[ike84]

Ike -

You are definitely right about that. Sorry that I digressed from your original question earlier. I have very little experience with older style carbeurated motors but from a basic principle standpoint, to have a single bank lean condition you are either going to have to have unmetered air in that banks intake or lack of fuel isolated to that bank. In this respect the M96 is somewhat unique. These motors just don't have the problems of blown intake seals or head gaskets (thank god). But they do have a screwy ass overly complicated vac layout (i said to STL recently that biggest beef with Porsche is that while I see beauty in simplicity, Porsche sees beauty in complexity lol)!

When the 1128 codes have come up in the past as isolated codes I think the majority of cases were tracked back to the vac-assisted components I was describing earlier. I think that is purely due to design - those components are normally routed back to the plenum boot, which is direct line of flow to the intake valves, so bank 1's pull is going to be largely diverted through those components. Things like hairline cracks in the vac reservoir or leaky check valves are nightmares - being inaccessible while the motor is in running condition they would be a nightmare to diagnose and fix.

I do think though that the overwhelming number of lean conditions is going to be due to unmetered air and not a lack of fuel. This is for 3 reasons - 1, the fuel pumps are more than up to the demand of our cars (they are the same for a 2.7 and 3.2, and although it is a different model number the t fuel delivery volume is the same for the 996 3.4) so even if they are running on the low side of pressure there is still more than enough oomph. 2nd, the dme can adjust a significant amount without throwing codes (I dont know the exact limits but running a 996 tune on a 2.7 motor my LTFTs are at -17% with no codes) so you would need a massive loss of fuel to cause this imbalance. third, the rails are in series, so you would never have low pressure on bank 1 and good pressure on bank 2. also, as a side note, every time I have read about a faulty injector (mine included) they have always been stuck open, not shut. an injector that won't open may be the exception here and create enough of an imbalance as to create a single bank lean but I haven't heard of this happening before.


so, with all this being said, here is the daignostic alogrithm i would likely recommend for this situation.

1 - vac test.

if vac test is abnormal, find the leak.

if vac test is normal, move on.

2 - fuel pressure test

if fuel pressure is normal, move on.

if fuel pressure is low, test the pump output and act accordingly.

3 - sensor replacements

this one is a bit tricky. theres no way that I know of to test/verify that the MAF and O2 sensors are working properly (except for voltage readouts which gets complicated).

but, pre and post cat sensors can be swapped. if its a bad sensor then the codes should change. if its only a single bank, sensors can be swapped from one bank to the other. Maf can be cleaned.

4 - if all else fails, investigate injectors

what do you think?

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