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Baffling Overheating Problem

1999 1500 model. 300k miles. Burns one quart of oil between oil changes. No head gasket leaks. We replaced a leaking hose, the fan clutch and the thermostat. It hovers around 210 degrees at steady highway speeds as long as I don’t run the AC and I leave the thermostat on hot. Any stop and start driving causes it to overheat.

Bad coolant pressure, or clogged radiator my first 2 thoughts.

Restricted radiator?

Get the engine hot. Then have someone take an infra-red thermal heat gun and move it across radiator cores from behind the radiator.

The engine doesn’t need to be running.

If some cores are cooler than others, it points to a restricted radiator.


Thanks. That was my duh moment of the afternoon as I was staring at my radiator and running through all the scenarios and I realized I had check everything except the radiator itself.

Next time it overheats, pull over at the next safe spot, keep the engine running, pop the hood. Are the radiator fans spinning to beat the band? They should be.


There’s only one radiator fan.

The OP stated that they’ve already replaced the FAN CLUTCH


How do you know? Do you mean there are none visible?

On some vehicles, it’s hard to tell if you need a head gasket until the head is off.

My 1998 Honda Civic, which I still drive, had a similar problem. It would occasionally run a little hot when I got off the highway (and my air conditioning was broken). Eventually, after ruling everything else out, I bit the bullet and had the head gasket job done, and it hasn’t overheated since.

Think of it this way: Whether the head gasket was the cause or not, the brief periods when the engine was hotter than usual very likely warped the head, even if it’s just by a very small fraction of an inch. Whether the head gasket was the original cause or not, you likely need a new head gasket now, which will likely include planing the head(s).

Good point. I haven’t ruled out the possibility that at 300,000 miles I may just need to drop in a new engine.

What was the original problem that caused the replacement of those parts?

It was overheating and while troubleshooting it we found and repaired a leak and replaced the thermostat just in case.

Was the overheating problem exactly the same before the repairs as it was afterwards?

Exactly the same

Auto parts stores usually stock an inexpensive chemical test kit which will quickly detect if overheated exhaust gasses are leaking into the coolant due to an internal head gasket breach.

This kit is $50.00. And the cheapest I could find.

So they’re not exactly cheap.


…and they might not indicate a head gasket leak if the part that is leaking doesn’t allow for contamination of the coolant.

I’m gonna try replacing the temperature coolant sensor and then flushing the radiator.

Don’t bother flushing the radiator, just replace it.


Yes. Flushing is a waste of time and a new radiator costs less than rodding out the old one.

Every caddy northstar with a bad head gasket always has a new radiator and WP. They change those before they give up. The OP’s case would be baffling if both those had already been replaced and it still overheats. But they have not Been changed yet.