RE: 505 Turbo smoke screen!!! and Fuel CUT

John Goodyear (jjg@optsys.com)
Thu, 13 Aug 1998 14:23:36 -0400

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505 owners Hit Fuel Cut!!!!!!

Alright now we are talking about a problem I can relate to real good.

First Consider yourselves lucky to have a fuel cut at 14.5 PSI.

The nitwits at Mitsubishi left mine at 9-10PSI on the 87 ECU.

Nothing hurts me more than to be screaming along at high RPM/Boost and have
that engine hit the proverbial WALL.

This fuel cut has left me with many hours of concern and inadequate
workarounds.

First Let me tell you about my system and we will see how much is in common.

The AFS/MAF (Air Flow Sensor/Mass Air Flow Sensor) of Karman Vortex type.
Monitors the amount of air flowing through it's metered portion and sends a
frequency to the ECU dependant on Volume of Air. Due to the set volume of
the unmetered side of the AFS, the ECU knows how much air is flowing through
the AFS, based on the amount of air going through the metered side. With
the total volume of air intake known the ECU can determine the boost level.

Too Much Air = Too Much Boost = Fuel Cut

StarQuest owners deal with this problem 2 ways. Electrically or physically,
some do both. Neither are up to my level of satisfaction.

First,

Electrically:
Put inline with the signal from the AFS a device that linearly reduces the
frequency output to fool the ECU into thinking it has less air going through
it. This will then in turn lean you out. This will also not eliminate fuel
cut just push it higher. This device is most commonly found as the HKS FCD
(Fuel Cut Defencer).

Physically:
Cut out the unmetered part of the AFS to Change the volume of it, thereby
allowing more air slip past undetected. This also tricks the ECU into
thinking less air is going through it then really is, thereby leaning you
out. I feel this method has one advantage. By slowly opening up the
unmetered part, a small section at a time, and trial and error. You can
slowly push the fuel cut level just above your predetermined boost level.
The less you modify the AFS, the more accurately the ECU can adjust injector
duration. This is the method I chose as the lesser evil. One word of
warning. I walk a fine line with this method and the right atmospheric
conditions coupled with a boost spike will produce a fuel cut. I should get
back in there and create a greater margin of safety or error (however you
want to look at it).

All who do this monitor air fuel ratio's religiously (or at least they
should) and compensate with larger injectors or Boost compensating fuel
pressure regulators.

I have talked to many other people including HKS on what I feel is a better
method. I was working with an EE to create a frequency comparator to hold
the AFS frequency at a level just below the fuel cut trigger level. Thereby
allowing the ECU feed as much gas as it's programming will allow at high
boost and be totally transparent to the ECU below that level.
I would then be able to go back to the stock AFS, get factory adjustments
during normal driving and know at WOT I was dumping the most gas allowable.

My EE seems to have disappeared and the project temporarily dead. If 505's
are in a similar boat as me, this might be a viable option for you to. I am
sure we have the talent on the list to create a frequency comparator.

Any Takers?

John