My sister has a 2002 Trailblazer with about 250K miles on it. It has the 4.2L I6 engine. A few days ago on her way back from a 100 mile trip, it suddenly started running hot, around 250° indicated on the gauge on the dash. She stopped at a rest area, and when restarting it a few minutes later, the "check gauges" light came on and the gauge was in the red zone. She opened the hood and let it cool off for a while. Then she restarted it and drove home, about 80 miles. The gauge shows around 250° the whole way back, but didn't go into the red and didn't turn on the "check gauges" light. The air temperature outside was around 85° at the time, and driving at night, so no solar heating, not pulling a trailer, vehicle moderately loaded with 3 passengers, no luggage or anything else.
After it cooled down overnight, she checked the antifreeze in the radiator and overflow jug. Both were full and clean. She has made a few shorter trips since then (5 to 10 miles each) and sees similar performance. It's like the thermostat is opening at 250° instead of 195°, both in town and on the highway. It doesn't boil over, and the cooling system is still full.
The fan clutch was replaced 2 or 3 years ago due to a worn bearing. I don't think this is the problem, as it runs hot on the highway, where the fan shouldn't be needed.
The water pump was replaced 2 years ago after it sprung a major leak. It is not leaking now and the bearing feels tight.
The thermostat is probably the original. I have read about replacing it, and it is not an easy job. I wouldn't mind replacing it, but I would like to be fairly confident that it will fix the problem. From what I have read, the most common failure mode for thermostats is that they stick open, not closed. I don't think it is stuck closed anyway, as it usually doesn't overheat, it just runs about 50° hotter than it should.
The check engine light is not on. I scanned it today, and there are no stored codes.
Maybe the gauge is malfunctioning? I don't think it is the stepper motor problem, as it returns to the lowest setting when the engine is shut off. Maybe the ECT sensor suddenly became inaccurate? Being a thermistor, I think it would have a lower resistance at higher temperatures. It doesn't appear to be shorted, as the gauge is not pegged, just reading close to the high end of the scale. My scan tool doesn't read engine temperature, but I am going to try to borrow one that does and test it.
Maybe the radiator is clogged? If so, I would think that would cause a gradual increase in operating temperature, not a sudden change to running hot. The antifreeze has been changed a few times over the years, and looks fairly clean, so I don't think it would have a lot of sludge buildup. I cleaned out a very small amount of sludge from the sealing surface in the radiator cap, but didn't see any in the radiator or overflow jug.
Maybe the thermostat is not holding pressure? That's probably the quickest and easiest thing to check. I have a pressure tester with various adapters, but the pump portion of it is broken (that's what I get for buying a cheap model at an auction...).
I have an infrared thermometer that I brought out there today. With the engine hot and just shut off, I measured around 200° at the top of the radiator (I didn't check the bottom of the radiator). The water inlet where the upper radiator hose connects to the right side of the engine, near the power steering pump, showed 230°. When I checked there, I thought that was the thermostat housing. After reading several forums here, I learned that the thermostat is not there, but is on the left side of the engine behind the alternator.
My sister lives 40 miles away, with no auto parts stores nearby, so I try to guess what tools and parts are needed and bring them along when I go there. I would like to find out what the most likely cause of this is, based on the symptoms (always running hot, around 250°, at least according to the gauge on the dash, with AC on or off).
What should I check next? I would guess:
(1) Check the temperature indicated in the computer with a scan tool to see if the engine is running as hot as the gauge indicates.
(2) If the gauge appears to be accurate, replace the thermostat, and maybe the ECT sensor, too. Flush cooling system and install new antifreeze. Maybe replace all the hoses, too. When the system is open, check for free flow through the radiator.
(3) If it is still running hot, replace the water pump, as it might not be circulating coolant adequately.
Does this sound like the right approach? Am I overlooking something? She has little money, so I would prefer to not start replacing parts and hope it fixes it. The thermostat and ECT sensor aren't that expensive, but are time-consuming to install. Again, I don't mind spending the time, but I would like to be fairly confident that it will fix the problem.
After it cooled down overnight, she checked the antifreeze in the radiator and overflow jug. Both were full and clean. She has made a few shorter trips since then (5 to 10 miles each) and sees similar performance. It's like the thermostat is opening at 250° instead of 195°, both in town and on the highway. It doesn't boil over, and the cooling system is still full.
The fan clutch was replaced 2 or 3 years ago due to a worn bearing. I don't think this is the problem, as it runs hot on the highway, where the fan shouldn't be needed.
The water pump was replaced 2 years ago after it sprung a major leak. It is not leaking now and the bearing feels tight.
The thermostat is probably the original. I have read about replacing it, and it is not an easy job. I wouldn't mind replacing it, but I would like to be fairly confident that it will fix the problem. From what I have read, the most common failure mode for thermostats is that they stick open, not closed. I don't think it is stuck closed anyway, as it usually doesn't overheat, it just runs about 50° hotter than it should.
The check engine light is not on. I scanned it today, and there are no stored codes.
Maybe the gauge is malfunctioning? I don't think it is the stepper motor problem, as it returns to the lowest setting when the engine is shut off. Maybe the ECT sensor suddenly became inaccurate? Being a thermistor, I think it would have a lower resistance at higher temperatures. It doesn't appear to be shorted, as the gauge is not pegged, just reading close to the high end of the scale. My scan tool doesn't read engine temperature, but I am going to try to borrow one that does and test it.
Maybe the radiator is clogged? If so, I would think that would cause a gradual increase in operating temperature, not a sudden change to running hot. The antifreeze has been changed a few times over the years, and looks fairly clean, so I don't think it would have a lot of sludge buildup. I cleaned out a very small amount of sludge from the sealing surface in the radiator cap, but didn't see any in the radiator or overflow jug.
Maybe the thermostat is not holding pressure? That's probably the quickest and easiest thing to check. I have a pressure tester with various adapters, but the pump portion of it is broken (that's what I get for buying a cheap model at an auction...).
I have an infrared thermometer that I brought out there today. With the engine hot and just shut off, I measured around 200° at the top of the radiator (I didn't check the bottom of the radiator). The water inlet where the upper radiator hose connects to the right side of the engine, near the power steering pump, showed 230°. When I checked there, I thought that was the thermostat housing. After reading several forums here, I learned that the thermostat is not there, but is on the left side of the engine behind the alternator.
My sister lives 40 miles away, with no auto parts stores nearby, so I try to guess what tools and parts are needed and bring them along when I go there. I would like to find out what the most likely cause of this is, based on the symptoms (always running hot, around 250°, at least according to the gauge on the dash, with AC on or off).
What should I check next? I would guess:
(1) Check the temperature indicated in the computer with a scan tool to see if the engine is running as hot as the gauge indicates.
(2) If the gauge appears to be accurate, replace the thermostat, and maybe the ECT sensor, too. Flush cooling system and install new antifreeze. Maybe replace all the hoses, too. When the system is open, check for free flow through the radiator.
(3) If it is still running hot, replace the water pump, as it might not be circulating coolant adequately.
Does this sound like the right approach? Am I overlooking something? She has little money, so I would prefer to not start replacing parts and hope it fixes it. The thermostat and ECT sensor aren't that expensive, but are time-consuming to install. Again, I don't mind spending the time, but I would like to be fairly confident that it will fix the problem.