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