Motor bucks slightly (long)

Brandon Rogers brogers at
Wed Nov 11 10:41:47 PST 2009

Experiencing a weird motor issue that has gotten worse in the last week.
Really got me scratching my head.
Over the last year or two my engine has bucked a little (on rare occasions)
when starting off.  This is when when the engine is cold (usually but NOT
always) and fully in gear and already moving a few (maybe 5 tops?) mph - not
during clutch engagement - but right after.  A few days ago I was going to
drive the car to work - backed out of the garage and the motor stalled with
car in neutral.  Re-started fine - but decided to take the S4 to work
instead.  So that night I went out to the garage - no codes - started fine -
backed out - no stall.  But it did buck a couple minutes later - in the
beginning when I took it for a spin.  It was a bigger buck compared to
"normal".  These aren't bang your head on the steering wheel bucks but
something is definitely going on.  Pulled codes again right away - nothing.
Drove around for 10 minutes - no more bucking.  Next day I decided to give
it another go.  Pulled codes again JIC - nothing.  Car started fine and all
that - but it did buck once while cold when I pulled onto the street.  Then
no more issues on the way in to work.  Continued to buck a little (say once)
within the first few minutes each tine I drove it.
Something is going on with my engine - here are my observations:
1.  Bucks once or twice when cold at low rpm in 1st - goes away when warm -
more often in the last few days.  It _has_ done it more when warm in the
last day or two.
2.  Very rich exhaust smell at idle - still
3.  Out of spec O2 Adaptation Values (I did put a new O2 sensor in - however
that is not a solution to out of spec adaptation per Bentley)
4.  Failed NOX emissions test - high NOX signals high exhaust temps (lean
run?) (later passed - just just barely)
As a side point to the data - I only get about 17 mpg - but I do likey the
boost (this motor is in an ur-quattro)  so I don't consider that too
So a few people suggested I pressure the system sounded like an air leak -
and I do agree.  So I did (again - had done it a few times).
So over the weekend I pressure tested the system - up to about 15 psi.
There was a slight leak in the rubber connector hose from the metal
crankcase breather (above the EM) to that black plastic thingie (TM) behind
the VC.
Some air coming out the oil fill cap.
So then I blocked the crankcase breather off.  No boost leaks in the intake
tract.  I used a stethoscope (hose) at all the junctions - can't miss an air
But I kept looking around - decided to pull off the small vacuum lines that
come off the back of the IM and use the mighty vac.  The first one held no
vacuum at all.  Well it turned out a hose had come off - but it was _AFTER_
a check valve - so no boost leak but was allowing incoming unmetered air
under vacuum conditions - which really seemed like the type of problem I was
experiencing.  Fixed it.  Other two were tight - held vacuum.
Unfortunately it did _not_ affect how the car ran. :-(((  Still had a couple
minor bucks when I drove the car in - then quite a few on the way home when
the engine was hot. Big disappointment.
I will replace the crankcase breather section and see if that does the
Anybody else have this issue before???  Any other ideas?

More information about the 200q20v mailing list