Air flow sensor

[pdmotors]

Hi all, not an exotic car but one thats doing my head in anyhow. Renault Laguna 1.9dci. Air flow sensor was nackered when i got it. I unplugged the sensor and as usual the car ran fine for a few weeks. Now the french bag of !&()%£! seems to be in limp mode, not venturing at all above 3200 rpm. Does anyone know of a way to permenantly bypass the sensor, as i cant thoil to spend £150ish on a Renault.

[stevieturbo]

The AFM is a pretty important item as far as modern cars are concerned. You cannot bypass it.

[350zwelgje]

Could be something else causing the limp mode. Have currently a disconnect AFM on my diesel (rover 75 - bmw origin), and it was and is a tip from the dealers as well and no problems. It is supposed to help diesels to be more environmental friendly as diesels do not need an AFM as there should be a lot of air and the amount of injected diesel (=quantity) determines the power. But as said before, it could be that you have a computer that needs this input, then the trick could be to find out the resistance or volts the ECU expects to run ok.

Rob

[CombeMarshal]

Have you thought that it might not be the AFM that is not faulty if there is still a fault, have you tried putting it back in?
Then Try ringing around some scrap yards

[andys2]

Try these guys:-

http://stores.ebay.co.uk/MIDDLESEX-TU

I've managed to pick up some useful parts from them, cheap and fast delivery too.

[stevieturbo]

350zwelgje said:

Could be something else causing the limp mode. Have currently a disconnect AFM on my diesel (rover 75 - bmw origin), and it was and is a tip from the dealers as well and no problems. It is supposed to help diesels to be more environmental friendly as diesels do not need an AFM as there should be a lot of air and the amount of injected diesel (=quantity) determines the power. But as said before, it could be that you have a computer that needs this input, then the trick could be to find out the resistance or volts the ECU expects to run ok. Rob

But if it doesnt know how much air its using, it cannot meter the fuelling, and more importantly the injection timing as well as it could do.
I would find it hard to believe manufacturers would fit an AFM if they though the car would runn better and cleaner without one.

Return voltage would continually change, depending on how much air it consumes.

[pdmotors]

Cheers guys, I was just being tight really i suppose i'll just have to fit one. I just thought you may be able to cast a little light on the problem as i don't generally have much to do with Renaults. Thanks for all your help guys

[350zwelgje]

stevieturbo said:

350zwelgje said: Could be something else causing the limp mode. Have currently a disconnect AFM on my diesel (rover 75 - bmw origin), and it was and is a tip from the dealers as well and no problems. It is supposed to help diesels to be more environmental friendly as diesels do not need an AFM as there should be a lot of air and the amount of injected diesel (=quantity) determines the power. But as said before, it could be that you have a computer that needs this input, then the trick could be to find out the resistance or volts the ECU expects to run ok. Rob But if it doesnt know how much air its using, it cannot meter the fuelling, and more importantly the injection timing as well as it could do. I would find it hard to believe manufacturers would fit an AFM if they though the car would runn better and cleaner without one. Return voltage would continually change, depending on how much air it consumes.

You are right if the system would be used on a diesel in the way you describe, but air is not one of the basic diesel engine design parameters for fuelling. In general they try to squeeze as much air in as possible, and always have too much and inject diesel in the quantity to get the required power. Total air volume in the engine is allowed to fluctuate as much as it likes until the 'smoke barrier' is reached. Lots of smoke is then visible, and that is called polution nowadays. Until now many use the info to adjust and improve on diesel exhaust emissions only, as a small correction to injection timing(s) or just a little bit of amount of diesel. So they use all the tricks up their sleeves possible to pass emission tests, as from the basic diesel concept it is very difficult to pass at all these days.

Rob

[jmcc500]

The Mass Air Flow (MAF) sensor is quite important for one reason not mentioned here. Basically, to reduce NOx emissions modern diesel engines have exhaust gas recirculation. The amount of exhaust gas is controlled by the MAF sensor - the more exhaust gas that is being recirculated, the less air there is going into the engine. Most diesels have a map of engine speed and fuel quantity which gives a MAF setpoint. Opening the EGR valve reduces the MAF until it reaches the setpoint, and then controls the valve to maintain the setpoint. Disconnecting this sensor will basically prevent the engine from meeting NOx emissions requirements, so on-board diagnostics (OBD) will detect this and put the car into limp home (detected because the MAF reading is consistently way off the setpoint, leading to a plausibility error).

The second reason (as mentioned) is to prevent over-fuelling during transients to try to minimise smoke under hard acceleration. In this instance there is something like a limit on the air fuel ratio, so until the turbo is spooled up and giving you enough air the fuel is limited.

There is no way of getting round the problem without getting a new MAF sensor (sorry!) as the OBD requires a feedback that ties up with the MAF demand map.

>> Edited by jmcc500 on Tuesday 14th March 23:49

[pdmotors]

Cheers for the feedback so far guys. I have fitted a new MAF sensor and the problem still persisted. I had the car plugged back in and the same fault code came up. I was told that it could be the air flow temp sender instead, so as they are only £20 I fitted one. To my dismay, the problem is still ongoing and i am running out of patience with the damn thing, not to mention money. I just wondered if anyone else had any more thoughts on the subject as it would be nice to know the true fault for future diagnosis. SOMEONE PLEASE HELP ME!!!
Cheers, Paul

[stevieturbo]

What sort of fault code scanner is being used, and does the operator know how to use it ??

Have you also considered it could be a wiring issue to/from the AFM ? Just because a fault reader says "xxx" doesnt mean it actually is that. It could be things around it, that affect its readings a lot, or its operation ( including wiring )

[pdmotors]

cheers stevie ill check out the wiring. i presume the guy does know his stuff though as hes been doing diagnosis for a long time now.