Questions After Smogging Car.......
 

Yesterday I brought my car (92 500E / M119) over to have it smogged, it passed without a problem but a couple of things cought my eye and I'd like to get a techs opinion.

I was comparing the results of this smog and my last smog a couple of years ago and I noticed a significant increase in the amount of HC's (Hyddrocarbons) especially at idle (643RPM). I also noted that my idle was lower than the last time I had it checked (667RPM). Are the two related? I want to keep the car at it's peak so knowing what the cause is would be helpful. Anyone have any ideas or suggestions where to look? I assume a higher HC amount means it's running rich?

Some history: The spark plugs are relatively new (4000 miles) and the oil was changed recently. The gas tank does have Techron to clean up the injectors and fuel system, can this be a possible source? The caps/rotors have not been changed and the car has had about 10K miles added since the last smog. The caps/rotors were changed at 44K, car has nearly 66K, is this a possible cause? O2 sensors are the same and I haven't done much else other than strict maintenance.

Is it possibly my idle control valve is worn?

Is it possible that using two different test stations can results in these kinds of measurement differences?

Any help would be appreciated.

placo1

You could run the same test, at the same station again every 5 minutes, with different results.
Idle speed, plus minus 25 - 40 rpm has no meaning.
HC's are caused by many things, such as ignition, overly rich mixture, overly lean mixture ( to the point where you'd have misfire ), a degrading catalytic converter, a lazy O2 sensor, slight amount of oil consumption. You want me to keep going?
My advice to you: You passed the test, don't worry - be happy ! ;)
It runs well ( starting, power, fuel enonomy ), LEAVE IT ALONE.

Afshin,

Another thing to bear in mind is that the prior test was most likely a simple high rpm/low rpm test and this year you went through the Dyno-test. The loaded dyno test seems to result in higher HC readings...at least it did for the 560, which still remained within limit.

Manny,

I know your right about leaving well enough alone. I guess my goal is to have the car operating at its best and catch problems before they become serious. I'm not stressed about it, just trying to give my wife excuses to spend more time in the garage.:D ;)


Mike,

Funny you mention the Dyno thing. The guy yesterday rolled the car up to the Dyno and I said "Oh BTW the car has traction control" and I told him it won't work on a two wheel dyno. He scratches his head and gives me this look like yeah whatever. Then he calls the local MB Tech (not sure which one) and asks him. This is what shocked me. The local MB tech suggested turning of the ASR via dash mount switch. He insisted that it exists even after I explained that it didn't. The guy spent 5 minutes looking for the switch and finally changed his attitude. He proceeded with the high/low idle test only. Very frustrating. I know it's not common knowledge but do these people deal with liars all day giving them the right to ignore customer comments?

Anyways the same test was performed although the RPM's were slightly lower than the last time. HC's on the higher RPM's were closer but still higher than last time. I wonder if elevation change and outside temps could have affected this as well?

Afshin,
How much higher was the HC count? Was it close to the max limit or just higher then before? In my experience, the most common reason for high HC is a cool catalytic converter. For instance, if the smog station you went to is close to your home AND you had to wait a while before they actually tested it. The cat needs to be fully warmed up to work correctly. I recommend a good 20 minutes of sustained (freeway) driving before pulling into the emissions testing shop. And if you need to wait more than 10 to 15 minutes, make sure the car idles for a while before they take measurements.

The next most common problem is a degraded O2 sensor.

Anyway...what were your actual numbers vs. the previous test vs. the max limit?

I'm not sure if your car has KE, but it is common for KE equipped engines to see higher HC as they age, and many are at the ragged edge or fail, marginally, on the 15 MPH portion of CA's ASM test.

I have two theories. The first is catalyst performance degradation, which requires higher catalyst temperature to achieve equal efficiency compared to when the catalyst was new. Since the coolest catalyst tempearture is at idle, the period the engine idles prior to the loaded test reduces catalyst efficiency. Once the loaded test starts the catalyst heats up, but the emissions from the first part of the test when oxidation efficiency is low may be high enough to exceed the standard, even if the test is run the full time duration. If low emissions are detected early in the test, the test terminates before the full time period, but if emissions are high the test runs the full (minute I think) duration, and the final report number is an average over the duration of the test.

