Just got my car back from a shop after the engine rebuild. On of the pistons (4th) had a huge hole in it and a couple of scratches on the cylinder walls. I have heard my engine detting before this happened but didn't taked it too serious.
After installing the Labda Link it show that the CO is aroud 5% (A/F ratio between 12 and 13) and I steel can hear the engine detting. Is this mixture considered to be lean?
The only mods to the engine are:
Apexi rear box and K&N panel filter.

P.S. Is it possible that the knock sensor is dead? It doesn't lower the boost or retard the timing....

Nick
MY99 UK

 

There is no ideal air/fuel ratio in practice.

You will see reference to the "stoichiometric" ratio which is the theoretical ideal ratio for complete combustion. This is 14.7:1 and is the ratio of air to fuel by WEIGHT (and why a car goes better when the air is damp/dense - more mixture in).

In practice the ideal fuel/air ratio varies hundreds of times per second depending on the engine's requirements at that time and is dependant on load, ambient and engine temperatures, petrol RON/MON rating, air density, cylinder head design/efficiency, ignition timing and a whole host of other factors.

Actual fuel/air ratios can be something like the following:
Start 1:3-4
Idle 1:6-10
Low speed running 1:10-13
Light load economy running 1:14.5-16
Heavy load performance running 1:12-14

Of course modern fuel injection systems work this out for you automatically (theoretically). The various sensors - and particularly the knock sensor as a last resort - should keep control of everything.

The Subaru system works by using a MAF (mass airflow sensor) which is relatively crude (or at least compared to some others such the most recent Bosch systems).

The 4th cylinder piston failure is well known and whilst I have not read the archives on this I would guess that it may be the complicated inlet manifold design of the boxer engine causes charge robbing/weak mixture on no4 cylinder (although I did read this starts as piston slap which likely a wear/oil starvation issue, but holed piston is ALWAYS det/weak mixture).

The MY99/00 engines are apparently particularly sensitive to oil contamination on the MAF sensor (revised design) and it is possible your K&N might have been over oiled causing the MAF to be contaminated thus causing the ECU to be fooled and weakening off the mixture and causing the problems.

You did not say whether the A/F ratio you mentioned is at idle whilst warm. If so, then the answer is probably yes to your question - it is too weak. CO I believe should be around 6-7% not 5%.

There are many others probably better qualified than me on this BBS to answer your question plus a read of the archives should help you. Also talk to someone like Pete Croney at Scoobysport or Possum Link guys (Bob Rawle or Scoobymania).

I would not run the car until the problem is diagnosed as unless the fault is fixed there is a good chance engine no2 will follow the same route as engine no1.....

[This message has been edited by Dave T-S (edited 30 September 2000).]

[This message has been edited by Dave T-S (edited 30 September 2000).]