Hi OM,
in normal operation the simple Cat flat seated check valve as opposed to the Bosch type tapered seat positive seal check valve the Cat unit is cheap.
The check valve is expected to maintain a slight pressure in the injection line and above the pump plunger during the down stroke of the Inj. Pump Plunger, or else a delay to the start of injection occurs that would vary in duration depending on the volume of fuel being injected at that time--the check valve helps keep a constant delay so actual start of injection/timing would be consistent--all from memory way back in the 1960's Apprentice schooling so, I do stand to be corrected.
Cheers,
Eddie B.

Agreed, the check valve would always see hydraulic back pressure below the cracking pressure. Any air on the discharge side of the check valve would compromise the hydraulic coupling between pump and injector.

Hi Team,
also meant to say in my description above that the Bosch style delivery valve has a positive guide to ensure it stays square to its tapered seat--I can see the Cat style valve being upset by turbulent gas bubble laden fuel causing it to flutter and tilt on its seat--not a positive sealing device.
I guess it should be remembered that this Cat style of delivery valve is being used primarily in a low pressure injection system with an expected test pressure opening nozzle valve of around 600 psi. as apposed to the Bosch style at around 1750 psi generally.
Cheers,
Eddie B.

"edb" wrote:

Hi Team,
also meant to say in my description above that the Bosch style delivery valve has a positive guide to ensure it stays square to its tapered seat--I can see the Cat style valve being upset by turbulent gas bubble laden fuel causing it to flutter and tilt on its seat--not a positive sealing device.
I guess it should be remembered that this Cat style of delivery valve is being used primarily in a low pressure injection system with an expected test pressure opening nozzle valve of around 600 psi. as apposed to the Bosch style at around 1750 psi generally.
Cheers,
Eddie B.

Eddie Thanks for the update... On this type of IP my local and only shop cannot test the top pumps as they rarely work on these only back in the 80s to early 90's did they see these old girls... I went ahead and bought all 6 new pumps.. My concern is the dozer has approx on chassis 28,000 hours on her. I have been told that 30,000 hours are the rebuild on this unit from some old timers... Basically what the shop can do is set the timing and replace the pumps that I bought and inspect the cam. Of course they will measure the yokes and visually inspect. What are your thoughts about the check valve and other problems with this type of pump? I am just trying to cross the fuel off the list as I go through this machine... The nozzles of course they can check on the injectors... Old Mag please chime in as well... Basically, the pumps on the machine are to old to test and the nozzles can be tested to make sure they are opening at 600psi per your specs.. Thanks guys for your advice...

Setting the lifter heights on a machine as old as that.....OFF the engine will not produce as good results as doing it with the pump housing installed ON the machine. There will be enough wear in the gears etc that when the lifters are set with a degree wheel off the machine, the timing will be delayed when installed on the machine and driven by the gear train with accumulated wear.......could be why the tractor was always a hard starter after the last overhaul.

"ccjersey" wrote:

Setting the lifter heights on a machine as old as that.....OFF the engine will not produce as good results as doing it with the pump housing installed ON the machine. There will be enough wear in the gears etc that when the lifters are set with a degree wheel off the machine, the timing will be delayed when installed on the machine and driven by the gear train with accumulated wear.......could be why the tractor was always a hard starter after the last overhaul.

So basically in a nut shell leave the pump on replace the top pumps and set the lifter heights and be done with it... As many more experienced people have said on here is she will run on 3-4 cylinders. Time to get her timed properly and move on... Thanks guys...

New pumps and tested injectors should get you where you want to be. Set lifter height on the engine. 600 psi is optimum breaking pressure on the injectors. Acceptable range is 300 to 600 psi.
Anything below 300 psi should be discarded. Proper spray pattern is a must, no dribbling.
It's fairly rare to have problems with the pump check valves. That's probably why it drives trouble shooters nuts when it happens.

Actually Cat specs call for 400 to 750 psi for a nozzle with a good spray pattern. Leak down specs 300 psi no drips 200 psi max drop in 30 seconds.
Bob

Per specs given in Cat Diesel Fuel Injection Test Apparatus Instructions.....
Acceptable range is 300 to 600 psi with 300 being the cut off for usage.
For leakage rate with pressure adjusted to 300 psi. If the pressure drop falls more than 100 psi in 30 seconds the nozzle should be discarded.

"Old wrote:

New pumps and tested injectors should get you where you want to be. Set lifter height on the engine. 600 psi is optimum breaking pressure on the injectors. Acceptable range is 300 to 600 psi.
Anything below 300 psi should be discarded. Proper spray pattern is a must, no dribbling.
It's fairly rare to have problems with the pump check valves. That's probably why it drives trouble shooters nuts when it happens.

Any idea how to just replace the pumps? It looks like the pumps are attached to the lifters or yokes... It does not show this in the service manual and I am not about to go yanking things apart... I have done plenty of that... Any advice would be helpful.... If I need certain tools please let me know...