M8540:Is my fuel filter diaphram stuck?

[alphamomega]

Howdy,
just replaced the fuel filter on my M8540 for the first time.
Manual says to prime the fuel lines by pulling and pushing the cap
on top of the filter housing up and down.

My cap will not go any higher that it is in the 'resting' position.
I've tried pulling it up with my fingers and gently prying it up with a pry bar.
I don't want to break the round plastic cap
but I simply cannot get the cap to go up at all.

There is a metal rod in the center of the cap.
I can push the cap down ~.5" and the rod will not go down with it
and protrudes ~.5 above the cap.
I can then push this rod down ~.5" and it will spring back up.
Cycling this seems to partially prime the system but doesn't seem to be doing it very well.

It appears the proper operation should be to pull the cap up
and the rod which is attached to the diaphram will go up with it.
Then when you push the cap down, or release the cap, the rod will
come down cycling the diaphram.

Two questions:
1. Is the cap supposed to come up from the 'resting' position?
2. Do I have to twist it or unscrew it to come up?
3. If its stuck, does anybody know how to unstick it without breaking the cap?

thanks

z

 

[North Idaho Wolfman]

Don't force it at all!

When you drained and cleaned the water seperator did you remember to open the fuel shut off?

Did you replace the fuel filter with a stock filter?

 

[alphamomega]

I only changed the fuel filter,
I didn't touch the separator.

I replaced the fuel filter with an after market filter.
It had an after market filter on it.

I did try lifting the cap with the filter removed.
Still did not move.

thanks

z

 

[tthorkil]

I suspect that the plastic cap has broken as the plastic cap is supposed to be attached to the top of the rod that connects to the diaphragm - when it is depressed the spring compresses and should return the plastic cap and rod to the top position so that you can keep pumping the diaphragm to help prime the fuel system. The pictures at the attached link on ebay show how it should look.
https://www.ebay.com/i/254254612330?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=254254612330&targetid=885798446591&device=c&mktype=pla&googleloc=9019545&poi=9014868&campaignid=9243453116&mkgroupid=94117041536&rlsatarget=pla-885798446591&abcId=1145977&merchantid=112314928&gclid=Cj0KCQjw1Iv0BRDaARIsAGTWD1szUqB5hk1ZtZqyfCVVv9bCdJJOO9fK2HkQNOOEiAdzNSCmPZ9T_OMaAjgREALw_wcB

 

[alphamomega]

Wow,
thanks for the reply and terrific picture.
My filter assembly looks just like the picture.
Now I know where to replace the whole assembly if its broken.

Like I mentioned in my description,
I can push the cap down,
but the rod in the center of the cap does not go down with the cap.
The cap goes down but the rod stays put,
eventually protruding above the cap by about .5".
With the cap pushed all the way down,
I can push the rod down with my finger and it will spring up again.
Thus I can try to prime the system by holding the cap all the way down
and cycling just the rod.
This seems to do kinda a half assed job of priming the system.
When I do this with the filter off,
a small amount of diesel does seem to get pumped.
Also, with the filter on, when I do this I can hear something happening
and the sound eventually changes
kinda like the diesel fuel has built up some pressure in the line.

So I guess its possible that something on the cap or rod broke and when
you push down on the cap it doesn't push down on the rod like its supposed to.

But a big question remains:
are you suppose to pull the cap up after in the resting position?
Or do you just push the cap down and let it come back to the resting position?

Another way to put it:
Do you cycle the diaphram by
pushing the cap down and letting it come up
or
pulling the cap up and then pushing it down.

thanks

z

 

[tthorkil]

If everything was as it should be, you would push the cap and rod down and the spring would return the cap and rod(as one unit) back to the top position.
The cap should be solidly attached to the rod - since the plastic cap seems to be broken you should be able to leave the cap in the down position and 'pump' the rod down and let it return to the top position and then depress again and again following the priming instructions.
Since it seems that the cap is separated from the rod, whether to leave the cap down or pull it back up is questionable as to which position would provide the best protection of the rod and diaphragm for priming the fuel system..
The position of the cap will have no affect on the fuel flow as it is just an aid in pumping the diaphragm to prime the fuel system.
You will have to use your best judgement since you can see the assembly.
You could remove the fuel filter assembly and get a better look at how the cap is supposed to be attached and possibly repair it - maybe a c-clip/e-clip or pin came out of position allowing the cap to slide down the shaft.
Good luck.

 

[alphamomega]

tthorkil
thanks a bunch.
I think you solved my problem.
I appreciate your help.
z

 

[Russell King]

I suggest you look at a parts diagram of the part to see if it can be repaired by replacing the plastic button. For the $90 cost I would probably replace that entire part if it can’t be repaired while you’re into it already.


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