Odd Bucking Phenomenon

[sdg3205]

My car is running really well except for this weird reoccurring "bucking" experience.

The car will run flawlessly for 45 minutes to an hour and then start exhibiting the typical collapsed/torn pick up hose symptoms; it will shake/buck violently if you try and accelerate hard. The harder you try and accelerate, the worse the bucking.

Here's where it gets interesting:

- it stops after 2500 RPM. I can mash the accelerator at 2600 RPM and get silky smooth RPMs to redline which would seem to eliminate a gas starvation problem.
- it isn't alleviated by a full tank of cool, fresh gas, which would seem to eliminate a fuel pick-up hose problem
- if I park the car for a while, it goes away and takes some time before it shows up again.

This spring I replaced all the ignition components, DMCNW did a fuel system overhaul. The only fuel system part I have no changed out is the fuel filter.

So far everyone I've talked to is stumped.

Throw some ideas at me, cause I'm all out!

[Bitsyncmaster]

Wonder if your having a low voltage problem. Did you happen to note what your dash voltage gauge read?

I would also suggest you test your pump voltage at a normal idle. There is another thread going on about fuel pump voltage. Mesure the voltage directly on the pump, don't use another ground for your meter.

[sdg3205]

Wonder if your having a low voltage problem. Did you happen to note what your dash voltage gauge read?

I would also suggest you test your pump voltage at a normal idle. There is another thread going on about fuel pump voltage.

Volt meter was reading 13 to 14v.

Brand new 140 amp alt from DMC reconditioned again in May due to a bent shaft.

I'll test the pump at idle first chance I get.

Thanks Dave

[David T]

Check the base timing and the vacuum and mechanical advance systems. Sounds like the spark is not advancing as it should. Also make sure the air sensor plate is not rubbing on the sides of the venturi and moves smoothly.
David Teitelbaum

[TTait]

A collapsed hose isn't the only thing that causes starvation like that, its also possible that debris in the tank is getting sucked against the pick up screen. Eventually after shutting down the suction subsides and the debris floats away, till it gets sucked back again.

With the 2500 rpm thing I don't think that's what is going on, but I wanted to mention it.

[TheDutchTexan]

Sounds like a MAF issue... I had it on my old peugeot 405...

[sdg3205]

Maf?

[A Van]

Could be lambda playing up, check the earth back right hand side of manifold, circuit runs a different path when engine is cold

[WelmoedJ]

Maf?

Mass Air Flow ;)

[Spittybug]

Check the base timing and the vacuum and mechanical advance systems. Sounds like the spark is not advancing as it should. Also make sure the air sensor plate is not rubbing on the sides of the venturi and moves smoothly.
David Teitelbaum

I'm leaning this way too. Sounds like ignition. Vacuum advance solenoid opening and closing perhaps? Broken mechanical advance springs? Arcing?

[sdg3205]

Thanks everyone, there are a lot of ideas here I would not have thought of.

So here's what I've got to check:

1. Fuel pump voltage

2. MAF - which sounds like it's as simple as removing the air box and checking the movement of the plate

3. Lambda - I'm 99% sure I have no ground on the right side of the manifold. Which harness does it originate from in the bulkhead? I can trace it and see if I grounded it to the left/drivers side of the manifold. Also, this was occurring before I fixed the lambda system a month ago. Still, I'd like to know what happened to that ground.

4. Base Timing - I'll have to check the manual on that one.

5. Vacuum Advance Solenoid - the solenoid on the back of the mixture unit? How does one test it?

6. Broken Mechanical Advance Springs - I've never heard of these, where are they located and what's the procedure?

7. Arcing - where might this occur? I'm running all new ignition components so hopefully it wouldn't be due to lack of insulation.

Thanks so much folks!

[sdg3205]

After searching for the right spring, shoving it (gently) into the pick up hose and checking everything in the tank, the car died on me again tonight.

Same symptoms - started bucking below 3000 RPM, but not above it, then when I came to a stop light it died and wouldn't start again.

I pulled off to the side, opened the engine compartment to help it cool down, took a piss and checked out a cool abandoned house. 15 minutes later, it fired back up and I limped home with the bucking awesomeness.

Obviously, the fuel lines in the tank are fine.

I'll have to investigate the other ideas above now. Damn this is annoying.

[AdmiralSenn]

I don't have a ton of advice here, but I suggest that if you do pull your distributor to inspect it, pay VERY CLOSE ATTENTION to the magnetic piece around the induction coil. 3416's magnet was cracked and it behaved almost identically to what you're seeing (stumbled like crazy and would then take off like a rocket if I asked it to above a certain engine speed, died at stoplights only to start up again as soon as it was cooled off).

What was happening was the magnets would get warm from the engine heat and expand enough for the one piece to move slightly. Sometimes this meant that it was just slightly disrupt the field and make it run rough. Other times the piece would actually wobble out of the frame slightly and kill the motor. Then it would cool off, and the magnets would stick themselves back together and everything was fine!

I wasted a looong time dealing with "fuel trouble" before I found this problem. Please note that this is very much a rare, oddball problem that's not very likely, but it's worth checking out if you take the distributor apart.

(Also, I read the title of this thread and thought "man, this guy's a potty mouth... and what's a benomenon?" I need sleep.)

[sdg3205]

Thanks Adam,

I appreciate all the pointers.

I changed out my fuel filter today as one of the last fuel issues I'm trying before moving onto ignition troubleshooting.

I just find it so weird.

[Kenny_Z]

Red did this and I found I had some bad grounds to the engine. I installed a new ground from the battery to the transmission (2ga) and then from the engine up to the bottom of the negative ground on the passenger side (4ga). This solved my stalling and bucking problem for less than 50 bucks. This bypassed the trailing arm ground and that braided ground strap. Because of the bad engine grounds I wasn't getting a strong enough spark, especially at idle. Basically the engine was flooding at idle and unless I gave it a few minutes for the fuel to evaporate I couldn't get her to crank.

[sdg3205]

Red did this and I found I had some bad grounds to the engine. I installed a new ground from the battery to the transmission (2ga) and then from the engine up to the bottom of the negative ground on the passenger side (4ga). This solved my stalling and bucking problem for less than 50 bucks. This bypassed the trailing arm ground and that braided ground strap. Because of the bad engine grounds I wasn't getting a strong enough spark, especially at idle. Basically the engine was flooding at idle and unless I gave it a few minutes for the fuel to evaporate I couldn't get her to crank.

Would this happen immediately when you wanted to drive the car? For this pattern to happen I need to drive the car at least an hour or more. That would mean I start out with strong spark and end up with nothing.

[Kenny_Z]

It happened within the first 20 minutes of driving during the hottest part of the summer. Cold startups were no problem but if she stalled at a light or when I pushed in the clutch I had a hard time getting the car to restart.