[s-cars] UrS4 running Lean fuel mixture

Michael Lardizabal mikellardizabal at yahoo.com
Fri Feb 13 05:29:12 PST 2009

Interestingly enough, there's a discussion thread in the BMW M5 forum about MAF's going bad.

Although the Bosch Maf in the e34 M5 & Porsche 928,911's of the 90's are similar to the UrS4, its not the same unit, but the metering is similar.

When the Bosch MAF goes south,usually attributed to age, it loses the ability to maintain the voltage needed to send the ECU the right  signals.,thus allowing the car to run very rich at idle and around town driving. 

Once at the 5-7500 rpm range a bad MAF can go dangerously lean. One guy put a hole right through his M5's block. So its kinda odd that you're just lean all the time.Hmmm...

Couple of things to check:

Battery voltage :
Voltage from idle to load. ( You need at least 14+ volts @ WOT IIRC ) The Walbro might be voltage starved giving impeded flow.

Stick a FP gauge before the rail.Make sure you've got at least 4 bar.

Injectors ? a bad batch of dirty gas can really do a number on flow rate. 

FWIW- You're running scary lean at boost. I'd keep the revs under 5K for now.

Walbro's are a hit & miss FP. Q control is moderate at best. Ask the Subie guys.

Good luck.
--- On Fri, 2/13/09, Dave Tremblay <da_trem at hotmail.com> wrote:
From: Dave Tremblay <da_trem at hotmail.com>
Subject: Re: [s-cars] UrS4 running Lean fuel mixture
To: "s-car-list" <s-car-list at audifans.com>
Date: Friday, February 13, 2009, 8:07 AM

No cold starts issues, same as with the old fuel pump.

Date: Thu, 12 Feb 2009 21:25:08 -0500Subject: Re: [s-cars] UrS4 running Lean
fuel mixtureFrom: abeckert at gmail.comTo: da_trem at hotmail.comCC:
s-car-list at audifans.comDid you do the walbro install?  Any issues with cold
On Thu, Feb 12, 2009 at 6:21 PM, Dave Tremblay <da_trem at hotmail.com>
Hi all,i'm having issues with fuel mixture with my  93' S4. About two
weeks ago i've noticed on my wideband lean mixture readings while cruising
and on boost.I've always seen 14.5-15 afr(cruise) and 11.5-11.8afr on boost.
Now what im seeing is 15.5-17(cruise) and 13.5-14(boost)The car as MRC stage 2,
K26#8 and a fmic for a long time now, never had a single problem with it. Motor
always run on 94 oct.Boost pressure is the same as before and it dosent miss or
anything..no leaks after the maf or after the turbo. Car is very well
maintained.Last work done recently:New 4 bar fprRelayed Fp255 walbro fuel pump,
about 2 months oldFuel filter as 20kNew O2 sensorNew hall senderThe ecu trows no
codes.Now my question is, could a bad maf cause these problems?...Your input on
this is greatly
listS-CAR-List at audifans.comhttp://www.audifans.com/mailman/listinfo/s-car-list

S-CAR-List mailing list
S-CAR-List at audifans.com


More information about the S-CAR-List mailing list