Bucking problem

DannyBNH · 06-26-2012, 05:40 PM

I've had this issue since I got the car last year. The PO said the problem was
"dirty injectors" or so he was told. He wasn't a wrench, he just drove it and changed the oil. I went to several bottles and it didn't seem to make a difference.

When I floor it, it will "buck" when you hit 3000 RPM.. In all gears. It bucks badly.. not good for the TT I'm sure. So for the most part I've been driving it easy since I got it, trying different things..

I've changed all the fuel lines/fuel filter did a tb/wp change. Put in a new "green wire". I had a few other suggestions including

Problem may be with the MAF
Might be the WOT switch
Might be the computer.
Possible Ignition ( wires or coil ).

Any suggestions on how I can test any of the above ( or anything else it might be) so I can definitively know what the problem is?

Like I mentioned above.. it seems to alway happen at 3000 RPM in all gears. Not sure if just a coincidence or what...

Thanks
-Dan

Pete R · Pete R's Garage

Does it clear up as the rpms rise past that or from 3k on?

DannyBNH · 06-26-2012, 06:15 PM

From 3k and up it will buck. When it starts to buck I immediately back off so I kind of drive it like an old lady.

It's one of those "I don't want to do that again" feelings when it bucks..

Pete R · Pete R's Garage

Have you done the typical "tune up" cap, rotor, plugs and wires? Could be the coil too. When it happens is there black smoke out the back. I'm thinking that you may not have enough spark to ignite all that fuel. Just a theory. I'm not real good with intake stuff.

Tom's 87 928 S4 · 06-27-2012, 03:42 AM

My 944 used to do that because it had oil leaking around the spark plugs. You might want to check the reference sensor also(CPS).

DannyBNH · 06-27-2012, 04:14 AM

I have not one the cap/rotor typical replacement yet. Aside from the bucking it idle's steady and responds well. I probably should though.. Right now I think I'm leaning towards a new coil or possible rebuilt MAF.

Is there a method of testing your plug wires?

harborman · harborman's Garage

Quote:

Originally Posted by DannyBNH

I have not one the cap/rotor typical replacement yet. Aside from the bucking it idle's steady and responds well. I probably should though.. Right now I think I'm leaning towards a new coil or possible rebuilt MAF.

Is there a method of testing your plug wires?

Old trick: Open hood, run car in the dark and look for arcing around wires, distributor cap(s). Make sure it is pretty dark out or in place with no lights on at night.

DannyBNH · 06-27-2012, 06:05 AM

Tried that. No fireflies with the tubes off.

harborman · harborman's Garage

Quote:

Originally Posted by DannyBNH

Tried that. No fireflies with the tubes off.

Other than that, replace the wires. But you were wondering how to test the wires. Have you pulled the plugs to see the condition?

DannyBNH · 06-27-2012, 09:14 AM

No I haven't. What would indicate they need replacing?

harborman · harborman's Garage

Quote:

Originally Posted by DannyBNH

No I haven't. What would indicate they need replacing?

The plugs can tell you a lot about how the motor is running. Maybe which cylinders are firing or not, oil consumption, etc. A matter of elimination to find your problem. Good luck.

harborman · harborman's Garage

Spark plug condition and how to read:

Verrill: Plug Color Chart

Opelotus · Opelotus's Garage

Is this a Euro or US spec car?

DannyBNH · 06-27-2012, 02:19 PM

It's an 84 US at.

jstobo · 06-27-2012, 03:07 PM

I hate to be a broken record but the problem is a classic symptom of dirty injectors. Pull and have the injectors cleaned before spending money replacing parts. I know its a pain but not that hard. On my 85 I kept thinking the liquid cleaners would clean the injectors. Couldn't get it to run right. Finally as a last resort, pulled the injectors and took them to a shop with an ASNU setup. My best injector was flowing 75% and there was only one. Several were at 40%. After cleaning(which includes a new screen at the top of the injector and new o-rings) all injectors flowed at spec. Put them in, said what the hell and started the car. Idle came in perfect. Car ran great and still does. Cost $25 per injector and had them in 4 hrs. Best $200 I spent on the car. CLEAN THOSE INJECTORS. At the minimum you will eliminate that as a problem.

Pete R · Pete R's Garage

If you don't know when they were last done I too would start with tune up (cap rotor plugs, possibly wires) and clean the injectors. very good place to start. I would do a tune up on any "new to me" car anyhow. All could be the problem and a cheap (relatively) place to start. You're injectors will be matched too. You wont have to worry about 1 injector being plugged, running that cylinder lean and wrecking the motor.

DannyBNH · 06-27-2012, 04:09 PM

I've already ordered the plugs/coil and rotor.
If that doesn't fix it I'll move on to the next thing.
I'm not opposed to sending the injectors out, but since I just did the fuel lines a little while ago I'd rather not take things apart again unless I have to.

Also just noticed that while idling just a few minutes ago in the driveway that if I rev the motor up to 3k it does it when stopped.

I'm betting it's eiither the coil, or the plug wires.
Either way, I can still drive it while I'm trying to figure it out. I can still do close to 100 before the tac hits 3K rpm

turborat · turborat's Garage

Spending a hour pulling the spark plugs and looking them over prior to spending a single dime will most often point one in the right direction towards diagnosis and needed parts when a misfire is detected. Spark plugs fail. At $3.00 to $10:00 each, its much cheaper than throwing $100s parts at the car and hoping something works.

Dean_Fuller · 09:33 AM

I had this problem on my 89 after I bought it. Ran fine a low RPM. Would sputter and cut out in a harder acceleration. I saw NO fireflies when checking to see if the wires were bad. I changed the wires anyway and the problem has been long gone.
Plug wires can not be expected to last 25 years without losing performance. They could be original and if so, need changing anyway...although pricey you would be set for the next couple decades wires wise.

My brother had this also with a Toyota and it was a really dirty fuel filter.

1982_928 · 1982_928's Garage

Another thing to do would be start the car (when its cold) and run water from a waterhose on all of the wires and all over the engine. It will jump sparks and "buck" if they are bad. Another thing it could be is a clogged fuel filter. It could possibly be suffocating because it can pull enough fuel through to supply it right up to that point.

If its the injectors, pour a can of seafoam in the tank, run it a bit, and let it sit for a day or two. Stuff does wonders.

If you have to pull the injectors, i wouldnt send them off and pay $200 for someone to pour carb cleaner in them and let them sit. Pull them and get some spray carb cleaner (B12 or seafoam preferably.. Seafoam would probably work best) and spray some down in the injector, set them upright and let them soak. Then get an air hose (Wouldnt suggest using a ton of air pressure when doing this)and blow them out. Maybe tap them on a solid surface (such as concrete) and that will knock some crap out as well. Repeat this as many times as crap comes out. When i got my 928, 7 of the 8 injectors were stopped up. Pulled them, sprayed some cleaner in them, soaked, blew them out (did this twice) threw them on the car, hits on all 8, and i have $200 more to spend on other parts.

Can you post us a video of it doing this?