MKI Adjustable Air/Fuel Mixture

From: Jorn Innset (jorni@sn.no)
Date: 02 Oct 96 21:27:28 +0000
Subject: MR2 MK1: Adjustable air/fuel mixture - a 'How-to'

EFI Performance Tuning
from the book 'Toyota Performance Handbook'
by Pat Braden ISBN: 0-87938-320-8

(Note 1: I take no responsibility of what you might do to your car
by following these instructions!!!)
(Note 2: I've not included non-MR2 relevant comments)

When  installing higher-performance camshafts in Toyotas equipped with electronic fuel injection (EFI), you'll have to increase the amount of fuel injected for the engine to work at full efficiency.

The  procedure for EFI mixture adjustment was supplied by TRD for race cars  *only*. It will cause a car to fail emission control tests and void its warranty, therefore I do *not* advise it for street car modification. Also, do not make this modification with the standard catalytic converter in place. A rich fuel mixture will cause the catalytic  converter to overheat, and may result in severe damage. Install a high-performance exhaust system first.

Follow these steps:

1.   Locate the water temperature sensor. On MR2s the sensor is located on the water neck at the rear of the engine.  The sensor is bright green with a rectangular base and has a two-prong plug with a rubber boot.

2.   With the engine off, disconnect the plug and unwrap the plastic tape at the rear of the boot. Pull the boot forward to expose the two wires under the boot. There should be a brown wire and a green wire (sometimes green with a white stripe). You want to tap into the green wire. You may do it here or by the computer (My note:  We did it at
the computer, i.e. the CPU in the boot). It's preferable to tap into the wire near the computer. You can trace the wire using an ohmmeter only if you disconnect the multi-plug containing the wire at the computer. See your workshop manual for the location of the computer in your car (Me again: It's in the boot!).

3.   Locate and cut the green (or green and white) wire and splice a 0-2,000 ohm linear (not logarithmic) variable resistor (also called a potentiometer) with a 1.25 watt capacity in series with the water temperature sensor. The resistor can be purchased at any electronic supply store for about £1.

4.   When wiring up the resistor there will be three prongs coming out of the resistor body. You will want to use the centre prong and one of  the side ones.  Attaching the wire to one of the side prongs will cause the resistance to increase when the knob is turned clockwise. The other prong will cause the resistance to increase when the knob is
turned counterclockwise.

If you mount the resistor in the dash, you'll be able to adjust the mixture while you drive. Use fourteen-gauge stranded wire to install the resistor and put some kind of pointer on the resistor knob to indicate its optimum position.

5.  Determine the optimum position by adjusting the resistor until rpm increases slightly. If the engine chokes, sputters and smokes or if the 'check engine' light comes on, it's too rich. Back off a little. You should feel a big difference between on and off.

6.   Next, locate the airflow meter. To find it, first find the air filter. On MR2s, the airflow meter will be the square metal box with a black plastic lid located on top of the air filter canister.

7.   Remove the black plastic lid of the airflow meter by cutting the silicone seal with a razor and gently prying it out.

8.   To adjust the airflow meter, begin by clearly and permanently marking the location of the hold-down clamp on the gear wheel. This enables you to return to the stock adjustment easily if necessary.

9.   Next, loosen the hold-down clamp set screw and rotate the gear wheel three to five teeth counterclockwise (one or two when TRD big injectors are fitted). Retighten the hold-down clamp set screw. Do not exceed five teeth  because the engine will bog between shifts. This adjustment will improve throttle response and allow the engine to
rev freely.

10.   Reseal the black plastic lid with silicone sealing compound. Check the integrity of the seal by looking at it carefully; the electronics are easily damaged by moisture.

I recommend this book heartily - it contains lots of information for
MK1 MR2-owners!

 Jorn Innset   jorni@sn.no

Visit Jorns website http://home.sn.no/~jorni  CHECK THIS LINK!
 

Reproduced with permission