As a Miata owner, you are, no doubt, aware of your many blessings. Your car is a reliable, nimble performer capable of giving you many years of trouble-free, pleasure-filled driving. Your sprightly performer asks very little of you: a Saturday afternoon washing, a tank of gas now and again, and a little regular maintenance.
One of the "hidden surprises" of Miata ownership is that so much of the maintenance that must be done by a certified mechanic on most cars is nothing more than an afternoon idyll for the diligent Miata owner. Why, I almost wince when I hear my fellow Miata Club members tell the all too familiar story of taking their little friend to a mechanic for simple maintenance and paying huge expenses only to find the work poorly done. In hope that more of my brethren will discover the simple pleasures and personal fulfillment of doing some of their own maintenance, I will give you step-by-step instructions for replacing the O2 sensor modulator. I am sure that when you read these simple instructions, you will want to do this little operation yourself. After doing this easy task, you will not only have restored your Miata to top working condition but you will have gained the confidence to take on the other easy periodic maintenance tasks that will keep your Miata humming well into the next decade.
When your Miata reaches a certain age, you will notice a bit of surging beyond 4000 RPMs in second and third gear. That, my Miata friend, is a certain warning that the 02 sensor modulator is becoming impacted. Time to act quickly! We want to replace the little bugger before complete impaction, otherwise the whole diode array, or even the array non-compete timing mechanism could go. We wouldn't want that to happen on a cold January night - would we?
First, go to the auto parts house and get a replacement modulator. (If your Miata has power windows, be sure to get the modulator with the tripole connectors.) Do not get the WkM 60 modulator kit unless you have an automatic transmission. While you are at the autoparts store, you should pick up a short spool of #22 copper coding wire - in green if they have it, however any color (other than black) is acceptable. You will need a 9mm quadraflex socket, so this would be a good time to add one to your toolbox if you do not already have one. (About $34.00 if you can find one here or 78,000 yen on any Japanese Internet site.)
While most of the work can be done under the dash or leaning over the drivers-side front fender, steps 5-7 will require working under the car so you will probably want to put your car on jackstands before you begin.
Step 1. Remove the negative pole of the battery. (See that wasn't so bad, was it?)
Step 2. Carefully remove cowl cover and cowl backing by removing the seven sheet metal screws that hold it in place and then "gently" turn the spring clamps counter clockwise 2 and 3/4 turns until they click free. Sometimes the spring clamps are a bit sticky. When this happens to me, I sometimes find a gentle tap with a ball peen hammer helps release the sticking clips. Be sure to secure the self-centering capett stays beneath the cowl backing. The remainder of the work inside the car is best done laying on your back with your head resting on the break peddle, your shoulders on the floor, and small of your back on the edge of the drivers seat.
Step 3. Now that the cowl backing is removed you can clearly see the electromotive assembly up and to the left. Remove the four Henry nuts that secure it to the relay sending unit housing.
Step 4 Inside the housing, which is secured with four Lexan snaps, the connections to the modulator are attached to poles 3,7,9, and 11 except on 1.6 engines and on later models with dynamic retard senders, in which case the red, fuchsia, and mauve positive return check wire is on pole 8 instead of 9. Remove the pink and yellow positive wire and the positive return check wires by applying pressure to the right side of the spreague clip. If the spreague clip does not detach easily (mine didn't) you may need to use quarter angle pliers to help it. You are now ready to begin the install.
Step 5. Turning your wheels to the far right will make it easier to get to the plenum shield and the BHP unit. If you are careful, and you have small hands, you will be able to remove the plenum shield with the BHP unit in place. I found it easier to simply remove the BHP unit, but if you do, be careful to torque it to proper specifications. A BHP with an over-torqued spline can make a simple modulator replacement into a major task. We wouldn't want that - would we?
Step 6. The plenum shield is attached with 9mm hex bolts. (These are on tight - you should have soaked them with WD 40 last night.) The upper inside hex bolt will require the quadraflex socket with an extension. DO NOT completely remove the hex bolts, or the self tightening hig-bessel washers will fall into the plenum shield and they can only be retrieved with a 12 inch or longer magnetic probe grip.
Step 7. Now simply release the guy clips from wire trolley and pull the positive wire and the positive return check wire through the wire tunnel. (By the way, you should have soldered #22 green coding wire to the end of the positive return check wire as a guide, otherwise sending the wires from the new modulator through the wire tunnel is nearly impossible. It requires a metric wire tunnel crawler which is only available in Japan.)
Step 8. Just below and aft of the reciprocating fuel impulse unit you will be able to feel four henry nuts. Loosen the two rearward henry nuts until you can turn the impulse unit a quarter turn clockwise. This will reveal the main bracket holding the modulator. By applying gentle pressure on the flagett of the modulator with our handy quarter angle pliers, you can pry the modulator out, being careful not to damage the nylon cushion stays.
Step 9. Replace the modulator and follow steps 8 through 1 in reverse order.
Congratulations. You have restored your Miata to peak performance and saved nearly $20 in the process. And you have the peace of mind of knowing that the job was done right. Should any little problem turn up later - a rattle in the plenum shield (indicating a hig-bissel washer you failed to secure) or a dangling spreague clip, you know exactly how to fix it.
So, my Miata friends, happy motoring to you all. In my next column I will tell you how to dial in your MSD (with Bipes mod) and LINK II as a replacement for the OBDII for maximum performance.