Another possibility is uneven injector flow rate due to deposit buildup or wear of the nozzle chatter valves. The O2 sensr(s) maintain an average stoichiometric fuel-air ratio, but if injector flow rate is unbalanced some cylinders will be rich and some lean, which will increase engine out emissions. Uneven injector flow can also cause the slight idle roughness that many of these engines develop as the miles and time add up. Flow balance at idle is a fundamental problem inherent to continuous flow injection systems, which is why Mercedes ultimately replaced it with a full electronic system with electric solenoid injectors.

I don't think there is anything specifically wrong with your car, but make sure you use non-resisitor plugs, which is the original type used by Mercedes. This will likely limit your choice to those recommended replacement plugs listed in the manual. Most replacment plugs other than Bosch or Beru at resistor type.

From you idle speed readings the nominal is probably 650, and the variation is nothing to be concerned about.

Make sure the car is as hot as possible for your next test. I recommend going to a "drive-through" test station when the line is short and preferably after some highway driving. Keep the engine at 2000 revs in neutral until the tech is ready to drive the car onto the rollers. The above will tend to increase catalyst bed temperature.

Another "trick" is to go on a rainy day (not too many this year) and ask them to run the "tire dry test". They will run the car on the rollers with no load at about 2000 RPM for 30 seconds to a minute, and this will heat up the converter enough to reduce the 15 MPH HC by 10 to 20 percent.

The 25 MPH HC is usually not a problem because the converter heats up during the 15 MPH test.

I'd appreciate it if you would post all the test result numbers/limits at both 15 and 25 MPH including the 02 and CO2 readings. With the numbers I can give you a better analysis, and it will help me in the informal research on how to keep these cars from busting the emission standards.

Duke

P.S. Reading your response to Mike's post it sounds like the problem was definitely too cold catalyst temperature, especially if the engine was shut off during the rain dance about traction control. Even if it was left idling, more than five minutes of idling is going to allow the catalyst to cool off, and this will increase emissions. I'd still like to see all the exhaust gas results/standards even it it was only a two-speed no load test.

Glen / Duke,

Well I can say for sure the car was warmed up completely considering it was running the entire time the guy questioned the ASR issue but it was idling with AC off so no load really.

The values were:

- Idle RPM Before: 664
- Idle RPM Now: 643

- %CO2 Before: 15.2
- %CO2 Now: 15.1
- %CO2 Limit: not given

- %O2 Before: 0
- %02 After: .2
- %02 Limit: not given

- HC Before: 5
- HC After: 73
- HC Limit: 120

- %CO Before: 0
- %CO After: .01
- %CO Limit: 1.0

******************************************
- 2590RPM Before
- 2519RPM Now

- %CO2 Before: 15.1
- %CO2 Now: 15.1
- %CO2 Limit not given

- %O2 Before: 0
- %02 After: .1
- %02 Limit: Not given

- HC Before: 7
- HC After: 19
- HC Limit: 140

- %CO Before: 0
- %CO After: .04
- %CO Limit: 1.0

The higher RPM's didn't change too much but as you can see the idle looks significantly worse. Note that none of these values are even 30% of the limits except for the HC's at idle which is about 55% of the limit. Do you still think it's an O2 Sensor going south?

Something else I should add, the first test was done about a week after I drove the car back from Seattle which was about 1k miles. The car hasn't had a trip like that since Morro Bay in June of last year. I know highway driving is good for the car but do long trips vs 30 minute trips make that much of a difference?

As I said previously, even if the engine was left idling during the ASR discussion, the catalyst will cool off enough to reduce oxidation efficiency. Note that the O2 content at idle was 0.2% percent versus 0% previously. Zero percent O2 indicates 100 percent oxidation efficiency as all the O2 is being consumed to oxidize HC and CO in the catalyst, and the emissions are very low.

During the 2500 RPM test the catalyst heats up, and efficiency is improved with only 0.1% O2 and lower emissions than at idle, and the difference between the recent and previous test is much less.

The 02 content is an important diagnostic tool. If any is left it may mean the catalyst is not operating at 100 percent efficiency, and a hotter catalyst will be more efficient. If O2 is 0.2% or less emission performance is usually within limits, and the lower the O2 reading the lower the emissions. If O2 level is 0.3% percent it might squeak by or marginally fail.

There is likely nothing wrong with your car. The catalyst was just too cold due to the excessive idle time. Keep this in mind for your next test. No long highway trips since the last test is not likely a factor.

