Land Rover Repair: 300tdi overheating, combustion gases, coolant temp

Question
Dear John

I have a 300 TDI that's over heating. I have pressure tested the system and no leaks indicated . I performed an internal leakage test with the colored chemical test kit to determine combustion gases leaking into the cooling system = test ok. I connected a pressure gage into the cooling system to see if there is an excessive pressure build up = ok. The radiator has been dismantled and cleaned internally & externally. I installed new thermostat.I went through every possible means Including near standing the vehicle on it's rear bumper to make sure that there was no air trapped in the system. New cooling fans ( elect) have been installed. with the engine idling for more than an hour = normal temperature. However when I road test the vehicle  after only two km the temp will shoot up to max.in seconds.The temp. between top & bottom rad varies constantly between 2   & 16 degrees. I fill the system by removing the filler plugs at the thermostat housing , rad and expansion tank. I have tried filling at each filler plug until the level is full at each. I removed the hose from the heater core & filled them.  I even blew into the expansion tank filler neck  until water spills out of the rad plug . close the rad plug and blow until water spills out of the thermostat plug & top up the level in the expansion tank to the half way level mark. Start the engine , run for 20 minutes and the water gurgles and spues out of the tank filler neck.  I opened the plug on the rad with engine running (just shortly after running and only warm coolant temp)  and the level has dropped substantially. the water pump was replaced several months ago - it's not leaking. Since the temp.varriation btwn top & bott rad ie. 14 -16 degrees  indicates water flow problem. however the temp diff will drop back to only 2-3 degrees indicating air flow problem. I suspect that this may be due to more of a water flow problem and that for some reason the coolant flow increases and thus drops the temp diff momentarily. My gut feeling is to change the pump, however I don't see any visible signs of a problem in this area ( bearings, seals & impeller) look good.   Since my tests indicate no problems with internal combustion leaks I am not convieced that there is a problem with the gasket or internal cracks  Any suggestions???

Mike  

Answer
It certainly sounds to me like a flow problem or a head gasket failure.  My guess is that there is a blockage, the pump impeller is cavitating, or there's a problem with something you have already done (i.e., what you thought was done is not really done)

I have seen head gasket failures that will not show up with that chemical test.  We have had better luck sniffing the air in the expansion tank with an exhaust analyser.  


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