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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