Overheating problems

Okay, this problem has started to occur lately and prior to a faulty thermostat change out, had never occured before.

Okay, here's the setup:

1990 D-150 regular cab long bed
3 speed automatic transmission
5.2L V8 318 Engine W/ AC

About a month ago, I replaced my thermostat as I had belief the old one was getting stuck in the open position and was a result was pumping cold coolant through the system during the winter months. However, I didn't ask for advice on exactly what I should put RVT sealant on and had put it on both sides of the gasket and around the outer edge of the thermostat itself (The bit that sits in the flange on the water neck that comes out of the engine).

I have had no problems until on my way back from a camping trip the engine temp started to go up while we were going downhill. It would slowly climb then suddenly drop back down to normal temp, then climb a bit more again, then drop, then it would sit at a higher temp, then drop again.

It kept this up and I wound up pulling off the first time to let it cool down. At this time I had been running my A/C and it was around 97 degrees out. Once the water in the overflow tank had stopped bubbling, I turned off the A/C and started back up and headed back onto the road. The engine continued to run hot and eventually overheated as I was creeping slowly up a hill. Fortunately, I got it off to the side and shut down before the water temp actually went into the Red (H zone) on the gauge, so the engine made it out without any major harm.

At that point I wound up talking to my brother and was then informed that I had botched the theromstat install.

I was fortunate at that point to come across the help of a farmer out in the area who allowed me to limp into their driveway and then drove me into town to get parts. At this time, I replaced the 1 month old botched thermostat with a new one and gasket that had a lower open temp to make sure that it was opening and getting water through. It was properly installed with the sealant in the appropriate areas and amounts. The truck ran a little warmer than usual, but didn't overheat on the way home after that.

A couple days ago, I noticed that in the evenign with a headwind the truck's water temp started to go slowly up again when the truck was being run at a constant 60 mph on the freeway for about 35-40 minutes. I wager it probably would have continued to slowly rise, but I had reached my destination and the truck had time to cool before being run again to repeat the process.

My question to the forum is this.

What is failing? The water pump is around 6 months old, having been replaced after failure the prior winter. The thermostat is new and installed properly with a lower inlet temp (Stock is 195F replacement is 180F) . I've also replaced the radiator cap incase the system was having trouble keeping pressure.

My only guess is that somehow the water pump was damaged or the 15 old Radiator is dying.

I would like your insights as to what might be the culprit component before I buy a new radiator. The sooner I settle this, the sooner I can replaced the coolant in my system as its predominatly water as the original mix had boiled mostly away out of the overflow tank and all over the highway that day.

bump

The rad.

You might have a rad... leak..i use to on mine. when the water ran out the therm. needle would go up and when i pressed the gas more it would go back down.

I would think the rad is plugged up at that age. When the water pump starts to squeal and leak from a bad bearing, that is the time to replace it. A new water pump will not pump anymore than an old one. The impeller is steel and pressed onto the shaft so the only thing to fail is the bearing and seal. Are you sure the thermostat is properly installed, with the spring pointing into the engine??

Radiator. Even a 10% reduction in flow can make you overheat on a really hot day.

I bet if you look inside you'll see some buildup. Time for a replacement.

Thermostat was installed correctly both times, the difference was there was a bead of sealant partially jamming the spring on the first one I installed (too much rvt in the wrong places.) Did find a new radiator to replace it with, $160, so I am happy. Now, to get the last bits and flush out the system and start it fresh once more.

Thanks for all your posts. I have since installed a new 3 channel radiator by Ready Rad as the replacement for the original. The stock radiator was only a 2 channel and a POS at that.

http://files.automotiveforums.com/gallery/watermark.php?file=/503/242315old_radiator-med.jpg

Odd enough, my truck is only a 3 speed, but it had the tranny cooling lines first routed through the truck radiator then through the tranny cooler (stock) on the front before making their return. Didn't know older Dodges had tranny coolers stock. The things you learn when ya do things yourself.

Anyway, figured out why it wasnt' cooling. If ya look at the bottom half of the radiator in the pick you'll see the dirt there. When I removed the cover that was installed around the A/C grill and the radiator I discovered about 5 lbs of grass, leaves, and even I think a dead mouse stuck between the A/C grill and the radiator. I was honestly suprised the truck hadn't overheated before now. Makes me wonder what the hell the state of Oregon was using it for before I bought it at an auction.

Long story short, the truck now runs nice and cool regardless of the outside temp, doing freeways speeds with the A/C and even sitting stuck in traffic in the same weather with the A/C on.

Thanks all for your help!