gavin lewis

I have a 2005 997S, 3.8, with VF Engineering supercharger, Fabspeed headers, high flow cats and x pipe, no other engine mods. Car is running rich , and when I pulled up Durametric I saw this.


Initially I thought bad sensors, and replaced them with new, but the sine wave activity is tracking with a functioning O2 sensor, and the new ones still read -50V. I checked to make sure they weren't tuned out by the previous owner, but when I unplugged them and ran it, it threw a code. I'm lost and don't even know where to begin looking. I've replaced the sensors (pre cat only), and I've done a smoke check to look for a MASSIVE air leak, but nothing shows. If anyone has any insight, I'd really appreciate it, because I'm about to start throwing money at it, and I hate to pay shop rates.

Thanks
Gavin

Petza914

Since yiu have a Durametric, log mean fuel trims for both banks and let's see what those look like. In a separate test, log MAF values so we can tell if the MAF is functioning. Also, where is the intake air temp info being captured. To be done right, it has to happen after the air is compressed by the supercharger so in one of the intake manifolds and then a sensor there is probably spliced into the wiring harness from the normal MAF sensor, since that's where the temp reading is done on normally aspirated cars. If VF is using the MAF temp sensor before the supercharger, the car will never run right or have the proper mixture, as the act od compressing the air by the supercharger generates heat.

gavin lewis

Thanks for the reply. I did have fuel trim mean values logged on that printout, they are about .95xxxx, and consistently that high. Air intake temperature is coming from the outlet of the intercooler, so it should be accurate after being compressed and cooled. This is really frustrating...

Petza914

Sorry, missed that. Yes, those numbers for fuel trim are really rich. When I had an AOS corrugated pipe leak with a harline crack, even at 0.98 it was too rich and both your banks are significantly richer than that. Have you ever replaced the AOS and the corrugated plastic lines that run to it.

Your smoke test didn't show any smoke emanating from anywhere. An intake or AOS system leak post MAF is my best guess for why the DME is enriching things so much. Have you had the plenum out at all. That's hard to get back in and sealed well.

gavin lewis

I've never had the plenum out, but I do know it's a pain, I had it off on my 996 to replace the AOS. I've never replaced the AOS, as the smoke check didn't show anything. Is -50V even a real reading on the O2 sensor though? I thought they read between .1 and .9, and were incapable of negative values. I can do another full smoke check today, as I own the machine, but the last one showed nothing.

Petza914

Pay particular attention to the plastic lines from the back of bank 1 to the AOS and the front of bank 2 around the outside of the intake manifold and back to the AOS. If yiu can manually spin the super hsrger pulley with the belt removed to help force the smoke all the way through the system, that's helpful too.

yelcab

You cannot get -50V out of a regular O2 sensor. Put a scope on the signal and see what the actual value is.

Mike Billings

I had the -50V O2 reading on the Durametric. Sorry I don't recall the details. So that particular problem is probably not due to your car.

Good suggestions otherwise.