Another overheating question

[argonaut]

When I bought my Blazer I noticed, among other things, that the water pump was leaking out of the weep hole. So I promptly replaced it, flushed the motor and radiator, and put in a new thermostat. I did check the thermostat in a pot of boiling water before installing it.

I then noticed that the freeze plugs on the sides of the block were leaking. I'm not ready to pull the motor and trans just yet, so I replaced them with the rubber type for the interim. I did notice a ton if gunk and corrosion built up in the lower portion of the water jacket when i replaced the freeze plugs. All the more reason to pull the motor, clean it and rebuild it.

Anyway, back to the point.

Before replacing the freeze plugs the engine would run cool while driving at speed. Then when idling around town it would slowly creep up hotter and hotter.

Now that the system no longer leaks is runs cool while on the highway, but after a few minutes on the street or idling the temp gauge will max out. I'v eonly taken the truck on one trip and back so i've just shut the truck off once the peg gets too hot.

What gives, this doesn't make any sense!

 

[4X4HIGH]

Air pocket trapped in the cooling system somewhere.

 

[argonaut]

Yeah, so I went out to fiddle with the truck. Coolant was low, apparently I didn't get the engine hot enough the first time i refilled it to open up the thermostat. So i ran it for a while and filled it up.
Drove to in n' out, it got hot, but then the thermostat finally it temperature and coolant went flowing through the upper radiator hose. Then it stayed cool while idling at the drive through.

However it still seems as though the motor gets overly hot before the thermostat opens, and its only a 160 degree thermostat.

 

[4X4HIGH]

IMHO a 160* is too cold of a t-stat, you should be using a 180* as was original if it was a carbed engine (from your profile pic i'm guessing this is for your 1st gen) or a 195* if it is a FI engine.

 

[British Slave]

Ya gotta remember...when the thermostat reaches it's set temp and opens, it doesn't instantly open, and the water starts flowing slowly. The water in the block is still heating. The gauge sees hot water for a couple of minutes till the water is cycled out of the block and cooled and the temp of the water flowing past the gauge has been cooled.
Something else to consider. Most cooling system are pretty effecient in that a thermostat set at 180*(example) will start to open at 180* and will open slowly till it is fully open. Water behind the thermostat will continue to heat till flow is established. Once flow is established, the water temp will normally fall back below 180* and hold fairly stable below the set temp. The thermostat remains open, only closing if the temp falls about 10* to 15* below set temp at which point it will begin to close, raises the temp and begins opening again, and the cycle continues.

 

[6.2Blazer]

Generally the engine getting hot while idling or slow speed driving, but fine when driving at speed, is an indication the engine fan clutch is not working properly. While driving on the highway the vehicle speed is sufficient to provide air flow through the radiator so the fan should really only kick on when idling to provide air flow. The other issue could be an air pocket which is causing reduced flow, and the higher engine speeds while on the highway is compensating for this.

The thermostat does sound suspect to me. With a properly working t-stat I never see any huge spikes in temperature before it opens.

 

[argonaut]

I ended up putting about another 3/4 of a gallon of water into the system between last night and this morning. I guess because of the cold weather its just taking a long time to heat the coolant up, even to 160, so the air pockets aren't coming out until i'm driving around. Drove it to work this morning and the overshoot was way less than before.

I also know for a fact that there is a ton of gunk and crap in that coolant system so I'm sure it is restricting flow through the thermostat and the radiator. It probably won't be until summer that I have a chance to pull the motor out to thoroughly clean it. I'm actually amazed at the build up. When I pulled the freeze plugs there was a solid wall of gunk behind them in the water jacket! I broke most of it up as best I could but I was horrified to see that level of buildup and corrosion.

I also think that the coolant flowing through the heater core is preventing the coolant in the intake manifold from heating up as quickly as it should. My HVAC control levers are broken so I think I need to search out and close off the valve to the ehater core manually.
I think it is actually a vacuum diaphragm valve. Does anyone know whether applying vacuum opens or closes the valve?

 

[argonaut]

Water pump, fan clutch and thermostat are all new.

Another note is that the PO, bless his/her soul, cut out the bottom section of the fan shroud right below the fan. This makes it great for removing the shroud without having to remove the fan, but I'm sure it seriously kills the efficiency of the fan at idle.

 

[dhcomp]

Water pump, fan clutch and thermostat are all new.

Another note is that the PO, bless his/her soul, cut out the bottom section of the fan shroud right below the fan. This makes it great for removing the shroud without having to remove the fan, but I'm sure it seriously kills the efficiency of the fan at idle.

That is going to be a major issue at idle/low speeds. Find a fan shroud, or mod a newer style one to work, and see if that fixes your problem.

 

[riverrat25]

i had a monte carlo do thatwith a new rebuilt engine i tried everything
to keep it cool then i put duel electric fans on it and it never run hot
again.