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I have a 2013 Sierra 3500 6.6L that is leaking coolant but only after about 1 - 1 1/2 hrs of the engine running. I can’t see where it’s coming from but it drips down from the front passenger side bumper area. Thanks y’all.

I have an interesting overheating problem with my 1990 C3500 Silverado 7.4L. I recently bought the truck from an uncle who, I'm guessing, screwed me on the deal... We use the truck to tow our travel trailer (22ft. roughly 5000lb). Prior to our first voyage out, we drove the truck around town to make sure everything worked well. After a few trips to our main hike spot I noticed the truck would run warm, 220-240, for a short time and then spike down to 210. I assumed it was the thermostat sticking. So I replaced the thermostat with a 180 degree unit. I topped off the coolant in the reservoir and it seemed to have fixed the problem. We went on our first trip, now pulling the trailer, and at our halfway point, I got off the freeway to fill up the truck. about 20 seconds after we dropped our speed below 30 mph the truck spiked, redlining above 260. After starting it up, it quickly started to overheat again until we hit the freeway speeds and all was good. This behavior continued up until we hit our destination. Thankfully, my mechanic brother-in-law was camping with us. He thought it was the fan clutch but after looking at the radiator, I was about 2.5 gallons low on coolant. Since we were out camping, I used the rest of the gallon I had and filled the rest up with water. Brother-in-law said the water pump looked like it was running good and it might have just been under-filled when my uncle replaced the radiator. Ran ok until we slowed down off the freeway. It looked like the coolant would get hot and boiling out of the reservoir. I just kept topping off the radiator and driving home (picked up coolant 50/50 on the way back) Looked at it at home and noticed a leak above the drain valve. Tightened the valve about 1/4 turn, filled up the radiator and again the truck ran fine until our next trip. Same problems, had someone bring out a new radiator cap since it looked like the coolant was not being pulled back into the radiator. Coming back we had the same problem. I since replaced the hose from the radiator to the reservoir and "burped" the system by running the truck and squeezing the upper radiator hose until the coolant level would no longer drop in the radiator. The oil looks really clean (not milky or showing signs of water/bubbles). Coolant looks clean and I have topped of the radiator last with concentrate coolant to account for the water I added before. So I don't think it's the head gasket. I rotated the fan with the truck off and it stopped immediately after letting go. There wasn't any play when I tried moving the fan around so I don't think it's the fan/fan clutch. I hear an electric fan on when it's overheating/hot and the fan clutch kick in when its really hot. Rather than purchasing a bunch of new parts I thought I would get on here and ask a couple questions and see if anyone might have some things to try out. Would having enough air in the cooling system keep the coolant from being sucked back into the radiator? Could the fan clutch not be engaging at a low enough temperature? If the radiator is a 2 core, would that cause the coolant to boil enough to cause an air gap that would then keep coolant from being pulled back into the radiator? This truck was originally purchased new by my grandpa in 1991. He was a mechanic and took amazing care of the thing (all receipts). My Uncle purchased it from him a couple years back and I believe it was just sitting on his property for most of the time.

LML has coolant coming out of reservoir outlet on top and side of reservoir