A couple days ago I noticed and audible air leak coming from my air compressor. I’ve been noticing over the last few months that at times I seemed to be losing air, but could never pinpoint the problem component. After starting and idling the truck for just a few minutes, I noticed the fairly loud leak and upon further inspection, I found where oil had apparently been blown through the side of the compressor head and onto the engine.

I called Peterbilt and they have a rebuilt compressor for under $1,000 (with exchange, of course). They also offer a seal replacement kit for around $150. The question is, should I replace the whole compressor or just the seals and gaskets and hope that solves the problem? The truck has about 765k miles on it and around 17k hours. I’m thinking that it would be less work to replace the whole thing than to take it off and open it up. I’ve never done either, but with instructions, could do either job, I’m sure. The secondary issue is with that many hours on the compressor, is it worth simply replacing seals and gaskets, or am I just prolonging the inevitable for a while?

I don't know which way would be best, just know my own experience. With only 2 or 300,000 miles on my ISX, I had to replace the compressor head because the control valve inside it was sticking and you can't replace just the valve. The mechanic told me this is common with the ISX. Don't know if this helps!

Removing and replacing times would be the same .....extra time added for rebuilding. What warranty does the factory rebuilt one carry? Doing it yourself may fix the problem without a warranty or it may not. Personally, I would go with the rebuilt one if it carries a warranty. And no, I am not a driver but sold auto/truck parts (yes heavy truck) for many years prior to getting into law enforcement. Driving is my next career.

Are you certain that it is the compressor or could it be a valve or air governor? I had a compressor go out on me in Kentucky on a weekend once. I managed to find a guy who helped me get a couple of parts and we rebuilt the head on the side of the road. I would not recommend it, but it didn't cost that much. It was a pain on the side of the road, but we got it done.

It’s nearly impossible to tell for sure where the leak is. On my ISX, the compressor is mounted close against the left side of the engine somewhat behind the power steering pump and just below the ECM. Wherever the leak is, it is on the back side of the compressor. I can’t really get my hand behind it because there’s not enough clearance, but I can see where a small coating of oil has been blown over the side of the engine behind the compressor. I started up the truck yesterday and let it worm up for about thirty minutes and when I shut it off there was no audible leak. Looking back on it, it seems that I only hear the leak when I start the truck up for just a few minutes to build up air pressure so I can move it to work on it. Even then, the leak is only audible very sporadically. I think I will let it go for now and just keep a close eye on it in the future. After taking off four weeks to buy my trailer and set my truck up (to the tune of about $7k and counting) I really don’t want to spend another $1k right now if it will wait for a few more months.

Before condemning the compressor, look at the valve mounted underneath. The "turbo air control valve". When it leaks it sounds as if it's coming from the compressor. It is almost impossible to get your hand on it to feel the air leak. There are two air lines going to it ( in and out) and hopefully one is leaking,(cheap fix) but the valve itself could be leaking. You can barley see it, if you can see it, you might try spraying some soapy water or Windex on it to look for bubbles, indicating an air leak. I am not trying to tell you that's where your leak is, but since you said you never really felt it, just heard it, you should rule it out first.

Thanks for the suggestion, moe. Would the turbo air control valve leak air even after the truck has been turned off? Would it mist oil onto the side of the engine near the top of the compressor? If the answers to both questions are “yes” then it might very well be the culprit. It only seems to leak until the truck warms up, and even then only occasionally. In light of the great amount of money I’ve already spent over the last month, I’ll probably chance it and hope it makes it for a few more months. If it becomes a persistent problem though, I’ll have little choice but to do something about it.

Yes; it would leak when engine is off, and would be more audible at that time. No; Unless you have oil in the air system it would not leak oil. If it was the air control valve, You should be able to run without any problem other than the air leak, which tends to be worse with engine off. Try to take notice if your secondary tank leaks down after sitting about an hour. One other thing you can look at is your primary air tank check valve. You can do this by disconnecting the main air line coming off the compressor. There should be no air coming back thru it.

The secondary tank empties pretty quickly when the audible leak is present from the area of the compressor. The primary tank loses air, but over a much longer period than the secondary.