when the pump went where would all the steam in the bonnet escape from? as there wasn't any obvious 'holes' in pipes or cap off the water pump?(that might be a very obvious question lol) can I ask how he proves to me that the HG has/hasn't gone?
When the pump went steam would have probably escaped from any 'weak' points in the system - the engine has become a boiler and as there is minimal (although, before it's pointed out,
some) circulation through the radiator to cool the water back down again. A lot of the steam would have escaped through the radiator cap which has a safety vent to stop pipes or the radiator bursting (if you look under the radiator cap you'll see it's quite chunky, there is a poppet valve in there).
The main issue though is that the cylinder heads can not cope with extreme heat stress and like all metals hit a point where they will expand, itself not an issue when it is controlled but hot-spots and irregular cooling will mean the head will warp (not dissimilar to leaving a vinyl record near a radiator) this means that the headgasket no longer seals properly. The headgasket exists to keep combustion gas, coolant and oil all separated from each other - if it doesn't seal you could end up with any mixture you choose. Often if you've got oil finding it's merry way in to the cooling system you'll get the dreaded 'mayonnaise' buildup, but it's as likely you'll get exhaust gasses finding their way in to the cooling system, and coolant leaking in to the cylinders.
To check, well there are loads of ways, but the more 'acid' tests would be;
-Use a test kit to check the coolant for traces of exhaust gas (pretty conclusive, but the test kits are a consumable);
- Use an MOT gas 'sniffer' to sniff the coolant expansion tank and see if it registers exhaust gas in the expansion tank;
- Pressure test the cooling system without the plugs in, if the pressure creeps down something is probably leaking in to the cylinders;
- Examine the plugs to see if they've got traces of coolant on them;
- Steam in the exhaust gas on a hot engine means that some coolant is likely to be finding it's way in to the engine, obviously water isn't flamable but generates steam, when combined with exhaust gas this looks like 'white smoke' and tends to plume constantly;
- Just general "vibes" - which can be a pretty useful indicator too.
I'd take two positives of any of those tests as a certainty all is not well. The biggest factor is that the engine is known to have overheated, itself a very common cause of headgasket failure.
