[Biturbos4] diverter valves, vacuum lines, DTC ramblings

[Ti Kan]

Hi all,

In the past year my S4 was having sporadic check engine light issues,
and VAG-COM invariably shows:

17538 - Fuel Trim: Bank 2 (Mult): System too Lean
        P1130 - 35-10 - - - Intermittent
17536 - Fuel Trim: Bank 1 (Mult): System too Lean
        P1128 - 35-10 - - - Intermittent

Since the check engine light usually goes away on its own and there
are no obvious seat-of-the-pants anomalies, I had put off diagnosing
further for some time.  Previous web searches seem to indicate that
these DTCs alone may indicate either a MAF sensor problem, or a
vaccum leak.  Since my car is a 2001.5 M-box 6-speed with the
Hitachi MAF (supposedly less problematic than the earlier Bosch units),
I thought that vaccum leak was probably the culprit.

I also had a pair of new Bosch 710N diverter valves sitting on my
shelf for ages, so this past weekend I got motivated to install them.
I figured that the original DVs at 68K miles were probably on their
last legs.  However according to common wisdom bad DVs don't usually
cause DTCs.

Well, during installation I was amused to find that the small vacuum
hoses to the DVs were different on each side.  The right (US passenger)
side is a short, molded rubber hose with an S-shaped curve, whereas
the left side is a thinner, straight cloth-braided hose.  The latter
looked to be in bad shape -- the outside is frayed in several places
(especially at the hose clamps) and who knows if there are small
micro-cracks in there somewhere.  Very likely source of vacuum leak!

I replaced that hose with a section of slightly thicker rubber hose I
had handy, installed the 710N DVs and buttoned everything up.

So far, so good, no check engine light or DTCs.  The car seems to
run a tad better too, but it could be my imagination.

So, if you ever get a check engine light with _only_ the above DTCs,
check that hose.  You might have the same problem as I did.  If these
DTCs are recorded in conjunction with other faults, then it's probably
something else.

P.S. I also had my front tie-rod ends and front brake discs/pads
replaced a few months ago.  It's unbelievable how much difference
that made.  The car feels like new again -- nice, tight, rattle-free
front end.

-Ti