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'93 S4 acceleration problem(stumble?)

Started by carbonfiber93, January 11, 2009, 02:22:04 pm

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My '93 S4 stumbles when I accelerate. It happens between 1500 and 2500 rpm regardless of engine/outside temp. It seems like the ignition cuts out briefly and comes back again. Also, at times, it seems like I have no boost. Other times it runs like a bat out of hell.  I'm thinking it might be a bad throttle position sensor, a bad oxygen sensor, or a bad mass airflow sensor. I'm leaning toward a bad oxygen sensor because I had a '92 Jeep Wrangler that had the same symptoms and the fix was a new oxygen sensor. Any help is greatly appreciated.



Jason C

January 11, 2009, 08:11:41 pm #1 Last Edit: January 12, 2009, 09:24:37 am by Jason C
first, check to see if all plugs are torqued to 21ft/lbs

if you still have symptoms, go to the AudiWorld FAQ and look for POS and Coil Pack issues.  It's a fairly common failure with the AAN.  The POS has extra channels, so it could be as simple as moving wires to the unused channels if the POS is the Fault. 

Also, don't rule out that it could be a boost leak.  One of the hoses could have the slighest rip or hole, or even a clamp could be loose. 
~Jason C
ANCA # 12209
another Audi comming soon
'96 Subaru SVX LSi AWD


if it stumbles at idle as well, your looking at a POS problem, like Jason said you can easily swap the channels or just even the 2 POS with each other.  If hesitation is only under acceleration, you have either a weak coil or a boost leak.
however you should check the plugs and the torque of them, check the plug connectors.

these are all pretty easy for you to diagnose and fix yourself with minimal tools in your garage.  the FAQ list on the audiworld forum is great, and you know we will help you with any questions you have in the process.

Does it still hesitate if you accelerate slowly? or ease into boost?  what are your boost numbers looking like?

throttle position sensor is the last thing your gonna replace most likely but you can clean it up if its gunky


What is the "POS" and how do I change the plugs? - I had to ask, I didn't want to tear things apart and not be able to put them back together.



The "POS" is the power output stage and there are 2 of tem located on the firewall underneath that black plastic cover. This is a common failure on the S cars of that vintage.  To change your plugs, you simply take off the coil cover on the valve cover and replace them.  Not really a huge deal.  Where are you located?  Maybe I can walk you through it.
2008 Spark Silver Impreza WRX STi
2004 Velocity Red Mica Mazdaspeed Miata

Theta Tau 87

Quote from: carbonfiber93 on January 19, 2009, 03:43:32 am
What is the "POS" and how do I change the plugs? - I had to ask, I didn't want to tear things apart and not be able to put them back together.


Your best resource for UrS issues is the AW UrS forum.  You could spend days reading the FAQ and never read anything that wasn't extremely useful for any UrS owner.  If it's not in the FAQ and you can't find it using the search function then ask the question on the forum.

Here's a link to the FAQ http://forums.audiworld.com/s4s6/msgs/149764.phtml

and the forum http://forums.audiworld.com/s4s6/
1995 UrS6
2002 S6 Avant (wife's)


Theta Tau, thanks for the links. I've got a new take on my acceleration problem. While accelerating up through the gears it seems as though the ignition cuts out between 2k and 3k rpm. When that happens I press the gas harder and the engine comes back to life and goes like mad(like it should). While I'm driving at a constant speed it drives normally for several seconds then feels as though the ignition cuts out(for a second or two)then drives normally. Also, the boost seems different every time I drive it. Some times it hauls ass, other times it seems like there is no boost at all.

Also, where can I find the breakdown of how many S4s were produced between '92 and '94 and how many were imported to the U.S.


Theta Tau 87

You really need to be on the AW UrS forum.  That forum is 100% dedicated to nothing but UrS cars.  Everything you could ever want to know is available by searching the archive of that forum or by asking questions there.
1995 UrS6
2002 S6 Avant (wife's)


I took a look at my vehicles records and found that the O2 sensor had never been replaced. Suspecting that I had a bad O2 sensor, I did some surfing and purchased an O2 from rock auto for $68.79 (plus shipping) and now my S4 runs great.


I would have guessed O2 sensor or BPV!!!

As for AW I am not a big fan...audifans.com s-car list has much better info IMHO. You are also welcome to shoot me an email if you ever have questions...I do have 200k+ of I5 20v turbo miles under my belt ;)
Shawn Olsen Austin Texas ACNA#1616
// 95 S6 RS2 // 02 A6 4.2 // 04 S4 4.2