1990 200 missing/lean under boost

[David M. Yentema]

Tony, from what I understand about o2 sensors, is the measure presence of oxygen, not fuel. A lean mixture will result in less oxygen, and a rich mixture in more. A misfire would mean more fuel is unburnt, resulting in less oxygen, showing lean. If I am wrong, please correct me..

As for the leakdown, I've pretty much confirmed a weak check valve, but it's an internal one, so i'm just dealing with it until it's time for a new pump. which it almost seems like it's about that time
----- Original Message -----
From: "Tony Hoffman" <auditony at gmail.com>
To: "David M. Yentema" <yentemad at rider.edu>
Cc: "quattro" <quattro at audifans.com>
Sent: Wednesday, June 8, 2011 6:54:12 PM GMT -05:00 US/Canada Eastern
Subject: Re: 1990 200 missing/lean under boost


I believe you are on the right path, chasing a lean mixture. A lean mixture will cause missing, which I believe is what you are feeling. If your engine is running normal and there is a miss, it will show rich on the meter, as there is now excess unburned fuel in the exhaust. 

Definately check that air passageway between the metering plate and the turbo. A leak there will cause a lean running condition. 

As for the vapor lock, I'd be looking at the check valve on the pump or one of the injectors may be leaking down after shutoff. The injectors are easy to check, the fuel pump check valve in a T44-not so much :( It involves a cold hand covered in fuel. 

Tony 


On Wed, Jun 8, 2011 at 4:54 PM, David M. Yentema < yentemad at rider.edu > wrote: 


This is an issue i've been dealing with for a while, Car is a 90 200 Quattro, MC-2, Ben Swann 2+T super chip (mac-14) 

basically what it's doing, it idles fine, starts up fine most of the time (accumulator was leaking, so I pulled it, made no real difference with starts, still vapour locks, but that's besides the point) and it will pull to about 6psi, but the second it goes over that, my NB shows full lean, and if I push a little further you can feel it miss slightly. I know a miss will show lean, and that actually running lean will show lean. 

I have tested and verified: 
ISV, IAT, Idle/WOT switch, ignition coil, cap, rotor, wires ohm out fine but they're old so I plan on replacing them anyway, pressure tested the intake manifold and fixed the few small leaks I found. 


I also bought an adjustable spark gap tester, adjusted it to the absolute max distance the coil would jump, and tested each of the wires, all of which were able to jump the gap no problem 


I'm running out of ideas here, and there are no codes being set. The only other idea I have is to tape off the intake snorkle and run pressure into the whole intake and see if anything is leaking (thinking being a vac leak before the turbo, but after the metering plate would cause a lean condition, especially as the turbo spools and sucks in more air.. 

any suggestions you guys can give would be awesome 

Thanks, 
Dave 
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-- 
David M. Yentema
Resident Advivor
Poyda C407
Rider University
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