Cummins 8.3 M923 not building air

Have a Cummins 8.3 in a 923 that won’t build air normally. Replaced air dryer with a new one. No moisture in system. Do the air compressors go out frequently on these? The engine appears to be a reconditioned one

builds air air slow and hard to reach 80 psi

Have you checked the unloader valve and governor ?
It looks like you have had this problem before.
[h=1]M939 No air pressure build up[/h]

My M923A2 wont build air pressure past 30-40 PSI in Primary/Secondary air tanks??

Different truck. That one was air dryer.

Thomas_G said:

Different truck. That one was air dryer.

Click to expand...

Take the discharge line off the back of the compressor and install an air hose and fitting with a gauge. And see what you have. If no pressure is there. You can try tapping on the unloader. Your gauge will wrap completely around. So it may not look like it has any pressure.

You might want to put a tee and a $5 125-135ish psi pop off valve before that hose, you should be able to hear the pressure and volume before you have time to look at your guage, if you compressor is ok.

Isn’t the unloaded valve in that aluminum cal head cover? I haven’t messed with a 8.3 at all

Thomas_G said:

Different truck. That one was air dryer.

Click to expand...

Same air system, same trouble shooting section in the same TM's.

Here is my standard advise. Follow the control line from the top of the air compressor head to the unloader control valve. Take both ends of that line lose and blow air through it. Then take the control off, clean and re-lube it with tacky grease (red) that is waterproof on all surfaces that can corrode and rust, especially around the spring.

Find a really small funnel or injector and put Marvel Mystery Oil or a good pneumatic oil into the head. There is a control piston that gets seized up with water and crud that holds the intake valve open and it won't let it close. Put the tube back on and work oil into that line at the top. 50/50% chance it works to free it up, so worth a try.

Re the control cleaning---keep track of the number of turns that the spring takes to take it out and put it at the same place when re-assembling. I first reset mine to about 125# and had the dryer relieving a lot so set it back to about 118.

I thought my unloader valve was bad. Turned out the compressor head gasket was brittle and had cracked in multiple places allowing the air to escape. The compressor is a pain to remove too...