3.2 engine jerking

[cancobra]

I have searched the forum for a similar issue to what I am having but have not come across anything yet.
I have a stock 3.2 in a 89 911 which has just had a major reconditioning which included externally new coil, fuel pump, distributor cap, wires, sensors, battery, fuel relay and also extensive internal mechanicals with the engine completely apart, essentially every thing replaced that needed to be replaced except pistons. Basically a rebuild.
I have 2 problems I just can't seem to solve. When driving the car at a constant rpm it seems to be jerking meaning it feels like someone tugging at the rear of the car lightly all the time. It is noticeable at all everyday rpm driving, for example 2000 to 4000. The car accelerates strong and without any problem.
The second problems occurs after about 10 minutes driving the car if I am in 4th gear and cruising the car around 3100 rpms it will give a noticeable jerk and the tachometer will drop to 2000 then come back up. I have attached a volt gauge in various locations to try and find out where this electrical problem is occurring. I have wired it into the fuel relay switch and at the fuel pump etc.
I have starting to think this may be a problem with the "computer" or "brain" as it is sometimes called. I wanted to avoid replacing it as I am told it does not fail very often.
Has anyone else experienced this kind of problem.

[boyt911sc]

You might not be aware of this protocol, it is almost mandatory for new members to post pictures of his/her car before being fully accepted in the club. Specially the kind of problem you are trying to fix. The earlier you post the pictures, the better.

Suggestions would be coming in and these guys are good. Ignore the protocol and you'll wonder why people ignoring this post. But there are members who are really nice and helpful to offer their expertise unconditionally.

Welcome to the forum!!!!!!!

Tony

[cancobra]

Sorry, Idid not know that. Here is a shot I have on my phone at the famous Canadian Wasaga Beach!

[Rick K]

Could be lots of things for sure, I'll throw out a few thoughts:

-Did you clean/replace the ICV when that work was completed?
-Check out the AFM - sometimes the track that the wiper arm travels on wears and causes your symptoms (at least the first symptoms described)
-Bad connections in the DME potentially?

[stlrj]

Seems like an ignition issue. Even new ignition cables could cause a problem if the plug connectors were not snapped on the spark plugs all the way.

Too bad these old engines are not equipped with on board diagnostics like modern cars made today. Even a slight miss like that would easily be diagnosed and could tell you which cylinder is misfiring.

I guess I'm spoiled...( where do I plug in my code reader?)


Cheers,

Joe

[CCM911]

Mine used to do the same thing. It cleared up whan I went through and cleared up all my vacuum leaks. I made sure that the intake manifold bolts were torqued properly, and I also renewd my fuel lines that run on top of the engine. And then it stopped that annoying "bucking" feeling.

[jcsjcs]

I am having a similar problem with my '84 911 Targa. At about 3000 RPM my car exhibits the same symptom of "jerking a bit" until i increase or decrease the RPM.

It's mostly annoying now - but i am concerned it may lead to other problems.

Check out this thread:
Question about cleaning AFM

I plan to take a look this weekend at my AFM (Air Flow Meter).

Will subscribe to this thread and hope we both can solve our problem!

[kidrock]

How recent is the rebuild? Often, an engine shop will request that you return after XXX kilometers or miles in order to re-torque several items.

Since what you describe is pretty much a process of elimination, it seems possible that a vacuum leak could be your issue. With the engine warm and running, try spraying some carb cleaner around each port, hose and connection where engine vacuum exists. If you find a change in your idle, you have found your culprit. Cheap, easy, quick and simple.

[Tippy]

Is it cyclic or random?

[cancobra]

Wow, a lot of knowledge floating around this forum!!!
The engine rebuild is at about 3000 miles now and the car went back at 2000 for re-torquing of everything needed.
No one seems to mention that computer box under the drivers seat which everything appears to run through.
I know I checked all the ignition wires making sure they were all snugged down but I never really thought of vacuum leaks. That is now something to look at to eliminate that from the list. I have spent about 2 months now replacing, changing and checking items to eliminate them. The more serious one I am worried about is that sudden drop in rpms because it is quite a noticeable hit when you are driving and I can't but help it is a warning to me that one day I am going to be blocking rush hour traffic with a car that won't run.
Thank you all so far who have taken the time to respond and I will try all these suggestions.

