75 Bus AFM adjustment/ stumble diagnosis help.

[tractoman]

My bus runs well but it has a hiccup or stumble randomly for a second, like it loses spark for a second or something electrical, not like fuel starvation. It is always when the engine is hot. It is a stock '75 L-Jet and it starts fine cold. I have tested the TS II and it is within spec, but I am going to replace it as I am suspicious it might be flaking when hot. I have also cracked open the AFM cover and inspected the tracks which look fine and clean. Reading another thread of Colins I tested it by nudging the wiper a little CW and CCW to check the idle mixture. The idle definately went up with a nudge CCW and manifiold vacuum increased also. On other cars I like to use my vacuum guage to dial in the mixture for highest vacuum. Is that OK here and am I safe to move the wiper a little CCW to get this dialed in. The mixture screw I have not messed with. I have checked connections, grounds and it has a new battery, fuel lines, injector seals, vacuum lines and positive cable.

[Amskeptic]

My bus runs well but it has a hiccup or stumble randomly for a second, like it loses spark for a second or something electrical, not like fuel starvation. It is always when the engine is hot. It starts fine cold.
The idle definately went up with a nudge CCW
The mixture screw I have not messed with.

Sounds like you need some idle mixture help. For fun, screw the mixture screw clockwise and see if you get the same vacuum improvement as your nudge. Leave it there and test drive for a couple of days and report back.
Colin

[tractoman]

I'll give it a shot this weekend and keep you posted. Any idea about the random hiccup? I doubt it's due to the mixture issue.

[vwlover77]

Have you checked the entire ignition system? Sparkplugs, wires, cap, rotor, and points all fresh, tight, and clean? No carbon tracking on the coil post, cap, etc? All connections to the coil in good shape? Coil clean? In-line fuse to the coil clean, tight and no corrosion?

That's all I've got!

[Amskeptic]

Any idea about the random hiccup? I doubt it's due to the mixture issue.

As per above. . . Unlike an analog reaction to insufficient or excessive fuel like with carburetors, when the FI mixture map is exceeded a very quick closed loop amplification occurs. If the AFM is reading x and the engine misfires due to insufficient mixture, that misfire drops the AFM wiper even more. The engine misfires due to leaner mixture yet. The AFM wiper drops still further. The engine misfires due to an even leaner mixture. The AFM wiper drops yet still even more further. Finally the engine gives up, the wiper shoots towards closed. At a ridiculously lean mixture, you will get a backfire up the intake tract. The wiper clangs closed and bounces towards open. Fuel is restored. The airflow picks up. The wiper goes towards richer. This all occurs in a very short period of time by the way.

So, do the mixture adjustment even before you follow Don's sensible Step Two, and let me know if the random hiccough disappeared. . .
Colin

[vwlover77]

Unlike an analog reaction to insufficient or excessive fuel like with carburetors, when the FI mixture map is exceeded a very quick closed loop amplification occurs. If the AFM is reading x and the engine misfires due to insufficient mixture, that misfire drops the AFM wiper even more. The engine misfires due to leaner mixture yet. The AFM wiper drops still further. The engine misfires due to an even leaner mixture. The AFM wiper drops yet still even more further. Finally the engine gives up, the wiper shoots towards closed. At a ridiculously lean mixture, you will get a backfire up the intake tract. The wiper clangs closed and bounces towards open. Fuel is restored. The airflow picks up. The wiper goes towards richer. This all occurs in a very short period of time by the way.

I'm trying to comprehend this. I can understand how a super-lean mixture would ignite prior to the intake valve being fully closed, causing a reverse flow of air in the intake tract. But how do small, initial misfires reduce the airflow? Are they also happening before the intake valve closes?

[Amskeptic]

I'm trying to comprehend this. I can understand how a super-lean mixture would ignite prior to the intake valve being fully closed, causing a reverse flow of air in the intake tract. But how do small, initial misfires reduce the airflow? Are they also happening before the intake valve closes?

It does not ignite prior to the intake valve opening. Lean mixtures are actually taking so long to burn that they exceed the entire exhaust stroke and are STILL burning when the intake opens on the next cycle. That is what causes the "backdraft" up into the intake tract.

Small misfires cool down the exhaust draft that pulls the intake charge in during overlap. This is what reduces the vacuum. Pull a spark plug wire with a vacuum gauge hooked up and see what happens.

A combustion pulse is around 1,000* as it is released from the combustion chamber when the exhaust valve opens. The exhaust valve opening ends the gas expansion pressure in the combustion chamber (it literally mandates the end of the power stroke) and the residual combustion expansion blasts into the exhaust pipe (part of why open exhaust is so LOUD). As the heat and pressure drop dramatically at the end of the exhaust stroke, both the piston going down and the intake valve opening and the effects of the prior hot exhaust pulse cooling down as it enters the exhaust pipe create much more vacuum than the simple physics of the piston going down.
Colin

[tractoman]

So, I got the time to work on the idle mixture. Originally, it was set about 4 1/4 turn out from seated. I ended up about 2 1/2 turns out from seated and was able to get the vacuum up to about 19 Hg and the idle sounded better, although the increase was very little in idle speed. I also replaced the TS II. Now when I nudge the wiper in the AFM CCW, I really get no change. CW also no change but it slows down sooner CW than CCW. I can nudge about a 1/16th to 1/8th of an inch either way and idle remains the same. Drove around with no incident so far with the hiccup although it was very random, like once every 30-40 miles. I'll keep you posted. Hopefully this summer Colin, you'll get to see it up close and personal and help get it all dialed in. That's it for now.