M931A2 won't air up over 80 psi

[Ue413]

Any thoughts on what to check? What I have done so far....

1. Checked for leaks....none found
2. Replaced drier unit with a known working unit
3. Disabled CTIS

just drove it a couple weeks ago with no problem, and if I rev her up to 2,000 rpm and hold it, can get up to ~70-80 psi, but cannot get over 90.

any recommendations?

 

[porkysplace]

Possibly the governor on the air compressor.

 

[Trailboss]

Try to air up the truck with another air compressor or truck to make sure you really don't have a leak and can get the pressure up to 120psi or so. Since it's a tractor, cycle the brake valve under the heater controls several times and make sure the trailer brake control lever is fully up. Also cycle the red gladhand valves at the front, rear and in front of the 5th wheel.

 

[SLOrazorsedge]

Wet tank PPV valve

 

[RobertoGatos]

Bad air compressor head gasket

 

[The HUlk]

Remove the small air line from the top of the compessor to remove the governor from the picture. If it still doesn't make enough pressure you have a supply and/or consumption problem. If the air builds over 120psi then you have a regulation problem.

 

[Ue413]

Thanks for all the advice.....won't have a chance to work on truck for a few days, but will report back with findings.

 

[M931]

check if there is coming air to the cooler-system, it can be the temp sensor to the fan.

 

[dilligaf13]

First step is to determine if there's any leaks in the air system This is done by building up the air pressure, either through the truck's air compressor or by utilizing shop air, and shutting the truck off. See if the system holds pressure or it bleeds off. If it bleeds off you've got a leak. If not THE VERY NEXT STEP is to determine if the compressor is producing enough air. A quick check is to start the truck up and quickly place your hand on the outlet tube (located at the top front of the air compressor, on the front of the unloader head). If it's not hot to the touch (like 200 degrees ++ hot) the compressor is not producing enough air. That is a pretty common issue with these trucks and you can either rebuild the unloader head or replace the unit. From there you can look at the other issues like the governor but leaks, compressor, air dryer and governor are the three most common issues (in that order). Good luck...

 

[Ue413]

Okay....worked on the truck some more today....replaced governor and same issue. Loosened bolts on top of compressor and gasket shot. Worked on removing back two bolts (closest to block) and couldn't pull them out due to injection pump.

do I need to remove entire compressor...or is there a trick with the bolts and injection pump?

 

[Plane Fast]

Look at the what have you done to your 5 ton thread. I talk about removing the unloader.
no you do not need to remove the compressor

 

[Ue413]

Thanks.... Looks like no secret, just patience

 

[Plane Fast]

Yep.. Bring the bolts up and tilt them to the rear and up. I had to remove the fuel line bracket as well. (One bolt)

 

[Ue413]

Cut out my own gasket and replaced blown gasket, and generated plenty of air. Unfortunately, my gasket material didn't hold, so purchased a rebuild kit and will replace gasket, o-rings, springs, etc. and when it arrives.

Thanks for for the help!

 

[Andyrv6av8r]

Cut out my own gasket and replaced blown gasket, and generated plenty of air. Unfortunately, my gasket material didn't hold, so purchased a rebuild kit and will replace gasket, o-rings, springs, etc. and when it arrives.

Thanks for for the help!

Been there, done that twice. I replaced 2 gaskets before I realized that the head was warped. Do yourself a favor and lapp the head on a perfectly flat surface before you reinstall with another gasket. The problem is the aluminum unloader head, not the steel jacketed head, gets warped over time.

 

[Ue413]

Great tip....had not thought of checking. I have a spare, so will swap head out straight away. Thanks again, you likely saved me another gasket and remove and replace!