Losing fuel pressure, thoughts?

[Sobek]

Originally Posted by 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.

Thanks for the suggestions! I actually readjusted my TV cable a few weeks back, it wasn't really out by much at all and it made no discernible difference to anything. A couple of days ago I disconnected the battery in the evening, discharged everything, and let it sit overnight. Hooked it all back up the following afternoon, but it was still running the same sadly.

I followed the instructions in this video to reset timing;

You Tube

and besides one moment of stupidity involving me trying to start it with a socket and wrench still on the crank bolt, all went just fine Sadly, again, runs exactly the same. The distributor was actually in REALLY good shape, next to no visible wear, no slop, I was actually surprised for 26 years and 220k kms (Jeep has 300k, replacement motor back in 2000 or 2001). Cam and crank sensors I replaced with OEM parts abouuuttt.... 2 years ago? Had a thread on it here actualyl. So I'm starting to feel like whatever has it down on power and running a bit rough / weak, has got to be ignition / electrical in nature. I checked fuel pressure again just to be certain, and it's still holding great pressure and starting INSTANTLY with barely even a single rotation before kicking off, so that part of things seems happy as. I contacted the vendor and they're sending me a new set of refurbished injectors too. I'm thinking I might put my stock units back in and run with those just to be SURE it's not an injector problem that I've added by swapping those out in amidst all my other work...

After 26 years, my stock injectors look pretty cruddy though. Any tips for hand cleaning those? Soak them in a fuel bath, gently scrub off the crap? Orrrr... ? Would love some suggestions

Basically what I have is bugger all power, like foot down LOTS of noise roaring thunder, and barely hitting 60km/h on a slight incline. But once every few months, for a run cycle or two, it'll just go nuts, like a rocket, throw you back in your seat on every take off. No check engine light any more, not even any codes. What I get is that from a cold start, it'll idle smooth, then you'll hear it gradually booogggg dooowwwwnnnn aaaand then pick up again. Then booggg doowwwwnnnn and pick up again, over and over until it's warmed up. It's far less noticeable once it's hot, but once it's hot it's rough, sounds kind of low and depressed, and thanks to my exhaust leaks I can hear it's a bit... ploppy. I pulled all spark plugs today while doing timing, and they're all lightly browned, none more rich or lean looking than the others. Listened to all injectors with a stethoscope, all firing strong and clear as I would expect. Zero vacuum leaks.

MAP sensor? TPS? Whatever it is, it's off enough to be messing up ignition, but not bad enough to throw any codes I have a sneaking suspicion I'm going to be deep diving into my wiring loom by the end of this

[alexbrown64]

I’ve reread your first post and it’s interesting. You have basically done all the things that i would of done. It’s also interesting that you say she runs beautifully some times. This tells us it’s probably not mechanical, like low compression for example.
I can offer two suggestions.
Hunt for another computer and swap it in. It may be running buggy and not letting your engine perform properly.
Keep an eye out for a OBD1 MT2500 like mine. Use it to check how all your individual sensors are running. Mine shows O2 sensor performance, open/closed loop function, map sensor readings, injector readings, timing advance, etc.

It must be very frustrating. Can you remember the last time it ran well, and then when it started to drop power?
Keep checking things, looking for clues and persisting. You will hit the nail on the head eventually.

Cheers,

[Sobek]

It's a bit of an odd one isn't it! You may be on to something with the replacement computer actually, come to think of it. A year ago, my aux fan started not working even I flicked over to Vent or A/C so I had to bypass that with a manual switch in the cabin (A/C not hooked up but just bypassed the compressor so the fan still works, been that way for a decade without issue). I'm told that's possible solder failure in the PCM. Then sometimes when it's wet or extremely humid weather, my wipers work all weird... some speeds not working, or they just go full speed on any speed setting, or they only wipe when I press the brake pedal haha. And there have been times when I've got a whole plethora of error codes, a bunch of which I KNOW weren't even possible / relevant at the time...

So maybe? Just maybe my PCM is on the fritz and it's dying a slow death. I see second hand ones are about a hundred dollars or more online. If anyone reading this is on or near the Sunshine Coast and has a spare PCM they wouldn't mind me plugging in for an hour or so to test with, that would mean a LOT!!

