Can a screenless LS1 MAF cause an HC emission failure


Dec 27, 2001
I've always been able to pass NJ state emissions in the past; typically without even being close to the limits. Essentially I just pop in a TT emissions chip and I'm good to go (functional Cat, EGR, etc.). No visual checks, just the tailpipe sniffer at idle and at "high idle" which is probably something like 2000 rpm. This year I failed: idle HC ppm was 459, CO% was 0.07, CO2% was 14.2 and O2% was 1.4. No NOx values. High idle was HC-24, CO-0.11, CO2-14.9, O2-0.2. Based on the HC/CO numbers, I thought I was running lean at idle. I adjusted my Translator Plus to put the Base MAF on 0 (no change) instead of the setting of 2 (lean) that it normally resides at with my Extender chip (which I use exclusively except for emissions testing). With the new Translator setting, I went back and failed worse. Idle values were HC-769, CO-0.46, CO2-13.5, 02-1.6. Fast idle values were HC-632, CO-0.16, CO2-6.8 and O2-10.5. For those that know, the only thing above the limit is HC which has a max standard of 220.

I haven't gotten a chance to pull the plugs to read them. The car otherwise runs nice and strong like normal.

The question I have relates to the screen in the LS1 MAF. A week before I went for emissions testing, I decided I was going to clean the MAF. When I removed it, I noticed the screen was damaged (still intact though). I don't think a foreign object hit it. I just suspect that since my MAF is positioned horizontally in the up pipe off my front mount intercooler, the "high pressure" air stream might have damaged it over time. Seeing that it was damaged, I simply removed it, used MAF cleaner spray on the elements and buttoned things back up again. I far as I can recall, this is the only thing different from previous years when I passed emissions w/o issue.

Anyone think the screenless MAF is the cause of my problem? I'm wondering if turbulent air is crossing the MAF and it's not reading properly for fuel management.
I also run the same setup as you, screen less maf ls1 with extender.

I don't have emissions testing like you do, but when I was told that I couldn't run a maf without a screen I contemplated putting a screen over the opening to reduce the turbulence that others said would be an issue.

I didn't follow through with doing that as my car runs fine without it. Bob Bailey also said this in response to my question in the thread [maf with no screen] it might run ok, perhaps a little lean at idle. It depends on things like the air filter and underhood turbulence.

I have a heavy duty piece of screen that I could try and see if that makes a difference, and maybe you could try it also to see if it makes a difference, but my understanding from what others have said is that it would make the biggest difference at idle.

Bob said it might run leaner at idle so it doesn't jive with your results on the emissions testing. It might help at high idle to have a screen and not having one might be throwing the high idle readings off enough to make you fail.

Get a piece of heavy duty course screen and see if that helps, and if it does report back so others may use that in the future if it works.

I figured out how the screen got damaged. I have the 3.5" MAF in the middle of my 3" up-pipe. The 3" up pipe slips inside the 3.5" MAF. It was this contact that damaged the screen. Anyway, since the screen was still intact and with it butting up against the up-pipe and inside of the MAF, it wasn't going anywhere. I put it back in. I also put my translator BASE MAF switch back to 2 (lean idle) like it was before. I hope to get a chance to take the car back for re-inspection sometime this coming week. I'll post here how I make out.
it is not a turbulence problem, the screen directs the air and hits the element differently when it is there causing more air to be "seen" by the sensor. Removing the screen will cause the car to run leaner (up to 15%) and somewhat erratic as in a rough idle or drivability issues
I took the car back today for re-inspection. With the screen in place, my numbers improved, but still failed. New numbers at idle: HC-302, CO-0.06, CO2-14.4 and O2-0.5. At fast idle: HC-52, CO-0.05, CO2-14.5, O2-0.3. I guess the screen made a positive contribution. Odd that the car is now setup the way it's been for years past when I passed inspection with no problem.
I finally passed. I retarded the timing 4deg via the Translator Plus. HC barely squeaked under at 217 at idle. All the other numbers were good at both idle and fast idle. Odd that I never had to mess with the timing in the past to get it to pass.