Avant died and reborn again...

Bernie Benz b.benz at charter.net
Sun Mar 7 11:43:52 EST 2004

Phil and list,

I used TAB instead of ISV, corrected  below.  Sorry about that.


> From: Phil Rose <pjrose at frontiernet.net>
> At 1:27 PM -0800 3/6/04, Bernie Benz wrote:
>> Phil, I'm not sure that I buy your suggested reason for stall after sudden
>> throttle closure under boost with a failed closed BPV.  The throttle plate
>> is a pressure balanced valve, i.e. equal plate area on either side of the
>> throttle shaft center line, thus it's position is unaffected by differing
>> pressure drops across the valve.  More likely IMO, just that the ECU is not
>> programmed to control idle in a timely manner, while the ISV is still
>> subject to boost pressure.
> You suggest that the stall is a "lean-out" situation caused by
> excessive (boost-derived) air being passed through the ISV?
No, because it is all metered air so the ECU should maintain the A/F per the
idle map.  But all this idle metered air must be passed thru the ISV,
inasmuch as the throttle valve is closed.  Assuming that the ISV is closed
during open throttle and boost, the ECU must sense a wind down to idle and
open the ISV in a timely manner wighout hunting or overshoot.  Not normally
a problem with the Motronic system with the Bosch rotary ISV, but this is a
big problem in the NA 7A's Hatiachi ECU and linear ISV, as used on the CQ
and 90Q-20V.  
> You could be right; I was just stating what I thought I'd read to be the
> "official" reasoning. I'll accept your explanation unless someone has
> evidence or better reasoning to the contrary.
That should be easy to come by.
>> If this throttle body had a throttle air bypass
>> adjustment aka the 7A, this problem could be adjusted out.
> What is  that?
So, to solve the stalling problem caused by the ECU/ISV transient
instability on a "drop throttle" condition, that being the shutting off of
all idle air, the 7A system incorporates a TAB (throttle air bypass)
adjustable screw valve in the throttle body which, in parallel with the ISV,
allows idle air around the closed throttle valve.  To adjust, one
disconnects the electrical input to the ISV, thus closing it (if it is in
clean and well lubed condition) and adjusts the TAB for a hot idle of
700/750 RPM.  This sets a lower limit idle speed which is independent of the
ECU controlled ISV idle speed of 900/950 RPM.  Result, no stalls for this
cause.  BTIT.

> Phil
> -- 
> Phil Rose
> Rochester, NY
> mailto:pjrose at frontiernet.net

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