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Old 03-29-2004, 12:06 PM
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FAIL Emission Test

What could be wrong with my 260E except that is too old (1987)?
HC(gpm) 3.96 (N=2.0)
CO(gpm) 86.9 (N=30.0)
I have the jpeg-picture of the report also that I can e-mail.

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FAIL Emission Test-vet_fail1.jpg  

Last edited by andrew1413; 03-29-2004 at 02:54 PM.
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Old 03-29-2004, 12:14 PM
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Join Date: May 2003
Location: DC Metro Area
Posts: 365
High Hydrocarbon reading sounds like there's unburned fuel exiting your exhaust.

Possibly an Oxygen sensor? When was the last time you had it replaced?

Have you noticed any recuction in mileage over the last few months / year? Any drivability problems?

1995 E420
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Old 03-29-2004, 12:55 PM
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Location: SoCal
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High HC and CO suggests rich mixture. First check would be whether loop control is on. If that all seems ok, then the cat could be bad. Too bad there in no NOx info.

'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7
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Old 03-29-2004, 03:15 PM
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I bought this car in November 2003. It was 181000 miles on it. Now - 187000.
I never replace the oxygen sensor. I changed an air filter, oil and oil filter.
It is leaking in the front of the engine - It is no spots on the parking place but it probably loosing oil during the run. I have to add the oil time to time (every ~800-1000 miles)

I droved my car to the test station at 65-70 mph and noticed that temperature was about 100 C, usually I drive 80-90--100 (if it is an empty highway) mph and the engine temperature stays under 100 (80-100 C). After the test it was under 120 C.

How can I check the loop control?
How to check the condition of the catalic converter (cat?)?
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Old 03-29-2004, 04:55 PM
Ali Al-Chalabi's Avatar
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Location: Knoxville, TN
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I would start my investigation with the oxygen sensor. As stated here, it is passing an excess of unburned fuel through the engine for some reason. What state was this emmissions inspection in?
Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record
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Old 03-29-2004, 05:35 PM
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State of Illinois

How can I check the oxygen sensor?

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Old 03-29-2004, 05:39 PM
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The oxygen sensor light is in the working condition - after an engine starts the light turns off.
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Old 03-30-2004, 02:48 PM
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This is the engine leak.
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Old 03-30-2004, 02:50 PM
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This is the crack on "air delivery tube" ??
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Old 03-30-2004, 07:26 PM
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Once I had the same problem (about 3 1/2 years ago), O2 light worked normally. But kept failing the NYC emission testing, even after new wires, rotor, sparkplugs and distributor cap.
I paid an extra "fee" to have them pass it for me.
I was getting very bad performance, (like 13 MPG) and had to take out the sparkplugs weekly for cleaning (black soot all over them).
Guess what?
The O2 sensor light turned on about 2 months later. Changed the O2 sensor, and VOILA!! perfect emissions testing every year since (you can find my latest results doing a search).
All this happened before I knew about this forum.
So take the advice, do a search on how to test the O2 sensor and if it fails the test, REPLACE IT!!!
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
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Old 03-31-2004, 11:00 AM
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Thanks for information.
How can I access the O2 sensor on 260E/300E to replace it? From inside or under the car?
How can I reach the sensor connector to test the sensor with a voltmeter? Is it inside the car?
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Old 03-31-2004, 08:50 PM
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Location: Near Raleigh, NC
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I will assume you did not the search.

The O2 sensor is under the car, next to the catalytic converter.
The connector is under the carpet, front passenger seat.

Do the search on the testing procedure, I know it has been posted numerous times.
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified
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Old 04-01-2004, 12:51 AM
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Join Date: Sep 2002
Location: Denmark
Posts: 412
Originally posted by andrew1413
This is the crack on "air delivery tube" ??
This is bad!
You'll need to replace the cracked "cam cover-to-air cleaner rubber-thingy".
It represents a vacuum leak on the vac. hose to your Idle Control Valve -which will result in a real crappy fuel economy -which will again toast your O2 sensor -which will cause even worse fuel economy -which again will most likely toast your catalythic converter..

