How do I fix high NOx levels. - PeachParts Mercedes-Benz Forum

dpetryk · #1 07-30-2002, 05:27 PM

My 91 420 sel failed the NOx portion of the emissions test. All the other emissions were well within specified limits. Does anyone know which emission related component is most responsible for the control of NOx? I suspect that the problem is related to the EGR system. How do I approach the problem and figure out what needs to be fixed?

My test results were:

allowed actual
---------- --------
HC 103 13 (hydrocarbons)
CO .57 .05 (carbon monoxide)
CO2 na 14.7 (carbon dioxide)
O2 na .1 (oxygen)
NOx 712 924 (nitrogen oxide) FAILED

engatwork · #2 07-30-2002, 05:46 PM

Dave - it's good to see you asking this time instead of providing some of the excellent help you have provided in the past on various subjects

. You may have already tried what I am going to suggest so disregard if you have.

Have you tried the test where you have the car hot/idling and put the Mighty Vac on the vacuum line to the egr and put some vacuum on the egr to make it open? If it almost stalls then the egr is working the way it is supposed to. The next step is to start troubleshooting the actual vacuum "signal" to the egr. I am not familiar enough with your application to help here but I'm sure someone will come along that can offer some advice.

I am not familiar with the emissions testing but are the test performed at idle. The egr valve is supposed to be closed (not recirculating) at idle.

dpetryk · #3 07-30-2002, 06:15 PM

Thanks Jim.

I have checked for movement of the EGR valve and it is moving and it holds vacuum. I also checked for presence of vacuum from the throttle body line and it shows vacuum when the throttle is opened. BUT -- when I apply vacuum to the EGR valve when the engine is idling, there is no difference in the way it runs. And as you suggested, I would expect the engine to run poorly. Its not doing that. So The actual exhaust gas circuit must be plugged or the valve is not really opening. So my next step is to pull the EGR valve off and see what is going on and check to see that the passages are clear.

I am just praying that my cat is not the problem. Is there any way to tell if the cat is working or not?

BTW these new emissions tests are really tuff. They run the car at speed on a dyno at about 25 MPH and measure the emissions under actual driving conditions. Last year it was only measured when the engine was not under load.

engatwork · #4 07-30-2002, 08:06 PM

If you have never cleaned it I'll bet you a cold one (when I am in the area or you are in mine) your egr line is plugged from the egr valve to the manifold. On the E320 the check engine light comes on whenever this happens. I used an old speedo cable to rotorooter it out and when I replaced the head gasket a month or so ago I thoroughly cleaned it out. The majority of the pluggage will be at the manifold end. Also, on the E320 the tube inserts into the manifold about an inch or so past the point that it bolts up. In talking with Scott P at the last ATL g2g he said that they usually just replace this line with a new one. The rotorooter trick does not clean it out completely and the problem comes back too soon. You should be able to ascertain if the egr valve is inoperative by doing a bench test on it. Put the vacuum to it and see if you can blow through it.
I have considered modifying my egr pipe with a straight "clean out" at the manifold end with something I can easily remove/replace - something like a plumbing clean out.

Oh yea - the recirculated exh gas is used to drive the temp down which results in reduced NOX emissions. The same technology is used on steam boilers.

dpetryk · #5 07-30-2002, 11:17 PM

OK Jim you are right on target so far. So far I owe you several cold ones.

I removed the EGR valve and sure enuf, as you predicted, it was plugged. I managed to clean it out and I put it back on the car. I tested it by idling the engine and using my vacuum pump to open the valve. I still did not notice any change in the idle. When the valve was off the engine, I started it to see if there was any air flow into the EGR pipe. There was some. The engine ran rough, but it did run. It sounds like I need to clean out the pipe next. The only problem is that it is a real b*tch to get the manifold end off. I think I have to remove the intake manifolds to get it off. I was wondering if I could roto-rooter the thing out from the valve end and then suck up the debris somehow. The last thing I want to do is remove all the intake stuff.

If I do manage to roto-tooter the thing out and I get some carbon junk sucked into the cylinders do you think that would hurt anything, or would it pass thru without harming anything?

Is an E320 a six cylinder engine?

MikeTangas · #6 07-30-2002, 11:29 PM

Yes Dave the 3,2 should be a six and the postings about the EGR tube and rotorooting show it is much easier to access than the eights. What Steve uses, according to his post (if I remember correctly), was an old length of speedo cable driven by a drill motor. The cable is flexible enough to make the bends and auger out the crud. Probably how you'll need to try unless you want to pull the intake, the fitting is well under the intake and nearly impossible to get to.

