1976 Bus - Test for AFM? ** It's still bucking!!**

[chitwnvw]

I had to run a new ground to the DR. I was getting bucking and fuel pump run on after shutoff. A new ground wire and all was good.

Congrats.

[Amskeptic]

Finally solved it
I've taken it on two long one day jaunts ... one was 150 miles, the other over 200. Ran great on both.

Congratulations.
Now which of the four possibilities was the actual cause?
Colin

[DurocShark]

This thread sounds a lot like the trouble I had that ended up being the wiring harness. Chased a lot of ghosts to find the problem was electrical.

[chitwnvw]

Hope you visit us more often Duroc.

[DurocShark]

Hope you visit us more often Duroc.

Thanks.

It seems the only time I come by here is when something's wrong. It's nice to just come hang out.

[TrollFromDownBelow]

Finally solved it
I've taken it on two long one day jaunts ... one was 150 miles, the other over 200. Ran great on both.

Congratulations.
Now which of the four possibilities was the actual cause?
Colin

I'm leaning towards a vacuum leak as the primary culprit... only because the gas mileage was affected so much (went from 12 mpg to 18 mpg). Either the loose S boot clamp, or the FI seal; possibly the intake gaskets. If mpg hadn't changed materially, would say poor ground on the double relay

[TrollFromDownBelow]

It's frickin' back! Took it for it's first real long drive (32 mile hwy trip to work) after it's winter hibernation.

Even though Colin and I thought we 'fixed' it two years ago, there were several variables we played with; so we aren't sure what really fixed it. So in reality, I'm back at square one. Good thing he will be here in 4 weeks.

If I can't get this resolved, I'm selling it. I've had this bus for 6 years. My goal has always been pretty modest; get it running reliably enough so I can take it to work on occasion, and take it up North to go camping a couple of times a year. I don't even want to count the hours / money /time i've put into this, and I'm not even close to obtaining this goal.

okay. Rant off.

[airkooledchris]

FWIW, when I last had a bus with a bucking issue, it was the wiper on the AFM itself.

I loosened the screws on the board, nudged it up a bit so it had a clean track, tightened it up and no more bucking.

try not to get too discouraged. not driving the bus on a regular basis may cause some fiddling once you get it back on the road, but give it a little time to work out the kinks and see if it continues.

[TrollFromDownBelow]

Nope. Not the AFM - It's got a brand new rebuilt. I'm going to get some new FI injector seals see if that fixes it.

[DurocShark]

Nope. Not the AFM - It's got a brand new rebuilt. I'm going to get some new FI injector seals see if that fixes it.

When it's rebuilt they don't replace the traces. Often they don't do anything with that piece. If they do anything at all they bend the arm in a dogleg.

Pop the cover and take a peek.

[TrollFromDownBelow]

^^^^Been there, done that, got the t-shirt, and it is in the hamper. . That part is new - it was one of the first things I checked when I got it.

The way it is acting, I would think it is starving for fuel. However, I think I need to turn my attention to possible electrical/mechanical gremlins.

[airkooledchris]

you have a brand new rebuilt AFM - do you still have the old one?

if you do, a quick swap would allow you to check two things.

does it do the same thing with the old AFM? - yes, then it's not the AFM itself, guaranteed.

I also like using a spare AFM to test for mixture. if your not sure if the mixture settings have anything to do with the problem, you can mess around with the settings on your spare/old AFM to see if it makes any difference. if it doesn't make any difference, no big deal, just swap back in your new one and your done. no worries about keeping careful track of which way you went lean/rich (though still try to get your spare back to it's original placement if you can, for next time.)

im sure you have been through all of the possible bucking causes already, but it wouldn't hurt to run back through them just in case.
swap out the fuel filter if it's been a while, in case it's getting clogged.

[TrollFromDownBelow]

I wish I knew definitively what fixed it before .. because the symptoms are exactly the same. Bottom line is I am going to have to start nearly from scratch from a diagnostic standpoint.

[BellePlaine]

^^^^Been there, done that, got the t-shirt, and it is in the hamper. . That part is new - it was one of the first things I checked when I got it.

The way it is acting, I would think it is starving for fuel. However, I think I need to turn my attention to possible electrical/mechanical gremlins.

Are you sure that your fuel tank is clean? I have a feeling that you might have some corrosion from the tank blocking the fuel filter. It kind of makes sense because your bus sat over the winter, you took it out on a 30 mile run (where it presumably ran fine) and now it's bucking and acting like it's starving for fuel. I would see what happens with a new fuel filter.

[TrollFromDownBelow]

I'll check the fuel filter, but seriously doubt it: my fuel filter was crystal clean last time this happened (maybe 3k ago?), and it has always been clean whenever checked or replaced in the last 6 years and 10k miles.

Im thinking the injectors are not getting the correct signals to fire... or MAYBE ignition related. If you read through the entire series, I did a pretty thorough process of elimination the first time this happened. I just used a qtr of a can of either looking for vac. leaks and found none...

really it is all conjecture until i do real real diagnosis