305 TBI Overheating

[WA1DH]

I have a 95 GMC K1500 pickup with a 305 TBI engine. It had a heater core leak for a while but was otherwise running fine. I swapped the heater core recently and topped off the radiator and overflow bottle. I drove it the other day (about 20 miles) and it reached normal operating temp with no overheating issues. After driving I checked coolant levels and found oddly that the overflow hadn't changed and the radiator was still full to the cap.

Today I took it a few miles down the road and the temp gauge started bouncing (quickly) into the red then back down around normal. I thought it was an electrical/ground issue by the way it was bouncing, so I kept driving. It went up to red, stayed there, then came back to 2/3 (hot), when I finally realized it was really overheating. The gauge was being very erratic, which is not normal. Since it was a short trip I was pretty much back home by now, I just cranked the heat all the way up (it was blowing hot air) to make it a few blocks home.

I opened the hood and found the top radiator hose (between thermostat outlet and radiator) was hot and the coolant inside boiling. I felt the radiator cap, which was cool to the touch, and felt the lower hose (radiator to water pump) and it was cool. Overflow bottle level is still at the cool mark and isn't boiling. Fast forward a few hours later, I open the radiator cap and the coolant is full to the top like it was before. I started the engine with the cap off and after a few minutes the coolant started surging out of the radiator, and the gauge was back toward 2/3 (hot).

I'm guessing I have either an air pocket or the thermostat just up and died. I changed the thermostat and water pump not too long ago. Any thoughts or suggestions? If I run it with the cap off to burp it the coolant surges out then goes back down.

Thanks.

 

[Wampahoofus]

Definitely thermostat, or the system is otherwise blocked. If you had an air pocket coolant would still be circulating and all radiator hoses would be hot.

 

[oldhp]

I always drill a 1/8" hole in the thermostat so air can burp before it opens. Good bet engine needs burped or stat not opening fully. Other thing to check, is the radiator full of junk at the bottom of the tank and lower cores. With that old of truck you know there is scale/crud from engine block in there somewhere. Flushing radiator will not get it all out either.

 

[WA1DH]

I'm going to pull the thermostat over the weekend, test it in boiling water, and drill the hole in it. Now that I think of it, all was fine until I revved it up higher than usual and I heard a gurgling at higher RPM. I'm guessing air bubble.

 

[Warstud]

Originally Posted By: WA1DH
all was fine until I revved it up higher than usual


Yep sounds like air in the system. Look for a bleeder screw...if none. Try raising the front end up. Either by parking on a hill or jacking the front end up. And bleed it again like you did before.

 

[Garak]

It's possibly air, but those generation 305s have fairly simple cooling systems. I have never had difficulty with air in them in many thermostat changes. I, too, would check the thermostat again and ensure of no blockages, as was suggested.

 

[Chris142]

A stats have an air bleeder. No reason to drill a hole in it. Usually the piston will Have a small "v"on one side and that's the bleeder.

 

[nwjones18]

Be sure to get OEM or Stant Superstant.. Just got done redoing a tough thermostat job on a Dodge Ram because of a piece of junk Motorad...

 

[WA1DH]

Had some time to work on it today. Pulled the old thermostat, didn't notice anything unusual about it. Changed it for a Stant superstat which I tested in boiling water. I also drilled the 1/8" hole in the trim of it. Installed, idled with radiator cap off for a while. It seemed to be warming up very slowly. As it got warmer the coolant would randomly rise and overflow, then fall. After it started doing this more quickly, I topped it off and popped the cap back on. Let it sit for a while, temp gauge still reading 1/3 (below normal). Took it out for a quick ride, and suddenly the temp gauge gets all erratic and swings to high. I stop, turn around, and now it goes back to normal. Ride down the road a bit more and it seems to be reading normal. I stopped briefly, got out and felt the hoses, not boiling. As I took off again temp started to get erratic more, swinging high then normal before settling. Park in driveway, open hood and find upper hose starting to boil. Check gauge and its in red.

I shut it down and let it cool a few minutes. I noticed this time (unlike before) the overflow bottle level has risen. After a few minutes restart and I take it for another quick ride and it's running hot (2/3 gauge). I shut it down and check with an IR thermometer, upper hose boiling, therm housing reading ~215F. Heater hose reading ~160F, lower radiator hose is showing ~90F. I stuck the IR beam through the fan shroud and found the radiator fins were cool on the bottom, ranging from 85F and gradually rising toward the top of the radiator to ~185F.

Another thing I noticed is that I was able to move the fan blades by hand right after shutting it down when it was running hot. The fan was spinning before I shut it down but it always spins from the shaft friction. I'm wondering if maybe my fan clutch is bad?

Any other thoughts? I don't think it's my water pump because I have blazing hot heat.

 

[Warstud]

Originally Posted By: Warstud
Originally Posted By: WA1DH
all was fine until I revved it up higher than usual


Yep sounds like air in the system. Look for a bleeder screw...if none. Try raising the front end up. Either by parking on a hill or jacking the front end up. And bleed it again like you did before.


Under the circumstance I would assume you still have air in the system.