Intake Air Sensor Woes?

raykinsella · Post by **raykinsella** » Mon Sep 01, 2008 5:43 pm

I noticed that my '74 412 started chugging out dark smoke at idle and generally running less than perfect. I also noticed that the fuel pump was now running beyond the 1-second it always does when turning on the ignition. It was running constantly. After looking at a lot of things I popped off the little hatch to the control area of my air intake sensor. There's a little spring-loaded thing in there that swings back and forth, depending on the rpms. Mine was stuck all the way over to the right; I gave it a little nudge and it popped back to normal. Result: the fuel pump no longer runs beyond the starting charge of 1 second, and the rough idle and smoke are gone. HOWEVER: when I give it gas it acts like it's starving for fuel and runs weakly, as if on less than four cylinders. But it idles pretty smoothly now.
Any ideas about this situation anybody? Thanks!

raykinsella · Post by **raykinsella** » Mon Sep 01, 2008 9:49 pm

UPDATE!
As it turns out I had a spare unit on hand - it came with the car with a whole bunch of other spare parts, so I know it is used but no idea of its condition. I installed it and turned the key on and right away the fuel pump started whirring away nonstop. BUT: when I started the engine it revved right up and it runs like a dream now. So I wonder what's up with fuel pump running like this now? Strange indeed.
I noticed that my new unit has the same VW/Bosch part# as the old one, but it also has an Audi# along with it, unlike the original. On the inside it looks a little different but is essentially the same, plugs right in, and like I said the car now runs GREAT.
Now I feel I need to get my old unit rebuilt, if possible, or start looking for a new one for a spare. Any tips about this?

raygreenwood · Post by **raygreenwood** » Tue Sep 02, 2008 12:35 pm

Slow down. Regardless of teh semi-happy outcome....the first rule with any EFI part (especially MAFs and MPS's)...is do NOT start swapping parts. Yes its gets you off the roadside and is the correct course of action in that case, but generally masks the "real" issue.

Ok...standard lecture done.

It sounds like you original MAF simply had a sticky flap. This can be from grime on the pivot...or from dust in the intake, debris...leaves...etc......or in worst case it can be from the wiper lever inside being cracked loose from its center axle mount.

As you know, the fuel pump only runs when you turn the key.... when airflow from immediate cranking lifts the flap from its stop....and moves the wiper contact from the parking location. After the key is released from start position...if the engine is running....the pump will continue to run as normal.

I would bet that the new part ...as most will...has differnt internal calibration (on the cogwheel inside of the afm)...from your original.

Since you had noticed that the original AFM had a stuck wiper contact...which means for 100% sure that either the AFM has : (1) a gummed up flap axle (2) a broken mainspring (somewhat rare but it happens)(3) or a broken loose contact wiper (rare)......I would start by removing the original AFM and first inspecting that the wiper contact moves when you move the flap with your finger. If that works, then visually check the at the clockspring inside is undamaged. Then if that is all correct...clean all moving parts with fast drying non-flammable contact cleaner.
My bet is that this fixes it.

What it appears is happening is that once you turn the engine off...something is preventing the flap from closing to its stop point.

Had you not seen anything getting stuck....like you did...Iwould suspect a bad pump timing circuit in the ECU.

Since the new AFM runs well....it simply means that the overall main setting of tension on the clockspring...is similar if not identical to what you had. However....this does not mean ...since you notcied that the internal configuration was different....that this unit was set up to use the same ECU. Ray

raykinsella · Post by **raykinsella** » Tue Sep 02, 2008 1:03 pm

Thanks, Ray
Before all this started the normal thing with my 412 id that when I turned the key on NOTHING happened, except the idiot light and fuel gauge came to life. No fuel pump sound at all. Obviously it came on once I turned the starter on, for the car started and ran(!) I also have a 1971 411 with the d-tronic sytem. In that car the pump charges for one second when turning the key on. Is that the normal thing? If so, then why was my 412 not doing anything when turning the key on? Oh yeah, when it's hot inside the 411 the fuel pump does NOT shut off. Is this a relay issue, the one under the dash by the headlight switch?

raygreenwood · Post by **raygreenwood** » Wed Sep 03, 2008 4:47 am

The pump not turning off is indeed usually a relay issue.

