My 320 '86 is destroying spark plugs..

[6inline]

Hi everyone,

I own a '86 320i (LPG) which cracked yesterday its third spark plug in 3 months and this time damages are big. Cracked piston and crack in the head's chambre (that's what I noticed for the moment at least. I haven't opened the bottom yet).
It has a '88 320 head on it - skimmed and machined in front of my eyes and new spark plugs. First cracked one was Bosch, then I decided to put NGK - last two cracked were NGK so I doubt it has anything to do whith spark plug quality hence the electrode ceramic coating is falling appart after 5k miles or so.
May be the LPG is producing more heat then petrol...? But then why it happens only now. It's running on LPG since 1995

Any comments or ideas on th issue would be appreciated

[BreadBin]

Does it do it when you are low on LPG? I suspect it is leaning out and melting the tips of the plugs. LPG is pressure fed so will do this when low.

[6inline]

I think there is gaz flow regulator to avoid this.
Actually I don't know when exactly the ceramic coating (the one in the chamber) is cracking
I can't figure out why it's doing this precisely these last three months

[Simon13]

something else has gone wrong, as it's obviously getting way too hot in the chambers

maybe this pressure regulator for the LPG is not functioning right and allowing enough LPG in, like said causing it to lean out too much

Each time the plugs have died have you been running on LPG?

[StuBeeDoo]

I'm new to the LPG game, but it definately sounds like it's running weak.
Is your system open loop or closed?
Stuart.

[Brianmoooore]

I've seen it recommended to run plugs one grade colder for LPG use.
LPG will ignite properly over a wider range of mixture ratios than petrol, so it's possible to run weak without knowing it.
This is why I've been recommending the AEB Leonardo/Millennium feedback controller for use with single point systems with its weak mixture warning option enabled.

[6inline]

Thanks for the feedback.
As I said it is difficult to say when plugs crack exactly as I only get aware of this either by misfire problems or the last time the engine stopped...i run mostly on gas though so this must be linked to the mixture ratio then

I've seen it recommended to run plugs one grade colder for LPG use.

Does one grade colder means lower number? Should I replace W 8 DC with W 7 DC or lower?

[Yaninnya]

Does one grade colder means lower number? Should I replace W 8 DC with W 7 DC or lower?

No, exactly oposite. W8DC you shoud replace with W9DC. But... Never use Bosch spark plugs. They are crap. Use Denso or at last NGK.
Jan

[6inline]

I bought another head and piston and will get new NGK plugs with '9' on them. Thanks

[Brianmoooore]

What sort of of LPG system do you have?
Beware the plug numbers - IIRC, one manufacturer goes the other way with the numbering system.
You want a plug that will dissipate heat one grade faster.

[6inline]

What sort of of LPG system do you have?

It is an open loop I found out.
As I went to a LPG conversion garage I saw an E32 '94 730IA V8 with gaz injection kit fitted....felt in love and bought it
I will abandon 6 in lines, for a while, this V8 sounds awsome

[Brianmoooore]

What sort of of LPG system do you have?

It is an open loop I found out.

This was probably the problem then - leaning out at high load.