'79 Fuel Injection Poor Performance Over 60mph

[garyt]

I finally watched the video and yes, you open the flap on the AFM by moving the wiper arm and it actually loses rpm, which is very strange, because opening the flap should let more air in (of course, as we all know), so this is definitely wrong. But really you should try pushing the flap with a stick to see if the wiper arm moves and the same thing happens. If it does, and the flap isn't stuck, it seems to me like an electrical problem, possibly with the wiper arm or its connections. Or possibly it is set so rich (idle mixture screw) that more air simply floods it?

[busdaddy]

Moving the flap has no effect on the amount of air entering the manifold, that's done by the throttle butterfly. The flap is a meter and moving it more than the existing airflow already has fools it into richening the mixture. Alternately vacuum leaks cause a lean mixture because more air is getting into the manifold than the flap has metered.
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[foxmon]

[garyt]

Yes busdaddy that's true, but the throttle butterfly only lets in the air (if no vacuum leaks) that the AFM flap lets in and which comes before it.

[aeromech]

[cmonSTART]

+1 on Amskeptic's page on the AFM. It helped me immensely this year getting things dialed in. Graph your mixture to start like he suggests so you have an idea what adjustments you're going to make.
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'78 Bus 2.0FI
de K1IGS

[garyt]

I suppose its correct though, if the throttle isn't opened the air flow flap wont have much effect, but here it is decreasing rpm which seems strange to me. And I assume it is also also true if he revs it and the throttle opens.

If that garage really did everything they said, then it isn't the fuel injection, but something in the engine wrong, because you got spark, compression and fuel.

[garyt]

LOL aero, oooo, no not really, kind of thinking out loud. He is right (as usual!).

[garyt]

foxmon, I read it ages ago, mine is now working perfectly after a lot of work and reading, and help from busdaddy.

[garyt]

"Alternately vacuum leaks cause a lean mixture because more air is getting into the manifold than the flap has metered."

Yes, but this does not get into the manifold as you have said,

"Moving the flap has no effect on the amount of air entering the manifold"

You mean entering the AFM not the manifold.

So it will richen or lean the mixture. Yep that is right, I was wrong. But because its not entering into the manifold, does it tell us anything?

Is it rich or lean? Pusing the flap and it slows could mean it is rich or lean, no? Or that the wiper arm is not working properly?

[garyt]

Just my two cents as they say

but... I suppose that IF it is a problem with the AFM (too rich or too lean) then I think you may be better to correct the idle mixture by the idle mixture screw while it is idling (that's what it is for after all).

- I think pushing the flap/wiper arm may not be very helpful because idle is where the fuel pump is switched on in a different way, because the wiper arm might be at rest and sending a different signal to the ECU than when it is moved off its base?

At idle, does the flap normally open a bit? Not necessarily because there is still a small opening which the Idle Mixture Screw deals with. But is the throttle butterfly (and so the entrance to the manifold) closed? No. Because the butterfly has to be open a bit to let some air in for combustion, and this is via the Air Bypass Screw, if not the car/bus wouldn't start at all.

So at all times there should be some air that can get through to the manifold and engine, via the Idle Mixture Screw and the Air Bypass Screw.

[garyt]

And, in fact, also the Auxiliary Air Valve, should be mostly open at start up.

But I'm talking about a 79 beetle FI maybe this bus FI is different, so in this case sorry if I confuse. I believe they are similar enough.

[Randy in Maine]

No they both are essentially the same system. In fact a lot of the parts are interchangeable. L-tronic is actually fairly simple compared to the newer FI systems. It does not have the ability to determine the "mass" of incoming air though and thuse there is no "mass air sensor". Strictly temperature.

It does not tolerate poor electrical connections or vacuum leaks.

[busdaddy]

[garyt]

Yep, that seems right busdaddy. Phew!

So I reckon the OP should make sure: the Idle Mixture Screw is about mid way, the Air Bypass Screw is about 4-6 turns out, that the AFM flap is free to move and not too tight or weak, and the wiper arm is contacting properly. Plus the ohms tests check out.

Like Hoody said I don't trust the renovated AFMs so this seems to me necessary whether you got two or not (how do they know one is a 'known good one"? Has the PO actually seen it working?)

I guess after all that at least you can rule out the AFM. LOL

BUT!! Maybe check out other mechanical stuff first, and read up on the L Jet and the potentiometer. Most electrical FI problems would affect all the cylinders not just 3, I think...

I suppose there's no fuel in the oil?

[garyt]

"It should, and the AFM measures that air as well since it's drawn from the S boot upstream of the throttle body."

I think you may be mistaken here: Yes upstream of the throttle body, but the S boot is after the AFM so it can't measure this air. At least on my car, the hose from the AAV goes in under the S boot.

I also don't have a temp sensor in my AFM (Cali), but maybe this bus has.

[SGKent]

Has the OP found all his vacuum leaks because trying to adjust a vacuum leak away is piling poorly fired bricks on a weak foundation.
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[busdaddy]

[timvw7476]

If this engine has spark & fuel & compression at cranking speed, I would
now check/swap out the #3 intake valve spring, I think it's spent and only
floating when at operating speeds. Maybe even broken. : (

[my59]