Bucking Issues

[EZ Gruv]

We've been toiling over this on that other site, but I thought a fresh set of eyes might be good, so here goes


1977 Type IV 2.0L, Stock FI

I began getting some bucking while driving. I'm talking very severe - throw you around in your seat - bucking. Mostly under load, but also while cruising. It felt exactly like when my coil went bad last year, so I stuck a new coil on there - no change.

I went ahead and drove it to work on Tuesday and had no issues at all either to work or on the way home.

Wednesday was a different story. On the way to work, the bus started majorly bucking again. Bad enough for me to pull over and start looking around at things. I barely made it into a parking lot. The motor was stalling now and I could barely keep it alive. I found no visual clues as to why this was happening, so I fired it back up and gingerly drove to work.

Wednesday afternoon, I didn't make it home. The bucking was just too bad, and I couldn't get it up to speed. A call to AAA got me home eventually.

Fast forward to Saturday, when I finally had time to do some work on it (I bicycled to work Thu and Fri). Here is a list of what I checked/did:

- Cleaned and tightened battery terminals/clamps
- New Points
- New Condenser
- New Cap
- New Rotor
- New Coil (a few days prior)
- Verified each spark plug was getting spark
- Cleaned/tightened battery ground strap connection
- Cleaned/tightened transmission ground strap
- Cleaned/tightened Double Relay ground
- Tested fuel pressure (good)
- New fuel filter (old one clear)
- New Temp Sensor II
- New air filter (just because)
- Set dwell and timing
- Checked every vacuum connection twice
- AFM wiring connection good - no wires backing out of the connector


After a couple days I found this:



I replaced the cracked connector, and the problem went away...for 2 days. This bucking is more like a hesitation now, and much less severe.
I checked all the other connectors, the are not damaged.


I took the bus by the mechanic. He drove it, and nothing happened (of course).

A couple days later, I drive across town...While on the freeway, I was cruising at 60mph, and my momentum just went away. I hadn't moved my right foot at all, but the power was gone and I was slowing. The engine did not stall, I could just hear and feel the RPMs dropping. I lifted my foot and put it back down on the pedal, and the power returned.

Any thoughts from the masses? I sure would appreciate them!

[skin daddio]

make sure the plates and everything in the distributor are greased and freely moving.

[vwlover77]

At one time, my bus had a similar problem, but only at highway speeds. It turned out that my brand new Bosch spark plug wires were defective. The wire running from the coil to the cap must have had a small break in it.

I looked in the engine compartment with the bus running in a really dark parking lot and the spark was "leaking out" all over the place at the coil connection.

Another new set of wires fixed it.

[bretski]

EZ - Looking at your laundry list above, I have a couple of questions. When you cleaned the ground on the double-relay, did you check the connectors? Any corrosion in the prongs or connectors themselves?

What has the ambient temperature been like when the bucking occurs? I know you live in Vegas, and it get's kinda hot there once in a while... :)

The spark plug wires could very well be the issue, considering how that connector looked. As you well know, new parts can still be crap. I'd also be thinking about fuel delivery. Have you checked the wiring to the fuel pump? I'd be suspicious of the ground wire in particular.

One of my personal mantras when troubleshooting fuel injection is to start with all the grounds, then go from there. That's one of the grounds you missed, looking at you list. One other ground that can cause all kinds of havoc is the little braid inside the distributor. Check that one carefully, too.

[EZ Gruv]

I did check the fuel pump connections and made sure they were tight. Also checked the connections withing the double relay.

Haven't check the braid in the distributor though.

Temperature-wise, the issue has happened anywhere between 80 and 110 ambient degrees, which is our low/high range right now.

[Gypsie]

When you say that the fuel filter was clear, I presume that you cut open the old one. Was there ANYTHING in it (crusty chunks...)?

Wondering if your fuel flow may be getting blocked by debris.

[EZ Gruv]

Yep, I cut it open. Nothing inside besides fuel.

[EZ Gruv]

Pulled the plugs out today:

1 & 2



3 & 4



Another view of #2 with some oil(?) on it...Remember, #2 is the same plug where the connector was cracked.




I installed new NGKs and new wires today - haven't driven it yet though to know if it made a difference.

