Pressurising Vogue

Hi everyone I have a 2003 4.4ltr petrol vogue LPG with the BMW engine that is losing coolant and overheating. The car runs fine, no white smoke or steam when driving and the temperature gauge reads as normal until I slow down, say after coming off a motorway or dual carriageway. Then it shoots up to the red within a couple of hundred yards then drops down again. I've had it in to a mate who is a mechanic and a guy I trust. Its using around two and half litres of water every 90 or so miles. He's looked on this forum and discounted several things such as heater o ring seals, loose pipes and tubes, viscous fan something or other. He's had it up on the ramps and crawled all over it to find out where its leaking from but he can't locate a leak.He has also fitted a new expansion tank filler cap. He is concerned about the amount of pressure retained in the cooling system and thinks that it may be a head gasket problem. So what I'm after is second opinions for him and also a decent straight up garage that he can speak to before I put it in for what he tells me is a big job. They won't touch them in his garage as they say they have trouble getting them right first time or any time to be honest. Any help would be appreciated as I really love this car but miss the feeling of being able to drive somewhere without thinking that I'm going to arrive there on the back of an RAC van.

I think he may be right by the sounds of it

If theres no external leak, then it must be going somewhere internally, and to me that sounds like head gasket

coolant sniff test with exhaust gas analyser - hydrocarbons jump = hgf

Thanks for backing up his thoughts on that. He's a more than competent mechanic but he wanted to know what people who work on these cars a lot had to say. Can anybody recommend a genuine garage for this work that won't try and pull the wool on me? Preferably somewhere in the north west but I don't mind travelling for the best service. If I can get there that is.

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