Motor bucks slightly (long)

Wed Nov 11 12:43:10 PST 2009

$30 from Shokan.  :-)

Sent from my iPhone

On Nov 11, 2009, at 2:32 PM, "Brandon Rogers" <brogers at terrix.com>  
wrote:

> Oh Snap!!!!
>
> Duh - just recalled I did pull codes one more time with motor still  
> running
> (after last time I drove it when it bucked a lot when hot) - it did  
> say
> "sporadic RPM sensor".
>
> Can't believe I forgot I did that!!!!!
>
> Now to swap in a known good.
>
> Thank you!
>
> Brandon
> I CC'd the list for archives sake
>
> -----Original Message-----
> From: David Sexton [mailto:sextondm at gmail.com]
> Sent: Wednesday, November 11, 2009 12:04 PM
> To: Brandon Rogers
> Subject: Re: Motor bucks slightly (long)
>
>
> Brandon,
> I had a similar experience with my '91 200 20V in the last few  
> months.  The
> problem got worse until my car would buck and then stall, and not  
> want to
> restart.
> Mine turned out to be the RPM Sensor going bad, and finally giving  
> it up.
> But it was characterized by that "buck" like you described.  I  
> didn't have
> any codes until the car finally wouldn't start.
> I ended up replacing the RPM Sensor and the Reference Sensor just for
> reliability sake.
> Good luck,
>
> Dave Sexton
>
>
>
> On Wed, Nov 11, 2009 at 9:41 AM, Brandon Rogers <brogers at terrix.com>  
> wrote:
>
>
> 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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