Alpine951

2 years now trying to figure out whats wrong with my car. currently a lumpy idle, wb afr shows 15.0 to 15.7 at idle on warmed up car. no ISV because it leaks (plugged port), idle is around 820 , sometimes less, can't increase idle speed at all when adjusting screw. If I unplug the O2 sensor the idle drops and afr goes in the 17's.

O2 sensor is new and has very few miles on it.

EDIT: afr is actually 15.5 to 15.9 and that is whith FQS at the 2nd setting.

Alpine951

actually, idle is more like 780. can't get above 800.

PorscheFanatic202

Have you checked for vacuum leaks? Does the TPS work properly?

Alpine951

checked many times. tested tps. replaced with used working tps.

Droops83

Well, the simple answer is that when you unplug the O2 sensor, the DME will go into open loop control and the engine will run according to whatever the base idle is set to (combination of base idle map in DME and idle adjustment screw and integrity of your intake system/health of the engine and sensors, etc). If everything is well, then the idle should not change by much, if at all.

I know you have been chasing this lean running condition for a long time. Is the lean condition only at idle? If so, have you ever smoke-tested the intake system (including boost pipes, etc). 17+ AFRs at idle mean that there is either a large vacuum leak or that some setting or input to the DME is way off. If it is lean across the board, I'd be looking more at fuel pressure/delivery volume and again at inputs to the DME.

If all of the above are OK, do you have the ability to reinstall a factory AFM to eliminate the possibility of a faulty aftermarket MAF sensor?

Alpine951

car has been pressure tested more than once, smoke tested at a Porsche shop. I have found some posts on here that the afr will richen when unplugging the O2 sensor. mine goes extremely lean. full pressure is good.. lean while cruising also.

Droops83

I think people say this because the base idle mixture is often set to be slightly richer than stoichiometric, but again, the mixture will go to whatever it is set to using the AFM bypass (could be lean or rich depending on how it is set, and if there are vacuum leaks, etc). There is no set "rule" about this.

Have you tried plugging a known-good factory AFM back in and re-checking the mixture (it should be relatively accurate at idle and light load conditions, even on a modified engine)? It could be that your aftermarket MAF sensor is out of range and is under-estimating the airflow into the engine. What sort of chips/software do you have, and how is the MAF signal being converted into a signal that your DME can interpret? It could be that your tune is off, or a wiring issue, etc.

Did the lean condition randomly occur one day, or has it persisted after some work was done to the engine? If it is the latter, I would take a close look at what exactly was done. The downside of adding aftermarket stuff is that diagnosis of such issues becomes more difficult (such as determining whether it is a fundamental issue like a vacuum leak, or something specific with one of the aftermarket parts). I wish I could offer more specific help, but it might be wise to get in touch with whoever makes your MAF kit and ask for specific advice about how to test the sensor if everything else checks out OK.