You see? Baaad crack!
How is the rest of your rubber connections for the vacuum system?

And as the others said: Start by testing your O2 sensor.

Never criticize someone until you've walked a mile in his shoes. That way, when you do, you're a mile away -and you have his shoes!

'86 300E optically converted to '95 E300 (Sold)
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Old 04-01-2004, 12:32 PM
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Technical Information Oxygen Sensor Diagnosis Generic Method

Here are some fast and reliable diagnostic procedures which you can use to check out most oxygen sensors. A great time to do this is when you are performing a tune-up.
The following symptoms will help tip you off to a failed oxygen sensor:
Surging and/or hesitation
Decline in fuel economy
Unacceptable exhaust emissions
Premature failure of the catalytic converter

You will need the following equipment:
A handheld volt meter (digital VOM)
A propane enrichment device
An oxygen sensor socket
The manufacturer's vehicle specific test instructions.

It should take less than 10 minutes to perform a diagnostic check on most vehicles.
1. Verify the basic engine parameters, per the manufacturer's specifications for the following: timing, integrity of the electrical system (supply voltage), fuel delivery mixture performance and internal mechanical considerations.
2. Treat the rich mixture performance as follows:
a. Disconnect the sensor lead to the control unit.
b. Run the engine at 2500 rpm.
c. Artificially enrich the fuel mixture by directing propane into the intake until the engine speed drops by 200 rpm. Or, if you're working on a vehicle with electronic fuel injection, you can remove and plug the vacuum line to the fuel pressure regulator.
d. If the voltmeter rapidly reads .9 volts, then the oxygen sensor is correctly sensing a rich mixture. But, if the voltmeter responds sluggishly, or if it stays below .8 volts, then the sensor should be replaced.
3. Test the lean mixture performance as follows:
a. Induce a small vacuum leak.
b. If the voltmeter rapidly drops to .2 volts or below in less than a second, then the oxygen sensor is correctly measuring the lean mixture. But, if the voltmeter responds sluggishly, or if it stays above .2 volts, then the sensor should be replaced.
4. Test dynamic performance as follows:
a. Reconnect the sensor lead.
b. Set the mixture to specification.
c. Run the engine at 1500 rpm.
d. The sensor output should fluctuate around .5 volts. If it doesn't, replace the sensor.
When performing diagnostic work on your customer's vehicle to determine the cause of a driveability problem or perhaps the reason for failing an emissions test, take the opportunity to check the operation of the oxygen sensor for proper functioning.
Recalling that an oxygen sensor will influence the air fuel mixture preparation only when it has reached proper operating temperature (at least 350oC), it is essential to first ensure that the engine and sensor are warm enough to allow operation in a "closed loop" condition. It may take as long as 2 1/2 minutes after cold start for proper exhaust temperature to be reached (somewhat shorter for heated-type oxygen sensors).
To check the performance of the oxygen sensor, run the vehicle engine at about 2000 rpm (or at normal cruise when working with a dynamometer) to ensure that the sensor remains hot throughout the test procedure. Do not remove or disconnect the sensor lead in order to test it as this will eliminate the "closed loop" signal to the electronic control unit and result in a non-cycling voltage condition. Using a correct electrical impedance test device as found with a laboratory type oscilloscope, connect your test leads so as to read voltage from the signal wire to the electronic control unit. With vehicles that use a heated oxygen sensor (three or four wire), it may be necessary to bridge the connector leads and tap into the signal wire with an appropriate test probe at the connector plug in order to obtain the signal. The oscilloscope will allow you to read the electrical response pattern of the oxygen sensor to changing exhaust gas oxygen content as a measure of its performance.
Before proceeding, be sure that you are using the correct measurement scale for your specific equipment as specified by the test equipment manufacturer. (Invariably, this will be a low voltage scale.)
A properly functioning oxygen sensor will exhibit a rapidly fluctuating voltage signal alternating between approximately .2 and .8 volts in response to varying residual oxygen content in the exhaust stream. Look to your scope's time reference line for a desired lean-to-rich and rich-to-lean time of less than 300 milliseconds. A response time greater than 300ms. means that the sensor should be replaced. It is important to recall that these values are valid only when checking a sensor operating in "closed loop" in a hot exhaust stream (350o-8OOoC). Sensor age degree of contamination, mixture setting, and exhaust temperature all have an effect on response time.
Without this rapid electrical response to changing exhaust composition, the control unit cannot accurately correct the fuel mixture. A sluggish sensor is either contaminated or beyond its intended service life and must be replaced. Additionally, check vehicle manufacturers' service recommendations and suggest replacement of the oxygen sensor at specified intervals.
Oxygen Sensor Problems
Clearly, the O2 sensor will slow with age, contamination or damage, decreasing it's reaction time to changes in the air/fuel ratio. This may cause higher emissions and greater fuel consumption.

