now sorted and the wheel arches look great  

[Fourpot]

Doh,

This 'fender flare' bit should have gone on my Cherokee XJ project thread...  I've got these cross-threaded.