Hi metisse1965,
what you have described is the hour/service meter. The fuel transfer pump pressure relief vave is just below that in a little housing where the fuel line from the tank comes in to the pump. You will need to turn off the tank tap and disconnect the fuel line--about 1" nut-- then take out 2 9/16" head bolts to acess the R/Valve and spring/stop. Usually the old plastic valves wear them selves and the housing and have gunk stuck under them. Some times a bit of emery with a hole to suit can be fixed over the end of a socket or pipe to twist the valve face against to get you going again tempory. Also check the fuel flow from the tank by turning on the tap and sight the flow rate, check and clean fittings, line, tap etc as needed. Refit as needed and bleed system as described by others.
Hope this helps,
Cheers,
EddieB.

What I always did with the housing if the relief valve had worn a groove in it is draw file the whole flat surface there that the valve seats against. All the later newer valves are made of a metal body & the end of the valve itself is a flat piece of rubber cemeted to the metal & makes it a little softer on the body it seats against.If your using the old spring & the fuel pressure is down then you can stretch the spring some to help the pressure to build up better.

We have a D318 (out of a crane I believe) that has a return line to the tank from the top rear side of the filter housing. So I belive you are correct, there could be a pressure relief valve in that location, but on a U series tractor, they were not installed that way, the relief valve is on the transfer pump inlet and dumps excess filtered fuel back into the pump inlet/line from tank.

The fuel flowing from an open bleed won't seem to be high pressure because it isn't being pumped by the injection pumps, only the transfer pump. If you loosen an injector line nut, that fuel will be under high pressure and spray out if you only slightly loosen the nut. don't get fuel injected in a finger.

I expect they are all easier to bleed with a full tank, but many folks report that a section of an innertube with the valve stem on it, clamped over the fuel tank filler opening with a large hose clamp can be used to pressurize the tank and make bleeding a lot quicker process.

do you have white smoke when you open the throttle?

CC on those older power units in shovels etc. if the fuel tank was lower than the engine & if I remember right was when the relief & return line was up on the top area of the filter base.

The "U" series tractors with factory direct electric start and hand primer pumps have the relief valve mounted at the top (rear) side of the filter housing with the return line tied into the supply line. Some enterprising folks have realized that having the valve in this location provides filtration to the recirculated fuel unlike the one mounted on the inlet that allows recirculation of unfiltered crud through the transfer pump.

Yes OM the Direct start also has the relief valve at the top of the filter housing too when it,s equipped with the hand primimg pump.

The old original valves were made of a hard black plastic but in the later years the replacement valve was a metal body with a rubber or neoprene type seal on them that did the sealing.I,d think Cat still sells them & yes clean up the seat on the body of the pump by draw filing it smooth.

All that means is use the file in a flat position to the part your working on so it keeps the surface flat & true as you remove the damaged surface. Use a fairly smooth file & not a rasp so the surface stays smooth as possible.If the area where the plunger (valve) seats has a round area worn in the body then it,s time to file the surface flat & smooth.

Hi Metisse1965,
there is an air bleed tap on top of the pony which must be bled to ensure the pony is full of coolant---maybe the pony has seized a piston and may unlock when cooled down.
Usually with a broken C/rod the engine may still turn a few degrees in either direction unless totally jamed up, a broken crank will show by the flywheel appearing to be loose in the main bearing--will wobble and move up and down -- but usually still turn. I hope it is only the piston!!
As for the lack of high pressure fuel at the injectors it may be that the fuel rack in the injection pump is is not able to move due to a stuck injection pump plunger. You can check this by removing the Fuel Injection Pump side cover and noting if all pump plungers rise and fall and the rack moves for and aft as the main is cranked over-- you can do this on decomp by turning the fan--. If you find a stuck plunger give it a gentle assist(up and down only) with a suitable pry bar--screw driver-- once the plunger will rise and fall on its own usually the rack will now move as required. DO NOT force the rack back and forth as you run the risk of turning the quadrant gear on the plunger and losing the correct orientation for the factory adjusted setting for fuel delivery volume for that and any other stuck plungers!!!! Get them to rise and fall first, by then the plungers should have loosened and be safely turnable.
Hope this helps ,
Eddie B.

Well old girl has been relucant to run on the main. Pony started in a breeze. After several weeks of fiddling w her I hired a Modern Cat Mechanic to help me. He thinks that the rack is stuck due to sitting for four years in my pole building. Anyhow while we where checking fuel pressure to the injectors and cranking the pony up to high revs the pony locked up.

Question: I am sure main will run once rack is freed up. Can a D-4 be pulled started by another Cat or possibly a wrecker? In case a serious buyer wants to see her run. PS Have 7 1/2 ft straight blade and 1 1/4 yard buck on this D-4 Crawler loader Ron Lapeer Michigan 810 664 9852 $1,850