bureau13

Searching back over the archives, there have been a lot of posts about surging or shuddering, some very similar to my issue, others not so much. There have also been a variety of solutions suggested and attempted, from motor mounts to fuel issues, etc. I don't necessarily see a definitive "Try this or this" type solution for my problem so I'll throw this out there:

The symptom is mainly light throttle, starting maybe 2600 and getting worse then fading away as I go through 3K rpm. Sometimes if I give it more gas, it seems to go away, but I would guess that may have more to do with getting out of that rev range. Sometimes I back off the gas a bit so the shuddering is less severe and then just wait 'til I'm out of the range.

It does NOT seem to get better or worse based on temperature (ambient or engine). It feels almost exactly like lugging the engine at too low an rpm, or the way you can sort of miss the shifting/clutch engagement so that right after you let the clutch out it jerks....only this doesn't seem connected in any way to me shifting or messing with the clutch. I can be fully in gear at 2K and everything is smooth, but as the revs climb, almost always I'll get this effect to a greater or lesser degree. Not 100% of the time, and sometimes worse than others, but more often than not, I'll get it at least a little.

Thoughts?

Lizard928

Install a WBO2 sensor. I recommend the Innovate MTX-L http://www.innovatemotorsports.com/products/MTXL.php

Then you can tell if it is going lean, or rich etc. and will help identify the problem.

Mongo

Could be ignition related. When was the last time the distributor caps and rotors were replaced? What about wires?

bureau13

I replaced the caps/rotors when I got the car, a few months ago, but the wires appear to be original. I do have a new set of wires but I haven't put them in yet. It would be ideal if that fixed the problem :-)

I've heard a number of people referring to that wideband O2...I know conceptually what it is but not what is involved in adding it. Time to do some reading....

GlennD

Wonder if this is similar to mine.

If you push the clutch in (from your text, I believe you have a standard), and wait a few seconds for the drive shaft (in the torque tube) to stop spinning, then rev the engine, do you still have the vibration?

With mine, the front bearing migrated back a bit resulting in a vibration starting around 2800-3000 rpm. The intensity of the vibration was not always consistent - similar to what you posted.

Cheers
Glenn

bureau13

I will try that...

yardpro

I would also suggest torque tube.
If your vibration is dependent on engine rpm and not road speed it has to be from the torque converter forward.
Since the torque tube spins at the same rpm as the motor is is very likely to be the cause of vibrations. It is a long shaft of steel spinning...
It diesnt take much for an ocular ion to start.

bureau13

I'm sad about this, but what you're saying about the torque tube makes a lot of sense. I really would rather it not be that of course...hmmm. Well I still hope it's a fuel or spark issue that I can clear up but I guess we'll see.

outbackgeorgia

Yard pro,
If it is a 5 speed, not exactly. The shaft is running at road speed, not engine speed if the clutch is disengaged and in gear. In neutral, with the clutch engaged, shaft is at engine speed. In neutral, clutch disengaged, shaft should coast down to zero rpm regardless of road speed.
Different than auto where shaft is always at engine rpm.
Dave

Chuck Schreiber

My GTS was doing the same thing.
Always between 2-3K RPM.

Felt like "Lugging" the engine or like I was tapping the throttle off and on very quickly.

Sean diagnosed as a temp II problem.

All fixed.

GregBBRD

Porsche ran these cars very lean at light throttle. I've had a few (one recently) of the '85/'86 vehicles that will "surge" between 2600 and 3100. Especially noticeable in the lower gears....like when driving the car in slow moving traffic.

As a test, try disconnecting the O2 sensor and see if it runs better in this rpm range.

If this helps, hook the O2 sensor back up and monitor the output from the O2 sensor to the brain. Pay special attention to the rpm range where it surges. If the O2 sensor fluctuates between .2 volts and .8 volts.....that is correct and tells you that the O2 sensor is correcting the mixture as it "thinks" it should. If it doesn't fluctuate and "locks in" at one end or the other....then you have a component failure, of some sort.

Report back with your results. I may have some additional ideas, depending on what voltage reading you are getting.

bureau13

OK, I will do this....currently I've got my interior partially torn out but I will hopefully get it back together tomorrow so I can drive it.

Can I disconnect the O2 at that green wire and barrel connector by the CE? That is the O2 sensor output and not the heater, right?

The Temp II is one of the sensors I haven't replaced yet, despite being on at least one person's list of "Things you need to replace in general when getting a car back on the road." It doesn't seem to get better/worse as the engine warms up, but I realize that doesn't mean it couldn't be the sensor. Greg's suggestion gives me something to try as a diagnostic tool, which I like much better than a blanket replacement of possibly OK parts.

The torque tube comments made sense to me, because it really does feel "mechanical," very similar to lugging the engine or screwing up the clutch engagement, etc...but it seems like this symptom can have different causes, so I'll get those test results as soon as I can.

John Speake

Another possibility is an injector not flowing properly, giving a weak mixture. This can give the seemingly intermittent symptoms you describe. These old injectors are getting past a satisfactory working life.

Greg sells a kit to swap in Type 2 4 hole injectors.

the flyin' scotsman

I had a bad shudder on a S4 I worked on.....it was the injectors.

On the other hand my GTS has a bad vibe that's the torque tube similar to what Glenn described; Im replacing the bearings (Constantine super brgs), drive shaft (Gregs 300m shaft) and any clutch items incuding the stub shaft (another Greg part).

As you can tell from the responses your car may have one of a few issues and will take extensive diagnosis......good luck.

hwyengr

Is there a firm consensus regarding the difference in latency when swapping injectors? I don't have a access to a Sharktuner, and as awesome as they are, it seems like it's a bit much to get one to change the latency setting in the LH once.