Friends, I know this low RPM bucking topic has been discussed before. Some even call it the “parking lot buckaroo,” but a remedy often remains unsolved. Now I have started to experience it as well, though I have an interesting twist that has not been mentioned in other threads. It has to do with DME chips. With the help of tuning experts, this clue might point to the remedy (at least in my case and maybe for others).
Background: a very well maintained 1987 Carrera with ~200K miles. The engine was rebuilt 10K ago, including the addition of a 964 cam, new catalytic converter, Fabspeed exhaust and a Steve Wong chip programmed for the 964 cam.
For the first 8K miles after rebuilding the car ran beautifully in all RPM ranges up to redline. Then about 2K miles ago, I started to notice a stumble that appeared under very light throttle in the 1600-1800 RPM range (negotiating parking lots, stop-and-go traffic, etc). This stumble did not happen 100% of the time but it was often. Other than that, the car still runs perfectly all the way to redline.
I started to investigate the typical culprits. This is what has been done with no appreciable change:
- Checked that the idle switch "clicks" the instant the throttle lever moves -- it does
- Replaced the throttle body with a newer one (the action felt rough)
- Re-tracked the original AFM
- Checked / torqued the intake manifold bolts to 18 lb/ft
- Changed the DME relay
- Replaced coil with a new Bosh unit.
- Had the DME gone through by Steve Wong
- Engine out: Smoke inspection. Replaced vacuum lines and connections behind engine near the fuel pressure regulator. Replaced the seal between throttle body and manifold. No other leaks found, including injectors and intake manifold. Plugs looked good as did the distributor cap.
- Engine in: stumble was still there. Swapped-out another AFM as a test with no change.
Before getting out-in-the-weeds any further, I decided to
remove the SWong chip and put the OEM back in. The stumble was completely gone! Okay, that narrows it down. Three Porsche experts I told about this said there must be a problem with the chip.
I called Steve Wong. He said it wasn’t his chip and the OEM chip is covering up for a lean surge. I sent the chip back to get the chip tested and/or programmed more like the OEM chip
only below 2K RPM for better drivability. A week later, I put the newly customized SWong chip back in.
It stumbles at 1600-1800 RPM just like before. Dang it!
Tuning experts: since the car runs beautifully with the OEM chip, and still stumbles with the modified SWong chip, where do I go next? I’m stumped.
Thanks for your input.