Advice on Air Meter

RobAMerc

I have been told that my Merc needs a new Auxilliary air valve (air flow meter I think) and thats whats causing the 12+- mpg and the fact she will not idle when cold.

A mechanic friend I recently met tells me that does not sound right, does anyone have any ideas?

Cheers

mustang 69

Yep I know of 2 Mercs with the same issues fixed by replacing the air sensor

MercMad

What Merc is it again ? Remind me of the model ? Is it a 280CE ?

An auxilarly air valve is a different component to an air flow meter so somebody is confused !

RobAMerc

Tis the Auxilliary air Valve I have been reliably informed, but I have also been told that that is the same as the air flow meter ? Perhaps not???

mike65

Auxilliary air Valve is a mechanical deivce connected to the fuel/air vacume system, the air flow meter meaures the air flow into the engine (I call it the throttle body myself!).

air valve


Mike.

MercMad

No they are definitely NOT the same thing, on ANY car, never were or will be !

The air flow meter literally does that, it measure the air flow. On your car it does so mechanically by detecting throttle plate position. This is relayed back and the appropriate ratio of fuel is supplied.

The auxilary air valve is a byepass that allows a measured amount of air in when the throttle is closed, so it can idle. Its an electrical disc switch and sometimes the hoses to and from this can be perished, a pinhole can cause problems !

May cause your idling problems, but not poor MPG, and most likely the problems will be there always !

If you issue is only when cold then you may have a defective cold start injector or warm up device !

AMurphy

MercMad wrote:

No they are definitely NOT the same thing, on ANY car, never were or will be !

The air flow meter literally does that, it measure the air flow. On your car it does so mechanically by detecting throttle plate position. This is relayed back and the appropriate ratio of fuel is supplied.

The auxilary air valve is a byepass that allows a measured amount of air in when the throttle is closed, so it can idle. Its an electrical disc switch and sometimes the hoses to and from this can be perished, a pinhole can cause problems !

May cause your idling problems, but not poor MPG, and most likely the problems will be there always !

If you issue is only when cold then you may have a defective cold start injector or warm up device !

Depends.
there are so many different types, designs and implementation of the same device over the years, that without some idea as to the year/model and EFI system, it is pretty much impossible determine exactly what may be amiss.

However, I'd read teh use of the word "auxilliary air" as above, essentially a choke when cold and an idle regulator when warmed, with nothing to do with the Air Flow sensor, which is a seperate item somewhere else in teh air intake.

anyway, it is it external as explained above and leaking, the fuel mix woule be lean, and if it has an O2 sensor, woudl try command the ECU to pump in more fuel to correct the mix. However, I'd not expect the O2 adjustment to be great. However, the ECU may be able to detect the disparity between the air flow sensor volume and the actual O2 sensor signal and put the whole system into a limp or open loop mode, "just to get you home".
So If your engine is kitted out as described above, a leak coudl cause mix problems, so look for cracked, broken and leaking air intake hoses, no matter what they are connected to.
These leaks woudl certainly cause cold ideling problems, as would this valve if it were stuck/gummed shut, so insure it is working as expected.

That aside, EFI temperature sensors that report the engine as cold, will drive the engine ECU/EFI into a rich mix mode.

So don't assume the air flow sensor is the only possible cause. However, a relatively simple hand held scan tool should be able to get any fault information on a recent vintage car.

mike65

RobAMerc has a 280ce about 1974/5 I think.

Mike.

RobAMerc

Yes its a w114 '74 280CE

Problem now is it has started breaking down a bit while running now too.
I hope its something wrong with the new Dist cap and HT leads!

Thanks for this guys, I might just wait and pick up a runner with lots of tin worm and cannibalize it.