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6/1991 V2500 Suburban 6.2L 4L80e large aftermarket oil and trans. coolers, Banks Kit (worth every penny) 135K miles

First a preface... I've been an auto mechanic for over 32 years so I have a clue...

OEM fan clutch had leaked out all of the oil by the time I bought this 'Burb. I replaced it with a Murray HD clutch because it was available local and anything was better than the dead one I took off. Every day driving is fine, temp comes up a bit on long hills on asphalt. Where the problem shows it's ugly head is on Poor Mountain Road, Poor Mtn. Rd. goes up the side of Poor Mtn. (imagine that...), starts out around 1k ft. elevation and ends up at the transmitter site at about 3.9k ft. Best guess is about four miles of dirt road. The first time I did that climb this spring it was about 50°F and by the time I made the top the temp. gauge read about 210°F, which I thought was unusual, but not dangerous. I did that pull again about a week ago with the ambient temp. closer to 90°F and the gauge got up to about 235°F. The coolant wasn't quite boiling, but I could tell it was close. I let it sit and idle for half an hour with the hood up, the temp. was down to under 210°F within ten minutes, and after half an hour there wasn't even any pressure on the system. None of this sounds too terrible, but I was running with the 'Burb mostly empty. I intend to tow with this thing, and it'll be loaded down pretty heavy.

Any recommendations? I've read that the Delco fan clutches aren't what they used to be... I really don't want to convert to electric fans, but I will if I have to...
 

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Clean your cooling stack
 

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Clean your cooling stack
Huh? Brand new coolers, brand new radiator, fan clutch isn't engaging and not pulling enough air at low speed / high load conditions... thought I made that clear.
 

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Huh? Brand new coolers, brand new radiator, fan clutch isn't engaging and not pulling enough air at low speed / high load conditions... thought I made that clear.
You mentioned oil and trans cooler in your post but not the new rad. Try installing a hayden HD fan clutch
 

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The Murray clutch that I have >is< a Hayden, just reboxed for O'Reilly's... already checked into that.
 

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Try using a laser thermometer and take alternate readings to verify the gauge is correct
 

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New fan clutches aren't always good. I had a bad one that would stick on most of the time for a SBC of the same era. If you can't hear it, it's not on.
 
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What belt setup do you have? If it's serpentine maybe try a DMax fan

 
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What belt setup do you have? If it's serpentine maybe try a DMax fan

Pretty sure the 91 still used v-belts. I also recall that replacing the stock fan with anything larger in diameter required some pretty serious shroud surgery, as the stock clearance was already pretty tight, especially at the bottom for reasons unknown.. (worn mounts, maybe).

x2 on the Hayden severe duty fan clutch. Nice piece, solid quality, and reasonable price. No mistaking when that baby engages, you'll know it. Helped a ton on long climbs at altitude on my old '85 NA 6.2.

There's also the clutch coil mod which is in the stickies, iirc. I definitely found that to be helpful. Why wait til you're already in the danger zone to start ramping up the airflow?

This suggestion is only based on an semi-educated guess, and I realize that the 6.2 does not have the 6.5TD's oil squirters to help cool the piston skirts (so maybe this is less helpful on the 6.2), but I wonder if simply adding some oil capacity and a bigger oil cooler might not give you some added heat dissipation capacity? Maybe a remote dual filter setup with the long filters... kill two birds with one stone.
 

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Yaa 6.2’s and hills ... most times your pride has to take a hit by slowing down, possibly dropping a gear ...

what stats are you running? I’m a firm believer in geni hm or Robert Shaw’s only.
 

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Yaa 6.2’s and hills ... most times your pride has to take a hit by slowing down, possibly dropping a gear ...

what stats are you running? I’m a firm believer in geni hm or Robert Shaw’s only.
Genuine ACDelco thermostat, and it's >not< a thermostat problem. It's 100% an airflow issue. I warrantied the fan clutch, the new one is on, and we'll see next week as it's supposed to be near 100°F here next week... Not looking forward to that in a non-A/C vehicle.
 

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There is a rear water cross over mod you can do if this doesn’t do the job ... it’s only some heater hose, shut off valve and a few other assorted nipple fittings
 
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