overheating with strange patterns

[outkast]

Hey guys, its been a while, but I'm having a problem with my 00 expy

125k
5.4
dual heat a/c
4x4

so the truck is overheating and I cant figure out why. It seems as if it will idle all day and its fine...I can cruise thru town at 30-40mph and its fine...but if I am on the hwy, then slow down (interchange or come into a town) it will get hot, when I accelerate back to hwy speeds, or if I sit in traffic for 30 mins, its fine until I accelerate back to speed. The gauge will go about 3/4 of the way to H, but no further. Shooting the t-stat housing with a gun shows 220-230 at the housing, 180 at the upper hose, and 160 ish at the resivoir.

I have flushed and filled the system, replaced the t-stat, and replaced the fan clutch....none have really worked. Also the truck only does this when ambient temps are above 83-85 degrees, so, heading to work in the AM when its 75 out, the truck is fine, but on the way home in the eve...it overheats. trip to work is aprox 30 miles.

any ideas? I'm stumped.

 

[Stoned06]

Have you verified the fan comes on when at operating temp? Sounds like the fan isn't working when stopping and it overheats. Might want to check the fan switch or relay.

 

[outkast]

its a direct drive fan, not electric, and yes 100% sure it works. plus I can idle all day without overheating, it gets hot when you take off to hwy speeds. I can drive thru town for an hour with no issues.

Also when it overheats, turning on both heaters and idling the truck around 2 grand will cool it back down within 30 seconds to a minute.

 

[ELVATO]

That's weird. There should be more air going through the radiator when at speed than what the fan can pull.

Here are the causes that the service manual lists for overheating:
-*Water thermostat.
-Water pump.
-*Internal engine coolant leak.
-Radiator.
-Heater core.
-*Cooling fan.
-Pressure relief cap.

The ones with * are the ones you already replaced. I'm guessing you would have noticed a coolant leak, yes? Maybe your radiator cap is bad? I figured they're pretty cheap.

If you peer inside your radiator, is the fluid clear (aside from the color of the coolant.) When's the last time you flushed your coolant?

 

[outkast]

I flushed it two weeks ago with two bottles of peak radiator flush and water...ran it for 4 days like that then replaced with 50/50.

before the flush the coolant was very rusty...I cant see into the radiator since the resivoir is all that is there to look at.

 

[ELVATO]

You put the same color coolant that was originally in there, yes?

 

[outkast]

yes, green.

Im thinking its either the cap, or a half clogged radiator....this is driving me nuts.

its been in the 90's here...it hasnt overheated in 4 days now....

 

[JUST4FUN]

here is my guess blokcage between condensor and radator, next weak coolant flow
bad tensioner

 

[outkast]

no blockage, but you may be on to somthing with weak coolant flow....care to elaborate?

by tensioner, I assume you mean a slipping belt, and I dont beleive that to be an issue.

 

[JUST4FUN]

the old coolant was rusty so, mabey it was acidtc and ate some of the water pump impeller finns . im just throwing this out there

 

[outkast]

water pump changed and a 180 thermostat installed.....still overheats?

WTF

 

[ELVATO]

You changed the cap?

When you start the Expy up, wait a couple of minutes. After 3-4 minutes, feel the inlet and outlet heat hoses (for the heater core). They should both be about the same temp.

On a related note, does your heat work?