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86 5kTQ breakup/tuning problems

I've been reading the quattro list for a while, and hope there's somebody
who can help shed some light on what's going on here.

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.

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.

More testing showed the check engine light always came on above 3K, and
went off below 3K rpm.  Even if you got it above 3K with a downshift
with your foot off the gas.  Also, the pressure guage should read 1.0
when the engine's not running - still 0.0 ... 

Finally, Anita at TAP said that she had shipped me the wrong wastegate
spring, and that I was running way too much boost. (I was dumb for not
having a mechanical guage on it right from the start).  

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.

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.

Now, with the correct spring and the MAC-11 computer, it runs about 10PSI
boost by the mechanical guage and only reads 1.3 or 1.4 bar on the guage 
in the dash.  Low RPM power is better than stock, but I'm still not getting
much power over 4000 or 4500.  I've done all the standard tune up stuff,
colder plugs (Bosch WR6DP), new plug wires.  Finally I took it to the 
dealer to have them check it.  They say all the sensors are working OK,
their suggestion was that the cat. conv. was breaking up, and under high
exhause flow conditions the particles are plugging up the outlet screen.
They said the CO adjustment and fuel pressures were right where they
should be.

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.

TAP is no help at all, the person I talked to before I bought the upgrade
was Ivor Wigham, who was very knowlegable.  Since I bought it he has not 
ever been around.  All I get is Annita who doesn't know one end up from the 
other, or Trevor Gurian who it the tech who does the PC board work on 
the computers, but doesn't seem to know engines much.

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

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.

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

Charlie Smith   charlie@elektro.cmhnet.org  614-471-1418  
                  Columbus Ohio   USA