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Stumbling problems... again. Also hard starting

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So for the last thousand or so miles my MJ has been having trouble starting. It cranks over, but it sometimes takes 10-15 seconds of cranking to actually fire, especially when hot. No trouble codes presented themselves, and beyond an inspection of ignition components, I couldn't think of anything except maybe that my 2-week-old (but 5,000 mile) fuel pump is dying and/or the janky hose-clamping to make the stupid Delphi pump fit properly was having problems. I haven't had time yet to go over the ignition components, but the spark plugs are new... although my buddy installed them while I was changing the fuel pumps and while I trust he's competent I never checked his work, but they're just spark plugs after all...

On my way to work today, it's starting stumbling under acceleration. My thoughts immediately jumped to the fuel pump again, but I checked codes again and this time pulled up a code 53, PCM fault.


So I work until 11:30 pm tonight, so tomorrow morning I'll be checking some stuff out. First is a hard look at the ignition system, which is all unknown because I just left it on the engine when I swapped it in last fall. It's run alright until now.

Then I'll make sure the connector for the ECU is clean and tight.

Then I'll check the O2 sensor wires (because I did use google just now, but not much came up).


Is there anything else I should do? Any other connections I should be looking at? Beyond the fuel pump and spark plugs (and two oil changes in the last month) the only new parts are a JY taillight harness yesterday, which I don't imagine would cause those kinds of problems.


The only thread on here I found specifically mentioning code 53 was my own... which brought back a whole wave of memories I'd rather not go through again, but the issue back then was the cracked flexplate, so I'll also be making sure all the bellhousing bolts are tight and taking a peak under the inspection cover. The newer flexplate is still sitting on the old engine in my garage so I can always put that in but really would rather not.

That thread also mentions a connection between the CPS and fuel gauge, and my fuel gauge has definitely been acting weird for the last few months. I didn't see anything visually wrong with my sending unit either when I did the pump, but I didn't take the time to put the voltmeter on it and check it out. Basically per the gauge it only fills to 3/4 (this was normal) and then it runs empty very quickly, with the gauge reading zero and the low-fuel light on after about 70-100 miles on the highway. So I'll also test the CPS as well. I've got at least one used one kicking around somewhere I think...

Actually thinking about it, I'm pretty sure I pulled the ECU out of the donor XJ before I scrapped it, so it's probably in a box in the basement. Before it comes to flexplates, I'll be giving that one a shot, assuming there are no issues running the '93 ECU in the '91.

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Yeah, I've seen both of those.

As a bit of an update I cleaned up the ecu connector and toughened up the bolt. I'm not sure about the coil, but the cap and rotor looked okay. Spark plugs are all good.

As a curiosity I hooked up the fuel pressure gauge and it seems to be reading mostly properly, but the needle is all over the place, like an old Chevy mechanical speedometer. Not sure if it's just the gauge (it's three weeks old and I only used it twice before!) or if my pressure is actually jumping up and down by 10psi several times per second... Does that mean anything to anyone?

I dug out the '93 ECU but I'm nervous to try it because the part number's different, not sure if it means anything though?

I also got underneath and it's hard to say but I think I hear the faint rattle of a broken flex plate. I'll have to pop off the inspection cover when I get a chance. Really don't want to do that one again though...

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Well to follow up on this, I think I've identified the problem. Still not sure how the code 53 fits in, but the other day on the way to work the truck was not doing great, every time I pushed the gas I had to back right out again or it would stall. Then when I was going home, it wouldn't start at all. It tried, coughed a couple times, nearly caught a couple times, but no go. I brought the '93's ecu, plugged it in, no go. Switched the '91 ecu back in, still no good. Unplugged half the sensors with the key on, plugged them back in, still no go. Hooked up the fuel pressure gauge, and no pressure at all, but the fuel pump was definitely kicking on. I got a little suspicious because I knew I was running a little low on gas, so I put in around 5L, just over a gallon. It fired right up. I then drove it to the gas station (stumbling the whole way) and filled it with another 70L. It runs perfectly with a full tank of gas.


75L is about 20L short of what the tank will take, and I've definitely run it lower than that in the past. So the janky-@$$ hose-clamping because of the stupid adapters that don't work that come with the Delphi pump that I put in on the side of the road are probably to blame, and I was correct when I assumed that not all the fuel going through the pump is also going through the sock from the bottom of the tank below the slosh pan.

Some of it is getting around the top of the adapter, about two inches above the bottom of the tank, and above the slosh pan. This means that when I get down to around a third of a tank or so, if all the gas sloshes (say during acceleration when it had stumbling problems), the pump is also/mostly sucking air.


So the solution is going to be keeping it above a half-tank until I can track down a better adapter. Seriously though, the one that came with the Delphi doesn't fit onto the pump in the only place it makes sense to put it, and the sock won't go into the end of it, so pretty much right now the pump is hose-clamped into the adapter to keep it sealed, except obviously it doesn't work that way...

Next time I'm getting the Bosch pump.

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Just the pump. Everything in there looked pretty minty, although when I get back in there I'm going to replace the rubber line, forgot about it this time until I had it apart on the side of the road.

I had 12V at the connector (ballast bypassed) and great continuity on the ground wire (pretty well 0 resistance although I couldn't figure out where it grounds in an HO. I know it's not behind the taillight, cause it ran fine with no taillight harness at all. The tail lights even worked without that ground hooked up when I plugged in the new harness, although they were super dim.

The main reason I'm wondering about the ground is because my gauge has started acting funky, going to empty and triggering the low-fuel light after 60-100 miles. I couldn't find anything visually wrong with the sender when I had it out, like I said everything looked mint in there. I blame in on winter-blend (no water-absorbing/ice-causing ethanol!) fuel for over half the year.

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It should, although they might not want to take it back considering I've done 6000 miles on it already, but I guess I don't need to tell them that, after all it's only been a few weeks...

I'll pay them a visit in the next couple days, see what they say. Canadian tire is usually pretty good about taking things back, no questions asked, just depends on which sacrificial kid is behind the counter.

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