Inconsistent hesitation and lunging (UPDATE: problem fixed?)

[locoqueso]

I decided to take a little road trip today into Wisconsin. It was a back roads trip where my speed stayed between 30-55mph without any problems. About two hours into the trip I decided to start heading back. About 30 minutes into the return trip I slowed down for a corner. Once around the corner I was doing about 30mph so I shifted into third gear. I accelerated to about 35 mph and then it started hesitating , bucking and lunging forward. The best I can describe it was like something was rapidly cutting the fuel on and off. I thought it was going to stall. This lasted for a couple seconds as I tried to accelerate and then I took my foot off the gas. I pulled it into neutral and coasted down to about 30mph. I put it back into third and accelerated. No hesitation this time and I was able to get back up to about 55mph.

All was well for about 20 minutes and then it happened again. I was accelerating and shifted to third at 30mph. Once I reached about 35mph it shuttered and lunged for a couple seconds until I let up on the gas pedal. A few seconds later I was able to accelerate past 35mph without any problems.

I changed my route so I could stay on roads between 45mph-55mph. At one point I may have felt a slight hesitation at 45mph after shifting into fourth gear, but I’m not sure. I believe I went another 20-30 minutes without any problems. I’m getting close to home by this point. At a stop light I go through the gears and now it happens again in second gear at 25mph. It hesitates and lunges forward hard enough to shake me. I continue accelerating and shifted into third. The bus continued to hesitate until I was over 35mph and then it mellowed out and operated like normal. I was able to make it home without it happening again.

To summarize, the hesitation and lunging occurred three times. Twice in third gear at about 35mph and once in second and third gear between 25-35mph. It didn’t happen at lower speeds in first gear and I don’t think it happened at higher speeds in fourth gear. The entire trip lasted about four hours and I spent most of the trip running between second, third and fourth gears with my speed ranging between 30-55mph.

My instinct tells me to change the fuel filter in case it’s clogged, but that’s just a guess. I’m not sure how the bus would act with a clogged fuel filter but I would expect the problem to be a little more consistent then I experienced.

Does anyone have any suggestions?

[bus71]

I'ts worth trying the filter, sometimes the crud moves around inside and blocks, then clears. Good luck!

[vdubyah73]

The harder you mash the the gas, the more fuel it asks for, the higher sustained RPM, the more fuel it asks for. You never stay mashed on the gas or sustain high RPM in 1st so it usually won't happen in 1st. Start with the filter. I don't know what you have for induction. If it's FI look for loose connections at fuel pump and double relay. If carbed and electric fuel pump look for same at fuel pump and the other end of the fuel pump power and ground wires. When you say hesitation I think you really mean cutting out. Two different things. Hesitation is when the engine bogs upon giving it gas and it just kinda delays it's reaction. Cutting out is when the engine literally cuts out and you have to do something to revive it.

[locoqueso]

Thanks for the replies...

I have the original FI. I'll change the filter and check for loose connections.

Based on those definitions I'm going to lean toward Hesitation as opposed to Cutting Out. The first two times I let off the gas right away so I only thought it might stall. The third time I ran it through 25mph-35mph, and although it was bucking, I didn't think it would actually die. I didn't have to do anything to revive it.

The breaks in acceleration were brief but enough to rock me forward and back. It was kind of like something was causing the flow of gas to randomly pulsate a couple times a second. My wife described it as coughing.

[vdubyah73]

Bad tank of gas?

[vwlover77]

Have you checked the black resistor track inside the AFM for wear-through that shows up as white/silver lines on the track?

The bucking/hesitation is a classic symptom of this problem.

[IFBwax]

This has been a problem for me on my brown bus off and on for a long time. Did you recently adjust your mixture? After checking everything what finally solved my bucking was leaning up the mixture about 3 clicks on the black wheel.

[Randy in Maine]

Possibly a Temp Sensor II also....

[misszora]

Miss Zora is sorta doing the same thing. LiveOnJG had a similar problem. My next step it replacing the throttle body gasket which is old and solidified. Looking forward to reading other solutions.

- paige

[bretski]

Are you still running the original coil? I had a similar problem about a year ago...hesitation at higher rpms. It turned out to be a dying coil. Replaced it with a "Bosch Blue", and the problem was solved.

[glasseye]

Or the condensor, like on my dual-carb last month. Similar symptoms, appeared out of nowhere. A new condensor solved it.

"Most fuel problems are really ignition problems" : my dad

[chitwnvw]

Gee whiz, could be just about anything.

[locoqueso]

Bad tank of gas?

About an hour into the trip I filled up with gas outside of a small resort town. Small place I never used before.

Have you checked the black resistor track inside the AFM(?)...

Not yet, but I will do that tonight.

Did you recently adjust your mixture?

Yes, about 700 miles ago in June. It was running lean so the mixture was adjusted rich. Outside of the problem I had yesterday, the bus has been running much better since the adjustments in June.

Are you still running the original coil?

Yes I am.

Gee whiz, could be just about anything.

Yikes!

[locoqueso]

Have you checked the black resistor track inside the AFM for wear-through that shows up as white/silver lines on the track?

The bucking/hesitation is a classic symptom of this problem.

I don’t have a digital camera but I’d say it looks similar to a picture Adventurewagen posted of their Air Flow Meter. There aren’t any white lines although I can tell where the two tracks are wearing. The tracks are shiny/reflective and could be considered very dark grey.

While looking at the tracks I noticed a few black specks in the AFM. They were near one of the rounded corners of the casing. Two of the large ones were about the size of poppy seeds. They easily crushed into a power like ash.

[vwlover77]

Sounds suspicious. Do you have access to a known-good AFM that you could swap in for a test?

One Bus owner I know had an AFM that showed no obvious wear-through on the tracks but had a bucking problem that was worse than yours appears to be. Replacing the AFM did the trick!