Have a 2003 Discovery II SE w/125k I have been trying to figure out a overheating problem that seems to occur when driving, but not idling ??

I have so far changed the water pump, all hoses, thermostat, bleeder T, Radiator, heat sensor. STILL OVERHEATS after driving about 20 miles

red light comes on thermostat, lift up hood and reservoir tank hissing (have replaced cap).

only thing I haven't changed is reservoir tank. Can that actually be causing this weird anomaly? If not, what could it be?

Thoughts?

Robert

 

Is it bled properly? Are your fans working (both electric and viscous)? Hoses routed correctly? Maybe you bought a bad part (new and broken is possible).

 

Maybe headgasket

 

Don't drive until resolved. There is a $50 or so chemical test you can buy at parts store that changes color if products of combustion are in coolant (HG). Waiting for temp gauge to rise is not a good plan when testing, the ECU is programmed to keep gauge at 50% until it is way too hot. A good idea is to use a scanner or ultra gauge to watch the temp. If it keeps climbing past 212, 215 - no point going to 235 - 284.

One thing you should check after all this work is the belt route for the serpantine belt. A 180F soft spring thermostat may help a little, but your description sounds more like a cracked block or HG leak.

 

125K - you are right on schedule for a head gasket.
When going fast - you are getting air pressure into the system from the combustion gasses.

I am betting dollars to donuts you have the water fall sound when taking off from red lights briskly..

Yes?

 

Wait..you did all that stuff and didn't suspect a head gasket failure? It happens on 95% of Disco IIs.....

 

Stop driving it, everytime your read light comes on you are closer to making a boat anchor out of the engine.
How mechanical are you?
Have you verified that the electric fan is working, have you tested the viscus clutch?
At this point you are looking at head gaskets for sure, if not a dropped sleeve for allowing the engine to go thru multiple red light warnings.