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S-10 Losing It's Cool

Q. This has been an ongoing battle for the past 2 years but I really started taking a hard look at it. I have a 1991 S-10, 2.5L, Manual Trans, 98,000 miles, and TBI. None of the extras. No A/C.

When I run on the highway the vehicle starts to overheat. If I let it go it gets up to 260 or maybe higher, so I turn on my heat to lower the temp. When I'm sitting still (vehicle running of course) the vehicle cools down after running on the highway. So far I've replaced the water pump, thermostat, flushed the cooling system, changed the radiator cap and changed the clutch for the cooling fan.

Still, the same as it was before. The radiator has good flow through it as well. I'm thinking it the sending unit, but I compared temps. with a thermometer and the gauge in the dash. They were pretty close to each other, but I'm wondering if it is giving a false reading as the temp gets higher. I've never blown a hose, lifted the relief valve on the rad. cap or overflowed the reservoir. It's really odd.

One thing I did notice is that when the engine is started, there is a great deal of air flow across the radiator, but as the engine warms up the air flow decreases. I found that rather odd. I would imagine that the clutch on the cooling fan would allow more air across the radiator as the temperature rises.

Thanks you for your time in advance,
Travis

A. Well Travis, you have a good candidate for a radiator rod job. Scale builds up in side the tabulator tubes and, over time, closes them up completely. This is like hardening of the arteries in a person. What they do is take the end tanks off the radiator and run thin rode throught the tubes to clear them out. This opens the tubes and brings the radiator back to full heat transfer capability.

Looking in the radiator neck is not really a good indication of coolant flow. It may look good from up there, but lower down it may not be passing through at all. What you can do to get an idea of how much blockage there may be is this; run the engine until it's hot and feel the front of the radiator (not between the fan and radiator unless extreme pain is something you enjoy). What you want to see is where the "cold" spots on the radiator are. "Cold" spots are areas where the tubes are blocked and there is no collant flow. If the "cold" spots are large, then it needs to be rodded out.

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