Wavering idle speed

[my2cta]

My daughter has a 1990 that has recently developed an idling problem. When returning to idle after driving, the speed drops to around 500 rpm, then rises to 1000, then drops to 500 and so on, until after a few times of this it settles at a steady 800, as it should. The number of times it can do this varies, from one or two to half a dozen or more. When started from cold the idle was a steady 1500.

1) This 'hunting' didn't happen until after she had to get the car jump started when she left her side lights on all day and the battery went flat. It happened immediately after that. Could the jump start have affected the ECU in some way that could cause this problem?

2) When I got out of the car last night and it was doing this, it smelled as though it was running rich. Smelled rich cruising up the hill too.

3) It hasn't had a service since she bought it - 8000 kms, may not have been done for a while before that either by the previous owner. Obviously needs its oil and filter changed urgently, but can lack of servicing affect the idle speed/mixture as well? Thought the ECU managed all that automatically.

I've just taken it out for a drive this morning. Apart from the idling problem everything else seems to be running perfectly. Engine picks up cleanly from low revs under load (accelerating from 2000 rpm in third up a steep hill) and from a spirited standing start through the gears, revving to 7000 in the first two. Also runs flawlessly at 3500 in top.

[Fletch]

If the car has its original panasonic AGM battery (or a genuine Mazda replacement), the following comments apply.... if its been replaced with a standard wet-cell, then ignore them.

Its not the jump-start, per say, but the running flat just before that....

The AGM batteries dont like to charge of the altenator like most car batteries do, and often wont come back to full life from just driving around after a jump-start. The battery needs a 24 hour trickle charge from a battery to restore full health.

Heres a thorough reference...
http://www.miata.net/garage/battery.html#orig

Fletch.

[my2cta]

It doesn't have the original or equivalent, it has a wet cell replacement. It was put on a trickle charger for the night after the jump start, around 14 hours. Apart from that it has just been getting recharged over the last few days by 25 minutes drive to and from her work each day.

[aidan crabtree]

Try this site http://www.miata.net/garage/KnowYourCar/S7_Idle.html

[GEEMCB]

Hi, Tim

Don't know if it helps or not, but check out this thread in idle droop - symptoms sound quite similar. The battery issue could possibly be a red herring ....or not !

http://www.mx5forum.co.nz/viewtopic.php?t=2867

Regards