300TDI overheating

[dero]

Having trouble with my 95 defender 300TDI.
It warms up & starts to overheat after aprox 15km at road speed .
Radiator was replaced aprox 20k ago, water pump aprox 10k ago .
Since having the recent trouble I have replaced the t'stat & bottom rad hose [ it had signs of ware from the top pulley , so replaced it to eliminate any doubt ] .
Cant see any sign of leaking anywhere .
When I remove the t;stat housing plug there is always air in it & needs topping up . I presume the overheating is because of the air, but don;t know why .
Any help would be much appreciated .

[justinc]

Does your heater get warm at all?.

If not or if is warm and goes cold after spirited driving and there is also excessive pressure in the expansion tank i would lean ever so slightly toward a leaking head gasket....

Also. Is it actuallygetting hot? The defender temp gauge can rise alarmingly due to poor earthing etc.
Jc

[AlecW]

There could be an airlock in the head and coolant needs to be bled to remove air pocket. However if the coolant has pressurized air coming out after running it for a while you may find it's a head gasket\warped head (exhaust gas pressurising the coolant system) from previous overheat.
Lets hope that isn't the case as expensive to fix. I have done 2 and same symptom.

Take to a mechanic and get them to do a pressure on coolant system 1st and see what comes back. If you have done a gasket there's a high chance the aluminium head has warped and needs to be skimmed, valves reground. This process of pulling apart, pressure testing head, skimming and valve re-seating (providing the head is still viable) and refittment is a costly exercise. Do not run it until is diagnosed as overheating a 300tdi is bad juju.

If you undo expansion tank is there air that rushes out after running it?

[dero]

heater has been bypassed for some time & if run to as hot as I dare, aprox 3/4 on the guage , the expansion tank boils over when the cap is cracked open .
Head gasket ?

[AlecW]

Head gasket mate. I feel for you, I really do....

[AlecW]

Looking at couple hours to get head off, then prob $200+ for pressure test/skim head. If head ok and can be skimmed then new headbolt set and correct tolerance gasket ie piston protrusion from head deck (Elring composite), manifold gasket and maybe rocker cover gasket then about 6hrs+ to refit and do valve clearances.

If need new head set then around $1300 for kit. I got maybe 8000km after skimming before mine went again so got a turner performance head from the UK.

It's about $2700 if you need a new head or around $1600 if can salvage existing going through mechanic in my exp. Obv this is a give or take thing but just to give you a idea for lack of surprise

[PAT303]

I'd replace the fan hub first. Pat

[TonyC]

heater has been bypassed for some time & if run to as hot as I dare, aprox 3/4 on the guage , the expansion tank boils over when the cap is cracked open .
Head gasket ?

The heater is part of the cooling circuit.
You can't just block the heater hoses, you must run a loop of hose from the outlet on the motor back to the inlet on the motor.

Tony

[AlecW]

I'm pretty sure that's what he means by "bypassed". Didn't say blocked. The fan hub would not account for expansion tank burbling after running it.

[PAT303]

It would if it was getting hot because the hub is U/S. Pat