Bottom Line
Maintenance Q&A
Troublesome temperatures

By Paul Abelson
Senior technical editor


Q: I own a 1996 Peterbilt 377 with a Series 60 Detroit Diesel 12.7 engine, and I pull flatbed loads throughout Nevada and over the mountains into nearby states. Some mountain summits are more than 8,000 feet. Nevada is dry and hot, with summer temps of 120 degrees. I often see the temperature gauge and pyrometer gauge going up and down almost in synch with the tachometer. The pyrometer will go up to 1,200 degrees and coolant above 220 degrees on 10- or 12-mile grades.

I checked with my local Detroit dealer. He suggested I check my charge air cooler, hoses and clamps, but I found nothing. I changed my air filter, but on the first grade I hit, my in-cab restriction gauge went to the No. 7 position, like a mild restriction. My engine runs great and doesn’t skip a beat. No smoke or loss of power, but no one can explain the temps running up.

I had my engine warning light come on, and the engine started to shut down. I always turn my fan on early when I traverse grades, no matter how small they might be.

I also routinely turn my heater on, with my blower on high to increase cooling. I have no leaks and no apparent problems with the 15-pound radiator cap, one-year-old green coolant, hoses or belts. The coolant level doesn’t seem to change. The thermostat checks out, and I can’t find anything else. I can understand the temps going up a little when you’re grossing 80,000 and climbing a 6- or 8-percent grade, but to consistently reach such high numbers concerns me.

I don’t understand it and don’t want to burn my engine up. I really like this truck and engine, but am deeply concerned that I might be doing something wrong or am overlooking something minor.

A: You were quite thorough with your checklist, covering most of the items I would have suggested. Pulling flatbed, you have added aerodynamic resistance compared with a van trailer. It puts an extra load on the engine, but not enough to hit the numbers you describe.

So I checked with Chuck Blake, senior technical sales support manager at Detroit Diesel. Chuck is a TMC Silver Spark Plug recipient for helping develop far too many Recommended Practices to mention here. Here’s what he suggests:

  • Make sure the fan shroud is in place.
  • Make sure the fan is centered and there is no excess clearance anywhere around it.
  • Clean the radiator and charge air cooler, both outside and inside.
  • Clean the fins on the air conditioner freon condenser, the charge air cooler and the radiator. Sometimes debris gets through one and plugs the second. Separate them and clean them.
  • Check fin integrity with the cooler tubes. A 12-year-old radiator may have some fins separated from the cooling tubes.
  • Check thermostats and seals to ensure there is no bypass when fully open.
  • Program the engine controller for a warning instead of a shutdown. With a good radiator cap and less than a 50/50 mix you can run 210 to 215 degrees. You may want to use minimal antifreeze mix for the lowest possible temperature you’ll encounter to get more heat transfer. Water is a better heat absorber than glycol.
  • Always use supplemental coolant additives, especially with green coolant.
  • Make sure there are no license plates or any unnecessary air flow obstructions on the grille.
  • Make sure all recirculation baffles are in place.