The signs of a failed O2 sensor are:
failed emissions test (high CO and/or HC typically)
damaged catalytic converter (from an over rich fuel mixture)
poor fuel mileage (caused by an over rich fuel mixture)
fouled spark plugs (caused by an over rich fuel mixture)
the car runs rough and has a sluggish performance.

Five Types of Oxygen Sensors
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Old 04-01-2004, 12:35 PM
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Posts: 77
Placing 1990 Mustang 302 O2 Sensor (Bosch) on MERCEDES

Using Generic O2 Sensors: a long-term fix. The $37 Bosch O2 sensor for the 1990 Ford Mustang 302 is available at many auto parts houses. It's applicable to many MB models as well as other marques, requiring only a connector change to be a direct replacement. After going this route once, I got to thinking about the wisdom of having to rewire these buggers when replacing every 60k miles or so. Hence an idea that worked for me the next time around: a permanent fix that makes future replacements a 5 minute snap!
Typical Parts Cost
1. O2 Sensor (from Mercedes-Benz dealers: ~$150.00) or
2. O2 Sensor (Bosch -- and equivalent to OE -- at independent parts houses: $37.00, for a 1990 Mustang 302).
3. Mating Connector (from a Ford dealer part number AUVC927082, cost me $9.06 for a 1990 Mustang 302)
1. Buy the Bosch O2 sensor at AutoZone or your favorite emporium for $37 or so.
2. Do not cut the leads. At the local Ford dealer, buy the mating connector for the Bosch sensor.
3. If you use the '90 Mustang 302 sensor, the connector is p/n AUVC927082. It has a weatherproof connector with O-ring seals. I paid $9.06 for mine, and the dealer got it the same day I ordered it.
4. Make a diagram of the original MB sensor's leads, then cut them close to the sensor.
5. Discard the old sensor.
6. Make a diagram of the new Bosch sensor and the new Ford connector's leads.
7. Slip heat shrink tubing over the leads of the Ford connector, then solder each wire to its mate on the connector you removed from the MB sensor.
8. Heat shrink the tubing until snug on the solder joints. (Doesn't have to be weather tight).
9. Install the wiring harness so that the new Ford weather tight connector is outside, near the O2 sensor's installed position, and the solder joints are inside the car. Plug the original end of the harness into the car's original connectors in the passenger foot well.
10. Screw the new generic O2 sensor into the exhaust line, then snap its connector into the Ford connector.
11. You have now completed the repair and modification unless your diagnostic unit needs to be reset before the Check Engine light will go off.


You now have a pigtail that goes permanently* between the generic Bosch O2 sensor and the MB connector inside the car. To replace with another generic sensor, just unplug the weather-tight Ford connector under the car and unscrew the old sensor, screw in the new sensor and plug it right in. You may not even have to jack up the car to do this.

*Should you or a future owner ever want to use the expensive OEM sensor, just unplug and remove your homemade pigtail and the OEM connectors snap right in place.

You now have the best of both worlds, and you only have to do the figuring-out and the soldering once.
Please let me know if you complete this procedure successfully; it took a while to type this, and I'd appreciate knowing when each person has completed the repair! Please e-mail me at

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