North American Turbocoupe Organization

Idle VAM Adjustment by Jeff K.
Pete D Offline
The following procedure was provided by Jeff K in this post;

Thanks Jeff.

Quote:Here is the info on idle VAM adjustment.....

First, a little history..... I replaced my VAM around 6 years ago due to a dead spot. The replacement (RockAuto) 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 was and still am running to keep up with the higher than stock boost levels. I didn't want to mess with the 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 EFFECTS 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 PCMs 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 thru 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 a vise grip on the screw head to pull out the plug. Take a long Allen wrench (I seem to remember 3/16", but I might be wrong here) 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 thru the VAM door and output voltage to go up. Turning CCW opens the bypass, causing output voltage (and reported air flow) to drop. My VAM, as it came from Rockauto, had a hot 1000 RPM no load idle voltage of 1.35 V. I adjusted the voltage down to .85 V by turning the bypass CCW to open the bypass and allow more air around the VAM door. After adjustment, just tap the plug back into place. After this adjustment, I had no more CEL at idle and cruise and WOT were not effected in any way.

FYI, to monitor VAM output voltage connect a voltmeter between the BK/W (-) and W/BK (+) wires at the VAM connector.

Jeff Korn
Pete Dunham


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