91 montero wierd fuel activity
 

I have a 91 montero rs with 200k. Car has only been mine for a month or so and I know it sat for a while before I got it. It has the original 6g72 and runs great, but seems to burn some oil. I've put about 1200 miles on it with no problems until now.
I ran it very low on gas a few days ago; didn't run out, but barely made it out of traffic to a gas station. Filled it about halfway up, started right back up, and drove another 5 miles or so to jobsite. Tried to start after about 10 minutes and it cranked and cranked but nothing. Finished work and tried to start again 4 hours later and still just cranked. Towed home and have now done a series of things to it with limited success:
-Checked all the fuses
-Pulled the fuel pump and it runs with current supplied directly to it
-Used the test circuit for the pump with it installed and it runs, although the first time I did this it just kicked on for a split second- there seems to be some question as to whether it should run continuously or just pressurize the system
-Swapped out the fuel relay, although the only one I could get was from a junkyard where I pulled it off another 91 with 200k miles, so who knows?
-Replaced the fuel filter and when I blew through it it wasn't clogged but very black looking fuel came out.
-Pulled the injectors and had them rebuilt/cleaned by an injector "Dr."

It now will turn on and idle fine for a minute or so, and then start to die, slowly. If I apply accelerator, it won't rev and then it dies.

I don't have a way to test fuel pressure. It is now delivering some clean fuel to the high pressure line on the firewall.
The pump doesn't turn on with the key in the on position- there seems to be some debate about this as well.
There is an audible click from the area of the fuel relay when key is turned to on and another about 10-15 seconds after turning to off.
My old relay does the same thing as the junkyard one so they're either both bad or both good.

1. What's wrong with my truck? Maybe that's too broad, though.
2. Should I just replace the pump, even though it's $150 and mine works to some extent?
3. Could it be the pressure regulator, and if so is it testable?
4. Do I need to try to clean out the tank somehow, or just be more careful to not let it run low?

thanks for your help

oh and I found a sweet mouse nest underneath the intake plenum and fuel rail. no damage/problems caused by it, but it explained the occasional whiff of mouse urine I thought I was smelling

no error codes.
throttle position sensor resistance is uniform.
fuel pressure regulator has vacuum going to it

anyone have an idea for me?
this is my only car so I need to fix it quicklike

Since you have already completed extensive diagnostics, one of the only options left that comes to mind is your fuel tank. Although the filter should have caught anything from the tank, it sounds like the filter was a bit on the old side...perhaps something got sucked up in the lines and is causing a blockage when giving more throtle, but when you have minimal fuel flowing it can pass the blockage. I know this sounds like a long shot, and a -ell of a lot of work...
New or cleaned fuel tank? Clean or replace fuel lines?
Sorry I can't be more helpful.
BTW what does your FSM say for diagnosing this sort of problem?

I just have the chilton manual, which is sort of sketchy on troubleshooting guidelines. I bet there is still some stuff in the gas tank but the strainer on the pump was intact and fairly clean so I can't see any chunky stuff making it's way into the lines. When I power the pump with the test wire it pumps a large volume of clean fuel to the high pressure line on the firewall. not sure at what pressure, but clearly more volume than the engine would need even at full throttle. I'll probably want to drop the tank and clean it out anyway, but that's not the immediate problem, I'm afraid. Probably taking it to a shop as I have checked everything I can think of and I'm losing money every day I can't use it for work. Anybody have a montero shop they like in Seattle?

I only paid $850 for the thing so I just hope they don't charge me a thousand to chase a ghosty problem.

Shop checked the fuel pressure and said it's running 20 psi and it should be 38. So yeah I probably should have paid the 150 and done it myself. It's going to be 320 now. Seems a lot for about 30 minutes of labor, what?

Are you assuming the fuel pump is bad?
If you want an OEM replacement, check out this site:

https://www.mitsubishiparts.com/oe_parts_catalog.html

$320 to check the fuel pressure...Ouch! Did they at least install a new pump?

So I think the pump stopped working as efficiently but still worked to push 20 psi. They said it should be 38 psi with engine running, so I can see why the injectors wouldn't like about half that pressure. They charged me 68 for diagnostic labor, 68 to install, and 178 for the pump, a NipponDenso. The oem pump from that site you linked was 415. A local parts store had a Carter pump for 150. I always kind of figured there wasn't much difference between the brands, but... I also always figured a pump like that either worked or didn't and that was that. So that's what I know. Truck runs great again, just idling high now, but that's probably just the throttle cable needing adjustment after I dismounted it to take apart intake stuff. Thanks for the advice.

Glad to hear the issue is resolved.
Although, it's too bad you didn't slap in an after-market yourself to save a bit of cash. At least your rig is back up and running, and your able to make some money again.