Help me diagnose these emission #'s - PeachParts Mercedes-Benz Forum

wbrian63 · #1 11-15-2007, 07:16 PM

'92 300SE. Runs fine but just squeaked by on my emissions test today.

Car was allowed to idle for an extended period - maybe almost 10 minutes (lazy inspector), so maybe that's why the readings are the way they are.

Outside temps were in the low-mid 60's.

I don't remember if he was this slow last year.

Last year's emission test results:
Low Speed 15mph
HC (ppm): 21 - max is 124
CO (%): .07 - max is .69
CO2 (%): 14.5 - not tested
O2 (%): .1 - not tested
NOx (ppm): 288 - max 937
Dilution (%): 14.6 - must be >6

High Speed 25mph
HC (ppm): 44 - max is 120
CO (%): .09 - max is .67
CO2 (%): 14.4 - not tested
O2 (%): .1 - not tested
NOx (ppm): 270 - max 848
Dilution (%): 14.5 - must be >6

This year
Low Speed 15mph
HC (ppm): 72 - max is 124
CO (%): .24 - max is .69
CO2 (%): 15.1 - not tested
O2 (%): .1 - not tested
NOx (ppm): 932 - max 937 - eek!
Dilution (%): 15.3 - must be >6

High Speed 25mph
HC (ppm): 112 - max is 120
CO (%): .24 - max is .67
CO2 (%): 15.5 - not tested
O2 (%): .1 - not tested
NOx (ppm): 637 - max 848
Dilution (%): 15.7 - must be >6

Something's definitely wrong since last year. I've put about 21K on the car. The plugs are new, and when recently changed, looked pretty good. The O2 sensor was replaced when the car failed to pass last year.

I've noticed the MPG is down somewhat from last year - about 1mpg less on average.

Any ideas?

I'll pull codes and see what's up - no check engine light on this car.

MB-Dude · #2 11-15-2007, 07:31 PM

I'm not an emissions expert, but have been doing a lot of research on an unrelated V12 problem. Anyway, if I understand what I'm reading from Bosch Fuel Injection & Engine Management manual (page 2-16), it suggests elevated NOx emissions can result from too lean an air-fuel mixture and elevated combustion temperatures. The section discusses the balance that needs to occur between too rich (elevated CO and HC emissions, but lowered NOx) and too lean (elevated NOx emissions, but lowered CO and HC). Apparently, there is a happy medium. Maybe you're just a little too lean.

Not sure if this will hinder or help, but it should give you a lead on where to go next - air-fuel mixture ratio. Good luck.

Cheers,
Jeff

manny · #3 11-15-2007, 08:50 PM

The first 2 things that come to mind, looking at your numbers would be EGR valve or possibly your catalytic converter is starting to say goodbye.
I would like to see measurements before & after the converter.
Easy for me to say, I have my own exhaustgas analyzer.

mbdoc · #4 11-16-2007, 08:00 AM

High NOx comes from HIGH combustion temps & lazy catalyst.

Possibly the EGR tube is partially blocked.

david s poole · #5 11-16-2007, 09:30 AM

it's a pity we don't have an accurate o2 reading as that would help considerably in the diagnosis.[narrow it down to one of two poss]

wbrian63 · #6 11-16-2007, 11:08 AM

David, what are you thinking the possibilities are? 1) the catalyst, 2) the egr? Or are you thinking of something else.

I'm going to check the EGR tube soon to see what's up there - if it's clogged then there's at least a contributor to the problem.

If I understand the process, the EGR contributes little to the emissions system at or just above idle, so if the tube is clogged/restricted, the impact would be most noticeable in the low-speed test, which is where I came close to failing.

Curiously, while watching the test, I saw something I don't remember seeing before. After the low-speed test @ 15 mph, there was period of time (about 90 seconds) where the computer requested a speed between 15 and 25 (I don't remember what it was) - then the computer requested the 25 mph run.

Maybe this is always the way the test runs, but I don't remember it being like that before.

patrick_mb · #7 11-20-2007, 07:43 PM

I went through this on a 300SEL about a year ago. Basically, I think worn valve guides and crusty old valve seals let enough oil go through the motor that it fouled the catalyst bed that deals with the NOx. There are two in a 3-way cat. One deals with CO and HC, one deals with the NOx.

Anyway, after replacing the valve seals, I made sure everything with the EGR, fuel injection, and ignition was up to snuff, cleaned the injectors, set the pressure in the fuel distributor, used some combustion chamber cleaner, even cheated a little bit on a second retest by backing out of the ignition timing by fooling with the resistor and the vacuum advance, and I couldn't get the car to pass the NOx portion of the test. I was hoping the cat wasn't smoked, but... finally I swapped out the cat, and with all that other stuff sorted out it blew right through the test with flying colors.

Oh, and I used a DEC cat, but it was a pile of trash. Mediocre welding, crimped bends, small tubing, bad fit between welded sections, non-OEM design with both downpipes merging into one small pipe before the cat, didn't even seal against the manifolds. And I had to put a spacer under the passenger side engine mount to lift the motor up a little bit so the forward downpipe didn't hit the crossmember on hard acceleration. Trash! I had to attack it with an angle grinder, die grinder, and MIG welder before the thing would bolt up and seal. Never again. After reading so much over the years about DEC quality, it was SUCH a letdown. Take a look at this. Seems like a nice deal, and at least it has two downpipes all the way to the cat:

eBay auction 190118202305

wbrian63 · #8 11-20-2007, 11:55 PM

I'm hoping that it's not the cat - the car doesn't use any oil to speak of and the plugs were clean when I changed them last.

I'm gonna try to check out the egr tube this weekend - we'll see what that turns up.

I've saved the link to the catalyst auction on e-bay - thanks for that.

david s poole · #9 11-21-2007, 10:49 AM

most of the time euro manufacturers don't use egr to kill nox[that is a chevy/ford trick]high nox is created by high combustion temps[lean mixture] and too much available 02[feeding egr back into intake substantially reduces available 02 and will hide a mixture problem]the available 02 is reduced as the mixtures move from lean to lambda[14.4-14.7 to 1 air to fuel ratio] a slightly lean mixture will also elevate hc as some small misfires take place.this is a 104 motor and your mixture is controlled by mas and electric injectors and fuel pressure reg.i suspect you are running a little lean.but hey it passed so this is management by design not crisis.

wbrian63 · #10 11-21-2007, 08:13 PM

I've yet to pull codes - this weekend should allow me some time.

My biggest concern is that I went from passing with flying colors last year to squeaking by this year. This with only about 20k miles of driving.

I guess it couldn't hurt to check the egr tube for clogging.

Codes will tell me something, if there are any to find.