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Discussion Starter #1
I recently picked up a 94 NAS 90. It's seems to be running pretty good but is losing coolant with no obvious leaks in the system. At highway speeds, it runs a little hotter than I'd expect but nothing too alarming. At idle or slower speeds, everything seems to stay in check on temperature. I'm guessing that I'm either losing coolant through a bad head gasket or it's escaping the expansion tank via a bad cap when it gets hot. Any other common places or things to check? Thanks for any suggestions.
 

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Discussion Starter #3
Temps have been measured by a dash temp gauge that is a nice aftermarket graduated one vs the stock one. I’ve also used my IR scanner and confirm similar results.

The pressure test is up next and will likely be completed in the next few days. Two tests will be done. In a static sense, pressure will be applied to the cooling system up to 15psi with the engine not running to see if leaks appear. Second, with the car hot and running, we’ll measure the operating pressure of the cooling system to see if combustion pressure is entering the system through a leaking head gasket.

Full report coming soon.
 

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Discussion Starter #4
The running pressure test was completed today.

Truck ran approximately 10 minutes at 0psi indicating a closed thermostat. Pressure increased slowly as the thermostat opened. Max recorded cooling system pressure was 8psi. Pressure decreased with higher engine speed and increased at lower speeds to the max of 8 pounds.

Upon engine stop, the max cooling system pressure of 8psi was held by the system for over 20 minutes before it was manually released to remove the test rig.

Max temp recorded from the test was a quite warm 220 degrees F.

No leaks were observed.

Does anyone know the expected cooling system pressure under normal operating conditions? The stock cap is supposed to hold 15psi but I didn’t see anything close to that in my test. I also did not witness the cap venting excess pressure during the test.

Static pressure test is up next. This will test for system leaks at the max pressure of 15psi. I’ll also test the pressure of the cap.

Any other ideas?
 

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Discussion Starter #6
Agreed. 220 was our preset cutoff point for testing. The gauge read 220, IR was 215 at the thermostat. Ambient was mid 70s today. In short, yes, the truck is overheating. Clutch fan appears to be working well. Regarding clogs or water pump inadequacy, that's why I'm asking about normal cooling system pressure. Along with the other tests coming, a good system flush is in order as well.
 

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perform a cylinder pressure test, it will assist in diagnosing internal coolant leak.
 

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This Guy covers pretty much everything.


Hopefully it's something external like a rad, fan or trapped air and not a head gasket, slipped liner or brackets block.
 

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I know not the same truck but I chased an overheating situation in my 2004 Discovery. After checking, chasing and ultimately replacing various cooling system parts, it was ultimately my radiator that was the culprit. Since replacing, Im at a consistent 188.6 degrees. Hopefully your issue is a simple fix like this. Best of luck.
 

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I had similar issues, and when I pulled out the heater matrix, the insulation/cushioning had disintegrated and the matrix had banged around a bit causing tiny cracks. The coolant was leaking out but not dripping. The matrix is inexpensive and relatively simple to pull and replace.
 

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Discussion Starter #13
Thanks for the assistance. I think I’ve located the issue. My heater box is shot! The heater controls are stuck and the heater core isn’t flowing freely. Hoping to tear it all apart sometime this weekend.
 

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Unless the rad is in good condition it may be the culprit and can lead to clogging the next matrix as well. Definitely replace all gaskets and seals within the heater box. I made the rookie move of assuming the flaps would be fine and had the pleasure of doing things twice.
 
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