Emissions. W124 E320 failing NOx
 

Hi all


Question for the collective wisdom out there. My 94 E320 has failed the emissions test in Arizona several times. These are the initial numbers and they have not changed much in any of the subsequent tests

Pollutant Reading Limit Result

Hydrocarbons 0.12 0.80 PASS
Carbon Monoxide 1.13 12.0 PASS
Oxides of Nitrogen 4.74 2.00 FAIL

For the initial test, I just took the car as is, no tuneup or anything. Car is/ was running great. Failed with the above numbers.

Then I did an oil change and tuneup. Essentially the same numbers. Car ran about the same, perhaps a little smoother. :o

I used Chevron fuel injector cleaner in every tank for a week or two, ran the tank low and added a bottle of HEET as prescribed in some other forums (non Mercedes). Essentially the same result. Slightly lower HC and CO numbers, but not noticably lower. Marginally lower at best. :mad:

Then I cleaned out/ rooted the EGR tube using the methodology outlined in several threads here and there. Essentially the same numbers. :furious3: :bigcry:


I'm loath to replace the catalytic converter ($1600 from the dealer, ouch!!!) or change any parts unless I can nail down the problem. Some have stated to just change the catalytic converter, but surely if that was the problem I should be failing the HC and CO portions of the test, no? Besides, I am not going to spend that much coin unless I absolutely know that that is the problem. If I even do. The car is worth perhaps that much with the extensive milage on it (170K.)

The culprits for high NOx are listed as mainly the EGR tube being blocked, lean mixture and high temps. Being that I unblocked the tube, I had the subsequent tests done with the heater in the on position, and the mixture is electronically controlled, I am at my wits end. When I did the EGR tube I did test the EGR valves operation by applying vacuum to it and it opened like it should.

I have been told to use high octane fuel ( is 91 going to make a difference? It's all I can find here) and take it there before the car has warmed up. My plan is to leave the car near the testing facility overnight and drive it there first thing in the morning. With maybe a bottle of octane booster in the tank.

Any pointers or advice is greatly appreciated.

If you pull vacuum on the EGR valve with the engine running, does the engine almost DIE?

If not, then back to cleaning OR replacing the EGR tube.

I did not test it while the car was running. I tested it when I removed it from the car to access the EGR tube.

I will have to figure out how to apply vacuum to it when the car is running. :confused:

I've always been told you want the motor good and hot for emissions testing.

Is your O2 newer?

High NOx only indicates EGR system failure. OR possibly, but unlikely, a bad cat. I'm pretty sure you can find good quality OEM cats for around $600-800 installed if you look other places than the dealership. Otherwise I would throw one of those universal cats (OK quality) for around $150-200 installed. Your old cat is worth a little over $100 to someone who uses them for their precious metals. The OE Benz cats have a lot of platinum in them which is why they are valuable.

even though you might not have a check engine light on, I would suggest testing your 02 sensor ... i remember seeing a test for it on this site somewhere

Since most benz motors are made to run on premium grade I would suggest running it yes. You can run lower grade gas but not supposed to.

I second the O2 sensor.

How old is the coil? maybe worth a shot better spark never hurt.

The O2 sensor has "little OR nothing" to do with this issue as the CO & HC's are within range...

High NOx on that car comes from either poor EGR flow OR weak catalyst.

IF fact, ONLY HIGH NOx reading cannot come from a non-working O2 sensor.

Thank you for your collective wisdom. I shall definitely re root the EGR tube.
I'm still not sure how I am going to apply vacuum to the EGR valve.

When I go, I will definitely add octane booster to a low fuel tank.

All you do to apply vaccuum to the EGR valve is disconnect the vaccuum hose, and suck.

You should be able to hear it snap shut when you remove vaccuum

I did remove the EGR valve and test it by sucking on the hose. It did open and close when vacuum was applied and removed. I didn't know if it was supposed to open in some special way like slowly, or snap open etc

The nut holding the EGR valve seems to be rounding off. Couldn't take it off.

