Bosch L Jetronic Surging

["Don Best" <bestdon@domain.elided>]

I am having a very difficult time trying to resolve a surging issue in my 
newly rebuilt 2.5 litre motor in my 1983 GTV6.  First the symptoms as shown 
by a Micro Dyno exhaust 02 monitor driven by the lambda sensor in the 
exhaust.  In closed loop operation the display shows that the mixture is 
changing from slightly rich- one green led lit- to so lean it is off scale.  
The rate of the cycling is approx one hertz at 2000 rpm and is dependent on 
engine speed- the cycling rate increases as engine rpm increases.  In open 
loop with the input to the control unit disconnected the mixture is steady, 
it is also steady in closed loop at Wide-Open Throttle.

The motor has about 1000 miles on the rebuild with low mileage piston, 
de-glazed liners, new rings and valve guides, refaced valves and seats.  
Compression is good and even across all cylinders.  So far I have 
substituted the control unit for another, changed the air flow meter and 
tested the air flow meter for ease of movement and for a steady and 
contiguous output voltage, tested and changed the Temp 2 sensor, tested and 
changed the aux. air device and installed a new universal replacement lambda 
sensor, the original sensor was dead.  There are no vacuum leaks as 
indicated by a vacuum gauge and as per the compressed air and soap leak test 
from the Bosch trouble-shooting guide.  I have even disconnected the other 
hoses to the plenum from the brake booster, vapour recovery system and 
crankcase breather and sealed the plenum orifices.  The fuel pump and 
pressure regulator pass all tests as per the manual and I have changed the 
new regulator as well.  Injectors were serviced and ultrasonically cleaned 
when the engine was rebuilt and I have tried two other sets just to be sure. 
 Camshafts are stock and cam timing and ignition timing are stock.  Open 
loop mixture has been set as per the manual and grounding the input lead to 
the controller results in a full rich mixture.  Substituting a voltage 
source as the lambda input to the controller allows the mixture to change as 
the voltage is adjusted and the exhaust 02 monitor shows a steady mixture.  
NONE of the changes has made any difference to the closed loop operation, 
except the rebuilt injectors do idle a bit smoother.  All electrical 
connectors have been cleaned and treated with De-Oxit.  Plugs, cap, rotor 
and high-tension leads have been changed.  A Sun scope indicates that 
cylinders 3 and 6 are a little rich on the rebuilt injectors but I have not 
redone the test with another injector set.

The car starts instantly when cold but takes some cranking when warm- 35-45 
degrees C.  When driven the car feels like it is running lean, power is down 
- it just does not feel right under less than WOT.

Any thoughts gentlemen?




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