[VWGirl]

I thought I was the only one with a bucking problem... I had it for 5 years... Off and on, it was spontaneous. Eventually I diagnosed a bad coil... so I replaced it, then the new junk from autozone started leaking on me and I got a new new one from advance... it still had problems... but not as bad.

I've since replaced my plugs and my injectors and a good 1000 miles no problems yet, but it can sometimes be 5000 miles between issues, so who knows.

One think I would check is you metal lines. I had a problem once before where my fuel pressure was good, but under load it was not able to get enough fuel and would cut out, buck or die... the inside of the metal lines were rusty and just wouldn't let enough fuel pass... but sitting there in the shop the pressure was fine. So maybe take those off and clean them out if your fix didnt work

[Amskeptic]

One thing I would check is you metal lines. I had a problem once before where my fuel pressure was good, but under load it was not able to get enough fuel and would cut out, buck or die... the inside of the metal lines were rusty and just wouldn't let enough fuel pass... but sitting there in the shop the pressure was fine.

Sitting there in your driveway. . . We had excellent pressure that dropped the instant we allowed flow. That was an extremely clogged supply line between the tank and the filter.
Colincerca2005

[VWGirl]

One thing I would check is you metal lines. I had a problem once before where my fuel pressure was good, but under load it was not able to get enough fuel and would cut out, buck or die... the inside of the metal lines were rusty and just wouldn't let enough fuel pass... but sitting there in the shop the pressure was fine.

Sitting there in your driveway. . . We had excellent pressure that dropped the instant we allowed flow. That was an extremely clogged supply line between the tank and the filter.
Colincerca2005

I vaguely recall us testing for fuel pressure.... but the first time you came the bus was in the back yard and it hadn't been on the road in a decade. the "varnish" in the tank was eating your hose to the pressure gauge. We fed fuel through the throttle body with a small cap of some sort just so we could get it to run on it's two cylinders... but I do remember the bus sounded GREAT when it was on two cylinders lol... it ran strong! Oh, and there was some guy that came over to help us? I think he drove a motorcycle? I have no idea who that was?

I thought the second time you came it was fixed? You drove it, remember? We worked on brakes. I have pics!

[Randy in Maine]

I would also leave my fuel pressure gauge hooked up on the fuel supply rail test port as you try to figure this one out.

[sgkent]

I think it is the AFM or decel valve. 1977 has no O2 sensor.

You lifted your foot and then put it back down - power came back.

Things that radically change when you did that:

Throttle closes and opens. Impact, minimal. All it does is control air flow. No switches etc on a 1977. But it does affect two or three other things.
*EGR if hooked up. EGR is mechanical. If it was defective opening and closing would not change the orifice sizes whether good or bad.
*Vacuum increases when you lift. This changes the FPR and the Decel valve. At most on the FPR you are looking at a small drop in fuel pressure. Decel is different, if the valve was opening on its own it would lean the mixture and kill the engine. Adding heavy vacuum might pull it enough to free it where it would close again.
*Closing the throttle also swung the AFM flap. That may have moved it to a spot where the wiper made contact again, the mixture changed and the engine came back to life.
*You also changed the signal going to the ECU. If it was starting to fail that could bring it back to life.

[EZ Gruv]

A little update from me....I drove the bus to Southern California for the Blackstar Campout, and had very few bucks along the way. The few I had were barely noticeable.

[EZ Gruv]

Aaaaand I'm back....

I've been keeping track of when the bus has hesitated, and it seems to happen just after I go over a bump with the right side tires (or both sides at once); nothing when I drive over a bump with the lefts only. In my mind, that means something on the right side of the engine compartment is loose. Make sense to you?

Today, driving home, I hit a few bumps and lost engine power (still had electrical power), and I'm coasting. I pick up my foot from the gas pedal and pump it, but power doesn't return. After maybe 30 seconds of this, the engie dies and my red lights come on. I'm able to fire it right back up and drive normally - until it happened again. Evil or Very Mad ...and again,,,and again!

Thankfully I could nurse it home. It took me 3 tries to get up the driveway!

So I'm sure it's related. If it wasn't 117 degrees right now I'd be out there checking connections, but that will have to wait for sunrise tomorrow (supposed to be even hotter!).

Open for thoughts...Thanks