Ok we all know it's summer and here in central AZ, today is between 113-118. A/C on, condenser fan's running, while driving the needle is south of the half way point, at a red light (most of the time) same thing, no movement.

But once in a while while at idle, the needle will start to climb to mid and a hair over. I turn the A/C off and bam, instantly goes back to where it should be. Also when I take off again with A/C on of course with the increased air flow it'll cool down again too. Again, doesn't happen all the time, any ideas? Time for a new T-Stat, water pump? The fan clutch is working properly and no, I've not taken the rad out and cleaned the channel but the rad is 3yrs or so new along with the coolant res. Any ideas will help.

 

A new t-stat is $10, start there.
When it starts to heat up are the a/c fans still running?
If you roll your window down you can hear them.

 

Yep they're running, they run as soon as the A/C fires up. I'll start with the T-Stat and work from there.

 

If truck is OK while runing at 45 mph+, seems like water pump is not number one suspect. Likewise, the thermostat seems to be doing a good job, and it really doesn't know what rpm or mph the truck is doing. But it is the cheapest thing to replace. While fighting overheating I put in a Stewart/Robert Shaw 180 degree one, high flow version, certainly cooled things off, may have cost me 1 mpg. Got it from Summitt Racing, there are other vendors. But when you say this happens while stopped, perhaps the fan clutch is not as good as you think; perhaps the front of the clutch is not getting to 170 degrees or warmer because of blocked air flow (mud clogged fins or junk between radiator and condenser). The fan clutch has to get above a certain temp for it to start to come back to more coupling (more cooling). Of course, when you shut off the engine when hot, the fan should not free wheel, that would indicate low fluid inside it. You could also check the radiator for hot or cold spots, might be sorta new, but if it is in good health the temps will be with 10 degrees across it (infrared thermometer). It does not get that hot here on the coast, seems like you might want to try one of those DIY Saudi Grilles - I plan to the next time I find a good grille at the salvage yard.

 

Sounds like it could be time to have the radiator cleaned. An inexpensive infrared temperature gun will make quick work of it.

 

Yep, thanks to you, as well as Savanna and Spike, I need to start with the basics first. T-Stat is cheap, flushing the system is cheap, checked the F/C and "seems" to be fine but yah, starting with the basics is the best start. I do know after a pretty deep inspection the heads have already been done, fresh RTV by the coil as well as around the plenum and what looks to be a brand new gasket around each intake manifold so I need to go back to the basics. THX.

 

If you have over 100,000 miles, don't waste your time flushing your radiator, take it out and have it rodded out. Then once you reinstall it, verify it the viscus clutch is working properly, if you don't know how, send me your number and I'll walk you tru it.
In the mean time, quit driving it or you will toast that engine.

 

Thanks Mike, I'll let it get to running temp then check to see if there's any play in the fan clutch. I'll also have the tank rodded out then change the t-stat.