Swiss964

Hi to all, I am new on this forum
I am a happy new owner of a 964 C2 cabrio manual from 1990, Anthracite grey with Burgundy leather seats. The car was prepared by RUF when it left the factory ( struts lowering, exhaust, chip, speedline wheels, steering wheel, short shifting, front spoiler lip, center console ), to my biggest surprise the car had a hard time to sell because of the RUF pedigree, it did it for me though, love at first sight!!!. the car also had top end rebuild and clutch job 40KMiles ago
I did own previously a 997c4s and I have to say that this is day and night, the 997 was fast but not fun, this has everything. it is as fast as the 997 and has the noise, old leather smell, oil smell also...and that air cooled flat 6


After driving around for a while and getting familiar with it I noticed that the car would buck when I would lift my foot of the pedal and get around the 1500 rpm mark, my classic porsche expert told me this is normal due to engine design, I do not believe this as it makes the car very uncomfortable to drive in a city, I also feel that if I push the throttle slightly the revs are not regular, and of course idle is hunting a bit +/-50rpm. I cannot believe Porsche designed something like this
I decided to start changing parts that could give a hard time to the motronic,
I replaced the RUF chip by a steve wong one (love it), replaced AFM, Idle control valve, O2 sensor, DME relay, HV coils, spark plugs, HV wires, caps and rotors, checked for leaks, cleaned intake, change air filter, fuel filter, checked fuel pressure, all 6 injectors are brand new, checked for error codes with scantool, I did a valve adjustment, changed oil (I use Porsche 10W60), checked oil thermostat, checked the distributor belt (car runs equally well one each HV system) checked WOT sensor, checked Idle switch sensor, checked resonance valve, cleaned flywheel hall sensor as it was full of dust...

To my big disappointment this has not improved my issue by 1%, the car still bucks when I pass the 1500 rpm mark

I am thinking that this bucking could be linked to a wrong ignition timing or a too poor fuel mixture, but I am short of ideas on what to check

Thanks in advance for your help

Jacke2c

I had this problem, used my Durametric to test it and didn't see anything but an occasional O2 sensor failure which I replaced. Did not help. I read about the grounds on our cars... I think there is about 8-10 different grounding points on our cars... like the hidden one behind the coils, and others for the lights, and the primary ground up by the starter that ties the chassis to the frame. I spent about 3-4 hour taking them loose, cleaning them, and then re-tightening them. For me this fixed the problem..... I believe that floating grounds (loose or corroded) cause voltage variations within the circuits. This variation will give you issues. Do you sometimes get the "Christmas tree" effect when you have this bucking? Mine did occasionally.

I have heard it suggested that another issue could be your clock and voltage issues.

Have you reset your computer? Best of luck, Jack

Goughary

If you have a single belt conversion, go back to a dual pulley oem setup. The alternator doesn't make enough amperage at under 3k rpm on the single pulley. That could be the main cause. This worked for me.

Coils, wires, dist caps could be your problem. Especially the coils. There is a good thread on switching to 993 coils going on at the moment. I haven't done this yet, but next set of coils will not be these terrible coils we normally use.

Also, since I you have cleaned your ISV (you said checked...I think you meant "cleaned"...if not, clean that out) anyway, check for vacuum leaks. There are so many ways vacuum leaks can occur here...but one of the simplest is through the intake stacks. They are plastic and tend to be over-tightened by the monkeys at shops. So over time, the plastic hardens and shrinks and then you develop cracks around the nuts that hold them down. Get out a flashlight and look around, if the cracks are on the outside of the nuts, you might not be leaking there, if the crack has spread to the inside of the nut, you very likely have a leak. You obviously can't see the inside and outside of each nut, but you may find a leak right away and find you need to replace all the stacks. As to vacuum lines and finding leaks..mamma mia....not easy. Lots of lines.

There also could be a fuel pressure issue. So you could look there.

And then there is the tsb on the wiring loom where it meets the afm. Make sure this has been done. It's easy to do and actually makes a difference. I didn't believe it. But it works.

Good luck tracking this down. There are many things that cause this low rpm bucking. One of them or maybe a few of them, will end up being the culprit...it is "normal" as your shop said, but only normal because so many of us have had to live with it till figuring it out...

You will have zero bucking. The car wasn't designed to not run at 1500 rpm. Don't worry!

Swiss964

Thanks for your answer
-The ISV is new, I found that there were 2 references in the Bosch spare part catalog and took the one that is appropriate to my 1990 with aluminum intake
-I am running on 2 alternator pulley
-coils are new, my old coils were showing 1Kohm less then the new ones
-I checked vacuum leaks with cigar smoke, I taped the intake at the AFM and blew the smoke through a silicone pipe in one of the small vacuum holes, there are leaks around the throttle and resonance flaps axles but I would think this is normal especially as I was putting nice pressure in, for the rest no leaks
-I checked fuel pressure, all ok, pressure behaves like it should be
-wiring loom mod has been done
-Regarding grounds, I did re-tightened the one behind the coils, I did monitor my battery voltage while driving and it is stable above 13v, never got christmas trees, but I get a foggy clock protective glass which indicates that there is moisture inside, but I doubt that would influence that specific RPM range if something was faulty

I will try to check my grounds tonight

Goughary

Wow you've covered a lot...that's awesome.

Moisture in the clock...you wouldn't think the clock would have anything to do with the car other than time....do a quick search on the clock and you'll find that so many people have found that to be the issue with all kinds of problems. I have not had to fix that yet (knock on wood)...

Good job on all the work so far. Car looks great.

g60pops

Sorry cannot help with your problem, but any chance of a pic of the Ruf centre console?

Swiss964

I am not sure this will show it well, nothing fancy, just have a large CD storage, quite ugly honestly
I did notice that all my relays under the passenger seat are also RUF labelled however I am not sure what they do differently from std Porsche ones

Ken D

Do you or a local repair shop have access to a Scantool/Bosch Hammer/T-OBD diagnostic tool which can interrogate the DME to check for faults?

Awhile back I had experienced low RPM running issues which were completely resolved when I replaced the ignition modules (see photo below, courtesy Jason Andreas). IIRC there is a test plan for this in the workshop manuals.

Swiss964

Hi Thanks for your reply
When you mean module, you mean the white part on the picture, not the coils
My coils are brand new
I do have scantool with TOBD and there are no faults reported
I tried to run on each coil separately and I do not notice any worsening of my issue

Ken D

Correct.

Swiss964

I have my doubts about defective IMs since the car works the same way when I run it independantly on each of them, it would mean the 2 IMs have the same rare issue...From what I have read in forums it is either black or white with those modules

Swiss964

I am slowly checking all my grounds as per the repair manual, did the one at the battery, lid hinge right, fuel filter, Between coils, there seems to be 2 under the driver seat + at the starter
Also, when I connect the scantool it displays 13.0 V, when the car is running, Is that not too low ? I have heard it should be 13.5V plus, i will check directly at the battery too

RallyeChris

Two grounds behind the alternator, too.

Swiss964

Did take out the alternator to get to the grounds behind, checked alternator brushes and one was close to gone, ordered the new brush/regualtor from Porsche 270 $, what a rip off!!!, after market cost 30$... anyway...
I took the opportunity to get a new coat on the cooling fan, fan is at the paint shop, will get it for next week end .
In the meantime, i found a post on a forum talking about issues linked to light weight flywheels, my problem soudns like it, however i have no clue how to check if I have a LWF, does someone know?

ducnine

Hey Swiss964...any more update on your bucking issues?