Emissions - high NOX

[unom]

Hi,

I'm having some trouble getting my '98 M3 through the Ontario etest. I passed HC and CO, but NO was about three times the limit.

I changed the catalytic converters and the after-cat O2 sensors (they came from a car that passed the test this week) and re-tested. HC and CO went down even more and are now almost zero. NO however is still high, but down to about 2 times the limit.

I've got a couple of ideas of where to go next, but I thought I'd keep them to myself so I don't steer your thinking one way or another.

What should my next steps be?

Thanks very much!

[LuxoM3]

Your M3... is it the US motor or Euro motor?
Any mods?

The after-cat sensors don't do that much when it comes to fuel mix - you want the PRE-cat O2 sensors, those are very active. Heck, swap sensors.

But given you have new catalytic convertors ($$$$) you really shouldn't be having any trouble with passing emissions.

IS your car nice and hot when testing?
M3's can NOT be emission tested when cold. They ALL fail if cold.

[unom]

Thanks for the reply. It's an unmodified US motor (well, PO put on a K&N cai)
The 'new' cats were from my 328is - that car passed a few days ago.
The car was really hot for the test - since NOx is the issue, I'm wondering if maybe too hot.
Thanks for the input. I'll test my pre-cat sensors.

[madviii]

http://www.theguestroom.net/manuals/Tuning%20Your%20MegaSquirt.htm

Look at the section on emissions....

I would say you need to richen it up a bit if nox is too high but co and hc are low. Can't tell you how to go about that though but you could use some old front 02 sensors (tend to read leaner as they get older causign the ecu to trim richer).

[MasterKwan]

Might want to check your fuel pressure too. Make sure your MAF to manifold boot isn't torn.

[unom]

Great ideas/resources. Thanks! BTW - I did do a lot of searching before I posted. I was 99% certain the the catalytic converter switch would clear things up.

[LuxoM3]

Some good reading about NOx - http://www.nichols.nu/tip766.htm

NOx is from high cyclinder temps... so the car might be running too lean.

Ditch the K&N and use the stock airbox, run some fuel system cleaner (get the injectors spraying more), and maybe the fuel filter too (if you can't remember when it was last replaced).

My theory is a little less air and a little more fuel will help bring that down.

[madviii]

Some good reading about NOx - http://www.nichols.nu/tip766.htm

NOx is from high cyclinder temps... so the car might be running too lean.

Ditch the K&N and use the stock airbox, run some fuel system cleaner (get the injectors spraying more), and maybe the fuel filter too (if you can't remember when it was last replaced).

My theory is a little less air and a little more fuel will help bring that down.

^ Exactly what I was thinking as well, however, if there are no codes and this is an obdII car then he is withing the trimming limits of the DME. All of the bad 02 sensors I have seen read lean rather than rich so you would think if anything he would be running a little rich if the sensors are old... THen again, just cause most start to read lean when they go bad doesn't mean all of them do :)

With OBDI it's easy, I feed the DME the narrow band signal from my wideband and then offset the narrow band signal to move the "stoich" point around.. Had to do this a couple months ago to get my car to pass. Just moving the point the DME thinks is stoic from 14.7 to 15.0 made a HUGE change in emmisions, HC and CO went way down but NOx went up so I moved the perceived stoic point back down to 14.85 and it passed just fine.

[LuxoM3]

Yeah OBD2 can be a PIA.

It's just enough information to let you know of an issue, but not enough to help you pinpoint it.

O2 sensors rarely flip the CE light until they are totally dead. I had weird WOT issues with my OBD2 M3 - no CE Light. But a code scan showed pre-cat reaction times for the O2 sensors.

The 90s are a whacky time for German electronics. I personally think the OBD2 cars ran better and got better mileage than the OBD1 cars but, sometimes I think the logic or code was hobbled together.

[unom]

Update. Replaced MAF boot, replaced MAF, replaced front O2 sensors, oil change, italian tune-up. Removed K&N and installed OEM airbox. NOx has gone down ever so slightly, but not nearly enough.

Latest readings are:

HC - 10ppm limit is 58
CO - .01% limit is .32
CO2 - 14.6% not part of gov't test
O2 - 0.1 % not part of gov't test
NOx - 910 limit is 435
I'm going to look at fuel pressure now. The spark plugs show I'm lean but they were very similar so I do not suspect a bad injector.

Again, I appreciate your input.

[LuxoM3]

Hmmm... silly question... what color are your fuel injectors? Green or Pink?

[unom]

Not a silly question, but they're pink. I had already checked that since the PO wasn't too careful about, well, anything. The car has had a tough previous life and in the three weeks I've owned it I think I've done more maintenance than in the previous three years. Sad really, but that's why it cost $2000:)

Changed the Evap Purge valve because codes were saying it was stuck closed. Not really a likely suspect but...

I passed today. I ended up paying for a diagnostic. They checked all the codes,vacuum hoses, fuel pressure, etc. They checked over the secondary air system as a long shot. They found that BMW had a recall to reflash the DME because cars were failing California emissions tests due to high NOx. My car is missing the sticker that says the update was done.

The car finally passed - they just put it on the test machine in off-line mode and with a combination of hot catalytics and the just-ever-so-right placement of the fan, it gave a reading of barely pass. They ran the test, and I'm good.

Here's a recap of what I did:
Catalytic converters, all four O2 sensors, MAF, MAF boot, Idle Control Valve hose, change K&N intake to stock box, oil change, Evap Purge valve.

My suspicion of what finally worked? I put the lowest octane gas I could find in the car. I know, I know. However, my thinking was that I had got NOx down from 2 times the limit to about 1.4 times the limit. That's not really very far to move, and a slight retard to engine timing would more than make up for that. Low octane gas = knock = knock sensor makes DME retard timing = low NOx.

Thanks for the suggestions! Time to go buy some octane boost and track time:)

[LuxoM3]

U - sheesh! What a PIA!!!

Well glad you got it roadworthy!