Charge Air Cooler

Care and feeding of the charge air cooler…. CAC.

The what?

Once again there’s a lot of stuff here many of you know, but I’m also trying to help the people that don’t know….. so bear with me.

If you look through the maintenance checklist that came with your Freightliner Chassis you’ll see that it’s on there, 09-02 the inspection and cleaning of, and quite regularly. Every service to be exact.

So what is it? First let’s talk about the turbo charger. Over simplified this is how the engine does it’s job: Inside the cylinder we introduce a mixture of diesel fuel and air, in the correct ratio, and when the piston moves up and creates pressure the mixture explodes and forces the piston back down…. The force of the explosion is translated into a rotational motion at the crankshaft, and passed on to the transmission and eventually the wheels. This is how we make the bus go.

Now, the bigger the explosion the more horsepower. So to move a 30,000 pound plus bus you can build a very large engine, but that’s more space and more weight. Or, you can force more fuel into the cylinder of a (relatively) smaller engine and create a more powerful explosion, but then you also have to put in more air to keep the proper combustion ratio. This is where the turbo comes in.

On it’s way out to the exhaust system the spent gases go through a chamber with a turbine (a fancy windmill with many blades) and make it spin. The turbine is mounted on a shaft and on the other end of that shaft is another turbine, but this one compresses the incoming clean combustion air. This is how we force more air into the cylinder. BUT, when you compress air you also heat it up, and hot air is less dense (read: contains less oxygen) than cold air. So to get even MORE oxygen into the cylinder we cool the air first. THIS is what happens in the charge air cooler.

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It looks just like a radiator. In fact it is about the size of the radiator and sits behind it. (As seen when standing behind the motorhome.) When you look at the shroud that contains the radiator from the inside you THINK you’re looking at the radiator but in fact you’re looking at the CAC. It’s a sandwich.

Now…. ever notice how the back of your bus is always a lot dirtier than the front? That’s because the air coming at the front keeps moving past the bus and most of the dirt is moving too fast to get a hold of us. In the back, however, there’s a lot of swirling going on and some of the air slows down enough for the dirt to escape and settle on our bus. There’s also a lot of swirling going on under the rear end, and then there’s that big fan sucking a lot of air into the engine compartment to blow over the CAC and the radiator to get rid of all that heat we generate. And a lot of dirt.

So that’s why we have to clean the CAC, regularly. Especially the folks running around dusty parts of the country.

As a thought…. sealing the underside of the engine compartment and putting a large air scoop on the rear of the roof running down into the engine bay would go a long way towards keeping it clean in there…. but (you guessed it) that’s more money, and it would add weight and take up space.

So we clean. And here comes the challenge: getting to it. Again Forest River didn’t do us any favors. As you know, standing behind the engine you can get to….nothing. A few reservoirs and dipsticks. That’s it.

So inside we go…..



Yeah, it’s dirty. So, on you knees you go and you peek in there…. and you see this:


So that’s the fan. And behind it the CAC. About 1/5th of the whole thing. And there’s (some) dirt. Doesn’t look too bad.

And then you look at the other side….


And you see mostly fan. You can turn the fan but there’s very little access to the CAC. Forget about the top and the bottom.

So let’s try to “inspect”. If you dive down deep enough you can look THROUGH the CAC and the radiator behind it and see daylight. (If it’s bright enough outside). If not you can place a light out there…..


There you go. Reasonably clean. And, that’s why the engine is still performing (getting enough COOLED air from the CAC to develop it’s horse power) and not overheating. (Radiator).

So now we clean……(WARNING… if you do this as described you WILL get wet)

As per recommendation I got a gallon of aviation Simple Green. Aluminum friendly. And I went out and got a 1 gallon plant sprayer…. like this one:

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I made a 50/50 mixture and then with a lot of squirming I was able to hit about 1/4 to 1/3 of the CAC from above, and better than half from the bottom, being seated, with some overlap in there. The wand on the sprayer bends, but it is tough enough to be used to move the fan around. All in all I squirted about 1/2 gallon on the thing. While I was at it I sprayed the radiator from the outside. Not ideal, but better than nothing. Then I waited, about 10 minutes from the start of application.

Then I got the hose out, with a wand with an articulating head on it…. (ACE hardware) and got back under there and rinsed with the head set to “Gentle Shower”.  (This is the part where you get wet). Then I stuck the wand up alongside the engine and pulled it up above and hit every spot I could sort of get to….

Warning: Do NOT hit any of this with high pressure. You WILL bend the “vanes” of the CAC and/or radiator and that greatly reduces the air flow and thus performance of both. So GENTLE rinse…….

Here is the “after”….

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Gave the block a quick rinse too….And the radiator from the outside.


So that’s it…..  Since it can’t be done very thoroughly I recommend doing it often…..