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Bucking Bus Revisited

Ahhh... it wouldn`t be the Guacamole Bus if it didn`t have this @!#$%^ intermittent bucking issue. The problem has been around pretty much since I bought the bus.

Some background is in order I suppose. Shortly after buying the Guac, I did a road trip with the family to California. The day we were supposed to leave for home the bus would act like the ignition was shut off when upshifting or when RPMs went over 3200 or so. The symptom would last for just a moment, then the bus would continue on like nothing happened. It reminds me of a cat that just fell off the bookcase, then sits there licking itself as if it meant to do that.

I replaced the points, condenser, plugs, cap, and rotor. None had any effect. I even bought a new coil from Autozone (read about that piece of crap here. I spent two days extra in CA trying to figure it out. Someone on the Type 2 list suggested replacing the fuel filter. A fuel injected bus is sensitive to fuel interruptions, and this was a logical place to go, even though the symptoms "felt" like they were electrical.

I replaced the filter and the large feeder hose to the pump and VOILA! The bus stopped its bucking! It made it back to CO without nary a whimper.

A month or so later the symptoms returned. I bought another fuel filter and swapped it in and the symptoms went away. This scenario repeated itself at least two dozen times over the next couple months, with me spending a small fortune on filters. I figured the tank was rusting out like the one in my 73 did and I just needed to pull it so I could have it relined. But just to be sure I opened up a filter to see what was causing the blockage.

As you can see, there was no blockage in the filter at all. Aside from a slight darkening in the half of the element that was constantly soaked with fuel, it`s clean and good as new.

Now I was really confused. Why would changing the filter resolve the issue? I looked for more clues.

One thing I found was some scorching in the distributor cap that appeared to be spark jumping from the rotor to the ground strap inside the distributor.

Again, I thought I had fixed it when I put a different distributor in. Not so.

I gave up for a while on fixing it, and just drove through the problem. Using some gas pedal massaging I found that I could keep driving whenever those symptoms appeared.

Until last week.

The bucking and cutting out got so bad that the bus was truly undriveable. I parked it and decided to leave it alone while I went over what I`d replaced and checked, and what I hadn`t.

While I had tested and inspected both the double relay and the AFM, I didn`t swap them out. So I figured this was a great time to try that. I asked Thesykboy to swing by and let me borrow the AFM and double relay from his bus for some testing. When he got here yesterday I quickly swapped in his double relay and started the bus. It idled for a minute than died.

Hmm... His relay must be bad (it was his spare). I tried the known good one from his running bus and got the same result. Then I put mine back in. Same result.


I gave up for the day and went inside. Today I went back out to start messing with things. I found if I wiggled the wires going to the thermotime switch I could stall the bus or keep it running indefinitely.

So the TT switch must not be grounding properly, or the wires are loose. There are a pair of crimp connectors on the wires to the TT switch, so those crimps could be bad as well.

I loosened the "T" to the brake booster and tried to remove the TT switch. Screw is frozen tight. But I noticed the TT switch bracket is just flopping around there.

AHA!!! That must be it! I wedged a screwdriver under the TT switch to make sure it was grounding and went for a drive. No symptoms. Yay! Since I can`t get that screw loose, I`m just wedging some steel underneath for now until I can get an impact driver to get it loose.

Now, before I get TOO excited, I need to remember that I`ve been here before. Let`s see if it lasts...