My '02 Blazer seems to be running hot. It's running at about 210 but goes higher when I stop. I checked temps with an IR thermometer and this doesn't seem too far off. It is around 90 today. I'm thinking that maybe the radiator is losing efficiency. I'm taking it to leave at my camp in NY so I don't think it's a problem in the short run but if it's a looming issue, I'd like to know about it. This is with the A/C on.

 

Running the AC on these trucks on full blast does put a lot of heat on the radiator, especially on a hot day. Seeing a small bump in temperature while stopped is normal for the way these are orginially configured because of the mechanical clutch fan design. The cooling fan is designed to engage it's clutch at a certain tempurature when at idle speeds so you get the same fan RPM sitting in traffic as you would on the highway being driven off the belt. Many enthusiasts see this as a flaw and swap for electric fans that are on at 100% all the time; that increases cooling efficiency and decreses load on the engine.

If you're seeing tempuratures regularly climbing above 210, then you do have a loss in cooling efficiency. The designed max operating temperature should be around 198 F. On a demanding hot day in my 2002 ZR2, tempuratures stay just under 200 F sitting in heavy traffic with the AC on full.

Here's what could be wrong: Your coolant isn't cycling, or the clutch fan has failed.

Here's the easy checks you can do.
First check your coolant quality. Pop off the radiator cap and check underneath it and the radiator port, and check inside the reservoir. If these things are super dirty, then there's a possibility your thermostat has failed because of dirty coolant and is stuck closed stopping the flow of coolant. This means changing out the coolant in the radiator (full flush if it's really dirty), replacing the thermostat, and replacing the radiator cap. Easy tasks to do and possibly the quick fix for you.

Next thing you can check is the clutch fan. Drive around normally with AC on until you see temperatures reach the trouble zone. With the AC still on, come to a complete stop at any traffic light and wait for green. If the clutch fan is working when you take off you should hear a loud fan roar. This is by design. If you don't hear that loud fan roar taking off from idle with temperatures at 190+, then the clutch in your cooling fan has failed and needs replacement.

If both the coolant quality and the clutch fan checks out, then it's likely a failed water pump. Check the water pump. If it looks super rusty/corroded, then you likely need to replace it.
Replacing the water pump was one of the first jobs I did on my 2002 ZR2 when I bought it at 120k miles.


Has the coolant in this Blazer ever been serviced in it's life?
If yes, when? The service manual recommends service every 100k miles.

If no...
I'd consider doing a full coolant flush. The original DexCool formula in these trucks was nortorious for turning into a thick syrup that would gum up the heater core and radiator (resulted in a class action against GM). If you find evidence of syrup-y coolant, then I would do a full cooling system overhaul: new radiator, replace all crusty hoses, and thermostat. I'd put the heater core on this list too, but it's in a really "fun" place to get to, so I typically don't recommend it unless you have time to decommission the truck for a while. Just make sure you flush the heater core before refilling with new coolant.

The new DexCool formula should be fine to use. I haven't seen any issues about it. You can buy it off . If you're cheap like I am, I switched my ZR2 to the https://www.walmart.com/ip/Super-Tech-Prediluted-50-50-Extended-Life-Antifreeze-Coolant-1-Gallon/299226991 after I overhual my cooling system. I drain and refill the radiator once a year with this stuff. The worst thing you can do is mix coolants. If you want to switch, make sure the system is fully flushed and you've got new hoses on everything. Else, stick with the DexCool.

 

Thanks, RedValor. I recently replaced the pump and drained the coolant to do so and put a thermostat in it. The vehicle is new to me but the PO was good on maintenance but I do not know if the coolant has been changed. It looks good. The fan engages at startup and lets up after a little while but I did not hear it run when the engine temp got hot. I thought that the fans locked up when they failed but if it gets hot I'll get out and see if it's locked. It's already in upstate, NY, where it rarely gets hot and probably won't be an issue unless something fails majorly. Under normal conditions it ran at 194 as per a scanner connected to it.

 

194 sounds perfectly normal with the AC on.
The fans might lock when they fail. I could be mistaken on that. I'm familiar with the clutch fan failing in my family's old '92 Chevy Astro and it wasn't cooling properly.

Here's a crazy thought when you have the opportunity to check. You could have a ton of road dust and debris clogging airflow between the radiator vents. Pop off the front grill and check. If it is dirty then give it a rinse with a high pressure garden hose. The grill is held in by plastic trim clips that just pop out. If the plastic is brittle it could break. The amber lights are mounted to the grill. Headlights are mounted to the frame.

 

Thanks again. I don't know why but checking the fins on the radiator didn't cross my mind, sometimes I just overlook the obvious. It seems to me that it's hotter under the hood than I am used to seeing on a vehicle. If the radiator is blocked up, it would inhibit airflow through the engine compartment too.