[cancobra]

AS far as I can tell it is random. I have a route I take every saturday and sunday morning where it takes me about 5 minutes from start up to get to a highway. When on the highway 9 or of 10 times this hard jerk with the rpm drop happens with a 1/2 mile range when I am in 4th or 5th gear at approximately 3100 rpms. This happens within a minute or so of getting on the highway. It sounds cyclic from this but on the past saturday it happened twice with a minute and did not happen on sunday on the same route.

[kidrock]

which sensors were replaced? Were the fuel injectors replaced as well?

Just taking guesses at this point...the symptoms you describe could be either fuel, spark or even air. Hunting down intermittent issues are difficult, especially for me. I usually get really pissed off and put off diagnosing the issue until it bites me in the ass.

BTW: VERY nice ride!!!

[James Brown]

is it a male engine?

[Tippy]

Did you replace the speed and reference sensors on the flywheel? They terminate by the last runner on the intake (#3 cylinder).

[Tippy]

is it a male engine?

Yes, 'cause it jerks, not b*tches.....

[DRACO A5OG]

Check the dizzy, I did not see it on your refresh list.

Remove the cap and check the rotor for abnormal wear.

Also remove the rotor ans see it there is any slop, axial and this case up and down play.

While running if rotor is bouncing up and down it will miss fire due to lack of spark.

In my case all the shims disintegrated coupled with the inner snap in bearing worn out. To be sure note the position of the dizzy or turn engine to TDC then removed dizzy to check for slop, the dizzy must have some resistance, if it spins freely it is time to add shims if you are lucky, if inner snap in bearings, it is toast.

To avoid this inthe future drop several drops of motor oil underneath the felt cover of the rotor receiver.

Jim

[Chuck.H]

If the tach drops rapidly while the car is in gear and in motion, then the speed sensor is bad IMHO.
This happened to me once, and what stuck in my mind is that the engine can't drop RPM that fast without depressing the clutch... try it, just take your foot off the gas at 4k rpm, it will just drop slowly as the car slows down.

If you replaced these recently, then maybe they just aren't spaced correcty.

HTH,
Chuck.H
'89 TurboLookTarga, 342k miles

[Quicksilver]

Originally Posted by Chuck.H

If the tach drops rapidly while the car is in gear and in motion, then the speed sensor is bad IMHO.
This happened to me once, and what stuck in my mind is that the engine can't drop RPM that fast without depressing the clutch... try it, just take your foot off the gas at 4k rpm, it will just drop slowly as the car slows down.

If you replaced these recently, then maybe they just aren't spaced correcty.

HTH,
Chuck.H
'89 TurboLookTarga, 342k miles

Chuck has hit the crux of it. The only way the tach can drop like that with the drivetrain engaged is if the DME thinks the engine has slowed that much, the DME is defective, or there is a major electrical fault. The DME provides the tach signal so it is saying that it stopped spinning which is the problem.

If you disconnect the AFM or remove the distributor, the DME will still show the speed the engine is spinning.

I'd look at the gaps on the reference sensors and their connectors. Because the engine is consistently running with very intermittent misses I'm not sure if testing the sensors with an oscilloscope would show the problem unless their output pulse is marginal. (Should be 2.5v)

The other things I would check is all the grounds. The ground on the left rear of the intake and the ground from the front of the trans to the body are the first that come to mind.

[Tippy]

Lastly, I'd add that my car had strikingly similar symptoms that I never solved. My thread is under the search of "Carrera" and "cliff". I wouldn't bother reading it unless you have nothing else better to do.

The closest theory I had for mine doing this was the alternator failing causing low voltage to the point the DME was acting up. Never had the chance to see but was quite sure it failed during the fiasco.

It was the only thing that made sense in hindsight.

[cancobra]

Again I want to thank all who have taken the time to respond. My plan is to make up a check list of all of the things suggested here that I have not already tried and start with the simple ones like the grounds etc. I have already spent a ton of money on throwing money at the problem.
Oddly the car did not do any of this before the engine and transmission were pulled out for rebuilding. Through the break in period it also did not happen but once I passed break in and started driving the car over 3000 rpms this inconsistent problem presented itself. So I am going to start to look at grounds, vacuum and even the alternator. I certainly will post here if I solve it.