swan

'ello all,

Clearly having a faulty MAF is a bad thing - what if there is NO MAF though? Then what happens? I'm guessing that some safe baseline figure in the ECU is used?

If there is no signal from the MAF (ie removed or disconnected) what does the car actually do? Will it even start/drive?

dowser

Idle poorly due to too much fuel, flash the CEL at you and completely fail to provide any spool because of not enough fuel...unless you've got a Link, then you'd use the MAP instead

Why do you think the MAF is bad?

Richard

swan

Richard, who no fuel? If the ECU is getting no signal, what does it do? From what you've said it overfuels at low rpm (idle) and then underfuels at higher rpm? This would mean it works on a standard level of fuelling irrespective of fuelling?

As I understand it the symptom of a MAF going bad is irregular idling.

ScoobyJawa

I unplugged my MAF when the car was idling and it stalled straight away!

ScoobyJawa

Richard's on the nail there

My car developed a bad idle (ie. up and down like a yoyo) after ECU reset. Merv at PE kindly whacked the Select Monitor on it and it was as rich on idle as you would expect with WOT CO2 wise. The ECU was having to pull fuel away to get the lambda to kick in. But the richness of WOT was only at idle, when I actually was at WOT the CO2 was way too low, car was too lean! Went well though! Fortunately Merv diagnosed it before the car went bang. phew!
Replaced the MAF with a new one and straight away the idle was normal. Select Monitor showed the ECU having to add fuel to get the lambda to kick in

[Edited by ScoobyJawa - 1/8/2003 3:59:00 PM]

swan

Why would the engine have to lean out the mixture to get the lambda to give readings? In a similar vein, why does performance increase if there is less fuel going into the system?

dowser

In regard to your second question - the Subaru (and most factory turbo applications) run pig rich for safety/longevity. A more efficient burn comes from a leaner charge mixture.

Richard

swan

If the engine runs too lean what happens? Not enough cooling because of less fuel means overheating? ie. when the MAF goes, the engine leans out and ....

dowser

You'll get detonation and a dead engine sooner or later worse case....

To confuse you some more; the MAF is used for all load related decisions, so not only are the fuel load zones used innapropriate...so are the ignition ones

So you end up with a engine running both fuelling and timing values programmed for a lower load value....too lean fuelling and too advanced ignition. Engine starts to det, ecu tries to pull timing via knock correction maps...which if you're lucky will just give you the timing you should have been running in the first place...but with much less fuel then expected....car continues to det (uncontrolled explosion of charge - it *should* burn slowly...relatively speaking).....mechanical failure isn't far away at this point

Supposedly the ecu can shut down boost if it's seeing too much det and on the knock correction limit (assuming it still has control of boost ), but I've never seen it.....and ain't brave enough to test!

Of the last 3 cars I've mapped, 2 needed new MAF's. The owners were unaware; one just thought he had a very fast unmodified car (around 240ps) - the MAF issue was obvious on this one during the health check.

The other read OK during the health check with stock boost, but after remapping car was giving it's knock correction maps a good workout - 20% more boost resulted in precisely .01v more output from the MAF (should normally be >.2v)

This was the first time I've seen a MAF go like this, and suspect if the owner had decided to fit an FCD and MBC, rather than the remap, his engine would have gone bang quite quickly - the car was faster than before and only someone familiar with the engines and what they should be doing would pick up the problems without the use of monitoring equipment.

Having said all the above, I've never known a car to terminate itself due to a slowly failing MAF - I'm sure it's happened, but I've never seen it. Switzerland don't get any imports, and have nice 98RON flowing through their veins

Richard

dowser

Faulty MAF and failed MAF are 2 different things. When they fail the engine generally dies - mine went wot in 3rd at 5k....I sh*t my pants such was the bang, flames and smoke out the rear

However, my car started straight away afterwards - With a CEL. It then seemingly runs a base fuel map regardless of load (which it no longer knows ), this is enough to let you drive the thing to a garage. I assumed it was doing some sort of TPS compensation, but WOT will be welcomed by coughing and spluttering, and idle will be waaaay too rich.

A slowly failing (or contaminated) MAF is harder to detect - the voltage slowy drops so less fuel goes in....performance then gets better until something goes bang

Richard

[Edited by dowser - 1/8/2003 3:32:32 PM]