I have a '94 D1 which runs fine for half the day, then will start randomly overheating. 1st, there isn't enough water, so after i release tons of steam pressure, add water, remove bubbles, it is fine. So i am guessing the fan clutch is working. Only one electric fan turns on, are they supposed to turn on at the same time? Only one turns on even when engine overheats. Also, the PO installed a racing aluminum pressure tank. It is half the size of factory pressure tank, and the cap is rated at 13Psi. (he stated that the factory ones kept cracking from pressure?) Does tank size matter that much? Also what is the correct cap rating? I fill it up, and no leaks that i can "see" while cold or hot. I doubt it is head gasket because no water in oil, and no white smoke out back. I am getting a radiator shops pressure tester to test the system, until then, is there any common problems i should check? Thank you.

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You should be able to hear the Whirrr of the Fan Clutch kicking in when its hot, it should be pretty noisy..

Have you replaced the thermostat?
Burped the system while the heater is running on high?
Are you burping it from the Radiator or that Aluminum resevoir?
Are you getting any gurgleing sounds behind the dash?

 

Fan clutch comes on when it is cold, not sure about when hot, I'll double check. I burp from the pressure tank, not the radiator. And yes there is gurgle sounds in heater core, but not as bad as before i burped it. Would one electric fan not working be enough? Or is the little bit of air in heater core enough to steam the system? If I do burp from the radiator... ...is it recommended that I use teflon tape on the plug?

 

I burped from the radiator but did not use any teflon tape.. I was very carefull when replacing it to get it tight enough not to leak, but not tight enough to strip the plastic threads.

Run the heater on high and let the engine go through a few heat cycles.. Thermostats are supposed to fail in the open position, but there would be no harm in replaceing it as well. Listen for that fan clutch while your at it... Good luck, you could also check to make sure the radiator has no debris in the fins, wash it down with a hose.

 

Your expansion tank is fine, not sure on the correct cap psi though.
Yes both electric fans are supposed to run,no one not running wont make it do this.
Is your fan belt routed correctly?
Properly belled the cooling system, read my sticky on the top of the page.
http://www.roverparts.com/TechTips/s...g_defender.cfm

If the belt is routed wrong the water pump will spin to slow, replace your thermostat, if the radiator cap is to low then it will boil over easily...just thinking out loud here...clogged radiator can cause this as well as a bad water pump, if the clutch fan were bad it could overheat under the right conditions...

 

Clutch fan kicks on at idle when warm, belt is routed correctly. On the side of the road because it over heated again, I positioned it so expansion tank was a little higher than all other places. I then turned heater on high with fan on high and filled expansion tank with water after it was done bubbling. I then ran truck at about 2K for a minute and a half. Added another pint of water. On the drive home, 15 miles mixed fast and stop and go, the needle didn't move a bit. I was in the drive way, letting it idle, and i kicked on the a/c with the fan on 2, and the truck immediately started to over heat. I shut the truck off, and the a/c pully is sort of hard to turn by hand, is the a/c compressor maybe going out? (water pump is good(had it off for timing chain), and thermostat is 1 month old)

 

The a/c compressor being good ro bad wont make it overheat, your radiator is clogged and it cant remove heat fast enough with the a/c on to cool the engine.

The a/c condensor, the radiator infront of the engine radiator, works by removing heat from the air inside the a/c system and dumping it infront of your engine radiator, so with the heat from the engine and the added heat from the a/c condensor your radiator cant keep up.
Flush your radiator and see if that helps, otherwise a new radiator is in order.

 

would a blown head gasket make it over heat? Someone told me that it might just be a small leak in cylinder gasket somewhere, that causes it to over heat. I think it is the radiator like you said though. I'll pull the grill, get the other fan working, and pull the radiator, and get a new rad cap

 

A blown head gasket shouldnt make it over heat but it can make it "boil", all the exhaust gases in the coolant can bubble and make it sound like it is boiling over when in fact it is at normal operating temp.
There is a test kit that you can buy, it is like the Ph strips that you use to test the Ph balance in a swimming pool, stick the strip into your coolant and if it turns color "X" you have exhaust gasses in your coolant, if it turns color "Y" you dont.
I do not know where you buy them, but most mechanics have them.

 

Often we see one way head gasket leaks, pressure from combustion displaces enough coolant to cause an air pocket under the thermostat. This causes the thermostat to not open..A simple test remove the thermostat drill a 1/8 to 3/16 hole in the outer flandge and re-install. This will prevent a build up of air under the thermostat and allow it to open like normal....This wont fix the head gasket but will allow the combustion gases to pass through to the over flow tank.