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Re: 86 5kTQ breakup/tuning problems

   I've got a 86 5kTQ that has a computer modified by Total Audi Performance,
   or maybe I should say screwed up by TAP.

   This all started with a stock motor that pulled OK clear through the
   RPM range.  The original computer was a MAC-10 unit.  I shipped the 
   computer to TAP, and they sent it back to me with their EPROM chip
   and a new wastegate spring.

The one time I talked to those folks (looking for help with a different
computer problem), the guy there said they replace the 28-pin 6840 chip,
and not the 24-pin 2532 EPROM. When I sputtered in disbelief, he double
checked and confirmed the 6840 -- "that 28-pin chip to the left of the
cpu". While it's possible that they indeed do perform some serious Black
Magic, I kinda concluded that they were full of bovine excrement . . .
(personal hypothesis not backed up by any solid facts one way or the ot-

   I installed both the computer and spring, and sent for a spin.  First
   impression was a LOT more power in the 2500 to 3500 range, but breaking
   up badly above 3500.  At this point, I noticed the "check engine" light
   came on above 3000 rpm, and that the pressure guage in the dash read 0.0
   all the time.

No idea on this. Is your FI system CIS or pulsed? (My [rapidly-growing] ex-
perience is with the '83 UrQ, with a MAC-02 ECU, which is CIS.)

Hmmm... is it a "burbling" sound, kinda stumbling, or just a "miss", as
in no spark/combustion? Wild guess might be running overrich under high
rpm/boost ("rich" is good thing, too much of good thing is bad thing).
If your system is CIS with a "Frequency Valve", you can monitor what the
computer is doing to the mixture trivially with a SunPro 7678 meter (my
car has a convenient two-wire test connector dangling by the coil...). If
it's pulsed, I have no idea what to say to ya...

You might be able to tell something by pulling the plugs right (as soon
as safely practical) after a high-speed run.

Or stick a voltmeter on the O2 sensor and see if it just pegs at +1 or so
volts (running rich) or near 0 volts (running lean) [if it's bouncing a-
round inbetween and you can't get a reading, that means the computer is
properly maintaining "ideal" mixture]. Typical ECU usage is to force the
mixture rich under max boost/rpm.

   To make a real long story shorter, the MAC-10 computer had a bad pressure
   sensor - whether caused by the high boost from the wrong spring, or whether
   caused by their mods - I don't know.  It worked before I sent it to them.

My (limited) experience says anything Audi Electrical is very fragile and
will break given the slightest opportunity. I find it easy to believe that
your original ECU did break all by itself.

   After shipping the computer back and forth a couple of times, they then
   sent me a MAC-11 computer that "had been modified to work like a MAC-10"
   whatever that means.

I'm curious -- is your MAC-10/11 ECU made by Hitachi? If so, and it's handy
can you pop the cover (5 screws on my MAC-02 unit) and tell me if there is
a xxx6802 (e.g., HD46802 P) chip in "upper right" corner, and an HD46506 chip
(squarish, not long rectangle like the 6802, if any) chip on a big squarish
daughter card sitting just "below" the 6802 (and just "above" the big air-
pressure transducer assembly mounted in the "front" of the ECU with the
nipple for the manifold vacuum/pressure line sticking out the front of the
box). [Be careful not to touch anything inside, as static electricity loves
to zap (very expen$ively!!) these chips!] How many "pins" in the klunking
wiring harness connector?

Also, when you have the cover off -- are there three wires running out of
the air pressure sensor box (green, and I think white and ???)? Do these
wires run "cleanly" and directly to the circuit board, or do they connect
up to some "stuff" that looks "after-the-fact" like resistor/capacitor/
potentiometer *not* mounted onto the circuit board but just soldered to
something on the circuit board on one end, and to one or more of the air
pressure transducer wires on the other. (E.g., has someone "hacked" into
the sensor directly, rather than actually modifying the EPROM program...)

Did they actually replace the EPROM (xxx2532-yy chip)? Does the soldering
look "different" for that chip and the other chips of similar construction
on the board? Is it in a socket? Did they return the original EPROM chip?

   Made sense at the time.  This afternoon, I pulled the cat. conv. off and
   ran it up the road with just the header pipe off the turbo.  It is real
   clear that it's breaking up above 4000 rpm, since I can now hear it.
   Doesn't always break up at the same point, but breaks up at a higher RPM
   under partial throttle than at full throttle.

Might also be timing map screwed up, and I have *NO* idea how you can test
that without a dyno!

   Does anybody have any ideas?  Is the MAC-11 computer likely to be 
   different enough from the MAC-10 to be causing this?  

Insufficient data...

What are the "part numbers" of the ECUs (as in 035 905 383F)?

   I've thought of buying another MAC-10 computer and installing the TAP 
   MAC-10 chip myself (I've got a copy of that EPROM and can burn another
   chip for the new MAC-10).  I hate the thought of what that will cost,
   even used.

My guess - $500 used (hopefully less, as 5KTQs are more "common"), and
around $1100 new [my '83 UrQ's MAC-02 ECU], from Linda@Carlson, or $1500
new (and up) from most other Audi vendors.

   I'm about at the end of things I can think of.

I'd try it "stock" first (you'll run into the overboost shutdown, but you
should be able to "gently" run it up to redline, using your foot to main-
tain a mere 5-6/whatever psi).

Are you in the New England area?