Losing fuel pressure, thoughts?

[Gam817]

Great info, i'll need to get a pressure gauge to check. However my fuel system is currently fault free Although i have updated to the 4 pintle injectors and will soon be upgrading to a Bosch fuel pump.

Based on this thread i have preemptively purchased the Delphi FPR. Did not realised they had issues! My is a 95 XJ sport btw with the the FPR at the rail. Hope the Delphi is ok?

[Sobek]

Gam817, I can't imagine you'll have any problems with the Delphi FPR, it's about as good as it gets. I just went with the SMP out of caution from some things I'd read, but I imagine failure rates, if any, are astronomically low anyway. You'll be good!

Thanks heaps for that information Alex! I always love some good old fashioned technical documents It does confirm that for one reason or another, I'm running at the same pressures as the OBD2 models do. I was doing some googling last night, looking at old forum posts and such, and just to add to the confusion I saw a few people talking about how they saw a whole heap of 95 build OBD1 XJ's come from the factory putting out the same fuel pressure as the OBD2 cars... So who knows what the real story is there!

Anyway, I think before I get too far ahead of myself, I'm going to go over my fuel system stem to stern and just make sure I haven't missed some obvious gremlin. I had fun reading a huge thread on NAXJA about some poor fellow who was going through something similar to me, and in the very end it turned out his PCM was dodgy! You just never know lol. Since I know I have one ever so slightly dribbly injector, I'll start with that and see what happens. I'll keep you guys posted when I have some answers, thank you again

[alexbrown64]

Nothing worse than a gremlin.
I had one a few months back and it ended up being a Chinese coil that i had fitted only 2 years earlier. I put the old one back in and she has been running smooth ever since.
Im lucky, as i have a great obd1 scanner for my XJ that just gives me so much information.. I can even have it on and read live data while i am driving as i have built it into my centre console.

Keep an eye on your codes everyday, in case one pops up again. Make sure you have a fresh rotor button, cap and coil fitted, as i have found these can really play havoc with intermittent stumbles.

You mentioned that your fuel pressure didn't change between idling with vac hose on and vac hose off. There is a problem there if this is the case.






Scanner is the Snap-on MT2500 with the correct cable and module for 95 XJ's. Ive found this scanner almost invaluable, as i have an ABS module for it too that has helped me cure problems with the brakes.

Cheers,

[awg]

logically, you would probably have to start by verifying FPR hose vacuum pressure is altering with throttle

not sure the best way..maybe a vacuum gauge with tee

dont think spit on the end of the tube will do

[Sobek]

Alex, that setup is incredible... I didn't even know we had that kind of live data possibility from the OBD1 XJ's! Your Heep is practically a luxury car compared to my horse feed mobile haha, I love it. For what it's worth, within the last 2 years / 3000km (since that's about all I've really done on it) I've replaced the following with factory or best-available alternatives;

Cap, rotor, plugs, leads, ign coil, cam sensor, crank sensor, MAP sensor, Throttle position sensor, O2 sensor, vacuum lines, fuel filter, fuel pump, fuel lines... Probably more, but my memory fails me lol. So barring any randomly failed parts for some reason, all things considered my ignition setup should be pretty healthy. The MAP sensor is probably 4 years old actually, it's the oldest part I replaced. Once in a blue moon I've had code 13 stating "No difference recognized between the engine MAP reading and the barometric (atmosphere) pressure reading from start-up." so I'm thinking my MAP sensor might be a bit dodgy?

Anyway, last night I actually managed to sort out both the fuel pressure loss, and the regulator pressure being stuck. It was indeed that one injector that was causing the pressure to drop when off. I put one of my old stock injectors back on that cylinder and sure enough, it held pressure over 20psi all night. As for the regulator vacuum... well, I was looking everything over again and again and it was staring me in the face - at some point in this motors' early life, someone installed a check valve in line on the pressure regulator's vacuum line. It was installed so that the valve sealed AGAINST the manifold, meaning it couldn't draw any vacuum against the pressure regulator. I removed that check valve and reconnected just the straight vacuum line, and sure enough my fuel pressure is spot on now, 41psi at idle, 49psi with the vac line disconnected. Who did this or why, I can't say. But from memory, that check valve has been there at LEAST since I started daily driving the XJ way back around 2005 or so. And given that this motor is a donor motor from a grand cherokee that we had swapped in in the late 90's when the stock motor kicked the bucket, I'm guessing whoever had that GC did it for... some reason.

The downside is now it's idling really rough and popping a little, but no codes whatsoever, so I've got something else going on there that I need to look into. Still feels waaaaaay down on power too. Could the MAP sensor cause rough running issues like this? All my vacuum lines are intact and unblocked, and short of some timing problem *somehow* I'm a little miffed as to what's got it running WORSE now that I've sorted the regulator and such. I've just done a long drive around town and the poor thing was coughing its guts up at every stop light, so I'll check it for codes again in a bit, but I'm not expecting much. It no longer smells overly lean or rich either, it just... smells. Like exhaust should normally smell. That's a bonus I guess!

[alexbrown64]

Glad you spotted that check valve anomaly.
You could reset your computer by disconnecting battery, turn lights on etc..
This will reset to factory specs as it may need it with the changes.
Also, replace each bit you removed one bit at a time to see which item affects performance.
To me, it sounds timing related. When I replaced my chirping distributor with a cheap Chinese one, it started popping and ran really gutless. I greased the shaft on my original one, swapped it back in and she purred like a kitten. I then bought another original Sagem dizzy, fitted a grease nipple and fitted that one. Runs beautifully. Each time, I just used the rotor button position as a reference and refitted the dizzy in the exact same position. Your dizzy may be worn, or need shaft regreasing.
Anyway, just another thing to check. Also, make sure the cam sensor is a good one, clean and seated correctly.
Another thing maybe that the single different injector might be upsetting the balance. Check the spark plug on that one too.

Don’t forget to do the TV cable. I thought my 4.0 was gutless as hell. I reset the TV cable and it was like a different Jeep. Google bleeping Jeep tv cable adjustment. Free, five minute instant power mod that I say is critical to performance.

Dizzy mod and tv cable are in my vsb14 thread.

Good luck and keep us posted.

[rainman]

ACTRON 9690 works well, would expect a snap-on scanner to do more though.
i have it in the cab plugged into a port on the dash.
with one of these, i can see what every sensor is doing in about 10 seconds- AIT, Coolant, tps map, iac, an analog voltmeter to test the tps sweep is still the best though. (no cam or crank sensor though) if yr gonna work on a 25year old jeep yr self a scanner is prolly the best thing you can do.

when i had an intermittent stumble i had a dizzy with alot of slop in the shaft. another tight dizzy and it went away.

X2 on resetting the computer.

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