[s-cars] Going insane... UrS4 falling on it's face at WOT

[Cody Forbes]

One note I have is to explain you AFR meter showing lean. A misfire will always cause an oxygen sensor to show lean. It's measuring oxygen content of the exhaust and without a combustion event burning up the oxygen the engine is just pumping fresh air through.

-Cody (mobile)

On Jul 17, 2011, at 12:20 PM, Darin Nederhoff <scarsgo at gmail.com> wrote:

> Hey gang,
> 
> My car has been driving me insane for a while now.  First of all the car is
> a '93 S4 with 170k and full RS2 spec build (cam, inj, fpr, maf, exh man,
> turbo, yadda yadda).  It started displaying symptoms of high boost misfire
> (above 16psi) which historically has meant a failing coil.  No biggie.
> Trouble is that all the coils, plugs, and power output stages are new or new
> within the last year (if that) as is the fuel pressure regulator, fuel
> filter, O2 sensor and wastegate frequency valve.  The car is still bucking
> like mad at WOT and I'm getting black smoke out the tailpipe at the same
> time.  The odd thing is that my A/F meter shows the car initially going rich
> under boost but then it swings right back to full lean on stumble.
> 
> I've already tried swapping MAFs with my father's S4 and got no change.
> Have tried hunting for boost leaks with nothing evident either.
> Arghhhhh
> 
> 
> For the nerds here, these are some of my test results thus far...
> 
> MOTRONIC IGNITION SYSTEM:
> 
> 1. Checked all resistance of all spark plug connectors. J28-7
> 1993 S4 (mine)  /  1994 S4 (mother's)
> 1 = 5010  /  4850
> 2 = 4600  /  4890
> 3 = 4520 /  4820
> 4 = 5000  /  4860
> 5 = 5190  /  5150
> 
> Replaced #2 with new = 5020
> Replaced#3 with new = 5100
> Reassembled and test drove; No Change
> 
> 2. Exchanged ignition coil mounting bracket with coils and spark plug
> connectors form 1994 S4 to 1993 S4. Test drove: No Change.
> Still misses, black smoke from tail pipe when it misses.
> 
> 3.   Switch back from Step 2. Disconnected harness connectors from
> all five fuel injectors. Installed spark plugs in the spark plug
> connectors and placed on the cylinder head cover. Operated
> starter. Strong spark at all spark plugs.
> 
> Tested each coil with gap tester set at 1.84mm (normal plug gap =
> 0.6 mm). Operated starter. Strong spark at all positions.
> 
> CHECKED FUEL SYSTEM PRESSURE: J24-13
> (Mine has RS2 FPR while 94 S4 has stock FPR)
> 
> 1.Removed fuse 17 and made remote fuel pressure switch.
> Removed vacuum hose from fuel pressure regulator and connected
> fuel pressure gauge. Activated pump until top pressure.
> 
> Top Pressure = 54 Bentley acceptable range = 58-61. To make
> sure I also tested the 1994 S4 = 58.
> 
> Immediately after fuel pump switched off, pressure = 50 Bentley
> acceptable range 49-54. 1194 S4 = 51.
> 
> 2. Inserted fuse and run engine at idle. Reconnected vacuum hose at
> fuel pressure regulator and observed drop in pressure 54 – 47 = 7.
> Acceptable range = 7. OK
> 
> 3. Stopped engine, timed 10 minutes. Residual pressure = 49.
> Acceptable for warm engine is 41 psi.
> 
> COMPRESSION TEST RESULTS:  (These concern me a bit?)
> 
>    #1    120 PSI
>    #2    115
>    #3    110
>    #4    115
>    #5    120
> 
> The Bentley lists the spec. range 131-189, difference between cylinders of
> less than 44 and the wear limit of 102.
> 
> 
> 
> Would appreciate other possible directions to go with this one?  I've never
> really been "stuck" with something on one of these cars in recent years but
> I'm not quite sure where to go from here.
> 
> Thanks in advance gang,
> 
> 
> 
> 
> -- 
> Darin Nederhoff
> http://S-CARS.ORG/site
> 1993 S4 //RS2 spec
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