Just thought i would let you know of a problem i had this week.
Replaced the original battery on my 1997 ZXi, The japs make them to last.
The battery was only removed for a minute or two whilst a new one was fitted.On driving home i noticed the revs ticking over at idle between 450 and 850 rpm.
Normally is runs sweetly at 750.
I have since found out that it is a common problem and what is happening is that the ECU is trying to find the optimum revs according to your car and driving habits. It takes around 30-40 miles to relearn this.
Anyway today its running as normal. Hope this helps anyone with the same problem
Steve..

this isnt a problem. It happens to most carswehn you disconnect the battery. The ECU has to readjust itself to the car.

This has reminded me of an interesting point,
Last year I witnessed a local garage / tuning shop, tune my friends CRX (modified with exhaust / filter, FSE Boost Valve etc) .Before he set the mixtures he disconneted the battery overnight and let the ECU resume default settings. The car was then put on the rolling road and run for 20 mins at high revs in all gears. He then set the mixtures etc.

When I asked him what it was all about he explained that most jap car ECU's have generic external parameters, like ambient air temprature, humidity, mean air pressure etc set in the ECU to allow the cars to deliver consistant performance under all atmospheric conditions in any country. The standard mapping is programmed with these values and installed in the cars at new.

The reasoning is basically, if you reset the ECU by disconnecting the battery, you can allow the ECU to remap its self, with new values using the actual measured and experienced, atmospheric conditions. If you run the car at high revs after letting the ECU reset, the ECU resamples specific parameters and creates a FAR (Fuel Air Ratio) map that is taylored to your specific state of engine tuning under actual atmospheric conditions for the country you are in.
I can understand how this all seems to work, as the average humidity, air temprature and pressures in the UK and most of Europe are far different from those found in the far east, or hotter countries where the universal average setting is determined.

The guy also suggested that even standard cars would benefit, as remapping also helps the ECU absorb and adjust for any minor mechanical variations and or wear between production engines.

The results on my friends CRX were quite stunning, i.e from 130bhp up to 175bhp just by mapping the engine to accomodate the modified parts in this manner.
I have however heard that the performance increase doesnt last, as the ECU can take reference readings to determine an average over several months, which modifies the mapping back to a more ecconomical state over a period of time. I also doubt if such massive gains would be seen on any of the mazda motors, as VTEC's are pretty unique and are well known to be good engines to squeeze more power from.

Could be food for thought, and worth looking into, for anyone who wishes to gain that little bit extra from their car, and get the full benefit of any modified parts, without the expense of a piggyback ECU.
I think the garage charges around £100 for the tune up and it takes around 1 hr. I dont think that there is anything special aboiut what he does, as the ECU takes care of the hard part. I have also heard that, anyone can do it, if they can find a bit of road long enough and free of cops, so they can give their car a thrashing after a ECU reset.

@ steve holt - I changed my battery on my 98 ZXi last month and noticed the same thing, but I believe that 850rpm is the default at idle.



Cheers