Have no idea what quantity of carbonized crud would be deposited in the engine, nor what effect it would have. I don't recall Steve stating any concerns about the same in his post about the process.

engatwork · #7 07-31-2002, 06:43 AM

The pipe on the 6 is a pain to get in/out too unless the cylinder head is off -

. Remove the egr valve and rotoroot it from the egr valve toward the intake. Make sure your speedo cable is such that it is flexible all the way to the end. On the 6 there is a 90 degree bend right before it gets into the manifold. If the speedo cable does not go around that bend it will not go all the way into the intake and not root it out like it needs. Again, the majority of the build up is at the intake manifold end of the pipe. I don't worry about vacuuming it out - just rod it out into the engine and then give it the old Italian tune-up when you finish.

Dave are you saying that you did NOT get the idle to change when you activated the egr during idle but with the egr valve off you did get the rough idle. hmmmmmmm, sounds like the pipe is clear because the rough idle with the open pipe is the same as a vacuum leak. When the egr valve was off were you able to blow through it when you put a vacuum on it to ascertain the egr valve was working properly. Let me think about this one some more as the day progresses

After thinking about it for a few minutes you may need to check the line from the exh manifold to the egr too.

dpetryk · #8 07-31-2002, 08:30 AM

Thanks guys.

My plan it to go the roto-rooter route. I think that there is also a 90 degree bend at the manifold end as well.

When I had the EGR valve off the engine and I started it, it ran rough. But Given the size of the EGR pipe I was surprised that it ran at all. The pipe is about 1/2" diameter. So my assumption is that there are still some significant blockages in the pipe. The EGR valve has been cleaned out and the short pipe from the exhaust manifold has been cleared out as well. The opening in the EGR valve is quite small as compared to the pipe. Are you sure the engine idle is affected to the degree you said?

Another suggestion was that I run that GM product "top engine cleaner" thru the engine to clear out carbon buildup in the combustion chambers. I used this stuff on my suburban several months ago and it did improve the idle smoothness. But I am not sure what the best way to apply it to the MB V8 would be. I have read that the best way is to get it into the air tubes that feed air into the injector sites. But I would think that the cleaner solution would not be injested equally amongst the cylinders. And I dont think that pouring it down the intake throat would work because then the solution would pool at the bottom of the manifold and would not go uphill in the intake manifold to get into the cylinders. This is not a problem in a chevy 350 because the manifild runs downhill.

Does anybody have any tricks to getting the MB engines to eat this stuff properly? I think that stevebfl has done it but I was not clear as to his application method.

So next step is to roto-root, then top end cleaner, and hopefully not a cat. If I am still failing then I probably dont have any choice about doing the cat.

engatwork · #9 08-01-2002, 08:31 PM

How are you coming with it Dave?

dpetryk · #10 08-02-2002, 08:51 AM

Not much progress since we talked last. I will attack it this weekend. I have been tied up with new sports season starting for my kids and lots of meetings.

My plan:

As you suggested, get a speedo cable and attach a small circular brush to it somehow. Using a vacuum cleaner, apply suction to the EGR pipe leading to the intake manifold while running the spinning brush down the pipe. I am concerned about the amount of carbon buildup that I see inside the pipe. And as you said the real heavy stuff is probably at the far end of the pipe. I have a spare 5.6 liter engine sitting around here so I will practice on that first. I have about a week on the retest time so there is some financial incentive to get it done this weekend. Also going to try to feed the engine some of that GM "top end cleaner".

In the meantime I have located a cat at a cost of $415 shipping included in case the worst happens. Dealer wanted $2000 for the same part.

Do you think with my NOx numbers being so high that the non-functional EGR would account for that. Or do you think that the cat is the primary cause?

engatwork · #11 08-02-2002, 08:57 AM

I'll put a six pack on the egr line being plugged

. Shiner's Bock please

Tinker · #12 08-02-2002, 11:04 AM

Dave,

My car also had high NOX levels during the biannual smog check on the dyno (CA). Its a 190e 16V with no EGR system. I ended up installing a new cat to make mine pass.

Tinker

jbaj007 · #13 08-02-2002, 01:58 PM

CAT controls HC emissions primarily.
EGR controls NOx primarily.
Therefore, smart money is on the EGR.

RunningTooHot · #14 08-02-2002, 03:01 PM

If you have some room to play with on the HC readings, then you can richen the mixture just a tad which will help lower the NOx readings - it's a tradeoff between the two. The richer mix will help lower the combustion temps and therfore lower the NOx a bit, but it will drive the HC up...

RTH

dpetryk · #15 08-02-2002, 03:45 PM

OK I have cleaned the EGR line. It was totally blocked. The Shiner Bock is in the cooler.

Good suggestion on the mixture but I cannot control the mixture since the O2 sensor and the associated control system will bring it back to normal. Unless I force the mechanical adjustment so far out so the Lambda control system cannot correct it.

So I am headed to get the car re-checked. If it still fails I guess I will go for the cat next. But if jbaj007 is correct I should pass now.

Thanks guys for all the wisdom.

Have a good weekend. I will let you know on Monday what happened.

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