N o r t h A m e r i c a n T u r b o c o u p e O r g a n i z a t i o n
V A M I D L E V O L T A G E A D J U S T M E N T
By Jeff Korn
I replaced my VAM around 6 years ago due to a dead spot on the old one. The replacement was
purchased at Rock Auto, and worked great at cruise and WOT but would set a CM 42 (O2 indicates
rich when closed loop expected / adaptive lean limit reached) at idle. I assumed the VAM was
indicating too much air flow (too high an output voltage) at idle, telling the PCM to inject
too much fuel, exceeding the adaptive limits of the PCM. This was made worse by the 43 psi
base fuel pressure I am running to keep up with the higher than stock boost levels. I didn't
want to mess with the VAM spring tension, as that often seems to lead to more bad things
happening, so I took an old VAM apart to look for another way.
On to the adjustment: FIRST NOTE THAT THIS ONLY AFFECTS VAM VOLTAGE OUTPUT AT IDLE AND VERY
LOW THROTTLE OPENINGS AND HAS VIRTUALLY NO EFFECT AT CRUISE AND WOT. How much difference this
makes above idle I don't really know, as our PCM's don't allow for real time monitoring of
STFT and LTFT, they only tell us when the adaptive lean and rich limits have been reached by
turning on the CEL.
The VAM has an air bypass adjustment that bypasses air around the VAM flapper door. More air
bypass means less air through the door and therefore lower air flow reported to the PCM.
Stand in front of the car and look down at the top of the VAM. At the upper right corner is
a plug around 1/2" in diameter. The bypass air adjustment is under this plug. To remove this
plug drill a small hole around 1/4" deep in the plug, screw in a sheet metal screw, and use
some vise-grip pliers on the screw head to pull out the plug. Take a long Allen wrench
(IIRC 3/16") and engage the adjuster at the bottom of the now-exposed hole. Turning the
Allen clockwise closes off the bypass, causing more air to flow through the VAM door and
output voltage to go up. Turning it CCW opens the bypass, causing output voltage (and reported
air flow) to drop. My VAM, as it came from Rock Auto, had a hot, 1000 RPM, no-load idle voltage
of 1.35 V. I adjusted the voltage down to 0.85 V by turning the bypass CCW to open the bypass
and allow more air around the VAM door.
After adjustment is complete, just tap the plug you removed back into place. After the adjustment,
I had no more CEL at idle or cruise and WOT was not affected in any way.
FYI, to monitor VAM output voltage connect a voltmeter between the BK/W (-) and W/BK (+) wires
at the VAM connector.
Page last edited March 18, 2015 @ 07:23 MDT