[Shotimes] 93 ATX ZEX

[Paul Nimz]

This I got from an AutoTap email.  It deals with OBD II but has some
interesting info on fuel trim tables.  The last paragraph was interesting.

Paul Nimz
'97 TR
'93 EG mtx



First let's take a quick look at what happens when you step on the gas. The
computer reads the throttle position (TPS), checks the coolant temperature
(ECT), checks the intake manifold air flow and pressure (MAF and MAP), air
temperature (IAT) and engine RPM. With all that info the computer then
performs a "look-up" function. Similar to finding a coordinate on a map,
except this map has many more variables than horizontal and vertical! Once
the computer finds the correct coordinate for all of those operating
conditions, it finds the result it was looking for - exactly how much fuel
to inject for that condition in order to get *almost* the perfect mixture,
giving you the best performance, economy and lowest emissions. All of this
is continuously recalculated, dozens of times per second.

Impressive! But why is it *almost* the perfect mixture? Because dozens of
things could influence your engine to the point where that
factory-programmed value isn't quite right. To "close the loop" of
information flow, there is a program running on your car's computer that
uses information from our friend the Oxygen Sensor to check if the mixture
is right where it needs to be. If it's a little off, the computer uses that
information to come up with a fudge factor. The name of that fudge factor is
Long Term Fuel Trim (LTFT). That value is the amount of correction that the
computer will automatically apply to the value that it looks-up. Measured in
percent, we typically see stay well under + or - 10% when driving. If it
gets over + or - 25%, the check engine light will turn on and a code is set
telling you that your engine is running either abnormally rich or lean.

I've simplified this just a bit. Here are a couple of the complications (and
there might be a few more that I haven't learned yet). There's another fuel
trim called Short Term Fuel Trim. There are also a bunch of those lookup
tables for different driving conditions, each one can have a separate fuel
trim fudge factor.

So - what does this mean to you? A few things.

First, when you get a code that your car is running too rich or lean, don't
rush out and buy new Oxygen Sensors. That's just shooting the messenger.
Those codes are usually caused by bad information from one of the other
sensors that the computer uses to do its lookup.

Using AutoTap to monitor your engine's Long Term Fuel Trims can give you
some insight into its health. Nice low numbers mean that your engine is
pretty content using factory-programmed values. Big numbers mean that, for
some reason, a big fudge factor is being applied.

Finally, you'll get the best engine power with a nice low fuel trim number.
Why? Because your engine quits paying attention to the Oxygen Sensor at full
throttle. It still applies the last known fuel trim correction, but since
maximum power requires a bit richer mixture than what is ideal for the
perfect mix of economy and emissions, you typically get the best performance
if a large correction doesn't need to be applied.

----- Original Message ----- 
From: "Anthony Clerkin" <Anthony.Clerkin@ihouse.com>
To: <shotimes@autox.team.net>
Sent: Thursday, March 18, 2004 8:40 AM
Subject: [Shotimes] 93 ATX ZEX


> Hey all,
>
>
>
> Thank you to those of you who responded to my air/fuel questions
> yesterday, Doug at Performance Plus was able to give me a great
> explanation of how standard O2 sensors and air fuel gauges work, if any
> one is curious, I'd be more than happy to spread the wealth.
>
>
>
> I got my Doug Louis custom built ATX with high stall converter put in
> yesterday, what an amazing transmission!  I would highly suggest it to
> anyone looking to get a rebuilt ATX!  So here is my dilemma:
>
>
>
> To zex or not to zex?  I currently have the jets for a 55 shot and a 75
> shot, do any of you with ATX's, especially if you have one of these Doug
> Louis specials, have experience with N2O either damaging your ATX or
> working great with your ATX?
>
>
>
> Thank You,
>
>
>
> Anthony.clerkin@ihouse.com
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