Failed Smog....again - PeachParts Mercedes-Benz Forum

glenmore · #1 03-06-2012, 10:49 PM

Failed another smog test with my 1991 300CE @180m. I failed once before 2/2008 @ 148m. That one was due to high NOx and I needed a new EGR vavle, solenoid and the tube cleaned out.

I took it in pretty hot and there was no wait so it went right to the test stand. My start had become a bit more stumbling as of late but was OK when hot.

At one time or another over the years I have changed the following:

IGNITION

Plugs
Coil
Battery
Cap
Rotor
Temp Switch

FUEL

Fuel accumulator
Pumps w/ check valves
Fuel Filter
Injectors and seals
Cold Start Valve
EHA o rings
Fuel Distributor o ring

ELECTRICAL

OVP
CPS

AIR

ICV
Air hoses under throttle body
Smog Pump w/check valve and hoses
Air filter
O2 sensor
EGR and solenoid

After each of these changes, the start was never any better, always with a short delay and stumble. Recently the start deteriorated to a stumbling chugging start particularly when hot. A new cold start valve seemed to bring me back to the before stumble start.

In reviewing my records, I've never changed the wires, so I really should try that.

Any other ideas?

Hit Man X · #2 03-06-2012, 11:53 PM

Early 104s are CIS right? Ever checked to see if the CO is set properly?

In my experience HC is unburned fuel and NOx is too lean of a mixture.

How old was the oil in the crankcase? This sounds trivial, but fresh oil helps pass too.

ps2cho · #3 03-07-2012, 01:33 AM

O2 levels are low, so cat was hot enough as you said.

I used to have high HC and high NOx. I replaced my fuel distributor, and that essentially brought my HC to zero, but my NOx is still high. I barely passed AZ smog, but since the last time I smogged (with the high HC) the distributor + EHA has been the only items I have changed since then. I had Larry @ CIS FlowTech rebuild the distributor and flow match the EHA to it.

I have always found that putting 4 bottles of ISO-heet into the gas tank has helped with #'s across the board every time so do that. Its like $5 total and can't hurt.

Here's the results for reference:

Don't have the image for the AZ Smog, and it uses different measurements, but the overall trend is the same -- NOx is still high, but my HC was super clean, so I think I'm just running lean with no unburnt fuel (since its running lean!) Not sure why though...

300TE // limit // result
HC: 0.18 // 1.60 // PASS
CO: 0.69 // 15.00 // PASS
NOx: 2.38 // 2.50 // PASS

mbdoc · #4 03-07-2012, 08:28 AM

The high HC at 15 MPH is most likely due to slight vacuum leaks in the intake system.

The very high NO is most likely that the EGR isn't functioning correctly.

glenmore · #5 03-07-2012, 11:31 AM

So there are 2 seperate vacuum systems? One for the door locks, rear head restraints, fuel door and seat backs and also to the tranny? run off the vacuum pump under the rear seat. And then there is engine vacuum which is a totally seperate system? I see one line to the breather hose from the fuel pressure regulator. There is vacuum in the brake booster servo line. Can anyone direct me to a "Vacuum for Dummies" to read?

Duke2.6 · #6 03-07-2012, 01:19 PM

[QUOTE=ps2cho;2897686]O2 levels are low, so cat was hot enough as you said.

No. If the catalyst is operating at maximum efficiency the measured O2 level should be zero. Catalyst "aging" issues increase the bed temperature required to achive maximum oxidation and reduction efficiency.

High HC at 15 MPH is typical of the cat bed being too cold. It heats up during this test, which results in passing the 25 MPH test. The NOx falilure at 15 MPH is marginal.

Shorting the R16/1 plug to reduce the rate of spark advance with engine revs and temporarily blocking the vacuum advance line to the EZL should yield an easy pass. Then if you want better low end torque and fuel economy, don't install the original resistor after the test.

To the OP: Search this forum for posts by me with search words like emissions, HC, CO, NOx, and R16/1.

Duke