Mack aset air compressor problem

[Jeffrey 34]

I have a 2006 Mack aset with air compressor popping off frequently,and there’s air coming out the air intake tube from compressor ,and it’s pushing oil to dryer also ,any help would be appreciated.

[Geoff Weeks]

way too little info to make much of a guess, but if only leaks out of the compressor side of the air intake once pressure has built to cut-out, then it would be the unloader in the compressor that is leaking. If it leaks back all the time the compressor is running, then it could be an intake valve.

Really need to know what compressor. 

Start by feeding shop air into the system and see where it is leaking out.

[Joey Mack]

I assume its a Wabco pump. Does it have a hard pipe on the discharge side ? 

[Jeffrey 34]

Yes it is a Wabco ,it has a big hard pipe that runs up to the air box just noticed a hole has rubbed in it when I cut the truck off air comes out the hole.

[Joey Mack]

Yep.. can you post a picture or pictures?  We can help more,  and it can help others .. 

[Jeffrey 34]

I’ll get some pictures in the morning,that hard pipe is where it gets it’s air intake from even though it has a whole rubbed in it there shouldn’t be any air coming out with truck switched off should it.

[Jeffrey 34]

This is the compressor that I mentioned in the post from yesterday that’s frequently popping off,it builds air pretty quick and keeps air pressure while driving pulled purge valve off and a good bit of oil drained out could I just change out unloading valve and be good didn’t want to replace compressor unless I had to.

[Jeffrey 34]

[Geoff Weeks]

Wabco SS318 compressor.

Most "compressor problems" are air system problems, not the compressor. People want to point the finger at one part, even if it is expensive, rather than take the time sort out why the compressor is popping off all the time, and passing oil.

Intake of the compressor fed from intake of a turbocharged engine, means that even on the intake (downstroke) the pressure above the piston is above the crankcase pressure. This helps reduce oil misting in the outlet because there is no "vacuum" trying to draw oil up past the rings. A N-A compressor inlet does see a vacuum above the piston and can, under some conditions draw oil up past the rings.

 However that is not how most oil makes it to the compressor outlet. The block of the compressor is not water cooled, or really cooled at all. Only the head has coolant passages. Bendix list the normal outlet temp as "less than 360F" this is after the air has passed through the restriction of the exh valve port. Air temp and any oil vapor will be above that in the cylinder of a working compressor. Even engine oil will vaporize at those temps. Some will start to oxidize into hard coke. Vapor will pass out and condense back to liquid.

  There is a reason that duty cycle is limited to 25% MAX with lower being better. When air is not being used, running down the highway, you should go 15 min between cycles minimum. Just sitting idling blowing off every 2 min means it is almost continuously compressing. THIS IS WHAT YOU NEED TO FIX! when that is taken care of, the oil in the drier will be gone also. 

Being a single cyl compressor, and coke that gets stuck on the exh valve seat would prevent it from compressing at all. So that is not your problem. However it does sound like there is some leakage back through the discharge (exh) valve if you hear air leaking back through the compressor when the truck is shut off. There may be some varnish build up preventing a good seal and allowing the air to leak back through. However, unless you address the root cause, you'll be right back where you started in short order.

How long does it leak? Could you be hearing the unloader leaking? The gov will supply air to the unloader when the system reaches full charge (120 +/-) and should stop supplying air once cut-in pressure is reached (usually 20-25 psi below cut-out pressure). If it only leaks for a short time, then put a pressure gauge on one of the unloader fitting on the gov, or the unloader line to the air drier and see if when that pressure drops to zero, that is when the air leak stops.

[Geoff Weeks]

This might help:

Wabco Compressor manual

[Mark T]

Been my experience EGR engines do not have air compressors fed from intake manifold air. Macks seem to draw fresh air from tapping into the ducting somewhere between the air cleaner and the turbo inlet. Usually with a rigid steel line and a couple little leghths or hose on both ends. Great idea as the rigid steel line often rotted or clamps rotted off it and it'd wear through and basically allow the compressor to have no filtration. On the other side  ???  those compressors are pretty much bulletproof other than maybe replacing the head assembly . That being said, still doesn't change that short times between cycles is air leaks somewhere after the air compressor discharge port.  They can be tricky. Sometimes you may think a little tiny leak means nothing.....but they do. 

[Geoff Weeks]

1 minute ago, Mark T said:

Been my experience EGR engines do not have air compressors fed from intake manifold air. 

Makes sense and shows how out of date my info is. I never even remember working on something with an EGR, even thou they may have been out in the time-period I worked commercially, they were still going to the dealer for warranty work at that time.

Air leaks are a PIA, they are not quick or 'sexy" to find or repair, just hard work with a lot of time chasing something you can barely hear, but they are what needs to be sorted out 1st before condemning big, relatively easy stuff to change.