# Thoughts on CAT excavator boom sticking



## viper6383 (Sep 26, 2008)

We bought an old CAT excavator. Most functions work great but the boom seems to stick which is going to be real frustrating when trying to operate the machine smoothly. If the bucket is just sitting on the ground and I try to boom up, it will growl a bit and kind of act like there is a load, then take off. If I try to go slow, it kind of jitters a bit. the cylinders are on Heim joints so they really should not bind. The only other thing is the main boom pivot pins but that seems hard to believe they would be that stiff, especially on this old machine. 

is there something I might take a look at in the hydro system? I am hoping there is something simple like a pilot valve issue or something that might cause this.


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## tgeb (Feb 9, 2006)

Sounds like air in the system to me.

Maybe others have a different opinion and/or advice as to how to remove the air in those Cats.


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## viper6383 (Sep 26, 2008)

I guess I would think with the cylinders leaking like they are, there would be no way to have air in the system, especially with them sitting vertical but I hope I am wrong. that would be an easy fix.


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## cexcavation (Apr 17, 2008)

been sittin with moisture in the Hyd. Fluid and you got a rough spot in the cylinder bore where the piston will stick is my guess.


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## tburritt (Dec 8, 2007)

Air would be your best problem but also if hoses are old or Overseas crap the inside lining of the hose have tendencies to co-laps (personal exp) although CAT is less likely to have as original but if a replacement hose very possible. The other exp has been valve body/spool issues with o-ring or trash in system. Filter and oil changes are important especially if you go through hoses (we did during demo and land clearing & grinding). I GOOD set of pressure gauges can help tremendously in trouble shooting if you want to do t yourself. If guys are in the hole or trench when this acts up you could have a bad outcome.
Troy B


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## ajbackhoe (Nov 26, 2006)

Maybe a spool valve sticking. I had the same problem with my Case 170B excavator. Spool valve had to be taken out and polished with emory cloth and it ran perfectly after that.

Alan


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## viper6383 (Sep 26, 2008)

pardon my ignorance but what is a spool valve? Is this the pilot valve body in the cab? 

Also, there are bleeders for the stick cylinder that would be super easy for pressure testing but I did not find any for the boom. Is there usually a bleeder or bleed screw somewhere near the cylinder? I do agree that pressure testing would likely figure this out. 

I FIRMLY agree that we cannot do precision work with it like this. I do not want someone around it when it does this.


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## S.R.E. (Apr 8, 2010)

viper6383 said:


> pardon my ignorance but what is a spool valve? Is this the pilot valve body in the cab?


No the spool valve is usually in the valve bank behind the swing motor.
Also a broken spool valve can cause you problems like, you boom down with a load and it takes off and then it acts normal.


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