Testing M42 air flow meter

[Dr Shuffles]

I'm having some mysterious engine problems, and I thought I'd traced it down to the potentiometer in the air flow meter. I followed the testing procedure in Bentley: measure the resistance across pins 1 and 2, and see if it varies as you move the vane (they didn't say what the resistance should be, just that it should change smoothly). The resistance didn't change at all, so I figured the meter was bad and ordered a rebuilt one. The resistance on that didn't change either. I sent it back, and ordered another from a different company - same thing.

I guess it's possible that I got two bum units in a row from two different sources, but it seems unlikely. Which makes me suspect that maybe I'm doing something wrong. If anybody who's reading has an M42, a multimeter, and a few spare minutes, can you see what results you get? You just need to unplug the cable from the AFM (the pins are numbered - if not, then 1 and 2 are the ones at the bottom) and unclip the top of the air cleaner box so you can reach inside the AFM and move the vane.

I found this thread, which describes the same problem, but doesn't really answer anything: http://www.318ti.org/forum/showthread.php?t=11740

[pdxmotorhead]

did you remove the hose from the meter to the throttle body and check for splits and cracks, also inspect the idle air control hose and its mate under the intake manifold...

Dave

[bazar01]

What problem are you exactly having?

The airflow meter has 4 pin connections according to the diagram.
The first variable resistor is at terminals 14 and 41 and the second is at terminals 14 and 76
The air temp sensor is at terminal 14 and 77

[Dr Shuffles]

Thanks. Since then, I've looked at two other circuit diagrams besides yours, and re-tested the sensor. It all points to one conclusion: I'm not stupid, just unlucky, and I really did get two bad AFMs in a row.

Instead of buying another one, I think I'll take a stab at opening up my old one and fixing the potentiometer. It's already broken, so I have nothing to lose.

I described my original problem here: http://www.318ti.org/forum/showthread.php?t=39314

[Dr Shuffles]

Originally Posted by pdxmotorhead

did you remove the hose from the meter to the throttle body and check for splits and cracks, also inspect the idle air control hose and its mate under the intake manifold...

Yes, I did. I also replaced both of the hoses connected to the ICV.

[bazar01]

Originally Posted by Dr Shuffles

Thanks. Since then, I've looked at two other circuit diagrams besides yours, and re-tested the sensor. It all points to one conclusion: I'm not stupid, just unlucky, and I really did get two bad AFMs in a row.

Instead of buying another one, I think I'll take a stab at opening up my old one and fixing the potentiometer. It's already broken, so I have nothing to lose.

I described my original problem here: http://www.318ti.org/forum/showthread.php?t=39314

It's worth a try to open what you have and take a look at the variable resistor carbon track or loose connections at the pins.
There is a black plastic cover that is sealed and glued to the aluminum housing with clear rtv. You have to cut thru that rtv to open the cover. Once open, you can see the guts and test for variable resitance and continuity on the pins.
Post pics.
Good luck.

[Dr Shuffles]

I haven't fixed the idle problem, but I did learn something important: the Bentley manual is lying. The circuitry inside doesn't look anything like their diagram. Which could mean that their method for testing the AFM is worthless as well. So now I have no idea if my AFM is broken or not.

[bazar01]

Air flow meters hardly breakdown.
I had a 944 S2 with over 220k miles on the original Bosch AFM. But at a certain mileage, the carbon tract wears out on the section that is mostly used. Usually it is the 2500-3500 rpm range. What I do is just bend the wiper arm so it wipes on a fresh carbon track. The carbon tract on you AFM does not look bad and I don't see any worn section at all.

You rough idle problem could be unmetered air. The fact the idle smoothens out with the AFM unplugged, it tells the ECU to disregard the AFM barn door opening which controls air fuel ration or fueling.

When you open the oil cap do you notice a change in idle rpm? When was the last time you checked the valve cover gasket?

[Dr Shuffles]

Originally Posted by bazar01

You rough idle problem could be unmetered air. The fact the idle smoothens out with the AFM unplugged, it tells the ECU to disregard the AFM barn door opening which controls air fuel ration or fueling.

I suspect you're right. It does look like a vacuum leak, I just can't find it. I might try to rig up a smoke machine. Or just replace the air hoses to the injectors, which are the only air hoses I haven't replaced yet.

Originally Posted by bazar01

When you open the oil cap do you notice a change in idle rpm?

When I take the cap off, the idle seems to get worse, but it's hard to tell.

Originally Posted by bazar01

When was the last time you checked the valve cover gasket?

Never. You think the gasket might be letting air into the crankcase?

[bazar01]

Originally Posted by Dr Shuffles

I suspect you're right. It does look like a vacuum leak, I just can't find it. I might try to rig up a smoke machine. Or just replace the air hoses to the injectors, which are the only air hoses I haven't replaced yet.



When I take the cap off, the idle seems to get worse, but it's hard to tell.
It shoudl stumble when you remove the oil cap.
If it is hard to tell, there is a vacuum leak.


Never. You think the gasket might be letting air into the crankcase?
We had to change the valve cover gasket at 205k miles since it will not hold idle and idle was rough. After changing the VC gasket, it still will not idle smooth and came to find out the new gasket was not lined up in the back side close to the firewall. After we got the VC gasket lined up, it idled smooth.

See repsonse in blue.

[Dr Shuffles]

Well, the VC gasket is definitely leaking at the lower edge. I can't see it, but I stuck my finger down there and there's fresh oil below the gasket. So I'll have to fix that no matter what.

Thanks for the help.

[pdxmotorhead]

They get really brittle so when you change have a shop vac ready and carefully remove it a piece at a time, if its gotten brittle... there are O rings around the spark plug holes. and O-ring gadgets around the bolts, you have to buy the o-rings separate from the gasket kit in some cases.

Not a hard job the rear lower bolt on the cover is a bast**D to get if you have big hands..

Dave