305 Overheating

[Flashback]

86 C10, 305 V8. 89k miles, mostly stock.

Truck started overheating, symptoms were a bad water pump. It would overheat under a load and go back to normal temp stopped at an idle. So I replaced the thermostat, since the truck came with a new one in box. Old one was a 160 unit, new one is OEM 195 degree unit. Still overheats, so I replaced the water pump. Still overheats, same symptoms. Radiator looks fairly new, still shiny. My dad is telling me to get the radiator pressure checked. Coolant is still green. I haven't checked the oil yet for signs of coolant. I just need suggestions at this point, kinda lost. Is it possible for a bad radiator cap to cause this problem? Or am I looking at headgaskets here? The #5 plug seems to be wet when I pull it.

[Captkaos]

Did it just start?
I would have the radiator checked.  Are you loosing coolant?  Does the exhaust smell sweet?

[Irish_Alley]

a bad cap would just cause the antifreeze to boil at a lower temp. the more pressure the higher the temp before boiling. now if you take the cap off while its running dose it boil or just flow? if it looks like its boiling it could be a cracked head or gasket. coolent going bad would cause it to freeze at a higher temp.

[Flashback]

It starts and drives around, but it'll peg the temp gauge out once I start going up hills, etc.

It was losing coolant before but the water pump was leaking. I figured it was toast. Right now I can't find any other leaks. Exhaust smells like old truck. I will investigate more tomorrow after class.

With the cap off I can see it flowing, I don't see any boiling.

[VileZambonie]

Make sure the lower hose is not collapsing. Check the temp of the radiator. If there's a sudden drop in temp it's clogged.

[beastie_3]

How fast are you traveling when going up hill? What is your fan/shroud setup?

[Josh454]

What shape is your fan clutch in?  I wouldn't be surprised though if you have a leaking head gasket....

[Flashback]

OK so here is a few more clues to help you guys help me.

Fan clutch is good. It has resistance when I try to spin it by hand when engine is off. From what I've read It disengages above 25-30 MPH. This is when the truck starts to overheat. When I stop and idle for 15 - 30 seconds, it cools back down to 210. So the fan is doing its job. I think Vile is right (of course) and my Dad as well, that there is a clog in the radiator and/or the lower hose is collapsing on me.

Also I checked the oil, no smell or sight of coolant. Coolant is free of oil as well. So HG theory is kaput.

[Irish_Alley]

Just cause there is no coolant in the oil doesn’t mean the heads not cracked. A cracked head can cause gases and all into the coolant leading to the rad. if you do a pressure check on the rad you will see if that's it do one with it running also to see if the gases are slipping pass into the rad

[Flashback]

WTH the new t-stat I put in isn't opening, the upper hose is empty and cold, except near the t-stat, like its barely letting any water thru. Fairly certain I put it in right.

[Josh454]

You can test the T-stat by putting it in a pan of water and see what temperature it opens.  If it's opening it sounds like you're getting combustion gases in the cooling system. 

[Jim Rockford]

tstats are pretty crappy nowadays, I'd just take it back and get another one, remember pointy side up.

[Irish_Alley]

just remove the t stat and see what it does

[4x4HighSierra]

i was having a similar problem with my old pos 305.  I bought two t stats and everytime it over heated when i put one in, so i boiled one of the new ones on the stove and it was stuck shut.  I went through 3 t stats until i found one that was working properly.

[Jim Rockford]

just remove the t stat and see what it does

That doesn't work because then the water doesn't stay in the radiator long enough to cool off.