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

[Darin Nederhoff]

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