My 81, since I got it, always has had a slight "bucking" sensation in the seat of the pants when accelerating. Last year I did an intake refresh on it to try to solve thinking it was small air leaks or sticking throttle plate, etc. but it still did it just the same after.

Here is some things Ive done/learned/checked

driving in lower gears- 1st, 2nd most noticeable
at higher speeds/momentum you cant feel
Car accelerates fine otherwise
Car only has 58K and its a 5 spd
driving at 2000 rpm and in 1st gear it bucks....you can feel in seat, jerks back/forward but not violently
no hesitation in acceleration, or decel
does not seem rpm dependent
does throughout rpm range as you accel
timing belt, WP, etc in last 1500 miles

did it before intake refresh which involved complete top end, all new hoses, etc. including fuel regulator and rear damper. Front damper checked out ok
tried two different AFM, and no difference
reset timing to 23 deg at 3000 rpm and idle speed to 750.
car starts immediately when cold or even after week long sitting.
Car starts but cranks slightly longer when hot
car idles at 750 hot or cold
temp sensor in spec
throttle plate switches work
New FR8DS plugs
injectors cleaned and serviced by witch hunter durrng intake R/R
Does it with O2 sensor connected or not
Fuel pump is only a few years old/very low miles, filter new
Green wire looks ok
fuel strainer in tank replaced last week.e
does it with vacuum line to interior hvac plugged off ( at blue/black one way valve)
Compression check- hot, throttle open- All cyl ~180 psi
engine ground is good, less than 1 ohm, no corrosion.
battery ground replaced with 4 AWG cable.
fuse panel ground clean

My next test is fuel pressure and flow rate.

any other ideas what may cause this?

ignition coil?
ignition control box?
could it be drivetrain related?

 

Driveability problems stink. Something is not quite right but almost good enough.

You have a long list of work done and that's both good and a problem. It's extra hard to find a fault with something that has been replaced recently. ("It can't be...") And then there's that lots of other parts have been touched.

Garage philosophy aside, I'll suggest that it's missing ignition. Use a timing light and check all wires. Look at the timing over an RPM range. The specs are in the manuals. Besides specific degrees advance, you want stability.

As for root cause, the green wire is always a suspect. Make sure it's routed away from the block. That is, zip-tied to the top of the main harness bundle.

Check the plug wires. Those are pricey to throw at a problem. Look for abrasions and make sure the ends are screwed onto the wires firmly. (The wires, on OEM sets, screw off from the ends) Make sure they're all seated well on cap and plugs.

I might go further and throw new plugs in. No real concern there but it gives a chance to look at the old plugs. You've got "fancy" plugs. I use the $2 Bosch copper versions.

BTW - Putting model info in your post is always good. I turn off signatures as some are just crap.

 

Granprix, my '81 seems to have similar problem. I never figured it out (10 yrs) and others that ride believe it is all in my head. Car has just about 80k. I am wondering if it might be a problem in distributor. To me it feels almost like a bad dist. on an old chevy. The '81 also has the advance/retard vacuum inside dist. Could this be causing problem? Mine had literally fallen off inside distributor. I never understood the function of this but was explained years ago (it had no effect on car stumble) I will be watching this thread.

 

My car does the exact same thing. Initially I thought it was my driving style but have come to the conclusion that it is something with the car. I figure I need new plug wires since they are original. I will check into that next spring. I will also follow this thread! Thanks for posting this issue!

 

My 85 Euro does something similar. I'll get it when going down a city street, 4th gear, ~30mph.
It takes so little throttle at that speed, any "touch" makes the car lurch forward, which then makes my foot come off the pedal, and then it slows (with a lurch), which rocks me forward onto the pedal and a lurch forward again. When I first experienced it, I looked around and found a small vac hose unhooked. Fixing that helped a little, but it will still do it on occasion.

 

Yup !.., me too, ongoing problem for quite some time as well, but not always ?

Ken
80 928 S
5 Speed

UK

 

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Get an O2 afr gauge. It will help show if it is leaning out. My 81 did that before I swapped the regulator and dampeners. She had a slight lean reading at tip in.

 

One of my 82 5 speeds did this - I put a new battery ground strap on and cleaned the CE panel with Deoxit. Removed all the fuses and relays - sprayed once and let it dry, sprayed a second time and let it dry. Put everything back.
Problem went away. Do not know if it was the cable or the CE panel.

 

I changed my ground strap per Roger recommend, but no change. I personally have not cleaned my CE panel but all electrical on my car is quite clean. Its been gone thru before.

The only damper I have not changed I ordered a new one to just rule it out. I will swap it later today.
I have another engine that I will try the distro from, but not another green wire.

anyone have a green wire that is known good that I could test?

Speedtoys drove the car today about 10 miles and felt it too. He and I agree that the "pulsing" is a drop in power, normal power, drop in power, normal power type of pattern versus an increase/drop (aka a surge) in power pattern. Happens at any RPM and thru range of acceleration in any gear.

 

60

 

Happens at those low speeds, at WOT as well, so I dont think its an O2 sensor issue, as it's not involved at that time.

I have my FP gauge ready for you, and we'll pull my WB02 out of the S4 for you to use, too.

 

Here is another thought (happens on my '81)

When coming to stop from cruising speed (60 mph or so) Let off gas and rpm drops to 400/500rpm then jumps back to 1500,
then down to 700,
up to 1200,
down to 900
and settles at just below 800 rpm (on tach)

I have not opened or done anything to AFM

I think a possibility is worn contact slide inside AFM and fix is changing contact point just a bit. I do not know if this would translate into surging under acceleration though.

 

GP, hoping you get to the bottom of this soon, following the thread.

Is it still possible that it is a vibration issue rather then hesitation? What is the motor mount status in the car?

I had a TPS going bad that had some of these qualities as well. I had WOT switch readings that were not in spec.

 

Has anyone considered that the cruise control may be involved in this?
It's a long shot but it could be lurking.