Any ideas?
I'm thinking loosen the EGR pipe at both ends and just root the pipe with the valve in situ. Obviously I will have to remove the allen bolt on the oil filter housing. I just don't know if I will have enough access.

Are you still using regular grade fuel?
It may not be you, but somebody somewhere with a name similar said something about your use of 87 octane on a road trip without any problems and your continued use of regular grade fuel.

Just saying, maybe the use of regular fuel over an extended period of time does have it's side effects. Food for thought is all. Would be interesting to see if that were the case.

That's true. Continual running of low grade gasoline is not good for the engine. Nowhere in the manual does it say that 87 octane is OK. It's not because Mercedes is trying to stick it to ya, these engines REQUIRE premium gasoline. Conversely, running 91 in your Honda when it only requires 87 is just as retarded.

Even though there is a knock protection system on these cars, there can still be detonation you are NOT hearing, causing engine damage!

That is/ was me, but I don't see how using regular gas regularly would affect NOx significantly. The difference in flame speed between premium and regular is micro milliseconds. The slightly slower burn of premium makes for a more "complete" burn and keeps combustion pressure on the piston for a fraction of a second longer, ergo more power.


To whit, I have only been using premium for at least a month and driving far more aggressively than the typical hypermiler driving I do. Going from circa 400 miles per tank to ~ 300 while paying more for that gas is a bitter pill to swallow. I have intended to get it done several times but I am only finally free this weekend. We will see if premium fuel makes a difference.

If there is a knock, the sensors should pick up on it and pull the timing, That is their function.

Only to a certain extent, once you have to much change the adjustment goes no further.

Yes. The knock protection systems on these cars are designed more for a "bad gas" scenario which is temporary. Not for continual usage of lower octane gasoline. Neanderthal, it's pay a little bit more now, or pay a lot more later (engine failure). Just sayin'! ;)

Also, it is well known, that pre-ignition will cause NOx to skyrocket. This is why you are failing emissions. Pre-ignition (detonation) causes much hotter combustion temperatures, which in turn increases NOx emissions.

Latest results. Run with a full tank of premium fuel (I can only find 91 octane,) AND a bottle of 104 octane booster. The previous tank of gas also was premium with a bottle of 104 octane booster. I also ran a bottle of Chevron fuel injector cleaner prior to that and have been running premium the whole of the last few months and driving a lot harder than usual, in case the car "learns" your driving behavior and replicates/ adapts to it as some of the newer ones do.

Pollutant Reading Limit Result

Hydrocarbons 2.19 0.80 FAIL
Carbon Monoxide 3.98 12.0 PASS
Oxides of Nitrogen 4.85 2.00 FAIL

I did reroot the EGR tube from both ends just 4 days ago. I did also spray and remove whatever tar/ carbon build up was partially blocking the intake end of the engine where the EGR tube is attached.

I had tested the EGR valve when I initially did the EGR pipe about 2 months ago. I admit to not testing it while the car was running as prescribed, but when I had tested it, it opened and closed with the application and removal of vacuum.

It would appear that using premium did absolutely nothing to improve the emissions results. And now hydrocarbons are also failing.

Prior to the test the temps were a little high and the overflow hose at the expansion tank is frayed, so it was spewing coolant out of there, and they wouldn't let me test with fluids coming from the car. My fault, I had slightly overfilled the cooling system. I presume that was preventing it from pressurising and ergo making it run hot.

I left, went elsewhere and parked noseup and "burped" the cooling system and returned to the emissions testing station whereupon it failed it's retest with the numbers posted above.

The tech blamed the hot temps (it was running at just about 100C) for the NOx which I thought was bullpoop since that is within the operating parameters for the car. I didn't state that, or inform her that the "high temperatures" stated in their paper work are high combustion temperatures.
The car did sit for about 5- 10 minutes before the test on the rollers, because they had to recalibrate the machine. I assume that is the cause of the HC failure, because the cat had started to cool down. But that is a wild guess at best.

While I love, absolutely love, my W124s, I am getting to the point that their foibles are starting to bother me. And this is just about the straw that is getting on my last nerve.

Just had a thought. What controls the EGR valve? Could that be the fault?