Fluctuating Coolant Temp

[import power on]

Did all the searches on this site along with google searches.

I have a 93 Navajo 4x4 4.0 V6
I have been driving it with two blown head gaskets for almost a year Having to replace the spark plugs every month or two from getting rusted out. It woudl still run great just start to miss from rusting out the plugs. Always kept it topped off with coolant/water and oil which is started to eat at the rate of one quart a week...leaking into the cooling system thru the heads.
All this time the coolant temp would go from halfway to cold and back up constantly. Figured it was the blown heads since this is common with blown heads.

I just had it all fixed. BOTH heads were cracked. It ran to the H on temp gauge more than once so I figured they would be bad. The gaskets were so eaten away the the water pasages in the Intake manifold were corroded and enlarged past the gasket so I also replaced the Manifold. New water pump, new thermostat new radiator cap. ALL new gaskets top to bottom.

Well, oil stays toped off never uses any now. Coolant is still fully topped off and pretty green. BUT the coolant temp still goes from halfway to COLD. Up, down, up, down. Constantly. And this is NOT the temp gauge. You can feel the heater output going from hot to cold.

System has been burped. Went thru that 10 times just to make sure. She is topped off. Up to operating temp let cool then check Radiator level.

HELP!

 

[Alec]

So you have totally new heads? My guess is that either the new thermostat is bad, or you did more damage that wasn't fixed.

 

[import power on]

Yep, totaly new heads they came with all new valves and everything. New thermostat. I guess it could be bad....Man why do they make the thermostat so hard to remove...

 

[SWLathrop]

Does it have the stock radiator in it? If not, and the radiator it too large, this will cause the cold/warm/cold/warm cycling. What temp stat are you running?

 

[Paul Gagnon]

Sounds like your engine is not full of coolant. Did you fill the block with coolant before puting the thermostat in? This is a must, simply filling the radiator will not fill the block and heads because the thermostat will be closed and prevent the flow of coolant.

 

[import power on]

Running stock temp I think it is a 190

Not the stock Radiator. Just using a stock replacement cap. Tried three now.

How do you fill the block if not thru the radiator?

Thanks!

 

[TPLYNCH]

How old is your radiator cap??? Did you install the thermostat with the weep hole in the 12 Oclock position?

 

[import power on]

New thermostat and cap was installed with new heads. Hole at 12 oclock. That is what is really pissing me off. Everything that normally causes this has been replaced.

 

[Kamaaina]

Just a long shot........do you have a clutch-type cooling fan? That is temperature controlled also and will cause fluctuations as it engages and dis-engages. Sounds like you changed everything possible.

 

[import power on]

Nope, Fan is turning whenever motor turns. No clutch.

 

[SWLathrop]

Well, if your radiator is too big (yes, it can be too big), you'll get such a temperature fluctuation. This is caused by the coolant staying in the radiator too long and cooling down more than it would under a "Normal" coolant flow rate.

 

[Paul Gagnon]

Originally posted by import power on
How do you fill the block if not thru the radiator?

You have to add coolant to the block and heads before installing the thermostat or afterwards through the cold(return) heater hose.

 

[import power on]

Originally posted by Paul Gagnon
You have to add coolant to the block and heads before installing the thermostat or afterwards through the cold(return) heater hose.

Meaning disconnect the hose (upper) from the radiator and filling there so that is flows into the block? And this can be done after the thermostat is in?

Radiator is not stock but the coolant temp used to stay rock steady back before the heads blew.

Thanks for all the ideas so far. I will try to fill the block coolant level. It does act like an air bubble.

 

[Paul Gagnon]

Not the upper radiator hose, the heater hose. If you don't want to remove the thermostat this is the only way to get water into the black. Other than that you will have to remove the thermostat to fill the block.

 

[rpenner54]

Paul can't you start the engine with the radiator cap open and watch the level go down as it gets hot and add as needed? This is what I have always done. I didn't realize that maybe I was doing it wrong???

 

[Paul Gagnon]

That's fine if you are changing the coolant because part of the process is to flush the cooling system. This leaves water in the block. If you completely drain the block and heads of water you need to get water back into them before installing the thermostat. If you don't do this the thermostat may never open and the reading on the temperature gauge is worthless.

 

[Paul Gagnon]

Okay, imagine it this way.

Take a straw (your engine block) and put your finger(the thermostat) over one end. Now submerge the other end into a glass of water(your radiator). If you keep your finger over the end of the straw no water will enter the straw no matter how much water is in the glass or even if you put it into a barrel full of water.

Remember that your cooling system is a closed and pressurized system and that thermostats only work when they are submerged.

 

[import power on]

VERY GOOD INFO!!! I will pull the Heater host tomorrow and attempt to fill the block...

I should do this with a cold engine correct? Engine Not running correct?

 

[rpenner54]

Wow I never knew that! VERY VERY good info Paul.

I need to change the T-Stat out on my F150 this weekend...