Duke

[side note to Duke]
Thanks for the great overview of how the cat and emissions system works! To further clarify, would you say it's pointless to let the car idle vs. leaving the engine off while waiting for the test?

Thanks.

Duke,

What if the idle test was done immediately after the higher RPM test (which it was). Would this point to degrading catalytic converters? I appreciate all your insite on this, it's a great way to learn more about the cars and like you my long term maintenance is what is important.

placo1, after having smogged many mercedes,there is something
about them that is much differant than most cars. the cats on these cars cool down fast. same as bmw.
the cats on mercedes seem to last almost forever.
if you are not sure about your car and its hc numbers, take it to a smog shop and have them precondition your cat.let them get it to
turn on. you will be amazed at how clean your car is at that point.
not sure if it's the surface area of the cat, but when it is fully fired up these german cats are great.
the smog numbers you have are from a cat that has cooled a little.that is why there is some 02 left over.your car is still to new, low miles, to have any real problems with the cat.
good luck and don't let these numbers worry you
rich

You absolutely DO NOT want to shut the engine down prior to the start of the emission test. As a minimum let it idle. Preferably keep it revving at 2000. Also, open the windows and sunroof with the A/C on max. Anything you can to to increase engine load will increase catalyst bed temperature, and you want the catalyst bed as hot as possible to maximize it's reaction efficiency.

Duke

Ah, yes! They do the idle test after the 2500 RPM test. It's been enough years since I had a two-speed no-load test that I forgot the 2500 RPM test is first.

In this case the catalyst temperature is an indirect issue, and my second proposed cause may be the primary reason why idle emissions were closer to the limit.

The injectors have a "chatter valve" that rapidly opens and closes during engine operation, so it's not really a "continuous flow" system, but meters fuel in continuous short bursts that average out to the proper steady flow rate. The "pulsed" delivery at the nozzle helps atomize the fuel for quicker vaporization, but it's a moving part that can wear, and since all will likely not wear at the same rate, the individual cylinder to cylinder and cycle to cycle fuel-air ratio scatter will increase with time, and the greatest scatter will occur at idle when injection pressure is lowest.

When my car was new its first emissions test yielded quite low HC. The second test in '93 at 49K miles showed greatly increased HC. (These were both two speed no-load tests.) This also correlated with the slight idle roughness that developed at about 40K miles, which is typical of many KE equipped engines. Injector flow scatter increases cyclic torque variation , which is what causes the slight idle roughness. Scatter will also increase engine out emissions, which places additional demand on the converter, and the result is higher tailpipe emissions unless the converter is VERY hot.

For this reason it is important to "manage" your emission test to ensure that the catalyst is as hot as possible before they start sampling emissions.

Since that test in '93 I've only accumulated 25K additional miles, so the degradation has not substantially increased, but I've been uncomfortably close to the limit a couple of times. The best numbers were in '01 when I went to a drive through test station on a rainy day (my test comes due in winter every other year) and asked them to do the tire dry test, which they did. I believe the tire dry test heated up the converter enough to significantly improve its performance. In '03 I went on a sunny day and was only one PPM below the HC limit at 15 MPH.

One sure fire way to increase converter temperature is to retard ignition timing as this will increase EGT. Unfortunately, on my 103 engine the initial timing is not adjustable. Through discussions on this forum I found that the rate of ignition advance can be altered by changing a resistor on a pigtail in the battery compartment. By slowing the rate of advance the ignition timing at speed under load will be less, so EGT will be higher and the converter will heat up quicker. As I recall, a shorting plug (zero resistance) will result in the slowest rate of timing increase with engine revs, so I plan to remove the OE resistor and short the plug with a piece of wire for my test next year and see what happens.

If I could change the initial timing I would just retard it 5-10 degrees and this should result in a noticeable reduction in emissions, but it can't be done on the 103 and many other Mercedes engines.

Duke

Duke,

Thank you again for all of the information!

I did a search on my engine to find out if it has the KE injection and I learned that is has the LH system. Would I continue to look at injector valve wear as a possible cause for the higher emissions? The car idle is good but then again I really don't have a new car to compare it to so who knows. Again I'm not stressed about this I just want to figure out what can cause the changes and what I can do to help for future tests and operation.

Rich,

I'm encouraged to hear the cats are not the issue as each costs about $1000:eek: . I'm thinking about taking the car in for further testing in the future but I really want to get a handle of what would cause the changes and if they can be addressed easily or not. I appreciate the info!