Fabspeed bypass - Causing low RPM Surging?

bornrich · 05-06-2005, 08:56 AM

My 81 911SC has a surging issue at light throttle (stop and go traffic - ~2-2.5k), the engine surges causing the car to jerk slightly. I can also hear this at idle, the idle goes up and down a few 100 RPMs with a regular pattern.
As a side not I have the idle stabilizer disconnected due to the RPMS hanging up. It is a minor problem but it is so much smoother with out the sensor, especially since I have to deal with traffic on my daily commute.

My tech (Bob Grigsby) tells me this is due to the location of the O2 sensor in the bypass only sensing on side of the engine. My Tech's explanation sounded logical since the bung locates the sensor in the path of the upper tube of the bypass thus getting only the mixture from one side of the engine. To eliminate this surging I have disconnected the O2 sensor but this causes poor gas mileage. I have had Bob check the mixture and anything else he cam think of that would cause this behavior. My gas mileage is poor but the car runs great.

I contacted Fabspeed and they said they have had no complaints and that my tech is incorrect in his evaluation.

Is there a fix for this behavior? Has anyone else experienced this surging with the Fabspeed bypass?

Scooter · 05-06-2005, 09:53 AM

Check the O2 sensor for damage. That is about the limit of my knowledge on this topic, but I do know that John Walker would be the guy that I would be talking to. Try shooting him a PM, maybe he could give you a second opinion (from your wrench, not me). Good luck and drive safe!

Bill Verburg · Bill Verburg's Garage

O₂ sensor would be the first thing I would look at also. Sounds like lean surge.

bornrich · 05-06-2005, 12:52 PM

I replaced the sensor when I initially installed the bypass in 02 and my tech replaced it again this past fall after my smog test and subsequent re-installation of the bypass. So I don't think it is the O2 sensor.

bornrich · 05-12-2005, 02:45 PM

Bump - help!

Paulporsche · 05-12-2005, 02:59 PM

A fluctuating idle usually indicates a too rich mixture. On my car (no O2 sensor) that same too rich mix also shows up as low speed, part throttle surging.

bornrich · 05-13-2005, 08:38 AM

My car does not surge with the O2 sensor disconnected (should be running richer). With the sensor connected it does surge. This is driving me crazy.
One message said it might be lean surge. That seems more likely, but then I know next to nothing about fuel injection. I will talk to my tech once again.