I've been reading this forum for awhile and figured I would try this post over hear.
I'm usually over on the Corvette forum, but you guy's are farther along with superchargers .. from what I've seen.

Sorry this is so long….
I’m having an odd problem with the oxygen sensor readings in park at idle. I posted the original details in the tech section (but its such an odd problem I didn’t expect much help) …. Also stumbling a little at low speeds.
http://forums.corvetteforum.com/zerothread?id=275056
In a nutshell the oxygen sensor reads lean at idle in park\neutral and operates what looks like normally in drive and out on the road. Tried all the things you would, no vacuum leaks, swapped oxygen sensor, MAF, ECM, etc. no change.

I figured out the cause, but need opinions on a solution……
Here is what’s new, I pulled the pipe connecting the supercharger to the MAF (let the supercharger blow into the air) and everything went back to normal the oxygen sensor started to cycle, block learn worked back from 160 to 132 after a few minuets.
I upgraded the supercharger early last year from an old Paxton SN93 to a new SN2000 with the high output impeller (6psi to 10psi) and at the time had a ported MAF with no screens, the car started running like %$#%&. Bought a new MAF and left the screens in and all seemed OK. Took the screens out (off the new MAF) over the winter and I’m back in trouble.
With the new supercharger there is a lot of air pumping out, even at idle, and this takes a 90 degree turn right into the MAF. I think all this turbulent air is effecting the MAF’s ability to read the airflow corretly, after all it works on the passing air cooling the wires.
I guess those screens are in for a reason afterall.

Is anyone else blowing through the MAF on an L98, have you ever looked at your oxygen sensor at idle, and does it do this. I can put the screens back in, maybe lengthen the pipe between the supercharger and the MAF. Maybe even put more straightening baffles in the pipe. Anyone sucking through (rather than blowing into) an MAF with a supercharger, and how does that work?

I’m worried, because they emission test my car off the dyno rollers (they are afraid of the posi-traction) with a two speed idle test. Doubt I will pass with the setup as it is.

http://www.corvetteforum.net/c4/iris/finalsig2.jpg

 

SUPERL98:

If I understand right, you have the MFA after the supercharger compressor. I have a SN2000 on my IROC and my MFA is in front of the compressor, my paxton instructions say the MFA must go in front of the compressor. The compressed air is very hot and the voluem of air will be too much compared to the TPS sensor voltage, do you get a coed 33, 34, 12? the wrong TPS voltage vs. MFA voltage can cause a stumble on off idle acceleration. It may be diffrent for the corvette but I wouldn't think so. You are way lean at 160 BLM, I would move the MFA to the inlet side first, scan it, and see where you are at.

The stumble might be your TPS adjustment, what voltage is it set at? Should be .54v

Do you have stock 22lbs injectors?
What is you fuel pressure at idle/WOT?

Do you have a adjustable fuel pressure regulator? if so, what pressure is it set at. I run mine at 46 psi with 30lbs injectors.

Do you have a coustom prom? are the injector constants programed right, 19lbs, 22lbs, 30lbs.


What is you o2 voltage? I belive it should be at 500mv at idle.



I think you MFA location is you biggest problem, I would start there. good luck, let us know how you make out.

cody

 

Yes, thanks, the MAF is after the supercharger, but it's been like that for about 6 years with no problems till now.
Stock injectors and the fuel pressure is set at 44psi (I have a perminant guage and it is adjustable)
Ox sensor cycles around 70 to 100 mv.
Walbro pump in the tank and no pressure drop at WOT (climbs with blower output)
The chip came from CarrolSuperchargers with the original kit.

Put the screens back in this weekend and it helped blm moved to about 150 and the oxygen sensor cycles abit, but not much. The stumble is much better, but the idle is still lean in park.
TPIS is about .56, and I took the IAC valve out and cleaned it, but it was very clean to begin with.

Did try a full tank of new gas, no help.

I'm convinced (again) that I might have a vacuum leak, but I'll be %$#% if I can find it!!! looked at all the hoses, even the brake booster. I hate the idea of using a propane tank and tube around the engine, but I might try that trick.

Also been told to look for an exhaust leak, but I can't see anyplace that could be.

 

the procharger systems blow through the MAF which is no big deal on stock type computer setups because the MAF doesnt really do anything after a certain point of airflow (off idle, full throttle). the prom guys in the DIY prom section of this board can tell you more about that.

 

blow through maf

 

Paxtons Y body kits blow through the MAF and Paxtons F body Kits draw through the MAF.
The screens are in the MAF to help induce a more even, uniform, and controlled turbulence. Removing the screens allows more airflow but also allows more caotic turbulence. The position of the MAF in the airduct will also affect turbulence of the air passing through the meter. You could try repositioning the MAF to the compressors inlet side, or to another location in the outlet side. It may also help to fabricate some vains inside the duct to straighten the air flow.

Also using a ported MAF will lean out your calibration further. The MAF only directly measures the air that passes through the venturi (over the hot wire) in the center of the meter. The air that passes though the meter, but passes around the outside of the venturi, between it and the MAF duct, is not directly measured, it is just a calibrated ratio of air. If you change the diameter of the MAF duct, or port the duct and/or the heatsink fins you will change the ratio and the calibration. More airflow around the venturi VS though the ventrui, leans out the calibration. This, I believe, is the most likely cause of the remaining (but lesser) lean condition after replacing the screens.
The problem with throttle tip-in stumble that you are experiencing is 99.9% likely do to the lean idle and off idle transition that you are suffering from.
The easiest fix may be replacing the MAF with a completely stock unit. A little trickier, would be to relocate the MAF to a less turbulent place in the intake ducting, and/or fabricate airflow straightening vanes inside the duct before the MAF. The further the MAF is from the impeller (on either the intake or outlet side) the less turbulence you will have at the meter. Another possible easy fix (but maybe not the best choice) may be increasing the fuel pressure with an adjustable regulator to compensate. The ECM will compensate when at cruse, but it may run a little too rich at WOT, so this probably isn't the best option (unless you are already running a little lean at WOT too). Finally, a custom ECM calibration with a little richer idle table may help.