M923A2 won't idle and dies when put into gear

[JimW]

Hello all. I have an issue with my BMY M923A2 that has been a pain in the rear. I have looked all over the forum and see a few threads that have almost the same issues that I'm having but most just seem to die off and never say what fixed the issue. I have had my truck since april of this year and this is the first issue with it and it's got me.

Basically the truck has ran great but over the summer I did not drive it much due to the fact summer here is 115 or so. I did start the truck every week to let it run make sure the batteries were charged and would move it around my yard to keep seals lubed. Then one day I started it and it died. I can keep it running as long as I keep the rpm's above 600. Once it goes below that it just dies like it was shut off. If you put it in gear forward or reverse it will also just die immediately. I thought it was the fuel filters so I changed both the canister one and the one on the engine. I did make sure the O ring and square washer on top of the canister was on correct and sitting in the recess spot (It wasn't correct before I changed the filter). This truck is one of those with the schrader valve on top of the canister and I made sure the cap was on tight and had a good rubber seal in it. I also checked that the stop solenoid was working correctly. I bled the system after the filter change using the primer and loosening the bleed screw on the IP between it and the engine. My son and I bled it until it was spraying fuel with no bubbles and then started the truck and it still died. We then repeated the process like it says in the TM and it sprayed immediately and never had any bubbles. We gave up for the night and made a second attempt yesterday. The truck still had prime as we repeated the bleeding process first thing.

I have read threads here where similar problems could be caused by the lockup switch but I am almost positive it's a fuel issue because of the idle issue and checked the lines and lift pump. Both of which are newer and were replaced by the previous owner about a year before I bought it but as a mechanic (On gas cars and 4x4's) I know new never assures good. However seeing it ran great and I even drove it across Arizona to bring it home I'd say they were ok.

What would be my next step as I'm stumped and really want to get it up and running before winter when our Tonto creek flows and cuts us off from civilization. As I got the truck to cross it if needed for food and medical.

Thank you all
Jim

 

[dmetalmiki]

If that truck is computer controlled, it is a fair bet the problem lies in the (Dreaded) E.C.U.

 

[JimW]

If that truck is computer controlled, it is a fair bet the problem lies in the (Dreaded) E.C.U.

If you're talking about the PCB black box would that cause it not to idle?

All I have seen on that is when it goes the truck won't start at all or act like it has dead batteries and stuff. My truck has power to all the gauges and lights and will start it just won't stay running without either pushing the pedal or using the hand throttle to get it above 600 rpms.

 

[CARMAN]

Should be all mechanical. No ecu.

 

[98G]

I think you're sucking air somewhere.

If the lift pump wasn't new-ish that'd be my first suspicion. It may still be the culprit.

Between the lift pump and the little filter on the block is a fitting with a fuel pressure sensor. Check that as its prone to crack and leak.

Check all the fittings between the tank and the small fuel filter on the block.

Check the check valve and screen on the return line where it comes out of the IP.

If the above doesn't diagnose it, you can make a hose that goes from the lift pump into a jug of diesel. If that fixes it, then at least you've narrowed down where the problem is. At that point I suspect the pickup straw in the fuel tank has a crack.

 

[JimW]

I didn't think of the screen on the return line. Once it stops raining here I'll check out the return line screen and all the fittings on the lines. I'm thinking and hoping that it will be that screen. As it did have a crap ton of crud in the canister filter and I can see that screen getting plugged with old diesel gunk. I did also put some archoil fuel treatment in the tank to help over the summer. It surely could have broke some gunk loose in the IP and clogged up the screen.

I'll update once I get it checked. Thank you

 

[JimW]

I got out to check on it today and found another issue. Seems some pack rats got in there this week and chewed the return line a little. So I am just going to change out all the fuel lines with Russell braided stainless steel lines and AN fittings, put on a dual spin on filter system and ordered a new check valve from TorkTek. Also I'm going to get a set of tubes for the tank from TedCat here. When I have the tank off to do the tubes I'll clean it on out and when done have a completely fresh fuel system.

Does anyone know if the fuel line fitting on the lift pump is AN style. Seeing I am out in the sticks I have to order everything in.

On a side issue today I had another issue pop up. I started the truck after checking all the lines and wanted to see if the rat chews leaked or anything and it died when I let my foot off the pedal as it has done so I cranked it again and it made a pop sound from the batt. Well seems the marine quick detach battery cable end that was on it (I hate those things) went bad and arked when I tried to crank the eng. I pulled it off and put a proper end on it and now the alt does not charge. Even with a digital volt meter the system was at 24.6 volts running where it was at 28.3 volts before. I checked all the wires and they are good. So I'm thinking it shorted the alt when it arked. Went ahead and ordered a new non military alt, pulley and bracket. Now only thing left to do is explain to my wife about the bill... I'm glad she's understanding

It seems this truck is surely a girl and after my wallet.

 

[74M35A2]

I agree, seems like fuel. I've helped Ted on the pickup tubes. The stock dip tube attachment fitting is inverted (female) flare, and Ted's are traditional AN (male flare). To help make it easier, Ted can send you what you would need in Parker Push-Lok fittings and hose, at a super reasonable price, to get you all the way to your lift pump, and back to the tank on return if you need. He sells them as just suction or both suction and return. The hydraulic hose shop is local and the counter guy Loren really likes us and knows his stuff. You will have to lift the bed or remove the tank to install the new stainless dip tube(s) due to their height and not being able to bend them. The stock copper ones can be curve-bent to be removed with the tank in place. Looks like you know this already. With the Push-Lok, it is tool-less to assemble, no clamps or swedging on of fittings. Great for fast field assembly as you are saying. After you upgrade/replace the fuel tubes and lines, it should be rock solid for at least another 30 years.

Yes, the stock military alternators are very sensitive and failure prone compared to the modern civilian stuff.