The L-jet system pump is not supposed to run when you turn the key. It is only supposed to run once the engine starts turning...so your L-jet was running correctly.
The D-jet is the one that has the pump timer circuit to give the pump a buzz when you turn tehe key but the engine has not yet been started. Ray

raykinsella · Post by **raykinsella** » Wed Sep 03, 2008 6:11 am

Are there two relays, one above the fuse panel and one bigger one behind the passenger back seat side panel? Are both bad or just the main one behind the panel? I do hear a clicking sound back there when I turn on the ignition.
So, when the original air flow meter was malfunctioning, aside from making the engine run badly, the pump was running non-stop. When I was able to get the meter unstuck the engine now idled okay but not at higher rpms, and now the pump did not run when turning the key. With the new meter installed the engine runs great at all rpms, but the pump runs nonstop again.
Is the meter sending some kind of signal to the pump relay, or to the "brain"?

raykinsella · Post by **raykinsella** » Fri Sep 26, 2008 5:34 pm

Well this is why fuel injection can be such a joy...
I just got a NOS air flow meter with the exact same Bosch part# as my original. After installing it I noticed that the fuel pump no longer turned on and ran constantly when turning on the ignition, so that is back to normal for an L-Jet. BUT: the engine starts okay but runs like crap again, missing badly and having no power whatsoever, stalling-out. I have installed all new plugs, wires, air filter, rotor, cap, points, condensor, and even adjusted the timing, all to no avail. The engine was running fine with the odd# replacement meter I had been using, but the fuel pump ran with the ignition on - plus there was a moaning sound coming out of the system somewhere when I turned the ignition on when the engine was cold.
So, something is going on with the meter - is there an adjustment I can make on it? Do I need to pry off the little black lid on it?
Befuddled in Seattle

wildthings · Post by **wildthings** » Fri Sep 26, 2008 10:18 pm

There are two things that turn the fuel pump on on an L-jet fuel T4. The first is the starter circuit. The same wire from the key which energizes the starter also triggers the fuel pump relay. The second is the set of points inside the AFM. Once the flapper moves a little the points should close triggering the fuel pump relay. The two different triggers are electrically separated from each other by a diode in the fuel pump relay. The points in the AFM can easily be adjusted by carefully bending the rod that opens and closes them. The points should open just as the wiper begins to move.

Buying a rebuilt AFM is often just a waste of money. They take an old unit clean the outside and maybe do the simplest of electrical checks. They then shove it into a box and charge a few hundred bucks for a units that is at best out of adjustment and maybe even worn beyond use.

raykinsella · Post by **raykinsella** » Sat Sep 27, 2008 6:32 am

Thanks for that, Wildthings. Yeah, with my original AFM the pump turned on when the engine started, as it is supposed to. Then, when I replaced my AFM with another used unit I had on hand, because the original was sticking and making the engine run badly, the fuel pump turned on as soon as I turned the ignition on - but the engine ran great with it. So now I have this new AFM, and the pump turns on only when the engine starts, and that is good. BUT the engine runs like crap again, too rich and missing, etc. Is there an adjustment I can make on the AFM? There's a screw under a rubber plug, and of course there's the black lid with all the electrics under it.

wildthings · Post by **wildthings** » Sat Sep 27, 2008 9:11 am

If your engine ran good with one of the old units I would use that one and just adjust the rod that opens and closes the trigger points for the fuel pump relay. Just pries off the black lid, identify the parts, and see what needs to be bent/adjusted to get the points to open and close at the right point, probably just bending the rod a little. Shouldn't take but a few minutes.

If you want to use your new unit you would also need to pries off its lid and check the condition of the resistance strip. It should look uniformly smooth and have no seriously worn areas. You also need to make sure the wiper stays on the resistance strip at both ends of its travel. You can set your mixture by adjusting the tension of the large coil spring by turning the cog wheel a notch or two at a time. All that takes is two screw drivers, one to hold the spring lock wire away from the cog and another to move the cog.

