I have a very weird issue that i cannot resolve.
My top hose blew whilst on the highway after getting a low coolant warning. I got recovered and arranged to replace the top hose.... whilst bleeding the system i also found a chafed expansion tank hose which I also replaced.

Now after following all the procedure for bleeding the engine it appears that all the air is removed. However once under load the engine overheats and starts knocking due to what I assume lack of cooling of the EOC..

I have run a radiator cleaner through it as I think the system is blocked, but this doesn't seem to have helped. I cannot get the bottom hoses off the radiator preventing me from removing the rad... but I have tried to blow water though the system through various hoses and it does seem blocked.

As the EOC doesn't seem to be getting cooled everything to me is pointing to an air lock but having bitten the bullet and put in the garage they have done a pressure and gas test and said that it is losing pressure internally in the engine so requires a full strip down and possible head gasket failure...

What I don't understand though, it there is no white smoke, no oil in water, no water in oil and performance seems fine until the engine gets too hot.....

HELP as I'm reluctant to accept its a blown engine etc.

Does it only overheat under load? 2006 4.2 SC

seems to be... I can have it idling outside the house after bleeding and it doesnt seem to get over hot... plus the gauge is bang on centre.

Had water pump impellers come adrift before causing similar issues. Would be fine until you drove vehicle. Would be checking that and also the thermostat before pulling the heads. 2006 4.2 SC

Many thanks Spud, I'll do that...

So, water pump fine and thermostat being changed...... will update once back together.....

Get a sniff test on the coolant done, should confirm whether gaskets are still intact or compromised

Just reread your op and it seems like the garage has already done that, sniff test, you don't want to accept it, soo try the test at another garage, then if you're not going to like to have the heads removed try some k seal repair ointment in there, nothing to lose at that stage.

Thanks Dinng1, I was thinking of using the sealant if they come back with the we were unable to fix and a potential for a massive bill.... always a last resort.....

Sorry Dinng1, their response was very vague about the sniff test.... didnt say it was the gaskets or not... just said they need to strip down and look at internals..... end of the day if its stripped down they might as well do the gaskets anyway as the labour charge i have to pay anyway..... just dont want to have a massive parts bill on top if i can help it.

How vague do you need to be on a sniff test? After all you're only testing for hydrocarbons in the coolant system. There either is or is not any. 2006 4.2 SC

I know, that's why I was nervous when they said it was head gasket, just as an excuse to charge the labour..... all i got from them was the pressure was leaking....

Find a different garage with a 4 gas analyser. 2006 4.2 SC

Thanks All, head gasket had indeed blown... but it didnt get me out of a hefty labour charge for rectification as they chose to remove the engine to do the work....... However, bitten the bullet and had it rectified.
Caused by the blown top pipe through age.