Motor bucks slightly (long)

robert weinberg centaurus3200 at
Wed Nov 11 15:07:13 PST 2009

doubt it's the computer. seriously, change all those $18-$35 sensors, one of them is at fault. the ones that are okay are still 18 years old and could crap out at any point. you can change most of them yourself. they are staring right at you when you open the hood.

i've noticed the audi ECU is not real good at throwing codes. maybe it has too much pride? LOL!


From: melvin guyer <melguy1 at>
To: Mike Miller <mikemilr at>; Brandon Rogers <brogers at>; 200 20v List <200q20v at>
Sent: Wed, November 11, 2009 2:46:52 PM
Subject: Re: Motor bucks slightly (long)

there must be an epidemic of bucking 200q20v's. My 200 began bucking about two months ago. It feels like a momentary power loss and the "buck" is the sudden resumption of power. I have taken the car to the Audi dealer for diagnosis and repair and after lots of time and $$ it still bucks. Typically it bucks two or three times within the first 10 minutes following a cold start. It bucks intermittently after that, both under acceleration and while running at constant moderate speed. It also occasionally stalls now too, after hard acceleration and then a let up on the gas pedal. The idle stabilizer valve has been replaced and the O2 sensor as well, but that hasn't helped. The Audi dealer now suggests that the problem lies with the electronic engine control module(?). The car has 89k miles and is in excellent condition, always dealer maintained. Any ideas that come up on how to fix this would be welcome.

mel guyer

From: Mike Miller <mikemilr at>
To: Brandon Rogers <brogers at>; 200 20v List <200q20v at>
Sent: Wed, November 11, 2009 4:00:06 PM
Subject: Re: Motor bucks slightly (long)

Where is that located?

I get the engine buck more so with engine warm than cold and more so in 
higher gears (4th 5th) than 1st thru 3rd. Boost has also dropped from 1.8 to 
1.5 indicated. Most of the boost drop occured after replacing the lower hose 
on the IM - the one the Bypass valve ties into - that section split. I've 
replaced bypass valve, MAF. plugs wires, and a few othere things. I've been 
thinking IAT sensor - where si that located?

mike miller

----- Original Message ----- 
From: Brandon Rogers
To: 'David Sexton'
Cc: 200q20v at
Sent: Wednesday, November 11, 2009 1:32 PM
Subject: RE: Motor bucks slightly (long)

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!

I CC'd the list for archives sake

-----Original Message-----
From: David Sexton [mailto:sextondm at]
Sent: Wednesday, November 11, 2009 12:04 PM
To: Brandon Rogers
Subject: Re: Motor bucks slightly (long)

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
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> wrote:

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?

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