M11 compressor passing oil

Have you checked the intake or cac hoses for oil? Not an M11 specialist, but I assume the intake air for the compressor comes from the intake manifold.

 

While you’re checking for oil, make sure there isn’t any restriction,kinks.

 

I have a few dumb questions for you.

1 - When they replaced the compressor the first time, did they also replace the governor?

1A - If they did replace it, did they replace it with a genuine Bendix governor and make sure the lines were correct to the right ports and blocked off the ports not needed?

2 - Did you replace the Air Compressor with a Bendix, Cummins, or Wabco rebuilt unit, or did you get an aftermarket rebuild?

3 - Did you drain the air tanks to see if there is oil in them?

4 - Have you removed the inlet and exhaust lines on the compressor to see if they are full of oil?

I don't buy the dealer's point that you need a rebuild, put it on a dyno and see the actual blowby.

I had two trucks that gave me utter grief that did a similar thing.

One shop totally screwed us over; they used the cheapest parts they could find (*while charging us OEM prices) and the truck had oil in the air system that had to be flushed out after 4000 miles.

The other shop did not put on the governor correctly, it was a cheap Chinese governor and had to have ports blocked off. I don't get how it happened, but it was pulling oil through the governor, and we replaced the air dryer before we could get it back to our own shop.

Both were problems that were resolved with genuine Cummins/Bendix/Wabco parts.

 

I’m not an expert or even that knowledgeable about how truck air systems work, but I’m going to take a *guess* and say the events of Jan 3rd, 2024 is where it started to go wrong here. I’d have taken it to a Kenworth or Cummins dealer right off the bat and then taken it right back to them if the problem wasn’t resolved with an OEM replacement compressor. Sounds like there are other parts that need replaced that must have gotten boogered up when the original compressor went bad, if everything *had* been hunky-dory up till that point. Like for instance I know when an A/C compressor goes bad, it’s pretty much mandatory to also replace the condenser as it will be clogged/plugged with junk from the compressor fragging, or you’re wasting your time and the system won’t work right when it goes back together.

 

Usually that means the compressor is worn out. It is possible they installed a defective compressor. Easy way to check is to take the braided steel line off of the compressor, put your thumb over the outlet on the compressor. If you can hold it over the outlet, your compressor is bad.

 

There are a lot of shady rebuilders out there. Quality control just wants them to last the warranty period. An OEM compressor is likely better quality but more expensive.
Something I was taught early in m mechanics career was that after installing a new compressor you should create an air leak then let the engine run for half an hour to forty five minutes with the compressor pumping. Crack a tank drain so the compressor can almost keep up.This helps seat the rings as a compressor runs unloaded most of the time.

 

There is some special air compressor for early m11’s most people are not aware of. I’m not an expert but @singlescrewshaker knows all about them…

not using this specific compressor or dryer or something causes them to pass oil.i think there’s even a sticker on the frame or dash or something?

 

That would be a solid explanation if true.