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saand 08-14-2010 10:17 AM

Cold engine start, high idle reduction mod
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 didnít find many detailed threads about how to fix this for older cars and this improvement isnít 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 donít 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 wouldnít 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 couldnít 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.

Frank Lee 08-14-2010 11:44 AM

1. Turn the key
2. Engine starts
3. Go.

There. Even better.

RobertSmalls 08-14-2010 12:48 PM

What's the minimum coolant temperature at which the car idles smoothly now? How cold is winter in Melbourne?

If you're trying to negotiate a driveway or parking lot with a stick shift that won't idle, you can have one foot on the clutch, one on the throttle, and use the handbrake if required.

Arragonis 08-14-2010 02:33 PM


Originally Posted by Frank Lee (Post 188837)
1. Turn the key
2. Engine starts
3. Go.

There. Even better.

Are you really telling me that burning petrol generates heat ? When did that come out.

Frank Lee 08-14-2010 03:59 PM

It's a very recent discovery so I'm not surprised it isn't common knowledge yet...

nateman2007 08-14-2010 04:12 PM

unless the temp is below freezing or you live by the on ramp by the freeway and will be accelerating rapidly, there is no reason to warm up.

Just drive off smoothly

saand 08-14-2010 09:23 PM

Robert the winters in Melbourne don’t get cold at all at worst overnight 0 degrees but it never snows so for those in colder climates you may have issues with this mod but a manual control of the electric idle control would work for colder climates

The engine idles smoothly even at its coldest the only problem is if the car is just sitting idling. Only an issue when moving slowly and taking the foot off the throttle at cold. I don’t know the temperature of the coolant where this doesn't happen, I have only ever had it happen when reversing down my driveway so it could be related to getting the oil in the engine moving around and warmed up a bit rather than the coolant. Either way as soon as I accelerate up to speed limit no longer have the issue, this seems to get things moving enough to stop stalling.

Frank I like your suggestion of start and go, my issue is that I have traffic lights about 100 meters away from my home on my trip to work so the car still idles high at the lights and if I catch them at the wrong time I’m idling high for about 3 minutes. That’s why I concentrated on this mod. For those of you out there that have many miles before you stop at a intersection this mod may not give you great gains.

Frank Lee 08-14-2010 11:03 PM

OIC. Perhaps a grill block would help in your situation?

brucepick 08-15-2010 07:21 AM

Good on reducing what looks like an excessive cold idle speed that was originally engineered into the car. If you increase the holes you left in the soda can material, you might not need to baby the accelerator to keep the engine going.

A grill block definitely helps. If you can find a way to read engine temp digitally I'd recommend that in conjunction with the grill block. That enables you to see if it's running a bit hotter than normal. It WILL run hotter, given enough grill block and warm or hot weather. Dash gauges are nice but typical factory ones don't give you precision information.

You can eliminate most cold idling by killing the engine when waiting at traffic lights. But you do need to be prompt about starting it up again, or you'll likely hear from the drivers behind you!

Arragonis 08-15-2010 03:21 PM

Be careful with lots of starts and stops especially if the engine is running rich. Petrol washes oil off the sides of the cylinders so you may be starting again with no lubrication. I think the start/stop systems fitted as OE wait until the engine is warm before starting to have an effect.

Another suggestion may be a block heater but they can be spendy.

Phantom 08-15-2010 05:44 PM

To keep it from stalling a bit when backing out of the drive way you could back in to the driveway when the engine is not and just drive forward when leaving.

saand 08-17-2010 04:28 AM

Thank you all for your replyís and suggestions.
For those interested. I already have a grill block which I put in for aero improvement rather than keeping in heat. I didn't notice much change in idle reduction. As brucepick suggests it looks like the high idle was designed into the car for some reason probably for the very worst case situation it might need the high idle. The thermo device that reduces the idle at temperature runs off a coolant line so the coolant has to get warm before the idle reduces which means the whole engine block has to get warm. This takes more than a few minutes even with a grill block unfortunately. I also toyed with the idea of a block heater but I donít think I could ever be bothered turning it on in the morning and at a guess the money used on electricity to heat up the engine would be comparable to the fuel saved.

Yes I could kill the engine at lights which would also get around my high idle problem but I donít want to risk having my car not start at the lights. It has been temperamental at times and I would hate to hold up traffic. But I did still think about this and I guess for those prepared to take the risk it would be worthwhile and if your worried about wear on your starter, brushes are cheap to get. From reports on various threads it seems not much else wears out (I could be wrong though).

I had also thought about backing into my drive which would help somewhat however im on a main road so very difficult to do and would waist more petrol waiting for cars to pass so I could back it in.

Phantom 08-17-2010 11:36 AM

I'm not sure how the Throttle body (TB) is set up on your car but there should be an idle air stop screw, the screw will adjust how far back the TB plate will close since the arm the throttle cable if connected to rests on the screw.

With this you have two options tighten or loosen it.
If you tighten it the TB plate will not close as much this can help your stalling problem when cold and a 1/4 turn might be all it needs, but this will slightly raise the idle speed as the TB will let more air in at 0% throttle.
You could loosen the screw (possibly) and that will close the TB more with 0% throttle. That could make the stalling problem a little worse but as you said that could be fixed by making the popcan hole larger. The good thing with this is it can slightly drop the idle speed at all temps after correction has ended. One issue that could come with this is the TB plate might stick a little when starting the car cold but a small turn of the screw can correct that if you backed it out to much.

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