Matt Monson, Trackrash, Techno Duck and
Cory M: Thanks for your time to offer thoughts.
QUICK ANSWERS: I spoke to
Steve Wong about my needs. He's 2 hours away from me so I was hoping to find a guru closer so I don’t have to flat-bed the car. I also spoke to another tuner,
Sal Carceller. Both Steve and Sal suggest I find someone here in the San Diego area who can verify the stoichiometric ratio on my rebuilt/modified 3.2 (964 cam and SW chip) is in the range of 13.8-14.2 at idle and 14.4 under steady-state normal load. (Note they both say it should NOT be 14.7.) One local San Diego guru I knew, Jae Lee, moved to Palm Springs. On
Black Forest, when I called them I didn't get a clear answer if they actually have an air/fuel ratio test gauge and/or have a way to test the car at idle and under load. All they said was "yeah we can do it." Guess I'll call them again soon.
I was keeping the nature of my original post simple. I didn't want to give the backstory because I didn’t want to waste your time. However, since some of you have asked for more info, here's why I am asking about air/fuel ratio: It's the old 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 that I've read. It has to do with DME chips.
THE CAR/ENGINE: a very well maintained 1987 Carrera with ~200K miles most of which I have put on (2nd owner). The engine was fully rebuilt 10K ago, which included a 964 cam, new catalytic converter, Fabspeed Maxflo one-in/two-out muffler, and a Steve Wong chip programmed for the 964 cam. This also included ancillaries like the O2 sensor, TPS, IAC, rebuilt distributor … the list goes on.
THE PROBLEM: 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 under very light throttle in 1st or 2nd in the 1600-1800 RPM range (parking lots, stop-and-go traffic, etc). This did not happen all the time, but it was quite often. Other than that, the car still runs perfectly all the way to redline.
TROUBLESHOOTING: I started to investigate the typical culprits. This, and more, have 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 old one's 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 another new Bosh unit
- Had the DME unit out and sent to Steve Wong for eval/repair
- Engine out: Smoke inspection. Replaced a few culprit 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. Gapped and replaced the spark plugs.
- Engine in: stumble was still there. Swapped-out another AFM as a test -- no change.
- I have also set/rechecked the idle per standard procedure. It’s set to 880-900.
A CLUE: Before getting out-in-the-weeds any further, I decided to remove the SWong chip and put the OEM back in. The stumble was gone! That narrows it down. Three local Porsche shop owners 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 a little closer to the OEM chip below 2K RPM for better drivability.
TESTING: I reinstalled the customized SWong chip back in. It stumbles at 1600-1800 RPM just like before. Dang it.
Since the car runs fine with the OEM chip, but now stumbles with the SWong chip, where do I go next? Speaking to both Steve Wong and Sal Carceller, I need to get the stoichiometric checked/set and drive tested before I go any further and this is why I asked for names in my general area. Thanks for reading.