Emission issues

[Disillusioned Dude]

Hi All,

Can someone tell me if my engine should or should not stop when i disconnect the MAF sensor please? I swear i heard the engine should stop when hot and the MAF is disconnected, but now i'm getting told allsorts and don't know which is correct!

The emissions were out of limit on mot Co2, HC And Lamda. I put cleaner through it and hammered it, cleaned PCV and tube. Changed oil, filter n plugs. Emissions were ok so i did the other mech work and took car back for re test now ALL emission levels failed! Even though they were right a week ago. If disconnecting the MAF doesn't stop the engine Do I neec a new one please folks. My head is battered! email richardarcher1310@gmail.com

Edited May 20, 2019 by FROSTYBALLS
Shouting removed

[Garth-1]

I recently replaced a faulty MAF sensor on my 2000 BMW 5-series and can tell you that the engine will definitely run, whether hot or cold, if the MAF sensor is disconnected.  If the sensor is disconnected the ECU simply guesses at what the fuel mixture should be in the light of what the other sensors are telling it (engine temp, rpm, lambdas).  In my case the mixture was weak, performance was poor but fuel economy was OK.  More commonly the mixture will be too rich, performance will be reasonable but fuel economy will be poor.

One caveat though.  I'm assuming the ECU in your 2000 Corolla is about as sophisticated as my BMW's.  If on the other hand it was programmed badly then it may not have the intelligence to take all relevant sensors into account and will simply give up if one of them is faulty or disconnected.  Trouble is, the manufacturers never reveal how their ECUs are actually programmed.

[Disillusioned Dude]

Thanks for reply Garth, I really need some one with a model close to this of mine i think. A guy on line shows a Hyundai test of MAF by disconnecting and stops engine if MAF is ok. He says if your car runs after disconnection then MAF is no good. There's a vid of it. (when hot)

My car did this when hot but not cold (on choke i guess) Now believe it or not i disconnect when hot and it just stumbles a bit and goes on running, BUT WHEN I RECONNECT MAF THAT WHAT STOPS THE ENGINE!?

I'm lost with it G I can't seem to find anyone with a definitive answer on my model. I been ill two years and the pot has a hole in the bottom , so to speak. Am utterly skint and got a car i thought i knew to get me back into work. Its forty quid for a "bootleg" MAF and thirty five for a scrapper (for god's sake) Don't wanna buy one and find it makes no difference type thing.

Dunno what the Avensis is like on this. The engine is same in 1.8 Celica of the same period Apparently this varies from one make to another and even models ie mine up to 2002 and maybe your Toyota at 2004?

Could you possibly put this question to the right people? Am only just online recently with a very old Laptop and got next to no idea what am doing. Come here for obvious reasons. If i can't get it through emissions and mot i can't afford get another motor.

Thanks again for the interest

Obv realise you will help if you can

[FROSTYBALLS]

Moved to the Corolla club. Further responses urgently needed.

[mrfixer]

If the ECU receives an airflow signal that is not credible (i.e. outside of expected normal range) then it will ignore the reading and use a substitute value based on RPM and throttle position. This keeps the car running. 

Very often a faulty airflow meter will give a credible but incorrect reading. The ECU will use the value and the car will run poorly. Often it will run better with the faulty airflow meter disconnect - although with poorer acceleration. The airflow meter is used mainly to control fuelling during transient conditions.

Before playing 'parts darts' you'd be best to get some diagnostic software and a lead to get some diagnostic data off the car - either a cheap OBDII reader or a version of Toyota Techstream (check out ebay).  There are literally 1001 things that could throw out your emissions..

[Devon Aygo]

As mrfixer says your car will run without a MAF connected, when disconnected the car runs on a "base" map ignoring the missing data.

Toyotas diagnostic:

 

[DeTomato]

I have a 2003 VVTLi. Disconnect the MAF with engine running and the engine will stop. However, it you have a loose air hose to the throttle body and filter, or there is a leak in any hose, the engine will continue to run, all be it poorly. Also check the pipes on the back right of the rocker cover connecting the pcv because this can allow air directly into the air pressure flowing behind the throttle body.