Date: Mon, 21 Nov 94 14:07:08 PST
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: Dave Kautz <dkautz@hpsidms1.sid.hp.com>
Subject: Lunch with Bentley
Okay, here's the scoop from Bentley.
First of all, what I said earlier about the throttle switch enriching the
mixture was correct ONLY if you have a fuel injected bus built before 1977.
The enriching function was "incorporated into the control unit" on later
models.
Here's the rap on oxygen sensors on '79 and later California air-cooleds.
"The oxygen sensor generates form 0-1 volt depending on sensor temperature
and exhaust gas composition. The varying voltage is measured by the injection
systems control unit which alter the injector-open-time to give low emissions.
The sensor operates neither below 300 degrees C nor when turned off at full
throttle by a microswitch on the throttle valve housing."
So there you are. What Bentley doen't say is WHY it is switched off at full
throttle. My guess is that the aforementioned enrichening would drive the O2
circuit to correct for it if the sensor wasn't switched out of the circuit,
but that's just a guess.
Response to David Carment on EGR:
Your remarks about when the EGR is on and off are backwards, I think. The EGR
on my '74 is off at idle and off at full throttle and switched on in between.
If the EGR valve is open at idle it behaves as a nasty vacuum leak and trashes
the idle. Mine is electrically switched by a cam on the right hand carb
linkage. Pushing the switch and opening the valve at idle speed cause the
engine to die. Incidentally, that's what the smog inspector did to make
sure the EGR was connected and working.
Back to Nitrogen oxides - I think that the combustion temperature is affected
by many things. For example, my '72 controlled NOX emissions by turning off
the vacuum advance when the engine was warm. I read that richening up the
mixture a little will cool the combustion temp somewhat. What I'm getting
at is that all these things are related: you may not want to
dis-connect your EGR because your injection control unit is setting the
mixture assuming that some of the molecules in the mixture don't have an
oxygen to contribue to combustion. The spark may be timed with the same
assumption.
Dave Kautz
dkautz@hpsidms1.sid.hp.com