DIY_EFI Digest V4 #721

[Jens Knickmeyer]

James,

first, there are two types of temperatur dependent resistors,
PTC (positive temp. coeeficient) and NTC (neg. temp. coeff.).
>From my experience (European cars only), NTCs are more
common in cars. You have to find out which type your car uses
(check: cold = high resistance, hot = low resistance --> NTC, 
else PTC). So you get an idea in which direction you have to tune
the resistor.

The whole idea of tuning your mixture by changing the resistor has
IMHO no chance when your car has a cat.conv. because the oxo sensor
will detect lean out and correct it. All you might get is an error in your
ECU's error memory (at least my ECU would react that way). This is only
when running closed loop, of course, running open loop will work well with
resistor adjustment.

A better way would be to adjust the signal of the oxo sensor - but that will
be more effort in electronics.

Just my 2 cent,

Jens Knickmeyer

Date: Sun, 26 Dec 1999 17:30:10 -0500
From: "James Ray Crenshaw" <james at emeraldis.com>
Subject: Mixture Tuning Chevy TBI circa 1990

I would like to try leaning the mixture JUST A BIT. Theory tells me ("I
think,
therefore I err") that some kind of resistor in-line from the coolant temp
sensor
(with a two wire plug, in intake manifold, just forward of the throttle
body) to
the ECM just MIGHT accomplish this. Of course, my being a gearhead, the
neatest thing would be doing this with the ability to tune the resistor...
assuming
it would work.

... 

I would also like to be able to adjust the idle speed. Can I MINUTELY fiddle
with the minimum air adjustment (adjusts throttle plate position) without
screwing something up? I only want to lower it about 100-150 RPMs.

What say ye?
jrc in SC

------------------------------