violent jerking

kwm · 10-04-2006, 06:45 AM

On about 3 or 4 occasions now I have experience violent jerking from my car and it seems to be while it is still cold. I start it up, drive off and when it does happen it seems to happen before I can get to 3rd gear. The part that leaves me guessing is that the jerks are not shown in the tach at all. Therefore it doesn't seem like a miss so I thought fuel flow was interrupted but then about that would bob my tach needle. So, then I thought it might be in the drive train but that leave me scratching the old head also? Any thoughts. What ever it is it shutters the whole car pretty violently 3 or 4 times then everything seems OK????????

DonMo · DonMo's Garage

Jerking like that can be fuel mixture when cold. Too rich causes a lot of problems on a cold car.

MichiganMat · MichiganMat's Garage

I think what you're referring to is called "bucking". Do a search.

JK-81SC · 10-04-2006, 08:12 AM

My car had a similar violent bucking behavior, and it was caused by a bad rev limiter relay. The relay is suposed to cut fuel flow above red-line, but mine was cutting fuel flow at random times while driving.

There are several things that could be causing the problem, so this is only one of many things to check. It is easy to disconnect the relay, once you find it. It's tucked way back in the trunk, on the drivers side, on the fender near the oil gauge.

Read more about it here:
rev limiter

kwm · 10-04-2006, 08:32 AM

Thanks. I have a MSD set up that I installed on the car. The rev limiter in this application if a white plastic piece that plugs into the side of the MSD box. I have increments of 3k 5k 6k 7k 8k. I had a 7k in at first but was scared it was too high so I went with the 6k. I am trying to remember if it ever bucked with the Bosch unit on it and I don't think that it did so maybe it is my MSD box. Do you think it will get worse? As it is now it is so sporadic and seems to only happen in single incidence for real short times. Maybe I am worried about something I shouldn't be.

kwm · 10-04-2006, 08:37 AM

JK- I read that other thread you supplied the link to. I am not wondering if my fuel pump isnt going out? Do you think that would produce what I am talking about?

JK-81SC · 10-04-2006, 08:57 AM

I'm not too familiar with an MSD install, but is your stock rev limiter still plugged in? Since you have the MSD rev limit protection, I would unplug the factory rev limit relay.

I had my fuel pump replaced by a shop that thought the bucking was caused by a bad fuel pump. The new pump didn't fix the problem, but unplugging the rev limit relay did. Maybe someone can give us insight into what happens when a fuel pump is going south.

kwm · 10-04-2006, 09:00 AM

Yhea good idea JK on the symptoms of a bad fuel pump. Since the Borsch box is totally out of the car would the stock rev limiter still need to be unpluged? YOu say it is behind my speed-o guage in the dash?

JK-81SC · 10-04-2006, 09:19 AM

It appears that removing the Bosch CDI has no effect on the rev limit relay, so if it is still plugged in I would unplug it.

cdi and rev limit

The relay in the area of the driver's side hood hinge mount on the body. You will see a 3x3x1 black relay with a white plug on the bottom, bolted to the side of the cowl. It is also visible thru the gas gauge hole in the dash with the gauge removed.

If that doesn't fix the bucking, then it's time to try some other ideas.

kwm · 10-04-2006, 09:27 AM

Thanks Jk. I have not gone to lunch yet do you think it is something I could get to and unplug in my parking garage in dress cloths? I have tried to get at my hazard light relay in that access port in the area you are talking about but is sounds like it is right there in the trunk compartment and not up in that access port through the firewall?

JK-81SC · 10-04-2006, 11:30 AM

I was out for lunch too. It's tucked kind of deep in the firewall/fender area, but it should be accessible. I'll look for a picture so you know what it looks like.

kwm · 10-04-2006, 11:35 AM

I found it over lunch and unpluged it but havent driven it yet. We will see if it was it or not.

JK-81SC · 10-04-2006, 11:53 AM

OK, let us know if that fixes it. If not, then others need to jump in to give you some more ideas on what to check.

kwm · 10-05-2006, 04:17 AM

Well at least so far..............IT WORKED.......... but it was a sporatic issue. Time will tell fellas but I do appreciate the input.

JK-81SC · 10-05-2006, 12:39 PM

Sounds good for now. Give us an update in a month.

I was very happy to get rid of the problem in my car. I think it was a big reason why the PO sold the car, he was going crazy trying to fix it and gave up.

kwm · 10-05-2006, 12:43 PM

JK-should I ask this but..... did you then tell the PO you fixed his car by unplugging one little item????? I bet that would piss a guy off..... Oh well he should have tried harder.

kwm · 10-05-2006, 12:44 PM

Oh yhea, my car worked fine this AM too and in the AM was usually when it bucked.

JK-81SC · 10-05-2006, 12:59 PM

I never saw the guy after I drove away, so he doesn't know. There was plenty of other 25 year old stuff for me to fix, besides the rev limiter problem. I paid a fair price, so we both were happy.

JimCulp · 10-05-2006, 02:52 PM

Must have been a Christi Lake film.

brett25 · brett25's Garage

"must have been a christy lake film"

LOL