Priming the C12

[JonM934]

I accidentally let some air into my fuel line and now the truck will not start. I think I found the priming pump but I do not know where the bleed screw is. Can anyone help me out with step by step instructions for priming the C12 with photos? Or if you have the MK23 repair manual I would love that!

 

[MAdams]

I changed both my fuel filters(pre filled them before install) and my truck fired right up. There should be plenty of good info on the internet about priming a C12. They're a popular engine

 

[JonM934]

I searched the internet but it seems these engines are not exactly like the civilian models. The youtube videos show engines that I see look close but it is not the same as my engine.

 

[snowtrac nome]

the fuel system is a common rail style so you just pump the primer and the air will return to the tank through the return line, you can also crank it to force fuel through, it is self bleeding but its hard on the starter.

 

[JonM934]

snowtrac nome, I agree with what you are saying. The problem is the pressure regulator will not allow fuel to pass until it has built up a lot of pressure. I tried to use the prime pump and I even backed out the regulator bolt to bypass it but the fuel is not flowing. It might be that the prime pump seals are shot. On the internet I see that is a common issue.

I also tried cranking and I cranked for a long time, pausing every 30 seconds as the manual says to. But still no luck.

I have found a mobile diesel mechanic that is an expert in Cat engines and he is going to stop by tomorrow. While he is here, I will ask him some other questions that I have.

 

[snowtrac nome]

I've never messed with the c 12 engines bit the c-9 s I messed with I have blead them that way. The excavators all had an electric pump on them to bleed them. if I had to I would likely crack a line where the fuel enters the head , once you have fuel to the gear pump it will push through about anything

 

[Nomadic]

FWIW, the C12 was used in RV's too.

 

[JonM934]

The mobile mechanic stopped by and we could not get the truck going. There was air getting into the line somewhere. It must be the water separator but we could not figure it out. He did tell me a lot of interesting things about the truck. One thing he pointed out to me was the Ether assist to get the truck started when it is cold. I always wondered my the truck was so easy to start when it is so fricking cold outside. Now I know why. I guess I need to replace the Ether can once in a while.

 

[JonM934]

I got my truck started and I can drive it again!!

So I found a lot of instructions on the internet and even in the MK23 reference manual that is in the upload section. Most of it was wrong. I needed to take bits and pieces from many sources to finally figure out the proper way to prime this machine. I will make a video about it because it is complex.

 

[wheelspinner]

I got my truck started and I can drive it again!!

So I found a lot of instructions on the internet and even in the MK23 reference manual that is in the upload section. Most of it was wrong. I needed to take bits and pieces from many sources to finally figure out the proper way to prime this machine. I will make a video about it because it is complex.

Did you do the video?

 

[SWATMOD]

Did you do the video?

Typical forum leach, wants everyone to help when they are in need, but once they get their problem solved, they disappear...

 

[Nathan P]

When I got my MTVR, the only way I could get it primed was to have someone pump the priming pump until the fuel began to bypass and keep pumping while I turned the engine over. Using this method, it started right up and I’ve never lost prime.

 

[IridiumRed]

I need to change fuel filters on my Mk25 in the next few days (just doing long term maintenance, shes working well but no idea how old these filters are, they are as she came from .gov). Am doing both filters fine filter on engine + coarser one/ water separator). I assume ill lose prime* when i do it. Ill make some mental notes on what it takes to restart it, write it up sometime.

Personally im planning on just using hand pump after filters are changed. The one on the engine, i may leave just lightly hand tight... and after some hand pumping, crack it loose, let air escape, retighten lightly, hand pump, break loose and let air out, repeat process until its just fuel coming out (i suspect it would be easy to get big air pocket in system, "burping" it like that might get it to where its mostly fuel, this has been my experience w other diesel engines, that you need way to get air out... which sometimes is essential, sometimes the engine can crank through the air, get running, bleed itself). Might also just leave engine filter barely cracked loose, pump until fuel comes out there.

Im not sure if the pump is upstream or downstream of the engine mounted filter in respect to fuel flow to the engine. Its something that i could figure out, just dont have it in front of me right now. But how its positioned could determine whether having filter slightly loose would suck air in or let air push out. I think its the latter.

If i was in a real hurry, id probably prefill the fuel filters (VERY CLEAN FUEL ONLY), install, pump until decently hard, then start engine let idle a bit...


