Jerking car...why?

I feel your problem is due to defective clutch plate . It needs to be replaced.

It looks like this one is already beaten to death, but ill throw another dart at the suggestion guessing game. My friends jeep did the same thing and it turned out to be that his differential had worn out and had too much slop in it.

The jerking stops when I step on the clutch.

This should be considered a good clue.

Does the engine still act up, just not transfering to the wheels, so you would be unable to feel it? -if this is true its not drivetrain related, IE carb/ignition/emissions ect (possibly a broken/missing spring?)

Seems to me if this is not the case and the engine runs smooth after depressing the clutch, it would suggest a drivetrain issue, IE u-joint/CV joint/tranny wear/clutch ect.

Diagnosing a problem is as simple or complex as you make it, just use simple logic to eliminate as many possibilities as you can.

As far as I can tell, the engine runs okay when I step on the clutch.

As far as I can tell, the engine runs okay when I step on the clutch.

And i would suspect that the engine also runs ok when revving up, and quickley letting off the gas. (I know things can act different weather its loaded or not)

If it were my car, i would not spend any more time/$ on the engine(fuel/ignition/emmisions ect), and start focusing my time on the drive train. there ways to check on components without tearing them apart.

If i were to try and find the culpret, i always start with the easy/cheap things to test. i read someone's post earlier about putting your car into gear and rocking it back and fourth checking for play...... and listning for a "ping/clunk" when shifting from forward gear to reverse gear, you will need to engauge the drivetrain in order for this to work. - these are both good ideas.

Many good shops will do a free "diagnosis" on your tranny/CV's/transfer cases/diffs ect, and many bad shops will have you replacing things that don't need. Shop around. In the end, my guess is..... somewhere between your wheels and engine. Without driving the car i would have a hard time getting any closer than that.

In the easy cheap inspection department you've overlooked a restriction in the exhaust system

You would think a restriction in the exhaust system would be present during acceleration, which has been stated to be ok.

Right. Exhaust restrictions are very sensitive to throttle position, and engine load.

The exhaust restriction thought is due an experience I had with a four cylinder motorcycle. The effect was that when using throttle the restriction was diminished by the force of the exhaust gases but when backing off the throttle sharply decreased the pressure it seemed to hiccup for lack of a better expression.

I can definately understand that one, I too have had similar experiences. When the exhaust is restricted to that point, the performance of the engine is sluggish to say the least. The engine has to be able to breathe properly for sure.

Does the exhaust smell of rotten egs when loaded down? if so, the CAT has seen better days, and should be replaced.

Hi Vulcan, if your not completely worn out from stomping through all the suggestions, and still no answers....

do a quick check to see if yours comes with a CMP sensor

that is a Cam Position Sensor - if you have one it may be going bad and depleting the fuel -to-spark ratio....

If this is the right answer at this point, what do I win?

Anyways you can easily replace this sensor if needed

GA

There is a crank shaft sensor that could produce the symptom too.

While I do agree that a cam, or crank sensor could cause the jerking, especially if you consider possible crankshaft end play. But, the cam and crank sensors would be used to adjust what? If the car has no fuel injection, or feedback carborator, what would it need a cam or crank sensor for? I believe we have determined that the car has a distributor. Your opinion is welcomed.

The brakes have been replaced; the carburetor has been cleaned; the distributor adjusted (eventually replaced when it developed a leak); fuel filter replaced as per schedule.

Did this trouble occur before any of these repairs?

Or which repair was done immediately before the trouble began?

Is there a noticeable change in engine sound when it the jerking is occurring?

Did this trouble occur before any of these repairs?

Yes, it did but it did not occur to me that it might be a problem with the engine or other. Since I was still learning to drive at the time, I put it into the category of "inexperienced driver".

I replaced the brakes because they were worn when I bought the thing. The carb was cleaned when the engine started to sound like it was missing or sputtering (this was about a month after I bought it). The distributor was adjusted at the same time. Fuel filter was replaced just because it was on the schedule.

Is there a noticeable change in engine sound when it the jerking is occurring?

None that I can tell. The engine's quite good. Doesn't make much noise ('cept when I open the hood, of course).

Vulcan, check out this post. This could well be your problem. It explain exactly why you have the problem when decelerating suddenly but not when easing off the gas slowly. Essentially, your car is bucking because it can't get gas fast enough to keep the engine from partially stalling when the car changes to coast too quickly. Also, disengaging the clutch (which both lowers fuel intake rate and enriches mixture) give the engine ignition and combustion a change to catch up to fuel (and oil) intake into the cylinders.

PS: this is the kind of existing fault in a car that a used car dealer usually is aware of...and can easily pass off to a customer who does not have the car thoroughly check by a mechanic before purchasing. New drivers are especially prone to be taken. If you've only had the car a short time...highly recommend you tell the seller you're not willing to keep spending money fixing his problem.

Vulcan, a thought: a broken or leaking vacuum line is a distinct possibility. When pressure is suddenly released from the throttle, vacuum spikes pulling air in to the engine, causing it to surge. This is normally masked by the intake of air and fuel through the carburetor.

Another possibility is the throttle return spring being weak from age.

