M923 Hard to start on any slight incline

[Doubled]

Cut the bell end off a valve stem. Take a short piece of 1/4" rubber hose and clamp the valve stem in one end of the hose. Remove one of the vent tubes and clamp the hose to the fuel tank. Crimp the vent line you took off with a pair of vise grips. Put about 10lbs of air in the tank via the valve stem. If you have a leak in your system it will start either spitting out air or fuel. Mine was a small pin hole in the primer pump plastic line.

 

[juanprado]

She starts !!!!!!

Well, My saga is finally over.

I charged all 4 batteries so they were 100%. I cranked the bolt on the filter housing an extra 1/8 to gorilla tight. Cracked the rubber line at the pump and filled with diesel all the way to the top.

I also rebuilt the primer pump by changing the 2 o-rings even though they were ok. I could feel more umpah with the new o rings. Just turn the outer nut counter clockwise, pull the plunger all the way out, change o rings and re-assemble. Super easy and the o-rings are standard size to find easily.

The big difference that might have made all this easier earlier is the post from lonekazoo. The operator Tm-10 says to crack open the drain cock on the bottom of the primer pump till you see air and diesel coming out with no air. I was pumping like a mad man with the valve open but never got anything to come out. But based on his post, The last thing I did was pump the primer about 20 times with the valve closed. I am not sure if this pump is supposed to pull or push?

Started on the 2nd try and all is good.

 

[Beerslayer]

Glad to hear you got it running. I changed the o-rings on the primer pump too but no joy.

So what do you think was the problem/fix?

 

[juanprado]

I am not really sure?

Maybe all the dry repeated cranking had finally purged all the air?

Maybe using the rebuilt primer pump closed pushed all the air out?

I don't think I had an air or fluid leak that I could detect. I let her idle up 20 min and took her out for a spin. All is good and she felt the same as far as power, no misses etc. Started her 3 times later and everything was good.

Maybe the mechanical Gods showed mercy on me.

 

[SteelHound]

Hey Lonekazoo,
Thanks for the words. I just recovered an M936 which barely runs. The only way I got it started was with my son blowing compressed air into one of the tank lines. After it started, I would have a 3 second delay to a new throttle setting. It wouldn't idle and the max RPM's were 1800. I still managed to get home where I parked it on an incline. Now it simple won't start.
So I'll try some of the suggestions posted here to find an air leak. I was curious though about the primer pump: My primer pump only has one line going into it from the tomp. There is a 1/4 inch fitting that is empty on the bottom. Does your primer pump have two lines attached or just the one?
BTW, the primer pump does nothing.
Thanks.

 

[Lonekazoo]

Hi SteelHound,

My M923 (and the other M939's that I've checked out) has a single line running from the IP to the primer pump, followed by a drain valve. The drain valve is missing on my truck, but all other trucks I've seen have one (looks to be the same as the air tank drain valves). Operating the primer pump draws fuel up to the IP from the water separator in case of starvation. If I remember correctly, the pull stroke does the pumping.

If that primer pump or the line associated with it has an air leak, it will eventually let the fuel drain back down to the water separator/filter and also introduce air into the line. This will cause difficulty starting, and (if it's bad enough like mine was) might cause rough idling and knocking. My idle was all over the place. Because of the negative pressure in the line, I never saw fuel leaking out anywhere, which made it tough to diagnose. Now that the problem is corrected, it starts immediately and idles normally. .

Good luck on tracking the problem down.

 

[juanprado]

On the bottom it takes a simple pipe thread drain cock. Any parts store or hardware would have it. I would put one back in as I wonder it that can be a source of the air to get in as it seals on the bottom.

The primer pump has 2 simple o rings that are easily obtained anywhere.

I am not sure if pumping it closed helped as I have not been able to tell if it pushes or pull? Maybe my repeated cranking it got all the air out. I never found an air leak or fuel leak and it has run fine ever since. I keep the tank full most of the time and since I live in flat land ( no hills or mountains in New Orleans except red ant mounds) , I don't park on an incline.

Maybe check the new 23 manuals and see if there is an ip pump trouble shooting flow chart?

 

[Lonekazoo]

Oh, and a little trick that I watched MarkMontana perform in the middle of Wyoming on a 923 to get it started when the primer pump was inoperative: Disconnect the line to the primer pump and backfill it with a small funnel and some fuel.

 

[SteelHound]

Thanks guys. This is great talking it over. So that little 1/4" thing on the bottom usually has an air shutoff in it. Makes sense. It seems if the o-rings in the primer pump are shot, this acts like an open tube. So its time to invest in a couple of o-rings. I also have an electric vacuum pump that will screw nicely into that 1/4" fitting which should give me a quick prime.
Thanks again