MKC

I have read the many posts about erratic idling, ICVs and faulty MAFs and have a question which is not answered in any of them.

My MY99 UK has been running perfectly until today. Suddenly, after booting it for several seconds (warm) the idle is hunting/pulsing between 1000 and 200 revs. Other than that, cold start and driving is perfect. I have left is several times and it will go on hunting for ages..

I have removed the MAF connector (note MAF replaced 11months ago) whilst its warm and hunting and it settles. Not a perfect idle for roughly between 850 and 950 with a v.slight hunting, but far better than before.

Reconnecting the MAF connector and the bad hunting starts again.

This appears to indicate that the MAF and not the Idle Control Valve is at fault. I sprayed the MAF with carb cleaner and it made no difference.

My query is, how come the car idles when the MAF is disconnected when the MAF is faulty but when the MAF is supposedly working ok, disconnecting it will stall the car !!??

The "removal of the maf connector test..." doesnt make sense.

Could a dirty ICV be the thing keeping it idling when the MAF connector is removed ? Is it worth me clening the ICV even though this maf connector removal test appears to indicate a faulty maf?

A technical explanation would be good...

Cheers...

Bob Rawle

"the removal of the maf connector" ... exactly, there isn't much sense to that test, the ecu is designed to run the car with the maf disconnected, it uses the map sensor, you can drive it, I could even map it to run like that but it would be very "course", the misnomer that its rev limited to 4k is not correct in this instance, its simply that they run so rich off the crude ve map that they choke up.

Its possible to do a detailed map up to 4k rpm then all you have is a proportional throttle fuel map to use, hence its crudeness.

It sounds as though your latest maf has given up, only a datalog can truly diagnose. Whats happened is the maf output has dropped off causing the ecu to try and use an area of the fuel map that's actually not there (fallen off the edge iyswim). Spraying the maf sensor will not help and its nothing to do with the idle stepper motor.

What induction setup do you have, if its not std air box based how is it mounted.

hope that helps the understanding.

bob

DaveC1

Could this cause the ecu to go into limp mode, thus loosing boost?

Bob Rawle

yes it could, if the ecu sees too much knock sensor output over a pre-determined number of drive cycles then it will drop boost to actuator control which means circa 0.6 bar only. It will also have dropped onto the safety maps and added fuel. Have to say that most of the ecu roms run the same maps for low and high det maps.

bob

DaveC1

Is there any other way of telling if you have dropped into limp mode, other than low boost?

MKC

Thanks for your help Bob. I didn't think the removal of the connector made sense. My boost has remained uneffected. The car drives as sweet as ever, it just idles badly when warm.

I have since swapped my MAF with my old one (for idle testing only) which never had a problem idling (but did hesitate at high revs and melt my piston once The problem was still there with my old MAF but fractionally different or a tiny bit better. I also took off the ICV and cleaned the black grime from the plunger thingy and with ignition on and off it seems to do what its supposed to do.

It may be worth noting that when I remove the connector from my old maf whilst its hunting, the car stalls and does not get better, but removing it from my current maf, improves the idling ??!! I tried this several times and got the same results.

I may get a new MAF anyway and the current ones a year old.

Can MAFs degrade further when left lying around, unused wrapped in plastic?

To conclude - As both MAFs do the same thing and as the old maf never idled badly before I presume that it must be something else. Are there any other sensors which could cause the warm idling problem but not affect normal running ?

Lambda sensor ? Any further advice would be useful.

Thanks and Regards,

MKC