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Sonoma overheats only when AC is on.

14K views 22 replies 13 participants last post by  Jayhawke 
#1 ·
I have a 96 GMC Sonoma 4x4 4.3L. Its overheating whenever i run the AC. If i turn the AC off the temp will drop 60 degrees within 5 minutes. Brand new radiator, cooling hoses. heater core is disconnected and hoses are bypassed back into the pump. so its not the heater core. r134a is in the right levels. and the cooling system seems to have pressure on both sides as the car is running so it doesnt seems like a blockage in the cooling. any ideas???? Im in Miami FL. so driving without AC in july is like driving through hell.
 
#2 ·
Check the water level again. Sometime it needs a burp. Take the R-cap off and wait till it get really hot. Want to see steam, almost. Looking for some movement. If it drop out of site. You'll know it need more. If there no movement. The Thermostat might be the problem. Also, some vehicles use the heater core as a second radiator.
 
#4 ·
it gets hotter quicker in traffic. however dont think its crud since everything has been replaced. the old rad was rusty inside but we replaced it and the hoses and added new 70% coolant instead of straight water.

coolant level is fine and circulating.

mechanic said he didnt think it was the thermostat but i wanted to remove it since i dont need a heater down here. so i am still thinking of pulling that. fan seems to be running strong and ac compressor clutch seems to be engaging fine. its driving me crazy. someone said it might be the engine but i have no oil lose so not sure if thats the problem
 
#13 ·
mechanic said he didnt think it was the thermostat but i wanted to remove it since i dont need a heater down here. so i am still thinking of pulling that.
That sounds like the old thermostat was never replaced. It could be stuck half open, not opening all the way, etc.

Most important, how do you know it's overheating? Temp gauge in the dash? Boiling over? Whats the temp gauge read?

When the engine gets hot does the a/c stop blowing cold?
 
#8 ·
no foam strips, yes the plastic shroud is in place. ive had the truck for a year and the ac worked fine for the first 6 months nothing changed when it started overheating that is visible. onlky reason i even changed the rad was cause the mechanic said that was the reason it was overheating but the new rad didnt fix it.
 
#12 ·
Wow, never heard that one before... the heater core sits in a plastic box with no airflow.
 
#16 ·
70/30 anti freeze is to strong and has a hard time giving up heat to radiator . always go 50/50 for best results ....when truck is over heating does the fan sound extremely loud ? if not you need a new clutch......i.e. airflow problem....IMO
 
#18 ·
I agree about an airflow issue. The condenser get pretty hot so if the air flow isn't there the low amount of hot air coming off the condenser isn't dropping the temp of the coolant in the radiator.
 
#20 ·
Did you ever figure out why your truck was getting hot ? I’m having same issues, replaced water pump, heater core, thermostat, took radiator off and cleaned, also cleaned a/c condenser, new fan clutch, I’ve let it sit and run for 2 hours with a/c OFF, never got over 195, turn a/c on and 240 within minutes. Turn back off 195. Before anyone says put gauge in it, well you don’t have to have a gauge when’s it’s boiling over, 212 plus.
 
#21 ·
OP hasn't been on this forum for almost 2 years, doubt he's getting on now.
 
#23 ·
1- was the a/c recharge done with gauges on both sides- or a single hose can fill?
2-was the block FLUSHED prior to refill with 50/50 mix ?
3- sitting in traffic(stopped),does your truck sound like a small plane at takeoff?
4- if the shroud is not sealed ALL the way around- you are just pulling air in NOT THRU the rad
5-position of a/c heat exchange- needs to be in the path of air flow at rad- not outside the shrouds footprint- or it doesn't get air cooling.
 
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