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Discussion Starter · #1 ·
Hi All,

I purchased the truck about a month ago – 2004 sierra 2500 6.0, 213000kms/132000mi. It was a truck and trailer (24’ fifth wheel) package, I had issues with it running warmer than it should have the day I brought it home but I found the coolant was low, and thought that was the problem. It never ran too warm unless towing the trailer. I ended up flushing the cooling system and putting fresh dexcool in and new thermostat, and filling it properly.

The first test drive after the fresh coolant/thermostat showed that the cooling was better with a full system (duh!) but the temp still climbed to 110C/230F when climbing one of the biggest hills here in flat Saskatchewan, towing the trailer. I was happier that it was cooler, but didn’t think it was quite right yet.

We took it camping this weekend (towing the trailer), on the way out we had a 50kmh/30mph head wind. With the wind the truck was working significantly harder than I had worked it before, but the temp needle never budged from “a needle width left of center”. I thought it was odd that the temp was solid when it was working so hard, after the temp was rising with less load on the test drive after the coolant refresh.

Fast forward 2 days to the trip home – no headwind this time and the first 90 minutes of the drive went great – temp needle not budging from the same position. We stopped for ~3 minutes for construction and as we got back up to speed (only 80kmh/50mph since it was still a construction zone) I notice the temp gauge was creeping up – not to alarming levels, but not solid like it had been even though we were going slow and the engine wasn’t working all that hard. It acted like that for probably 20 minutes. Then it started to rapidly drop back to “just left of center”, slowly creep back up (te, climbing faster with more load on the engine) then rapidly drop back to “just left of center” again. It did that 4 or 5 times before settling at the “just left of center” position and not budging for the rest of the drive home.

Any ideas what could cause this inconsistent cooling? It seems that is has periods where it cools poorly, in between stretches where it cools properly. Could the new thermostat be sticking? I am fairly certain that the fan clutch does not engage fully when warm, but could that cause it to be inconsistent? I wouldn’t think so, but I’m all ears to hear experiences that would prove me wrong. I intend to replace the fan clutch soon regardless, but we have another trip coming up in a few weeks and I would prefer not to worry about something like this.

A few notes - front of rad was pretty clean (probably thanks to the chrome grill cover that was on the truck) and the coolant level has not dropped since I finished the flush and fill - I have been checking frequently.

thanks all,
 

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Discussion Starter · #2 ·
Anyone had similar symptoms caused by a faulty fan clutch? one that would sometimes engage properly and pull air, but then randomly stop engaging? The thermostat sticking seems a more likely scenario to me, but I was hoping not to drain the coolant again, its kind of a pain with no rad petcock.
 

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A couple things come to mind. Can you hook up a cheap scanner and monitor the actual coolant temperature? Maybe the gauge is a bit flaky. You should be easily able to hear the fan when it cuts it, if it’s cutting in. The rapidly dropping temperature may indicate a sticky thermostat, what brand of thermostat did you use? The radiator is also now the better part of 20 years old and my not have the same efficiency it one did. I have removed the chrome grill cover on mine when towing in extreme heat.

I forgot to ask what hill you were pulling? There aren’t many around here.


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Discussion Starter · #4 ·
Thanks randomledders.

I have a scanner and when i have been using it, it has shown exactly what the gauge is showing, so I'm thinking the gauge is okay.

The thermostat came in a "cooling depot" box, but the thermostat was stamped "Motorad". It is what auto parts plus gave me.

The hill that I have been referencing is the Lumsden hill on highway 11. I know it is really a small hill compared to what most other places have, but it is the biggest near me. The Fort Quappelle hill would be similar, I climber it on the camping trip this past weekend.
 

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Discussion Starter · #6 ·
Interesting Walkers, care to share why you think that?

I have a new thermostat and new fan clutch ordered, both GM parts. I will likely be replacing them both at the same time so if the issue is either of them I won't know which one it was.

Another note to share. I drove the truck on the highway for ~20 minutes yesterday evening, and noticed that the temp gauge never showed that fully up to temp, until I was back in the city streets, driving slower, stopping and starting etc. I haven't seen it struggle to reach operating temp before, thought it was worth sharing.
 
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