I just don't get this. My 2001 Blazer 4 dr with 4.3 has some problems when the weather is very warm. Just had the Heat Temperature Sensor replaced after looking up the codes that were pulled. One showed that the above sensor had to be replaced. This did not cure running problem. While traveling a speed of 60 to 65 MPH for a period on a hot day with A/C running, the Blazer "bucks" and the heat gauge in the dash goes to right showing very hot temps and then back to the left showing less-than-normal low temps. This problem get progressively worse and our speed slows until we shut off the A/C. After a brief time of driving with the A/C off the Blazer will usually return back to normal conditions. Upon checking the radiator fluid when this "bucking" starts I find normal water levels for the radiator with no apparent overheating going on. the Computer, however, which lies atop the radiator overflow reservoir is so hot that you cannot keep your hand on it without being burned.

What can be going on here? After replacing the HTS I flushed and replaced Dexcool and thermostat. No change in condition was noticed following this.

Did you check coolant level on a cold vehicle? Radiator and Reservoir?

What you are describing certainly sounds like an air pocket, caused by a leak....

Thanks for the comeback. I have changed the coolant and am in the process of cleaning out the cooling system with distilled water. I flushed it with Prestone Flush. I am sure that the system did not have an airlock since this problem has been intermittent for the past few months. I am wondering if bad gas is the culprit since the blazer seemed not to do this with a higher octane gas. My description of the engine "bucking" is probably a misfire but this only happens when the A/C is on during 90 degree temperatures outside.

The more load you put on the engine, like having the A/C on, the greater the chance for a misfire....have you checked for a misfire code?

Also, have you checked to see if your cooling fan is operational? With the A/C running, and no fan, that will increase coolant temp also....While driving on the highway, the natural airflow will remove the heat form the condenser and add that heat to the radiator...

Thanks for your comeback. I added distilled water to the radiator and ran it until there was no airlocks apparent. Put in some Heat in the fuel tank and filled up with 89 octane. Ran great for about an hour at 60 to 65 MPH. All-of-a-sudden it began to misfire and slow down. It did not quit but we pulled over to let it cool. No problem noted with the radiator and the blazer ran fine. About 20 miles further it began to misfire so we pulled over and shut it off. No battery power and engine would not turn over. Jumped started it and drove home with heat gauge totally pointing to the left (cool). What has the heat gauge got to do with this problem since when this problem occurs it goes from left to right and back and forth.

The problem of the heat gauge fluctuation combined with a misfire was finally solved with the replacement of the heat temp sensor electrical connector. It was found to be the problem whenever the engine would heat up and open the circuit. It was replaced and now all is well. More-than-likely it was the culprit all along and not the heat temp sensor but as they say, that is history. Thanks for your advice. I will keep a watch for any signs of an air pocket since I had to by-pass the leaking heater core until the weather cools enough to allow the garage to cool down.