[saand]

A while ago I noticed my car was using a lot of fuel just idling high when it was trying to warm up so I decided to make it more efficient. I didn’t find many detailed threads about how to fix this for older cars and this improvement isn’t in the list of 65 mods so read below to see how I did it.

I initially thought my car relied only on the electromechanical air bypass for cold idle compensation but after a bit of research with the workshop manual and a test I found that disabling the electric idle control did nothing for the high idle at start up.

My car is a 91 year Mazda 626 which has a thermo mechanical device which allows extra air to flow until it has warmed up. I believe newer cars only use a electro mechanical bypass device which is controlled by the ECU.

So the only way for me to make this improvement was to block the bypass air flow physically. The idle control module has both the electromechanical and thermo mechanical device integrated. It bolts onto the throttle body and has separated chambers for the ECU controlled electromechanical air bypass and the thermo mechanical device. This meant I could just block off the thermo mechanical bypass which would stop the high idle at startup and would allow the car to still maintain idle with ECU compensation if required.

That is exactly what I ended up doing, I tried aluminium foil at first however that just ripped apart due to the high suction. I then looked for some metal shim which I don’t have lying around but then I saw on an old thread someone did the same thing with a soda can. So I cut up a soda can cut it into the right shape to cover just the thermo mechanical air bypass section. I also poked a few small holes in the soda can metal so that some air would still bypass which would mean my car wouldn’t stall out as soon as I take my foot of the pedal. I screwed the idle control module back onto the throttle body with the metal sandwiched between the idle control module and throttle body. This held the soda can metal in place and the gasket seeled around the metal as well.

I did a quick test, the car starts which I was worried about, the idle then goes immediately down to almost stalling but at a good idle level, I have found that now after the mod the ECU uses the electro mechanical bypass to compensate for the lack of thermo mechanical air bypass.

After driving with this mod in place for a month or so I have had some small issues, when the car has started at cold in the first 30 seconds it can be prone to stalling. This happens if I use the throttle pedal to move the car down the drive way and then I lift the throttle off. Under this condition the ECU cant compensate quick enough to pick up the idle again so the car stalls out. I have gotten around this by using 2 feet one for throttle one for break (yes I have an auto) and this solution works well for me. If I couldn’t do it this way I could have opened up the idle control module and put a few more holes in my soda can metal which would fix the issue. I had also thought of manually controlling the electromechanical idle control valve which would be easy as well with a bit of electronics.

So now that the mod is done I am seeing a good improvement in efficiency, I do many small trips (2 or 3 a day) so the cold idle was using up a lot of fuel for me. I have not done detailed measurements but it looks like about a improvement of 0.5 L per 100km (1.5 MPG) for my tank fill ups.

So all up definitely a worthwhile modification. If I originally knew how to do it this mod would have taken about 1 hour but for anyone trying this you will have to take the time to work out how your idle control module works and which chamber does the cold idle compensation.