Discovery Overheats W/ A/C on?
 

This thing is driving me crazy. This truck can run all day without the a/c on and NEVER overheat but if you turn on the a/c it will overheat in about 5 minutes no matter how fast or slow you are going. Water pump has been replaced right along with the fan clutch and belt. Engine was rebuilt 13k miles ago. I am not losing any antifreeze and not burning oil and not getting antifreeze in my oil. I first figured out that the a/c was causing it because engine got 3/4 up in temp and I turned off the a/c and temp went back down. Waited a little bit and turned a/c back on and temp rose back up and kept going so I shut the a/c off and the temp dropped back down after a little bit. There isn't any belt sqeeling but could my tensioner be to blame? I hate throwing parts at a truck trying to figure out what is wrong. HELP!!!

Does the electric fan in front of the AC condenser turn on when your AC comes on? Hose off the outside of your radiator and flush your coolant system. You sure the clutch fan is working?

Fan clutch is brand new and that fan really gets it BUT at highways speeds how could you tell? I know it only overheats when the a/c is on. We drove it 3000 miles a week ago without it overheating once. We took a little road trip. Didn't use the a/c once but this weekend it was a little damn hot so we turned the a/c on and it overheats. I pulled over and the elctic fan is kickin ass. I rev the engine and I hear the fan WHOOOOSH. I do not know if the engine is really getting hot or if it electrical. It is just pissin me off. I also replaced the temp gauge sender because it got busted while installing the engine. They used OEM Land Rover parts. I may go ahead and replace my tensioner and go from there. They are $100 from Advance Auto Parts. Couldn't hurt. This thing is driving me crazy!

I'm not sure your tensioner has anything to do with it why do you suspect that? Is your belt too loose and slipping? Call roverguy.com and get a cheap used one. I assume you have replaced the thermostat and that the viscous fan is installed the right way.

If your radiator is limping by then the stress put on the system by the AC could definitely be enough to push things over the edge. I would still flush out your coolant and even run some of the radiator cleaning product through it.

The tensioner doesn't have anything to do with it. Did they replace your thermostat when they did your engine? When did this start? You need to give us more history on this engine replacement. When did this happen? Was the ac causing overheating prior? Also how many miles, model and year? Radiators always rear thier heads this time of year. Sounds like you have a restricted radiator.

Members I implore you to put your year, model and mileage in your signature. It saves us asking the question and you having to type it in everytime. It is essential information!!

I was having the same problem, replaced the thermostat on mine and now no overheating. Seems I still still had the original thermostat on my 99 D1 with 125,000 miles. Mine seem to be stuck shut and wouldn't open even as the engine got hot. Glad I can turn my a/c back on! Hope this helps.

You need to verify if the electric fans are running. They should come on within minutes of turning on your a/s and or while driving around town on a warm day.

It is a 98 Land Rover Discovery. I have been trying to figure out what is wrong with this thing for a while. The thermostat has been replaced and coolant flushed many times. truck has 130k miles on it. Oil pump went down and caused motor to fail. It has always kinda had temp problems (every since I have had it) but never let it get to the red. I do not know if it is actually overheating. One thing I noticed this morning is it got over half way hot and I turned my hazard lights on and the needle moved with the lights. Every time the lghts flash the needle jumps up a little hotter. damn thing drivin me crazy.

Start the truck and turn on the a/c and then go stand in front of the truck, are the electric fans running?
Not only will you be able to hear them but you will see them as well.
This we need to know before we can help you.

Fans are coming on and running like crazy even after you kill the truck when its hot.