Hey guys I just had an overheat experience.

I saw the needle climbing the temp gauge and I immediately pulled over. It didn't reach the red mark but it did rise. I was previously going speeds of about 35-45 mph. I was not low on coolant as I had just checked before I left. I had only been driving the truck for about 15 minutes and I was just about to take off from a stop light when I saw the needle rising.

When I looked under the hood there was water coming out of the overflow valve and coolant was boiling in the reservoir. The upper hoses also felt warmer than usual.

I turned the truck off and let it sit for about an hour and then I added a small amount of water and coolant. I was able to drive home normally and I revved the motor up in park to 2500 rpms and the needle on the temp gauge did not rise.

I had the thermostat replaced about a year and a half ago. I've heard that they only last about 2 years, does this sound like the problem?

As a side note I recently bypassed the heater core. However I experienced this same situation about a month ago before the core was bypassed, so I'm not sure that it's just air in the system.

 

Coolant boiling would be steam bubbles (antifreeze plus 15 PSI coolant cap yields a boiling point of about 265 F), so if coolant temp gauge was not up, could indicate a large steam pocket at the top of the intake, keeping the engine coolant sensor out of the hot water. Try to bleed your system, you should not have any water rushing sounds under the dash, because heater core is bypassed. So you did not know air was still trapped in there.

Could also be head gasket, if returns, you could buy a $50 chemical test for exhaust gas in coolant from parts store to be sure. When you bypassed the heater, is there a "loop" where coolant can still flow, just not thru the heater core? That would be good, as compared to "stubbed off".

 

you really need to undo the heater core bypass and route the cooling system as it came from the factory. At idle, the heater core is the only source of cooling, the thermostat closes and diverts almost all coolant through the heater core for cooling. The thermostat will open when the engine gets up to about 1000 rpm. That is why reving the engine at idle acutally moves more coolant through the radiator and you will get potential cooling. You can take the thermostat out and drop it in a heater pot and see if it opens at the proper temp. You can do a search and find the temps where it starts to open and when it is fully open, Rave also lists the data. Check the holes in the thermostat and make sure that they are not clogged. As mentioned, make sure that the cooling system is properly bleed of all air. Good luck, Phil

 

Well, IMHO the bypass thermostat is there to make the heater core get heat sooner for the passengers. From a cold start, it is correct that the core is all the cooling going on, A sample of the heated coolant comes down the top leg of the stat thru metering holes and mixes with cold return coolant from the radiator, trying to get the stat to 180F so it can start to open. It will fully open at 204 F. However, before it opens at all when cold, mom's taxi may be a high priority milk run, so we are not waiting in the driveway to warm up. We crank up, and move off smartly up the street. The water pump pressure increases with rpm. The spring loaded bypass plate on the top of the stat (which has the metering holes in it) is pushed down, which allows pressure to flow from the engine back thru the water pump intake. If this did not happen heater core would not take the increased pressure and leak. If you were watching coolant flow, it would go from a trickle (the flow thru metering holes) to a surge (the flow thru the bypass). The mechanics of a bypass thermostat are that as the main stat opens, it forces the bypass to re-seat, so there is only one main path.

The same style stats (without the honkin' container) can be found in some Detroit pickups, and other cars, and imports (even like an old Mercedes).

Until the thermostat section gets warmed up with hot coolant from the engine via sample holes or the top bypass plate moving, there won't be any appreciable flow thru the radiator, so not a lot of cooling, and what little is provided by the heater core. However, in cold start, the goal is to get the engine temp up to open the stat. Would be the same in a D1 where stat is mounted in the engine, it has to get hot to open main flow. Once open, it usually modulates slowly open or partial closed, like a variable speed drill, not a on/off thermostat at your home HVAC unit.

You do need that trickle flow thru the heater core, or the home brew bypass plumbing, or it wil take too long for coolant inside the stat remote chamber to warm up and open.

Here's pix of stat, note metering holes. Also, for other viewers, attached D2 cooling flow chart.

 

As mentioned, first hook up your heater then there are steps to take to determine the over heat fault. First, before replacing parts, do a coolant pressure test of both the engine and cap, if it fails might want to do the chemical block test, if that fails, then it is time for head gaskets.
If the pressure test and the coolant block test pass then either your t/stat, radiator is plugged, electric fan is not working or your viscous clutch is bad.
Check in that order or waste your time.