[urq] Motor bucks slightly (long)

Mike Sylvester mike at urq20v.com
Thu Nov 12 04:57:57 PST 2009

Interestingly enough, I have a similar problem with the RS2'ed 3B in my
For me, it almost always happens at light throttle.  Never at an idle
and very rarely (twice) under heavy throttle.  Only when the throttle is
just off the idle stop.

I also am achieving less boost than I should.  I believe the MTM RS2
software should allow 26psi, I get about 18-20 and it can be a choppy

I have never gotten any codes.

Every sensor on the engine is new except the IAT.

I've pressure tested to 10psi, but can't go any higher until I make a
different fixture to attach the hose from my compressor.  What I am
using pops off the MAF hose at about 15psi.

Under full throttle the air/fuel meter always shows rich, which you want
rather than lean, but it could show a boost leak.  I cant hear any boost
leak, but my exhaust is too loud to hear anything else.



> -------- Original Message --------
> Subject: Motor bucks slightly (long)
> From: "Brandon Rogers" <brogers at terrix.com>
> Date: Wed, November 11, 2009 1:41 pm
> To: <200q20v at audifans.com>
> Guys-
> 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
> much....
> 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
> leak. 
> 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
> trick.  
> Anybody else have this issue before???  Any other ideas?
> Brandon
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