Issues with idle

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: 2007 NC 2.0 Sport
  2. I’m based near: Farnham
  3. I’m looking for technical help or recommendations on: issues with idle

Hi everyone

Knows there’s a few threads on this topic but they’re slightly different to my issue.

Car had a new battery put in just over a week ago, ran just fine and started fine until yesterday. When starting the idle jumps around a bit, goes down by around 100rpm at least and feels like it’ll stall. Not every time but a few times now.

Went out this evening put it in reverse and used the clutch to back up. CEL blinked on and went away and again the idle jumped around a bit. Also does this randomly when at lights etc in neutral.

From what I’ve read it could be

  • dirty throttle body
  • MAF sensor
  • Potentially o2 sensor?
  • Sparks and coils
  • Might just need to re learn throttle position etc since battery was disconnected

I’ve driven about 150-175 miles since the battery was replaced so I wouldn’t have thought it’d be the ecu re learning.

Anything else I should take a look at tomorrow?

Also if wanted to ‘force’ the ecu to re learn should I disconnect the battery first?

Thanks!

2

I think the ECU learning procedure is done at idle not whilst driving.
Threads on forum for this.

1 Like
3

Hi

For some reason MK3’s are very sensitive to electrical supply.
When you changed the battery did you thoroughly clean both the terminal connections (inside the clamps) and ensure they are really tight?
Also check that the battery negative is connected tightly to the body and no corrosion is present.
Check the braided earth strap that goes to the front of the engine (mine was in a poor state.
Finally, if you have a multimeter check the voltage at the battery immediately after it is started and ensure you are getting a good 14V.
Good luck.

2 Likes
4

Let it tickover for 20 minutes.
If it came on with the battery change then It’s unlikely to be anything other than the learning process.

5

So I didn’t actually change the battery myself, had a garage do it when doing a bit of work on it.

Will check the connections this evening as well as the braided line. Also have a multimeter so can check voltage too.

Starts and runs just fine it’s very random when it decides to fluctuate.

Thanks for your help!

6

Brill can give this a go tonight. Obviously since the battery was changed I’ve been driving it around. Reckon I should disconnect the battery first and start from square one and leave it to idle after reconnecting it?

7

For how long it takes to disconnect the neg, then it won’t hurt. The first time I did mine, it took 2 x 10 mile journeys to correct itself. But every subsequent removal, I’ve just let it tickover for 20 minutes ish.

2 Likes
8

Awesome will give this a try and see what’s what after then, cheers!

9

So I’ve run my obd scanner on it, no codes at all.

Battery voltage is fine when off, cranking and at idle

Checking the live data it’s not picking up a reading from the front o2 sensor. Guessing this would be faulty wiring or a busted sensor. Weird thing is that it’s not bringing up a code and no CEL.

The throttle position relative is 2.4% which I’ve read is slightly high, anyone know any more on this?

Throttle position absolute is 11.8% - I have no clue what this should be.

RPM sat around 900-960 which is slightly high.

Intake, MAF and timing are all fine.

Long term fuel trims sat at about -6 which I believe is running rich. Read that as long as it’s below -10 that’s fine but will investigate further.

Going to check the o2 sensor when I rev the engine to see if it reads any voltage.

Any tips? And anyone know anything about throttle position values?

Thanks!