300E Failed Smog Check!!! -- Causes?

[dordal]

I just got back from getting a smog check, and my car failed. Any idea on what the problem is? Here are the failed items:

2500 RPM Test
HC PPM: 208 (should be < 130)
CO %: 1.15 (should be < 1.00)

Idle Test
HC PPM: 287 (should be < 100)
CO %: 2.65 (should be < 1.00)

It also failed the EGR test.

The car is a 1993 300E 2.8 w/ 175K miles. I replaced the 02 sensor about 20K miles ago. The only other noticable problem the car has is a slightly rough idle.

So what is this? Bad EGR valve? Defective O2 sensor? Gas milage is still good...

[mbdoc]

On that car when the CO is high the most likely cause is the O2 sensor not controlling the mixture.
You also need to find out why the engine has a rough idle, that is also a reason for high HC's.

[GregS]

My car was at the limit of hydrocarbons when it was last smogged and my mechanic said this is often caused by a catalytic converter on its last legs.

Greg
'84 300D
'90 300CE

[jsmith]

have you changed your sparkplugs recently?

Yeah mine did the same thing about a year ago, I think mines the Catalytic Converter, I didn't feel like dropping that much money into my car. So I just paid the $300 for a "certified" tune-up and they had to pass me. Altho when i came back in, for the second time, they had to check if I actually got the stuff put in, so I suppose that takes out giving a gas station $20 for the papers saying I spent $300 on a tune-up, oh well...btw, it still works for tint! give'em $20 and they'll look the other way on the dark tint.
So if everything dosn't work, get the tune-up.
Good Luck

[mpnye]

FIRST THINGS FIRST . . .

Start by curing the EGR problem . . . then proceed to the NEXT phase . . .

That actaully very well be my current problem...But when I asked about it, I was told I didn't have an EGR valve, seemed a bit odd, but I just said "OK". EGR also has to do with Gas Milage correct? I posted a thread awhile back asking about it, and someone also said i didn't have one.
SOOO, maybe i should just go outside a check it out when I get home.
Yeah, first things first, i agree

[dordal]

I'll have to look at the 02 sensor again -- the sensor itself was working last time I tested it, but the heater wasn't working. Perhaps there is a break in the wire. Anybody know where (i.e. what pin) the sensor goes into the computer? I could at least test to see if there is continuity.

[suginami]

Your EGR problem is likely a blocked "hot inlet pipe" to the intake manifold.

It is a well-documented problem on these M104 engines.

Search the archives and you'll see tons of posts on the problem.

The blockage is where the pipe meets the intake manifold.

You can either ream out the pipe, or replace it.

Stevebfl (Steve Brotherton) also points out the problem with pictures in the DIY link at the top of the homepage under the article Evaluating Electronic Engine Controls, or something like that.

[dordal]

Thanks Paul. I'll take a look and see what I can do.

BTW, that new throttle actuator you sent me is still working fine. I took apart the old one, and you wouldn't _believe_ the poor condition of the insulation in the control cable. I cut it open and it just flaked off in my hand. I'm working on a way to replace the cable (easier said than done b/c of the customer mercedes connector); I'll post pictures on here for everyone as soon as I figure it out.

[mbdoc]

EGR flow has no effect on CO or HC but will cause the NOx to rise!
High CO readings come from excessive fuel. High HC comes from un-burnt fuel, ie; misfire.

[suginami]

Obviosly take M.B.Doc's advice before mine!

However, it wouldn't be a bad idea to take a look at your egr valve and the hot inlet pipe.

Stevebfl has posted that he reams out this pipe as part of the 60,000 mile service.

There was also a TSB on the original EGR valves that commented on a sticking problem.

[dordal]

MBDoc-

So you're saying the CO is caused by an excessively rich mixture; possibly caused by a bad 02 sensor. The hydrocarbons are caused by another problem, but probably not the EGR system. The other problem may be a misfire... (it doesn't really feel like a misfire -- anything else it could be?)

So, did I get that right?

[ctaylor738]

Could a malfunctioning O2 sensor cause a 3x HC problem? Don't think it can swing it that far. This is more like a misfire and unburned fuel getting into the exhaust.

Given that the problem gets better as the RPM's come up, suggest vacuum leak caused by injector seal(s) causing a lean misfire at idle. Could also be plugs, wire, cap, etc.

[pmckechnie]

If the EGR valve has a small piece of carbon in the valve seat, it can cause
Poor idle
Unburned fuel from misfire
Rich mixture (the exhause diluts intake air, so less oxygen + same fuel = rich mixture)
Also it will cause a failure of the EGR valve. It is tested by closing the valve and checking the vacuum and mixture, then it open the valve and checks again. It should see a change. It won't if the valve is already open and stays that way.
SO replace the valve, clean out the EGR passages, and retest.


Paul