Maintenance Q&A
Whistling engines and failing AC

By Paul Abelson, Senior technical consultant

I have a 2010 Cummins-powered International ProStar with a whistle that’s driving me crazy. It comes on at about 1,500 rpm and gets louder on through 1,600, which is about as high as I ever go. I’ve had it to several dealers who couldn’t duplicate the problem. Can you help solve this?

I checked with some friends at the Technology and Maintenance Council who run Navistars with Cummins engines. Only a few had the problem, but the solution is fairly simple. There is a thin metal gasket between the turbo and the exhaust manifolds. When the gasket blows, being all metal, it produces a narrow, irregular crack. As rpm climbs, so does back pressure. Eventually, the exhaust gas opens the blown gasket and a portion of the exhaust creates the whistling sound you heard.

After we talked, you had the gasket replaced, making sure all four manifold bolts were torqued to specification. You reported that the whistle is gone, and fuel mileage has improved by 0.1 mpg.

My 2007 Freightliner has a Detroit Series 60 12.7-liter engine. The air conditioner doesn’t keep the cab cool anymore. On a hot day it starts out fine, but after about an hour or less, the air stops blowing and the cab gets sweaty hot. If I stop for a lunch break, the air conditioner will work fine for a while; then it heats up again.

I tried adding a can of Freon, but that didn’t help. My dealer wants to charge a small fortune just to diagnose it. He says EPA regulations require him to drain the Freon and vacuum out the system before he can even look at it. Is this true? Is there anything I can do myself?

Yes it is, and yes there is. The EPA does require authorized repair shops to evacuate the refrigerant from the system before stating any work on an air conditioning or refrigeration system. The rule applies to all air conditioning units, whether mobile or stationary. Many still call it Freon (R-12) but the refrigerant in use today is R-134a, far less harmful to the environment. The recovered refrigerant can be reused, and you should be charged only for any replacement needed to make up for leakage. Adding a can or two (never more) should be considered a short-term fix only, but you can legally do it to your own truck.

The good news is that you should not need a qualified repair shop. You have classic symptoms of an air conditioner freezing up. This happens with increasing frequency as a truck ages.

The system works as a heat pump. It draws heat from the cab and releases it to the atmosphere through the condenser. The refrigerant carries the heat in a continuous cycle.

Cool liquid (the refrigerant) flows into an evaporator in the cab. There, heat is removed from air when it flows around the evaporator and absorbed by the refrigerant heating the liquid refrigerant to the point it returns to a gas state. The gas version then goes through the compressor to the condenser where it again becomes liquid.

Chilled air cannot hold as much moisture as warm air, so excess moisture condenses on the fins of the evaporator and drips to the bottom of the evaporator. The water then drains out the bottom through a hose. That’s why you see water under vehicles on hot, humid days.

Over time, debris such as dry leaves and mud can get into the case and clog the hose. The condensed water builds up in the hose and evaporator, where it freezes. Eventually, the ice builds and air will no longer flow around the condenser’s fins. Then you lose all cold airflow. When you stop, you give the ice a chance to melt, allowing air to flow over the condenser’s fins once again. When you run the air conditioner again, the process of freezing up starts all over again.

To correct the problem without paying dealer rates, wait until the air conditioner has returned to ambient temperature. Disconnect the drain hose and blow it out, or replace it. If you can without damaging the fins, probe gently inside the evaporator case to see if there is any debris built up near the drain hole. With the drainage clear, your problem should be fixed. If not, you probably will need to take it to a dealer.

A driver called me on my CB and told me my trailer lights were flickering. Last September, I checked my lights for corrosion so I wouldn’t have problems this winter, but sure enough I have problems now. When I checked, there were places where it looked like the insulation melted off the wire. Do you have any hints to prevent this?

The most effective, but also most expensive, way to avoid corrosion damage is to rewire the entire trailer with a sealed harness, modular connectors and LED lights. But there are simpler solutions.

It sounds as if when you rewired your lights, you used generic “wire” in the gauge sizes you needed. Most people and many fleets do. But there are important differences in types of wire. Wires with twisted strands will wick corrosive spray under the insulation. Copper corrodes and becomes more resistant to current. That heats the wire.

Unless you specify a higher-temperature wire, you get GPT wire with vinyl insulation. It is rated at 185 degrees. Your wire likely heated beyond that.

Polyethylene insulation is rated at 257 degrees. Each polyethylene-insulated wire gauge size is available in three thicknesses designated TXL, GXL and SXL. Those designations indicate the insulation thickness, ranging from thin for flexibility to thickest for more protection. The polyethylene is more abrasion resistant, too. The price differences are minor compared to the benefits.

And finally, here’s a tip from a TMC member. FlexSeal spray watertight coating is available almost anywhere. Use it to seal electrical repairs from corrosive chemicals. LL

(Editor's note: When sending in questions for Maintenance Q&A, please include the make and model of your truck and your vehicle identification number (VIN) as well as your contact information. Paul Abelson tries to respond to every question, whether it's published or not. Send questions to