Emissions Test - What causes increase of HC?
 

The car: 1994 C280 with 95,000 miles.

I just did my emissions certification on the car, as we have to do every two years. I passed, but when I compared the results to the test from two years ago, my HC ppm had increased dramatically. I had 17 ppm at idle and 33 ppm at 25 mph (I'm allowed up to 55 before I fail). Two years ago it was 0 and 0. What could cause such a large increase in HC? I brought the car into the test hot, having done a 20 mile run up the highway to heat it up for the test. I have a failed air pump, but as the car was started hot (it was on the test apparatus within 5 minutes of arriving) I don't think that has anything to do with it. I also intermittently get a code for a failed purge valve, but I just clean it out with Arthur Dalton's trick when I get the code.

Any ideas? If I increase the same amount in two years, I'll fail then. The plugs have less than 7,000 miles on them, as do the plug wires/boots and the MAF.

Thanks,

Jonathan

when did you last change the oil?

The simple answer?
1. Ignition related items ( plugs, etc. )
2. oil consumption ( minute amounts ).
3. Catalytic converter degrading.
Remember you're reading parts per million.
Just because your numbers are up from 2 years ago does NOT mean they will continue to climb. ;)

What grade of fuel have you been using?

David: the oil is changed religiously every three months, which amounts to about 1500 miles per oil change.

Manny: I refreshed all the ignition related items a little over a year ago, which amounts to about 7000 miles (plugs, wires and the three plungers below the coils). The car doesn't use any measurable amount of oil between oil changes. I hope that doesn't mean a catalytic converter, but with my luck it probably will. Its always the most expensive thing.

Rick: Premium fuel only, 91 Octane.

You worry too much. :D
If the car was tested again tomorrow, your HC's could be down ( or up ) 10, 20, 30 ppm.
How do I know,........I test exhaust emissions every day ( with my own 5-gas analyzer). ;)

Is there any chance it could be the O2 sensor(s) "getting old"?

Mike

Temperature of system?
 

We have a 97 C280 with the inline 6, which have gone through several emission tests.
My experience is:
Cold -when I drove the car short trip (say, 10km) to test the HC was high.
Warm - when I drove it longer say 30km on highway to test the HC was always very low.
I believe the catalytic converter needs to warm up to perform well.



CF

failed test
 

I failed my inspection test,because my HC is reading 119 and HO% is reading 0.83, what can i do to fix that?

Year & model of car would help. ;)

failed test
 

its a c280 mercedes 1995

Lots of questions:
1. What are your limits for HC and CO ?
2. When was the last time you did a really good tune-up?
3. Do you have a " Check Engine " light on ?
4. Has the Oxygen sensor ever been replaced ?
5. How is your fuel economy ?
6. How does the vehicle generally drive ?

reply
 

1)none of the light are lighted up,
2)1 and a half year ago
3)no its not
4)No
5)prety good
6)perfectly

My vote:

O2 sensor.

mpolli

Based on the age of the car, I'm tempted to agree with you but, it bothers me that he does'nt have the Check Engine light on. :confused:
Possibly the catalytic converter " going south "?

Could be. However, often the O2 sensor failure is not detected by the computer. It can get sluggish. As far as a shotgun approach it is cheaper to try than the cat. Also I think it is more likely. Most cats are replaced because they rattle apart as far as I know. As long as the O2 sensor is fairly cheap I would throw one in. The Bosch ones are kind of spendy but the universal ones are 30.00-45.00. But I don't know how you pick a universal one since I have seen 1 wire, 2 wire, 3 wire and 4 wire... Maybe he could get the O2 sensor tested for a reasonable price?

Mike

Jonathan, those increases are relatively marginal. Could be plenty of factors, most probably minor. If you did a lot of city driving recently, versus a long highway trip, etc.

When I bought my E430 this past April, it scored perfect 0's. Earlier, the previous owner did a test and got some low numbers for HC and CO. So the car actually 'improved' over time, without having any work done to the car.

I wouldn't worry about those numbers at all. You could always do a follow-up test in a few months. If the numbers keep getting worse, you could have an issue somewhere, maybe the cats. Just my $0.03.

can the knock sensor malfunctioning hurt your HC as well?

That's a trick question.
It's ( knock sensor ) job is to retard ignition timing when it " hears " knock.
If it did not work, and the timing was way too far advanced, the HC's would likely be elevated. ;)