Regards Dragon

Ask a Nissan mechanic online now www.nissan.justanswer.com/?r=gacar&JCRN=Nissan%20Mechanics&gclid=CMquxsD1zZYCFQZqswodvhfe3A

Back to the easy cheap tests again. Why don't you check the carburetor mounting bolts. It was a common problem on early seventies GM four barrel Rochesters. It would cause engine speed to rise up when it would suck air onder the carb base. As the engine speed would raise, the engine vacuum would increase. As vacuum increased, the carb would suck the carb back down. Then the engine speed would slow down again, and the act would repeat.

I think a little face time with the previous owner may be helpful.

Okay guys, I have to thank all of you for your attempts to help. I've learned a lot of things on this thread and I really appreciate it.

The car's still running and the symptoms haven't changed one bit. I'll check what I can without spending too much first. Who knows? I might get lucky!

regards,

Vulcan

"Who knows? I might get lucky!"

And perhaps after that you will be able to fix the car. Most vehicles produced have a lable under the hood that list what emission equipment is used on it. If you can find that it will give some clues.

Have you been able to hook up the vacuum gauge yet?

Try looking at your Flywheel, maybe its not Balanced

Thanks for the suggestion, but an unbalanced flywheel would probably result in continuous vibration, not just when I lift my foot off the accelerator.

Okay guys, I've got an update.

I've been searching and searching for a repair manual and gotten nowhere. Then I changed tack and started searching for "jerking car" or "jerking engine". 'Turns out that this is not uncommon.

In one website (can't remember which now), the symptoms described exactly matched mine. The suggestion was to check the vacuum lines/hoses/tubes. I opened up the front and stuck my head in. Initially, I couldn't find anything. Then I thought I might be able to hear a leak if it's running so I started the engine. Sure enough, I found one. It was going into the distributor. A friend called it the "vacuum advance".

Lacking a rubber hose, I went to my neighbor (who is a pneumatic salesman) and asked if I could have a small length of a plastic high-temperature hose and slipped it in. The first thing I noticed was that the idle rate dropped. I tried going for the idle adjustment but it was a tight fit so I took it to my mechanic who adjusted it for me.

Disappointingly, the jerking was still there. My mechanic scratched his head and said it could be a number of things (just like the posts in this thread ). He wanted to keep the car in for a few days but I needed it for a small trip so it stayed out of the shop.

This afternoon, I tried checking again. Maybe there was another leak that I didn't see. Peering this way and that and sweating in the hot afternoon temperature, I paused for a while to gather my senses. Then I spotted that tube that I had replaced.

It was still intact. However, there was another port on the distributor that I had noticed before but since nothing was connected to it, I assumed it was supposed to be left open. The term "vacuum advance" piqued my interest since it sounded like it would affect the timing of the engine. On a hunch, I transferred the tube to that port and restarted the engine.

Oh man, the difference! The engine purred like it had never before! I quickly jumped in and took it for a test drive. The jerking was gone and the experience was so pleasurable, I drove around for about an hour before heading home. The wife had started to wonder where I had gone to and was amused to see me so happy!

It says a lot about the mechanics I've been taking the car to when they don't notice these things. Of course, the problem was that the hose had been connected into the wrong port from day one (when I bought it). A Nissan service center might have noticed but those people are expensive.

Anyway, problem solved. And I didn't have to spend a cent! Thanks guys!

regards,

Vulcan

Well I am glad your jerking car has quit jerking.

The brakes have been replaced; the carburetor has been cleaned; the distributor adjusted (eventually replaced when it developed a leak);

My guesses were wrong on your car. I'm sorry. But my #1 rule of auto repairs seems correct. The last one that touched it screwed it up.

Lets hope that is the end of you car problems.

But my #1 rule of auto repairs seems correct. The last one that touched it screwed it up.

And... my #1 rule also seems correct. It is more difficult to diagnose an engine problem online, compared to when one is leaning over it.

The last one that touched it screwed it up.

Right. That open port still interests me though. I can't find where it's supposed to be connected to. The previous owner probably messed around with it and changed a few things here and there. I could try finding a similar model and compare but I don't know anyone with the same engine.

And, yes. I also hope that's the end of my car's problems.

Thanks.

In the years just before electronics were used to allow US cars to run smoothly, many devices were used to help meet emission standards of the time.

One of these devices was a dual diaphragm distributer. One vacuum port was used to advance the timing. The other was used to retard the timing. The diaphragms were stacked one behind the other. All that i remember used the port closest to the centerline of the distributer to retard, and the outer one to advance.

The method of use was to connect the retard port to manifold vacuum, and the advance port to the ported vacuum location. In case this is not familiar terminology to you, Ported vacuum is a port on the carburetor that does not receive vacuum until the carburetor is opened a certain amount.

The way I remember the timing sequence was to connect the retard connections, then set the timing according to spec. That way the engine is running at it's designed timing when no load was on it. When driven in such a way as to need increased power, the ported vacuum would advance the timing for additional power. But if too much load was placed on the engine the vacuum would drop off, and the vacuum advance would not work. This is where the vacuum retard would take over. When the vacuum would drop off, the vacuum retard would also stop working. This would allow the timing to advance again for the necessary power. Got that? Explain it back to me now. Try setting it that way, and see your results. Only if you have access to a timing light. Is that clear now?