Motor bucks slightly (long)

jlagnese at jlagnese at
Wed Nov 11 11:47:24 PST 2009

My 20V does an occcasional buck at highway speeds. I
commute 144 miles daily. I have been unable to
diagnose either. The best was to change plugs one
time and problem disappeard for a few years. Then
the plug change didn't help.
---- Original message ----

  Date: Wed, 11 Nov 2009 11:41:47 -0700
  From: "Brandon Rogers" <brogers at>
  Subject: Motor bucks slightly (long)
  To: <200q20v at>
  >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
  >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
  >200q20v mailing list

More information about the 200q20v mailing list