ANYWAYS! Ill be doing this soon, ill take mental notes on what worked, and ill share them here Ill follow through on that. Just be good information for us to have available. Trying to prime diesel engines can, as a whole, be a frustrating and variable process from one model of truck to the next. Some needing very particular steps, others not caring much...some in between


Notes -
By losing prime in this case (swapping fuel filters), the fuel system will prob be dry from engine mounted filter back to the tank, but there should still be fuel in a lot of the engine. Fuel pump to injectors should still be full. This is not a "complete loss of prime" like if you ran engine out of fuel or took a bunch of fuel system components apart and let them drain... and who knows what condition the OP's truck was in. Not sure if there are anti drainback valves in the fuel system / where, but wouldnt surprise me. That could influence some of these things...

Above when i just say "pump" im referring to hand pump

 

[dlynes]

IridiumRed - Did your truck fire up well after changing your fuel filters?

 

[JonM934]

I found that the Tech Manual had the best description of how to prime the fuel system. The only issue I see is that the arrow pointing to the bleed screw is pointing to the right side of the fuel mechanism but the bleed screw is really on the top. One the side of the bleed screw body it is labeled "BLEED".

 

[Marquette]

Since I read through this thread recently I will add some of my experiences.

I've run an MTVR MK29 out of fuel on the road, cranked for at least 30 seconds while empty of fuel, and it started right back up after adding fuel and priming with minimal pumping from hand pump.

Changing filters:
I've changed fuel filters several different ways on trucks that were running without issues.:
- One at a time, filled each with fuel, minimal pumps on hand primer, truck starts right up, idles and has no issues.

- Two at a time, filled each with fuel, minimal pumps on hand primer, truck starts right up, idles and has no issues.

Changing filters on a truck that wasn't previously running:
- Two at a time, left both dry, attempted to prime with hand primer. When pumping hand primer it would only feel like it was pulling a suction in the pull direction and would snap back toward the block, again like it only had suction. When pushing the pump, it felt like nothing was behind it like it wasn't pushing any fuel.
-Removed all lines from tank and used compressed air to make sure passages were clear.
-Removed return line from tank, plugged off tank vent, and used 10psi compressed air in return fitting on tank to pressurize tank to attempt to push fuel into filters (vent on fuel filter base was removed, drain on water separator was open). Tank pressurized but fuel blew back out of the return fitting on tank after removing air.
-Removed supply line from tank, pressurized at return fitting, fuel blew back out of return fitting after removing air.
-Removed more fittings on the fuel supply line side, and found fuel check valve on supply line (pictured below). Swing door check valve was fully gummed up and stuck shut with very sticky material, (brake cleaner wouldn't even dissolve it). The check valve has a small flap door (not pictured) with O-ring on it, and a tiny spring.
-With the amount of suction built up on the hand primer, I'd think it would have pulled that door open to suck fuel from the tank but it wasn't working.

"Swing Check Valves have a spring-loaded, trapdoor style valve. The valve opens when system pressure approaches 1/2 psi to permit full flow with low pressure drop. As system pressure is relieved, the valve closes, retaining fluids upstream.
These in line check valves are designed with a low crack pressure spring especially for diesel and gasoline engine fuel lines"

-After cleaning the check valve, reinstalling everything, closing all open ports and fittings, the hand primer eventually pumped the system up, felt normal like it was pulling and pushing fuel. Truck was cranked for ~20 seconds, and hand pump feels solid. Unfortunately this truck isn't recognizing it's ECM so it did not start.

From my experiences with multiple MTVR C12 trucks, they are easy to prime and get started after working on the fuel system, with minimal checks of a few small components.

The female end of the check valve below unscrews from the housing. You can see an aluminum sealing ring between the valve and the housing.

 

[IridiumRed]

Lots of good info here.

Anyone know what the part # is for that check valve / where to get another? I know that on my truck i have a crack in one of those fittings (almost certainly from someone overtightening at some point... which, for once, was NOT me this time).

Not sure if its the valve or one of the fittings/adapters that screw into it. I disassembled that section of fittings once, found crack in one part. Dont recall which part was cracked, but it might have been the check valve. Not sure if i even noticed that it was a check valve at the time. But wouldnt mind having another check valve on hand if it is indeed that which is cracked. Its really ghetto, but short term i fixed the leak by wrapping externally with tape + hose clamp. (Truck is still sitting in shop, dont have on road yet, but seemed to run fine/not leak after my short term fix.. but do want something more permanent).

Conceptually i do like the idea of having a check valve... prevent drainback / loss of prime if there is any air leak in system.

 

[Marquette]

Parker 2650, 3/8" size. About $50 online

 

[IridiumRed]

Thanks! I just orderdd / received one of these (parker check valve) Havent put it in yet, but it certainly looks correct.

Btw, before hearing about the parker version, i did find the oshkosh version. I believe the oshkosh part# was 123125a
Now, being labeled oshkosh makes it cost several times more than the parker version... and a lot harder to find. So i went w parker.

Just thought id share that piece of info since i had it available... and before i forgot it