Revs Dropping on Idle after battery change
 

Just got new battery fitted about an hour ago to my P1 and it won't idle correctly now.

Revs sit at about 900 then drop to almost a stall....... then back up to 900 again and it keeps doing this over n over again.

What's that all about?

http://bbs.scoobynet.co.uk/showthrea...3&page=1&pp=20

Try some of the suggestions in my thread. My car is kinda knackered atm :D:D LOL

MAFFA

I had exactly the same problem after a battery change with my RB5.

Turned out it was the MAF sensor. I'd get that checked first.

My money's on MAF sensor as well. When you replaced the battery, the ECU would more than likely have reset. If the MAF sensor is failing, it will have been doing so for a while now, but you wouldn't necessarily have noticed it, as the ECU will have learnt its way around the problem. By resetting the ECU, all the corrections will have been lost...

MAF

It seems to have sorted itself out now...all seems to be running smoothly again.

Do I still need to investigate the MAF issue?

Almost certainly maf.

If you have a multimeter you can check this by the following:

Connect multimeter between the signal wire of the lambda sensor and earth and you should get a reading of >1 volt approx 0.8v that may dance about a bit.. If you have never found the signal wire of the lambda probe you will find 3 wires going to it, live earth and the other is the signal wire.

Re-set the ECU (disconnect the battery for 20 mins or so) then start the car and hold the revs about 2500rpm >3000rpm (this is sometimes more difficult to do than you think) for 20 seconds. Then let the car drop to idle. The car will hopefully try and hold idle. Now, watch the multimeter you will see it rise and fall a bit but after about 30 seconds you wil probably find that the multimeter holds about 0.9v and the car is very rich. This is a faulty lambda

For the techies out there - if you don't already know this but if you do I apologise - after dropping down to idle the Air fuel correction value will try and pull this fuel back from the DC of the injectors. It struggles due to the faulty MAF value metering too much air (and corresponding fuel) as it can only pull 25% off the value of that particular load site. It trys 3 times before basically giving up. Lambda registers approx 0.9v showing a rich mixture.

I have a great Deltadash log of this happening if anyone wants me to send it to them.

Regards

Gavin Wallace

Edited for the above posts benifit - The ecu will adapt over time to compensate for the extra fuel at idle.. Disconnect your battery and do what I've said above, that's if you want to have a P1 left... I've got heaps of MAF in stock if you need one fast?

Or a simpler way with reduced risk of being misled by a faulty lamda, is just measure the maf voltage when running.
1.0-1.3v is normal for a P1, 1.6-1.7v is a typical faulty maf reading.

Andy

Also worth remembering to use a digital not analogue volt meter, the input impedance of an analogue volt meter could damage a lambda sensor by drawing too much current. Just in case anyone has an old fashioned volt meter ;)

many thanks, will try this over the weekend.

P.S. where is the MAF and what does it look like? (this might be a good start)

This thread should help...

My P1 was doing exactly the same after the battery was disconnected, fitted a new MAF sensor and now its running as sweet as again!

Fitted new MAF today (supplied by Wallaceperformance) Disconnected battery for approx 30 mins.

Reconnected battery, started her up and straight away got a smooth idle speed.

Worth the money for the piece of mind in my opinion.

Nice one :)