Fuel system issues

[Zxjunkie]

1990 mm, 2.4L. Truck was running great, no misfires or shakes. This post kind of has two questions. It was running great, and then it wouldn't rev. It would get to maybe 2000rpm and back down, kind of like it would at redline. But at 2k. Did a little digging, and found that if I unplug the ISC motor sensor, it revs perfect. Drove it like that for a day or so. Had it idling, about to charge the a/c, and it died. Hit the starter, and it runs for about 3 seconds, badly, and dies. Try a few more times, and if it starts, it does that same thing. It has spark, and the fuel pump is running. Timing was just done, and it has perfect compression. If I spray carb cleaner through the intake, it runs perfect. The second I stop, it dies. Another symptom of no fuel that it has is that if you try revving it while it is attempting to run on it's own, dies instantly. My guess is that the injectors aren't firing. Is there a fuse for this? The interior panel only shows accessories, and the fuse link box only has ign, head, and fuse b. Any ideas?

[geezer101]

I think this is gonna be either a fuel system sensor (throttle position, MAF etc) a clogged fuel filter, fuel pressure regulator or the EGR is carbon'd up. Run a diagnostic on the ECU and see what error codes it throws. Be aware that the ECU's are known to fail and it'll cause no end of running faults. They are easy enough to repair and diagnose (a visual inspection of the circuit board will uncover leaking or swollen capacitors)

[Zxjunkie]

I've already cracked the ecu open, no bulged/leaking caps. It has already been remanufactured. Apparently these trucks do not prime the fuel pump, as the engine control relay fuel pump side only clicks once I hit the starter. Fuel tank was removed and cleaned, and the pump was replaced, along with a line flush and a replacement fuel filter and fresh gas. It gets fuel, just not very much. I didn't see a pressure testing port, so I can't verify fuel pressure. I also do not have an obd1 reader, and also have no idea where the test connector is.

[finalfighter]

i had the same thing happen with my 1990 mm a while back. it was the fuel pump. i could spray fuel or starter fluid down the intake and it would run but when i would stop it stopped running. funny thing was i had replaced the fuel pump with a napa brand pump less than 6 months back after the original one failed. ended up putting another oem pump back in and have not had problems sense.

[Zxjunkie]

Hmm. I can hear it running when I jump the relay. Hard to say that it is good for sure without being able to do a pressure test. Pulled the fuel line going into the rail and it is for sure getting fuel.

[geezer101]

A youtube vid on pulling codes from a Mighty Max - https://www.youtube.com/watch?v=XTkL2nvulWw (this guys' truck has a rats nest of wiring under the dash... wonder if that's the problem?)

Easy to do, just need a list of codes for a reference.

[Zxjunkie]

Okay. Went and found an analog multimeter. Hooked it up and flipped the key. I have no idea how to decode the sweeps. There are some short pauses which I assume are breaks between the codes. This is what I get
L
LSL
LSSSS

If those dont sound right, i can upload a video of the meter. It loops through that same pattern endlessly

[85Ram50]

Try looking at this http://www.mightyram50.net/vbulletin...ents:ECU+Codes
That is one of the threads in the forum Wiki ECU & Components

[Zxjunkie]

I get codes 21 and 24. Coolant temp sensor and tdc sensor.

[geezer101]

See if you are getting voltage/signal from the ICM in the distributor. It controls both spark and injector pulse sequence.

[Zxjunkie]

There are 4 wires, a red, a black, a white and a yellow. Which one am I testing for signal?

[Zxjunkie]

Also, I have some news that may point us in the right direction. My brother went out to charge the battery on it, and hooked the charger up backwards for about 10 seconds. Then he corrected it, and charged it for a while. I got in it the next day, it fired and ran, and then it died. That brings us to the problem I'm having now. I've checked the three main fuses on the battery terminal, and the three fusible links along with all of the under dash fuses. Are there any more?

[claych]

Forgive the poor picture, just for my own clarification, You checked all the fuse links in locations A & B below ?
And, how did you check the links ?
Thx!

[Zxjunkie]

Don't see a diagram.. I only found the one set of fusible links on the drivers fender by the firewall. I checked for continuity across them. I took a video of the raw fuel feed before the fuel filter. Can you guys tell if it is enough? Doesnt look like much gas to me. https://youtu.be/9DKo2R-Tl7I

[claych]

Very Humble Apologies Zx,
Seems photos have randomly disappeared since the update...

testing,testing, No such attachments. They did not load for some reason. Make another post and go on. Just ignore it.

Is that video of gravity flow ? or with battery voltage applied to the fuel pump test terminal ???
Thx1

[jospiv]

I'm with you. Can't find that fuel pump check connector or even the blue + black wires.

[Wejimed]

Any update on this I'm have the same issue?

[camoit]

Any update on this I'm have the same issue?

This thread is from 2022. Date code is in the left corner next to the username.