Readings: Standard Current

HC (ppm) 114----22 ====> pass
CO (%) 0.63---0.01 ====> pass
NOx(ppm) 796----2405 ====> fail!

Tested but not graded:

CO2(%) 13.2 (seems almost perfect)
O2(%) 20.8 (very high?)

I understand high NOx readings are caused by an unusually rich air/fuel mixture setting, high combustion temps and/or malfunctioning EGR.

I'm wondering if a 'dragging' parking brake would cause such high NOx readings. (high combustion temp) It sometimes doesn't release all the way on my car and the inspector did test the brake. It's since been adjusted and the engine operating temps are otherwise normal.

The car is a new to me '94 C280 and I've read about the EGR tube clogging problems. It has 101,000 miles on the clock. The very low CO and HC makes me believe I need to richen the mixture -- OR could extra load from a dragging parking brake cause this??

As far as driveability, the engine runs and idles superbly so this emissions failure has got me wondering.

Any insight would be very much appreciated!

__________________
-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi

Those numbers were decipherable...........until the O2 came in at 20.8%!!!!!!!!

Does your engine not run on oxygen? I would've thought the tester would pick this up. Some oxygen in the exhuast indicates a fault but i can't be that high with the engine running, no matter how badly in runs. This means either the reading is false or you have a leak in your exhaust which can allow alot of O2 into the system.

The CO readings so that it could be richened up a bit.

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Alex.

MB Tech
Sydney, Australia
Volvo 122S
W201 190D 2.5 manual
W202 C240
W203 C32

There's definitely something wrong - 20.8% oxygen is the normal amount of oxygen present in air. Exhaust from an engine should definitely contain less than this, because the engine uses quite a bit of oxygen to burn fuel.

Perhaps the test rig was screwed up? You'd need a hell of an exhaust leak to pass almost no exhaust gas through the tailpipe - and they probably check for a pressurized exhaust during the inspection.

Troy
1995 E420 90k

Yes, there is something obviously wrong with the O2 reading. Air only has about 18 percent O2. The exhaust should have zero to a few tenths percent.

What kind of test was this? I/M 240? ASM? If NOx was measured it must be a loaded test. Can you briefly explain the procedure? Was there another set of numbers from a second test at different speed/load?

High NOx can be caused by lean, not rich mixtures, however, I don't think mixture is the problem. You need to do a thorough functional inspection/test of the EGR system - both the valve itself and the control system. It's not uncommon to have EGR system problems at this mileage on any car.

A dragging parking brake could cause high NOx because it places more load on the engine, but would probably not increase it this much.

Duke

These numbers are very wrong.
Reminds me of an instance ( about a year ago ), when I did some testing ( with my own 5-gas analyzer ), at a customers location.
MY machine said there was 0.8% oxygen present in the exhaust of the vehicle I tested, his machine said there was 250 % oxygen.
Convinced him to have HIS machine repaired.

__________________
2007 C 230 Sport.

I appreciate all the informative replies, thanks!

Duke, the test was done on a dynometer at 2 speeds. The results I posted were at high speed (25mph), and the low speed (15mph) results were virtually the same so I didn't post them.

I will definitely check the EGR valve and tube for proper operation and passage and ask the testing station about the high oxygen readings -- there are NO exhaust leaks so I'm at a loss as to that anomalous reading.

Also of note, the wife had mid-grade fuel in the system at time of test (89 octane).

Again, thanks to all for the replies.

__________________
-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi

Your test sounds like the California ASM (acceleration simulation mode) test. The car is tested at 15 and 25 MPH using a load that simulates acceleration, which is more than what is required to maintain steady speed.

If the engine detonates during the test, NOx will skyrocket, however, if the temperatures were mild, which they usually are this time of year in Houston, I doubt that the engine would detonate as the load is not that great relative to engine output potential. If you don't notice detonation in normal driving, then it will likely not detonate during the test.

My last several tests have been with regular unleaded with no detonation related problems. My model does not have a detonation sensor, but I believe your model does. If detonation is detected the system will retard timing, and retarded timing reduces NOx since it reduces peak flame front temperature.
However, just to eliminate any potential unknowns, it wouldn't be a bad idea to fill it up with premium for the next test.

You should take your test report back to the shop and point out the O2 reading (it should be in the range of zero to a few tenths percent.)
As a minimum they should retest the car at no charge to you, and if the O2 readings are still preposterous, they have an equipment problem. If they give you the runaround, take the issue up with your state authorities who oversee emission testing. Such a huge and obvious error in O2 reading casts a pale on the validity of the entire test!

Duke

Yes Duke the TX testing procedure is called an ASM-2 (performed on '95 and older cars, OBDII on '96 and newer) -- I suppose identical to the CA test.

I just phoned the testing facility about the high O2 readings and he told me that the value is taken from the ambient air, not at the tailpipe.

So that explains it. I asked him why and he didn't know and he referred me to the Texas DPS.

Time to take a look at the EGR valve.

__________________
-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi

I have no idea why they would report the percent O2 in the ambient air. It's not directly relevent to emission testing and it's pretty constant at most places on earth. IIRC air is about 20 percent O2, 78 percent N2, and two percent other gases - CO2, argon, etc. Our CA test reports list percent O2 in the exhaust, which is a useful diagnositic tool for evaluating emission system performance.

You might want to check with your state emission test authorities to check out this claim. I doesn't make any sense to me.

Let us know if you find out anymore about this. It's rather odd.

Duke

Alright, I've narrowed this problem down to the EGR itself, or blockage of the EGR tube to intake -- I'm betting on the blocked tube.

However, I try to follow the EGR tube around the head and over to intake side but I can't see where it attaches back into the intake. I've seen it in other 104 pictures but maybe my 104.941 C280 is different?

Could someone please give me an approximate location of where I should be looking?

Wow, I sure do miss working on my 103 -- not enough room to do anything on this car!

Also, I'm perplexed as to why the check engine light hasn't tripped. Will it only light if the EGR valve itself gets stuck open, or should it light up also with a clogged tube.

Thanks again all!

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-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi

I applied vacuum to EGR @ idle and although the EGR held the vacuum there was no stumble. Looks like my EGR tube is blocked.

Dealer wants 2.5 hrs labor ($97 per) to replace tube.

Indy said he'd clean it out for $200, but I can DO that!

Sooo, to anyone that's done this on a 104.941 engine (C280), how did you tackle this? Is it easier to clean it at the intake or did you 'snake' it at the EGR side?

If I decide to go the snake route, how long is the tube so I'll know how long of a cable/wire/other device I'll need for the job.

Thanks!

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-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi

i need to get my car tested this month and i was thinking of leaning it out a bit, just for the test. under what conditions would this help me pass? the car really just needs a new exhaust system because i think the cat has gone bad, however i dont have the $1k to drop on a new exhaust from timevalve.

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Jonathan
1998 S600
2001 E430
1994 E320
1991 560SEL
1994 Turbo R

You can't "lean it out". The 02 sensor and electronics will maintain an average stoichiometric mixture assuming everything is in proper working order.

The initial mechanical mixture adjustment as determined by testing the "duty cycle" just establishes a basic mechanical mixture setting near stoichiometric so the lamda system,s control authority is not pushed to the limit.

M103s tend to have high emissions and you must manage your test. I've discussed this in many, many posts.

Duke