Has huge "U" blade attached . wont lift it when it gets hot .
Three things come to mind....lift cylinders need repacked, control valve issue or weak hydraulic pump. First test raise it all the way up, shut off engine and stand on the hood between the lift cylinders and listen to each one. If they’re bypassing you’ll hear it. Second test it to raise the dozer with the blade and see if it stays up or rather how long until they bleed off and it settles down. What kind of shape are the filters in? Oil clean and level correct? Next step will be a gauge to check hydraulic pump output pressure. Usually the best way to hit relief is to deadhead the ripper if so equipped. All these tests are done at operating temp to replicate the symptoms. If the pump is stuffed and weak the next step is to determine why. Not just swap and go on. Oil samples my give you an idea. Pull the filters and look inside the pleats, maybe drain the tank, remove the cover off the side and see what kind of trash you have in the system.
We had a mechanic out He felt that he was seeing cavitation and recommended changing the pumps . which we did. No joy . the ripper and tilt circuits are no probelm . When the temperature gets gets up around 200 f the blade lift starts to fail .If I try to lift the blade while sitting still I can feel it try to lift but then bypass . I believe it is a relief valve problem. I wil try on monday to listen for the lift cylinders to bypass
The same test that will check pump output will check relief pressure. Seems like one of the tests helps isolate the relief valve but it’s been awhile since I did it. With a S/N I can pull up the procedures for it. Changing pumps without some other tests is not how I’d go about it. They aren’t cheap and generally there’s an underlying cause for their failure. Are there any dents or damage to the lift cylinders? Mainly the barrels? Doesn’t take much damage at all to end up wiping a piston out and contaminating a system. Cavitation can be a suction hose or some other issue as well, not necessarily a pump problem. The two times I’ve dealt with similar issues it was on a D11N. Both times it was pump not making pressure which was ultimately due to contamination related failure. One pump was scrapped because of a damaged lift cylinder making metal. It was a weeks worth of work cleaning the system. Were the pumps new or reman from Cat? Or of unknown condition? If you follow the Cat diagnostic procedures it sure helps narrow things down. With a S/N I can provide them.
A D9L is over thirty years old. Do you have any history for the machine? Another thing to check would be the quick drop valves in the lift cylinders.
engage the tilt at the same time you engage the lift I have had the resolver ball in the control do some things like this even though most times it will keep it from lifting at all. if it lifts with the tilt or ripper bottomed out pull the control valve and reseal it
Maybe use a different mechanic in the future. 'He felt' should be 'he eliminated' by following diagnostic procedure steps AND found the problem(s). If the lift cylinders are by-passing oil (usually one) you can fell a temp difference with your fingers or use a heat gun. If you're in the Riverside-San Bernardino area PM and I'll give a couple phone numbers for experienced mechanics.
Pump cavitation is a symptom, not a cause. I would start at the tank and work your way through the system. Check the suction inlet from the hydraulic reservoir. I don't know about the D9L, but some hydraulic systems will have an suction filter, and most will have an inlet screen. Check to see if the pump is getting a steady supply of oil.
D9L has ONE screen mounted inside of the hydraulic tank around (outside of) the filter element. Oil to the pumps is gravity feed, e.g. pumps are lower than the fender mounted hyd-tank. (Designed to prevent cavitation) Possible cavitation: rags, plastic bag in the tank? That would effect the ripper lift-tilt and dozer tilt circuits also and hydraulic oil temp wouldn't have an effect /cool oil or hot oil. 15 minutes to remove, change filter element and visually inspect the hydraulic oil for> metal particles, dirt etc and reinstall. 10 minutes to cut filter element and inspect for > metal particles, dirt etc. PS: use a lock-out tank filler cap...couple of hands full of DG, sand, dirt will wipe out the pumps etc.
If it’s like a D11N the filters are cartridge style and easy to inspect upon removal.