Chevrolet Repair: 88 Chevy S10 overheating, losing coolant, head gasket

Question
Hi,

I have an 88 Chevy S-10 Pickup 4WD, Automatic, 2.8L engine. My problem is that when I drive the truck it wants to overheat. If I drive it through the city it runs warm but not really hot. But when I drive it at or around 55 MPH it wants to get really hot on the verge of overheating. I can run the Heater and it will help but sometimes still overheats.  I have replace many things, the ecm, thermostat, coolant tempurature sensor. I have also drained the fluids and flushed the radiator. Any ideas on what would cause this to happen.

Thanks,
Chris

Answer
You may have a bad head gasket, very common on the old 2.8's.  Are you losing coolant?  Pull the spark plugs and see if any coolant come out, if so you have a bad gasket.  Also you said that you flushed the radiator, but what does the outside of it look like?  Is it blocked by bugs or road debris?  Try blowing the hose of water over the front of it to clean it off.  Check also to see if the fan clutch is locked up.  It may be locked up causing the water pump to do more work, and thus over heating
Jimmy