Vanagon EuroVan
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Date:         Mon, 27 Feb 2006 13:13:48 -0500
Reply-To:     Geza Polony <gezapolony@SBCGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Geza Polony <gezapolony@SBCGLOBAL.NET>
Subject:      Tuning question--long, sorry!

I've been trying to tune my '84 Westy, and reading about others' attempts to do the same, for a few months now, and I've come up with a question I can't answer. It regards the input of the various sensors to the ECU and the way the ECU determines fuel/air mixture.

Let's say you have two identical, 22 year old Vanagons next to each other, and you're trying to tune both. The voltage from the sensors is going to be different for the same conditions, just because of age and manufacturing variability. So at 950 rpm at 185 degrees F (or whatever) the voltage from the AFM, the O2 sensor, the Temp II, the intake air temp sensor, and so forth, are going to be different from van one to van two.

There's no way around this, if only because you can't get "correct" resistance specs for the potentiometer in the AFM.

In practise, to set the FI mixture, mechanics put a sniffer up the exhaust pipe and turn the AFM screw, etc., until the readings are within specs.

But this doesn't really tune the engine at all temps, RPMs, loads, etc. It just sets it for one condition.

The sensors in the two vans are going to be putting out different voltages for 4150 RPM, 190 degrees F water temp, 85 degrees F air temp, open throttle, etc. So the ECU will be sending a different mixture to the FI's, FOR THE SAME CONDITIONS.

This leads me to believe that the ECU is in reality a relatively crude device, with huge margins for error from the sensors. Otherwise, the vans just wouldn't run. Or they would run at one set of condition, but not at another.

This in turn leads to relatively inefficient operation, in terms of engine output and gas mileage.

The variability of the sensors's output, take together, also makes these cars exceedingly difficult to tune accurately. And this isn't even taking into account the ignition!

Am I on track with my thinking here? Is there something I'm missing about the fuel management system?

Thanks for reading

Geza


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