2002 Trailblazer 4.2 running hot at highway speeds - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum



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Go Back   Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum > 2002 - 2009 TrailBlazer/Envoy Tech > 02-09 Vortec 4.2L I-6 Engine and Drivetrain > OEM Issues

OEM Issues Original Equipment Repairs and/or Problems

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  #1  
Old 07-26-2012, 12:14 AM
davertb davertb is offline
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2002 Chevy TrailBlazer LT
Pewter Metallic 4.2L I6 4X4
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2002 Trailblazer 4.2 running hot at highway speeds

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.
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  #2  
Old 07-26-2012, 08:06 AM
Happy_Trails Happy_Trails is offline
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2004 Chevy TrailBlazer LS EXT
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Hello, I had the exact same issue and it ended up being the clutch fan. Mine only ran hot (240-250) on the highway. Daily driving didn't seem to cause it to rise above normal.
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  #3  
Old 07-26-2012, 11:04 PM
davertb davertb is offline
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2002 Chevy TrailBlazer LT
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I could see a bad fan causing overheating at low speeds (city driving), but I can't see where a bad fan would cause an engine to run hot at highway speeds, when there should be more than adequate air flow to keep the radiator cool without the fan running.
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  #4  
Old 07-27-2012, 09:12 PM
davertb davertb is offline
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I did a little more testing today. I hooked up a code scanner that shows the engine temperature. It is consistent with what the gauge on the dash shows, both with a cold engine and hot, so the gauge appears to be working normally. I let it run until it was up to 230F and watched both gauges. After running, I checked the temperatures with an infrared gun. The water outlet to the upper radiator hose showed 230F, and about the same on the top of the radiator near the cap. The lower hose, about 6" from the thermostat, showed 120F. It appears the thermostat is not opening at all. I just ordered a new thermostat and, as recommended in the forums here even though it appears to be working, a coolant temperature sensor, as it is right next to the thermostat, behind the alternator. I will try to find time this weekend to install them and get the (hopefully) final outcome. I am 95% confident that this will fix the overheating problem.

Thanks to the Advance Auto Parts discount code I found on slickdeals, I got the thermostat, coolant temp sensor, 3 gallons of full-strength antifreeze (2 to use and an extra to have on hand), an AC Delco oil filter, and 2 wax applicator pads for $61.xx plus tax after the $40 discount off a $100 purchase, plus a 5% cashback bonus through shopdiscover.
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  #5  
Old 07-27-2012, 09:23 PM
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ylab ylab is offline
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The factory fan can actually restrict airflow at high speed, if it isn't turning fast enough. Just look at the fan and space, or lack of which exists between the blades. With most designs or just about any e-fan performance isn't an issue, but our fans is one of the exceptions....the price of an under rated cooling system. - We really need a double row radiator.
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  #6  
Old 07-27-2012, 10:00 PM
Vonhendon Vonhendon is offline
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Quote:
Originally Posted by ylab View Post
The factory fan can actually restrict airflow at high speed, if it isn't turning fast enough. Just look at the fan and space, or lack of which exists between the blades. With most designs or just about any e-fan performance isn't an issue, but our fans is one of the exceptions....the price of an under rated cooling system. - We really need a double row radiator.
I had a GM Tech. tell me the other day that these fan's were junk, and when they crap most people get Efan's.
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  #7  
Old 07-27-2012, 10:10 PM
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ylab ylab is offline
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The latest clutch design is better, especially with the pcm updates. Shame GM did put a better cooling system on these things....the 4.2 makes a LOT of heat. I'd give even money the OP has a failing clutch.
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  #8  
Old 07-28-2012, 08:24 AM
TrailblazrI6 TrailblazrI6 is offline
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2002 Chevy TrailBlazer LT
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Good idea on changing out the t-stat and ECT. 250,000 miles is ALOT of miles on the originals! It is very probable that it is sticking closed, resulting in the overheat condition.

If you need help changing out the t-stat, check the forums here. Its been covered a lot and for a first timer, can be frustrating. Good Luck!

Cross your fingers and hope she didnt damage anything but running it hot for so long. Those temperatures are not friendly to the head/head gaskets/block.
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  #9  
Old 07-30-2012, 08:26 AM
davertb davertb is offline
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2002 Chevy TrailBlazer LT
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It's fixed! The new thermostat fixed it. I installed the new thermostat and ECT sensor yesterday. It took me a lot longer than the 3 hours that sounds typical from other posts. That bottom bolt on the alternator is sure hard to get to, especially with the 2 AC lines right in front of it. Anyway, it is running cool now, with the temp gauge staying about a needle width below the 210 mark on the dash gauge.

I don't see how the thermostat could possibly be removed going through the wheel well, as suggested in one of the other topics here suggested. The gap to get to it is only about 2" wide. How would you ever get your arm in there to remove it? I don't see any way to get to it without removing the alternator.

Is there a special tool to install the ECT sensor? I used an 18mm deep socket to remove the old one, but I cut the wires off to get the socket over the sensor. NAPA said they had an 18mm sensor socket for $10, but it turned out to be a 22mm O2 sensor socket after a special trip into town, and an hour wasted. I ended up using an 18mm open end wrench on the new one, but I don't like tightening a brass sensor with a wrench on only 2 sides of the hex.
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