Here's a little background...

Replaced the engine 2000 miles ago with a rebuilt engine because the old engine wiped the bottom end bearings. The new water pump in the new engine died and blew the head gasket on the passenger side. The head gasket was replaced as was the thermostat, radiator cap, radiator, water pump and fan clutch. The system is full of fresh / new coolant.

For the first week or two after the head gasket job, everything was fine but now the overheating problems are back. Sitting in the driveway with an infrared thermometer, I can see the thermostat housing climb to 225' and then drop to around 180 before slowly climbing back to 225 or so. When the stat is closed, the radiator is cold and the thermometer tells me that I have a good drop in temp across the radiator when the stat is open. 150 or so in the in tank and 82 degrees at the bottom of the out tank.

When I drive the truck, the temperature rises past the midway (208') mark on the gage and the farthest I've let it get is about 3/4 of the way up the gage. When I pulled over however, I noticed that the upper radiator hose is sucked in as if there's vacuum in the system. I squeezed the hose with my hand and it opened up and the temp dropped.

Just this morning, before taking it for this test drive, I burped the system and had it running for several minutes without any bubbles coming out before I put the cap on. When the cap is off, the temp stays low and the radiator never seems to get hot.

So I'm thinking of replacing the cap and thermostat again and the upper radiator hose, but I don't want to throw parts at it, I'd prefer to have some idea of what the problem is first.

If anyone can suggest what I can do to test / check or if you've seen the problem before and know how to fix it, I'd love to hear it. Problems with this truck have been going on since March and I'm about ready to trade it in if I can't just drive the thing without worrying about it overheating.

 

Im having the over heating problem but no vacum, it hink mine is head gaskets.

But as for parts the only new part i would buy is the thermostat, but then again yours is doing the same thing as mine(delay in the thermostat opening) and ive replaced 2 both do the same thing.

 

Check that the hose from the reservoir isn't clogged

 

Vent tube too

 

Try new radiator cap

 

I don't think those hoses should collapse like that. Meaning, you might want to change the hose for sure. I've heard but never experienced this, that if hoses are doing that, they have been weakened and need replaced. Just a thought.

 

Very very weak lol the hoses are meant to hold tons of pressure

 

The bottom one yeah, the top one not so much.

 

Ok, so I replaced the thermostat and the cap (again) and am still having problems. Now, the hose is no longer collapsing, but it's still trying to overheat.

In the driveway, the temp goes to 200' and will stay there at idle all day. When driving, the temp will climb and if it gets to 3/4 up the gage, I put it in neutral for a couple seconds and the temp starts to come back down. Almost like there's too much pressure behind the stat for it to open when the rpms are up. Once this happens, it will pretty much maintain a little above half of the gage unless I have to stop at a light, then when I accelerate from the light, the temp will climb again. On the highway, it will stay consistent above the middle, but not quite to the 3/4 mark.

This is the second Bosch water pump I've put in the truck. The first one I know was bad because the boss for the idler pulley was machined incorrectly and needed to be shimmed to line up with the belt plus the impeller was metal unlike the OEM pump that had a plastic impeller. The beginning of all of my cooling issues is when this pump failed and the impeller actually broke off of the shaft. I'm wondering if this one could also be bad and possibly the hole that the thermostat plugs when it's open is machined incorrectly and the stat isn't closing it. I never get good heat out of the heater and I flushed the heater core and have great flow thru it.

So, is there a way to test the water pump other than replacing it and hoping that fixes the problem? Does anyone have a suggestion on what else the problem could be? The upper and lower hoses are all that's left to replace in the cooling system and I'm getting really close to replacing those just for the hell of it. And another new water pump.

 

Can you run without the thermostat to see where your temps end up? Just more info to consider. I wonder something has changed about your new engine that a stock thermostat is getting jammed closed.

Now for the uncomfortable question. I my car I know it's possible to put the thermostat in upside down. Is there a chance it's in wrong?