It is best to have some kind of exhaust analizer when adjusting the spring tension, but can be done by feel. The screw under the rubber cap needs to be adjusted after the spring tension is, it only affects the idle mixture.

raykinsella · Post by **raykinsella** » Sat Sep 27, 2008 6:26 pm

I'm pretty sure the mixture is too rich, both at idle and running ahead - the inside of the tailpipe turned black from just a few minutes of running. So for the idle screw do I turn it clockwise or the other way to lean in out? And which way the cog for the same result?
If I go back to the old unit I'll turn on the key to get the pump running and then look at the AFM for a little rod that shouldn't be touching anything and then bend it away a bit to see that the pump turns off - yes?
You know, with the used unit that turns the pump on right away, the car started a lot faster, after only a second of cranking (when cold).
I have a new cold start valve, new pressure regulator and new fuel pump relay in addition to all the new tune-up parts. I'll get onto the car again tomorrow and let you know how it goes - hopefully well!
Thanks again for your help. I really appreciate it.

wildthings · Post by **wildthings** » Sun Sep 28, 2008 10:26 pm

raykinsella wrote: If I go back to the old unit I'll turn on the key to get the pump running and then look at the AFM for a little rod that shouldn't be touching anything and then bend it away a bit to see that the pump turns off - yes?

Yes

raygreenwood · Post by **raygreenwood** » Mon Sep 29, 2008 5:21 am

I am not convinced that you do not have several small problems stacking up. For instance...if you have a malfunctioning fue lpressure regulator....an AFM that is perfect will run badly. Conversely....One that is not perfect may run correctly in certain ranges but not in others.

What is your fuel pressure like at idle and at WOT?
Also....adjusting the AFM is the very last thing you should ever be doing....if it is the same part # and spec that is supoosed to be with the system. This is not to say that the cannot or should not be adjusted. The question you should be asking....is what ELSE went wrong to make the original AFM setting....now be out of spec.

The problem with adjusting the AFM is that it is a progressive spring. Addiong tension to one location increases tension across the whole range removing tension does the same. It has to be a delicate balance or else it runs well in oen range and either crap or dangerously in others.

What about the condition of sensor 1 and 2?

Its worth it to bear in mind that the AFM flap does not supply all of the fuel for immediate enrichment on acceleration. The fuel pressure regulator does a fair portion of that.

I beleive the original mystery was ...why did the car no longer function the same with the original AFM when it was put back together. Quit swapping parts and slow down and trouble shoot.

This has all of the earmarks of a vacuum leak or a wiring harness issue. Ray

raykinsella · Post by **raykinsella** » Mon Sep 29, 2008 5:53 pm

Thanks for stopping by, Ray.
I do have a brand-new fuel pressure regulator, among other things.
I think it is the AFM because when I install the used one I have the engine runs fine - except that the fuel pump turns on and runs when I turn on the ignition. But Wildthings told me how to fix that.
Because I found an rebuilt AFM on ebay I went ahead and bought it and switched out the used one. The fuel pump thing stopped alright, but now the engine runs badly and is way too rich, so I'm thinking the new/rebuilt AFM is not set correctly.
I haven't had the chance to work on the car in the last couple of days, so I still can't say what I've found since my last post.
The original AFM was sticking - maybe I'll just clean and lube the flaps on it and reinstall it. The new/rebuilt one looks real nice, as if it was new, at least on the outside. It would be nice to get that one working right.

raygreenwood · Post by **raygreenwood** » Tue Sep 30, 2008 2:37 pm

Yes...the original was probbaly just sticking....but as wildthings noted...it may not just be the flap sticking...it could be a messd up wiper contact inside. Its worth checking to see if it can be repaired. Usually the wiper either dies from sheer miles and wear...or the box leaks and water corrodes the parts and circuits.

Yes...almost every rebuilt AFM must be adjusted.

What I am really getting at....is to go ahead and check your fuel pressure at idle and WOT, check your vacuum signature at idle and WOT, check your sensor 1 and 2 both cold and hot. The last thing you want to do....is adjust your AFM....when something else is off. because then when you set those parts straight....the AFM will bewrongly adjusted. Ray