Gx5 FF rotary screw compressor knocking...

Started working on a Gx5 ff compressor that hasn't been serviced for 5 years and only runs occasionally.
The oil view glass was covered in sludge. I removed all the oil and replaced the belt, oil separator and fuel filter . I replaced the oil and ran the compressor, it started filling with air and I watched the view glass that I cleaned and the clean oil I installed started getting dark. I then shut the compressor off after 3 minutes to check oil level after running machine. The oil level was fine but looked dark already again and 1/4 full. I drained some oil out and added some more clean oil. Then started the compressor again, but now the motor runs and the compressor doesn't gain anymore air pressure and stays at 100psi. Air compressor keeps running and no pressure builds up and you hear a knocking noise now. Any suggestions would help.

Thanks !

Comments for Gx5 FF rotary screw compressor knocking...

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Jun 20, 2017
compressor knocking...
by: Doug in s.d.ca.


On oil, given the mild use, maybe crud settled in the OT and/or the cooler?

The knock - well, what is? no belt no noise? Have/had you rechecked the tension?

The low pressure - The manual says that's the blow-off (Y1) valve stuck open.

Have fun and keep us posted.

Jun 20, 2017
by: Carl

Oil should be replaced every 2000 running hours or every year on these machines. If it isn't then it degrades and the properties in it do not do what they should (Cool, lubricate etc.) It can turn like toffee.

So when you find a compressor like you have, you must do a complete oil replacement, what I mean by this is drain the oil tank, cooler, air-end, oil pipes as far as possible, replace the oil filter, separator, thermo valve.

Then fill with new oil, making sure you put some into the air-end as it will be dry, run for 30 minutes, then do another complete oil drain, do this 2-3 times. You need to get rid of the bad oil asap.

The knocking you hear may be the air-end bearings, this should improve the more you run it with good oil. It sounds like it's recoverable but it will have done some damage due to the oil going bad.

Hope that helps

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