Lean?

**myne** · 12-02-2008, 11:17 PM

Just pulled a head off a 20yo 12v 626 and to my astonishment, the head is in excellent condition internally.

Absolutely zero corrosion whatsoever. looks flat at a glance and generally looks good.

However, it didnt take long to find out why the engine had shat itself one fine day on the freeway.

Around 3 of the spark plugs, the aluminium, which elsewhere is smooth and relatively clean, is almost grainy and eroded. It looks somewhat like sand grains and gives the area the appearance of being a poor casting. It is definitely not a casting defect though.

Around the exhaust valve seat of cylinder 1 the same thing, only the grainyness has surprisingly extended AROUND the steel valve seat and through to the exhaust port.

Yes, there is a hole in the roof of the conbustion chamber directly to the exhaust port.
Otherwise, all chambers look fairly normal.

The only thing I can think of that would cause this is long term errosion from running lean.

But I'm puzzled why it is so specific.
Wouldnt the damage be less specific? Shouldnt the whole head look like it was made from sand?

And I guess the last question, has anyone had a head like this successfully repaired?

Poor pic, but this was the best of 6 :/

http://www.users.on.net/~jvizard/myne/12022008125.jpg

**abently** · 13-02-2008, 04:09 PM

You really need better pics of what your describing........

You can tune the Idle mixture on these vehicles so I'ld hazard a guess its slightly too lean or been run on standard unleaded sometime in its life assuming that's its an early leaded model.

**oldcorollas** · 13-02-2008, 04:30 PM

you might have just been unlucky with a local casting defect.. a bit of core shifting off or whatever., or some part of the melt with high impurity..
also it may have started as a small crack, and then been burnt thru over time to give that shape (as happens with subaru heads)..

do they have a habit of cracking? if you ring around head workshops/wreckers, do they have a reputation for doing heads?

(you know.. abently might be right for once... now that leaded is no longer available... the engine might have been run on unleaded at some time in it's life!!!

)

**swan86** · 13-02-2008, 05:50 PM

Question, What spark plugs were you running? If they were the wrong heat range ie too hot, it can cause blistering on the insulator, too high plug temps can also cause pre-ignition to occur. As HT leads wear out at different, times this can effect different plugs and thus one cylinder may be effected.
Any chance of posting a pic of the plugs?

**wa5** · 13-02-2008, 10:15 PM

pretty sure that 12 valve engines came after 86 (introduction of ULP) ... this thing has probably got EFI (MYNE?). is there a chance that it has had an air leak somewhere in the inlet manifold..