Hessitation in Late Model Subarus

[CLsnyder]

----- Original Message -----
From: GARY <hobiegary at earthlink.net>
To: EFI DIY mailing list <DIY_EFI at efi332.eng.ohio-state.edu>
Sent: Friday, May 07, 1999 6:33 PM
Subject: Hessitation in Late Model Subarus


> Hello,
>
> A lot of people have had a problem with a stuttering acceleration in
> Subarus.  I offered up an hypothesis along with a fix for the problem.
> My hypothesis was first rejected by Subaru of America, then later
> supported in a Technical Service Bulletin.
>
> Rather than go into detail here, I'll supply the URL for my website on
> the matter. When you go there, scroll down for a lot of links.
> http://home.earthlink.net/~hobiegary/hesitation.htm
>
> Well even though Subaru claims to have a fix for the problem via a new
> ECU, I have yet to hear of anyone who could jump through all the
> necessary hoops to get a new ECU *and* find out that the new one
> actually cures the problem.  So I am not holding my breath on that note.
>
> What I am doing however, is planning on an improvement on my primary
> fix.  The ECU is a OBD2 type with EPROM.  It can be "reset," to loose
> stored trouble logs, by disconnecting the battery for about a half
> hour.  I need to electronically dampen or trim or filter (or whatever it
> is called!) the signal from the knock sensor to the ECU.
>
> What is happening is that the ECU is interpreting non detonation noises
> as detonation and reacting accordingly.  This leads to piss-poor
> performance to say the least.  I have gone the mechanical route,
> buffering the signal by padding the mechanical interface between the
> sensor and the engine block.
>
> What I would like to do is simply reduce the sensitivity of the sensor.
> That is what I did mechanically and it seemed to work pretty good.  The
> problem though is that it is a little iffy and esoteric and problematic
> and lacking consistency.  It is not solid state enough to suit me.  I'd
> rather dampen the signal electrically.
>
> I tried hooking up a variable capacitor in parallel with it.  I thought
> that this would let any voltage spikes bleed (partially) to ground and
> lessen the signal strength arriving at the ECU.  I had trouble with this
> method because as soon as I turned the capacitor a certain amount, I'd
> get a trouble code and would have to shut down and restart.  I'll guess
> that the ECU sends a monitor current through the sensor and detects when
> it is not at the proper impedance.  It didn't seem to help either.  I am
> not convinced that I was doing the wrong thing either.  I just could not
> get any positive results.
>
> I also wonder if I rather should be using a variable inductor in series
> with the sensor.  Wouldn't this choke a signal without greatly affecting
> monitor current?  I guess it would depend if the monitor is looking at
> impedance with a/c or looking at resistance with d/c.
>
> Comments?  Any other ways I should consider reducing the amplitude of
> this signal?  What about the ECU, is there anything I can do there to
> help the situation?
>
> I thank you for your attention Fellows and thank you in advance for any
> suggestions.
>
> --
> GARY       mailto:hobiegary at earthlink.net
> Have you thought about a high resistance pot set up as a voltage divider
on the sensor? Cut the output down to about 80% and see if it still false
triggers. Then take it back up to the point where the performance "just"
starts to deteriorate. I have not tried this, but as the sensor is just a
microphone, I figure a volume control should do the trick.