I can't recall exactly when it last decided to drive like a race car, but it would have been within the last 3 months. I distinctly recall it started with a persistently high idle, like 1500rpm, as if the TPS was on the fritz. It was doing that a whole bunch around that time, which is what eventually prompted me to replace it... But it was just nuts, at 2000 rpm I was spinning tyres going uphill. Today I'm holding 2000rpm on that same hill and craaawwwwllliinnngg up through 40-50kph hahaha. But yes, I'll keep persisting. I might dig out my old stock MAP sensor and hook that up, see what happens. I'll try anything at this point. Essentially, any change is a good sign, because at least I'll know I'm on to something. I will absolutely keep the peepers hooked on those MT2500's and see what I can do, because I've always wanted more detailed information for my XJ. I have a full computer setup for my 95 year 7 series (still OBD1) and man, I can drive around with that laptop hooked up and see more information and do more with it than you'd know what to do with! Having even a fraction of that degree of information for the XJ would be priceless... It's definitely saved my bacon countless times on the BMW's.

Sorry, I know I write stories, I can't help it. I'm too thorough for my own good sometimes!! But again, I really appreciate the insight, you never know what you might miss or forget or ignore

[awg]

I would be inclined to do try and do what I did,

which in your case would mean finding a '95 and stripping all the OEM sensors and computers

( get the throttle body with sensors complete)

Not my idea, I bought all this stuff of ex-listers from this forum

If you want to keep your XJ for unlimited years, I personally cant see anyway around it, as if you have just one or two issues, the amount of time and money spent fault finding can be brutal

probably more economical yet, buy a non-rego goer to serve as a test bed, if you have space

[rainman]

try disconnecting the oxygen sensor to see if anything changes.

also, disconnect and block the vac line going to the the evap canister.

where is the exhaust leaking from?

i too have a set of working but second-hand sensors including PCM and alternator under the back seat. as well as a spark tester, and noid light.


how does it drive if u pull the fuse for the TCM and take it for a spin?

when confronted with bat shit crazy gremlins check the earths

[rainman]

[Sobek]

Originally Posted by rainman

try disconnecting the oxygen sensor to see if anything changes.

also, disconnect and block the vac line going to the the evap canister.

where is the exhaust leaking from?

i too have a set of working but second-hand sensors including PCM and alternator under the back seat. as well as a spark tester, and noid light.


how does it drive if u pull the fuse for the TCM and take it for a spin?

when confronted with bat shit crazy gremlins check the earths

That's bizarre, you read my mind! I haven't been on in a while and have been dealing with that 'life' thing, so I had to put the XJ stuff aside for a bit, but I'm back to it now. 2 Days ago actually I stuck my head under the hood and went into hard thinking mode, and the oxygen sensor was one of the first things that came to mind. I mentioned early on how I would very rarely also get an oxygen sensor malfunction code alongside the TPS or injector misfire codes... I didn't think TOO much of it at the time, as with so many codes being thrown I had suspicions of a wiring issue or one main problem creating a bunch of false errors (VERY used to this sort of thing on the BMW's, one bad sensor will snowball down to 10 codes for 10 unrelated things, it's bizarre how it all ties in together).

So I did my key test for codes, nothing. Fair enough, so I unplugged the O2 sensor, fired it up, and it runs perfect, smooth, not so much as a hiccup. Like, balance a coin on the motor smooth. Let it warm up, took it for a test drive, still gutless as hell (and probably more so with no O2 sensor at all), and as expected smelling pig rich too. I still half suspect it's not JUST the O2 sensor, but for now I'll run with it. Would you agree, just replace it and go from there? Do we have any preferences to brand? Rockauto has NTK, Denso, Bosch, Delphi and AC Delco (AC Delco has a delay). Reading online, half the people seem to go Denso, the other half Bosch, with a whole lot of people swearing the Bosch sensors don't play well with our XJ's. The sensors have different heads too, the Denso being the cylindrical piece with perforated holes all around, the Bosch having the conal shaped head with slits up the length. Just curious

And man, THANK YOU for that ground information! That is beyond invaluable! Many months ago I was digging around online for some basic ground point information and I was surprised how hard it was to get any simple list or diagram, all I could get was random forum posts of people going "Yeah there's a couple of grounds here, couple there, check under the dash etc". Not the MOST helpful...

Thanks again, hopefully close to sorting this out hey