Motor bucks slightly (long)

Bernie Benz b.benz at
Fri Nov 20 13:50:20 PST 2009

Could well be plugs, or just one plug. Those F5DPOR plugs last for  
years, about 10 or so.
Years ago I had a random miss, replaced one plug at a time until I  
found the bad one. An open connection between the center conductor  
and the platinum tip. The rest have >100K on them.


On Nov 20, 2009, at 11:53 AM, Brandon Rogers wrote:

> Well here's my update - took me a while as I haven't been driving  
> that car -
> but I replaced the rpm sensor - and the car stalled as I pulled out  
> of the
> garage - so that was not a good start - but I took it for a spin  
> anyway - it
> did buck a little bit on a test drive.  SO I got back and decided  
> to swap
> plugs - as somebody here had mentioned that, too.
> I had the normal F5DPOR plugs in there - about 30,000 miles on  
> them.  I had
> some F6DTC plugs in the garage - the guys at 034 use them - say  
> they work
> well but don't last as long.  Fine by me.  Put them in and no  
> bucking yet.
> Only 1 day into testing but the bucking had become pretty  
> regular....I'm
> highly confident it would have happened by now.
> Now to put my old rpm sensor back in and see what, if anything,  
> happens.
> Brandon
> -----Original Message-----
> From: Brandon Rogers [mailto:brogers at]
> Sent: Wednesday, November 11, 2009 11:42 AM
> To: '200q20v at'
> Subject: Motor bucks slightly (long)
> 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  
> 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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