Engine: TDI: C70 coupe exhaust valve burn out

[andygoldsmth]

Help please. I have a 2.0L 20v LPT turbo C70 coupe with B5204T engine and auto transmission. It is professionally converted to run on LPG (Liquid Propane Gas) - the system is multipoint injection. The car has done about 115,000 miles and has been very well looked after by previous owners. The LPG system was fitted about 50,000miles ago.

My problem is that on two recent occasions an exhaust valve has burnt out

The first was in No2 cylinder - had the head off checked all the other valves and all the seats carefully and no problems found. The valve was quite badly burnt. Replaced the defective valve with a Volvo spare part then lapped all exhaust valves before refitting head. Checked compression before firing up for first time it was bang on on all cylinders and the engine was running sweetly.

About 200 miles later (using 50% time on petrol and 50% on LPG) another valve failed, this time in No3 cylinder. Head off again, just the one valve burnt with less burn damage this time (because I was on to the missfire earlier).

I thought that Volvo 5 cylinder cars dont burn out valves? The spares man certianly expressed suprise that I needed to replace one.

Not really used the car since because I dont understand the problem and taking the head off is doing my back in! Have taken the car to a garage to have the engine management codes read and the mechanic thinks that it could be valve seat recession- he thinks that the valves might not be fully closing on the cam backstroke due to this recession. One of the reasons why he thinks this is happening is that the car is running rich at tick over, with the mixture control device working overtime in an effort to reduce the fuel, yet the system clears itself when the revs are increased and the mixture returns to normal. Otherwise the engine appears to be running fine and there are no standing fault codes other than that associated with high mixture at tick over.

He has asked me to get a second opinion on this but there are no independent Volvo specialists in my area and I dont want to take it to a main dealer just yet - I want to keep the lid on costs!

Has anyone got any ideas? Is there a way of measuring for valve seat recession? Do Volvo 5 cylinder enginers suffer from valve seat recession when run on LPG, particularly if fitted with a turbo? There is some evidence that LPG does cause valve burn out on some engines but all the LPG specialists I have spoken to have not experienced it on a Volvo.

Or am I missing the point and is it something other than the LPG?

Any thoughts on this would be appreciated