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Discussion Starter #1
hey guys, I am new to forums so I will try to give as much info as I can.

I have a 2008 silverado 1500, 5.3l, 4wd. Has an 8inch fabtech lift. 35inch toyos, edge evolution programmer. Throttle body spacer

So my problem is, if it is above 85degrees outside and driving on the highway my truck will overheat. It does fine thru town, its only on the highway.
I have replaced, waterpump twice. First time because it was seeping out the weep hole and second time to rule out bad pump just cause.
I have replaced the thermostat, even boiled it to ensure it was opening. Upper an lower radiator hoses, and the radiator. When the radiator was out, i cleaned out the ac condenser with air, there is no bent fins or bugs any longer.
I did a compression test, cylinder leak down an checked coolant for exhaust gas.
Passed all 3.
I have tested my fans, both electric fans work high and low.

Any suggestions are appreciated
 

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2000 Silverado Z71 4x4 5.3L 460k+ miles w/ GM rebuilt motor and trans
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When you tested the fans, did you engage them with the thermostat or manually jumper them?

I've only ever had a truck do better on the highway and overheat in town, so this is a new one...
 

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Normally at highway speeds the air rushing through the rad will keep it cool; however, you said only when air temp is above 85, so that might be too hot to adequately cool.

With Jacob's suggestion in mind, i might consider a temporary switch, wired into the cab, that I could use to manually turn on the fans.
If the temp climbs, turn on the switch, if the temp falls, you know it's a fan problem.
 

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If the temp climbs, turn on the switch, if the temp falls, you know it's a fan problem.
^^^X2 - Try that, I'd almost bet the PCM has a setting where it turns the fans off when above a certain speed like 45MPH. If so, then the fans are just blocking air flow.
 

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Discussion Starter #5
I engaged the fans by removing the relay and using a jumper wire.
I would think a truck should be fine in 85°F temperatures?
Also when the truck is acting up, I will pull over on the highway and the fans are on. Sometimes low speed and sometimes they are on high.

I will wire them to a switch and give it a try.
Thanks guys
 

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Well if you tried the jumper at the relay, that would be the same as the switch.
I assume you jumpered the high speed relay socket.
And left the jumper in place while you did a test drive
 

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Discussion Starter #7
I did not leave it in and drive, but that would be a lot less work.
I think i will go that route.
Thanks
 

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Is it actually hot, or is it only the gauge showing high temp?

The fans haven't been previously messed with and are wired backwards now, are they? If they turn the wrong way, the air movement through the radiator would stop at a certain speed range.

It might be running real lean at highway speed. That could drive up the engine temp.
 

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Discussion Starter #9
I have also replaced the temperature sensor, the stock gauce and my programmer both show its hot.
Fans appear to be factory wired and are sucking air thru radiator.

It does with any tune, and with the AFM activated or turned off
 
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