5 ton fuel tank selector valve cleaning/rebuild

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AMGeneral

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Well,After searching the threads and getting questions from other members,I decided to cover the cleaning /rebuild of the fuel tank selector valve used on the M39,M800 and possibly M900 series 5 tons.

First off,this valve is a common air leak in the fuel system on the 800 series trucks with the NHC 250 Cummins and dual fuel tanks(wreckers,tractors,dumps).

Removal is pretty starightfoward,first remove the tool box under the drivers side of the cab,then disconnect the 6 lines that are attached to the valve itself,then remove the screw attaching the seletor lever to the valve stem itself,afterwards remove the 2 bolts attaching the valve to the bracket under the cab.

You should now be holding the valve in your hand with 6 short rubber hoses attached,this is a perfect time to replace any rotten or cracked hoses here,the 3 larger diameter ones are the suction hoses for each of the 2 lines from the tank and the 1 going to the filter housing.


Secure the valve in a vise(carefully as not to crack or distort the housing) and unscrew the top cap surrounding the valve stem,there should be 2 O rings here,1 directly over the threads and 1 surrounding the valve stem itself,these are the 2 you will need to replace.

Under the cap,there should be a small flat washer,a spring and a C shaped limiting washer for the valve itself, after removing these,the valve itself should now be able to be pulled free of it's bore,it is a tapered fit so it may require some pressure while twisting the valve back and forth.

Hopefully you are now holding the valve in your hand and looking at the bore of the valve body,if not you may have to soak this aera repeated times with you favorite penetrating oil(Kroil,WD40,PB Blaster,etc) until it comes free.

Basically clean everything up till it's nice and shiny and make sure the valve and bore fit correctly( not egg shaped or scored up) if everything looks good,reassemble it in reverse order with the 2 new O rings and a little grease,and install it in the truck.

**** Steel wool and carb cleaner work great at cleaning the valve. Make sure all traces of the wool are removed before reassembly ****

Pic 1- Valve after removal from the bracket,showing the cap covering the valve stem
Pic 2- cap removed,showing the stem O ring,washer,spring and C shaped limiting washer
Pic 3- closer view,with stem O ring removed
Pic 4- view,with close up of the C shaped limiting washer
Pic 5- showing top of valve and valvebody,with C washer removed
Pic 6- Fit of valve into valve body
Pic 7- Valve body inside view with valve removed
Pic 8- Parts removed from valve,left to right,Valve,C shaped limiting washer,spring,flat washer,valve body cap,valve operating handle and retaining screw.
 

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doghead

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#9 and #25 O-rings( I have this written on a note, found long ago with a search), I believe is correct.
 

tractors0130

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Just got through replacing the O-rings on the selector valve on the M818 as well as replacing the badly cracked hoses on the selector valve. The selector valve was by-passed before and only drawing from the left tank.

I used the Parker brand "push-lock" 1/2" and 3/8" hose on the fittings and ran the left tank supply hose directly to the selector valve from the tank.

The truck started right back up and never coughed when switching over to the right tank.
 

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CGarbee

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Sigh... Now that everything has been explained in detail (along with photos), I have no excuse to not go out and fix the valve on the dump and wrecker...
Perhaps I can talk AMGeneral into making a house call since I also need to "debirdcage" the cable on the rear winch of the wrecker still... :)
 

joesco

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Excellent thread, thanks! One question, do you need to completely disconnect the lines and remove the valve to replace the O - rings? My lines appear to be in good shape. ALSO, have you folks had the short hose fail that runs from the plunger hand pump to the IP? I am still get some air in my M931 fuel system after I changed the fuel filter. Pretty darn frustrating!
 

73m819

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Just a side note, the 39 series has a DIFFERENT change over valve, it is a single fueling port with two tank lines instead a double port, four tank lines.
 

doghead

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The O-rings can be replaced without removing the lines from the valve.

The primer line can leak too.
 
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tractors0130

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On the M818, you can loosen the brass head on the selector valve after the handle is removed and then unscrew it the rest of the way after you drop it down. Not sure if the M931 valve is identical or not. I took the valve all the way out because my hoses were all in bad shape and I wanted to ensure that there was no debris in the valve.
 

joesco

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Thanks guys, my project for tomorrow before another foot of snow hits on Wednesday. I have read in a couple of threads that some folks remove the fuel filter top wing nut pet cock and replaced it with a plug???
 

73m819

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The petcock is there so you can open the lower petcock after the top one is opened so you can drain water/crud from the filter.
 

tractors0130

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On one of the trucks I have now, it has a Schrader valve on top of the fuel filter instead of a petcock valve. It works well to bleed the air out when you pressure the fuel tank with about 5# of air pressure after a filter change out. You can also pressure up the fuel tank to find suspect air leaks in the fuel hoses, be sure to not overpressure your tank.
 

Floridianson

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On the 809 it's is so much easer if you remove the foot step too. Five min. four bolts and you will thank me.
 
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AMGeneral

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Just a side note, the 39 series has a DIFFERENT change over valve, it is a single fueling port with two tank lines instead a double port, four tank lines.
Single supply and 2 from the tank with no returns?

Must be for a gasser,all multis with dual tanks had a transfer pump and the Macks had the same as the 800 series.
 

joesco

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Hummm, I like the idea of a Schrader valve, it may be a bit more "air" leak - proof!
 

patracy

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Hummm, I like the idea of a Schrader valve, it may be a bit more "air" leak - proof!
Wouldn't think that'd be wise. That pump is all the way up at the engine and having to suck through a real long straw already. Adding a schrader valve would impede flow drastically.
 

joesco

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Thanks, tomorrow I will just replace the petcock. So far, the only weak link that I have found in my M931 is fuel delivery, specifically the petcocks, selector valve and the canister fuel filter housing. I am beginning to wonder if the truck maybe sucking air through the canister bolt. I am almost tempted to convert to a spin-on setup fuel filter. It was the best mod that I did on my deuce!!
 
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