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Old 03-23-2003, 08:04 PM
Registered User
Join Date: Mar 2003
Location: Southern California
Posts: 2,010
190E2.6 idle roughness and emissions issues

Subject vehicle is an '88 190E2.6 with a (very rare) five speed
manual, 73K miles, original owner.

The following questions/ issues are probably common to all
models with the 103 engine, and probaby the 102 engines,

Idle roughness has been an issue since about 40K miles and
was an issue with my former '84 190E 2.3. My suspicion is that
it may be inherent to the CIS-E system as the low pressure at
the injectors at idle tends to amplify any small variation in flow
characteristics resulting in uneven fuel distribution. This is an
issue with the old Rochester continuous flow FI system on
vintage Corvettes.

Does anyone have an explanation for the idle roughness and
a solution?

Second issue is degrading emissions performance in California
ASM (acceleration simulation mode) loaded dynamometer
emission testing. As with most cars the toughest bogey is
hydrocarbons at 15 MPH.

My first ASM test, 1/99, yielded 121 ppm HC at 15 MPH against
a limit of 141 at 63K miles.

Prior to the 1/01 test I installed a new Bosch cap and rotor
and a new set of H9DC plugs and the performance improved
to 87 ppm against the same standard (68K miles).

I removed the plugs following the test and reinstalled them
prior to the 1/03 test. The 1/03 test HC count increased to 113
PPM against a standard that was tightened to 116.

For two tests in a row my car has been tagged to go to a test
only station because it has been identified as a high emitter,
and this may be the case as the average for my year group is
about half of what my car is measuring.

A '90 300SE was also there at the same time and the owner
squeaked by with 1 PPM to spare although I think the limit for
that model was slightly different thnt my car. In any event, it
appears that this engine family has high emissions, and I
figure I'm going to fail the next test.

In asking around I have heard that the MBZ converter system
lacks longevity, and it's also been suggested that the O2
sensor can drift out of calibration causing a richer than
necessary mixture. (My O2 sensor is original.)

Does anyone has any insight into these issues?


P. S. It's interesting that NOx is quite low given that this
engine does not have EGR, but I believe it was added for the
'89 model year. The low NOx could be the result of a richer
than stoichiometric mixture.
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Old 03-23-2003, 10:10 PM
Posts: n/a

Have you checked the voltage coming out of the 02 sensor?

Search the archives for this one.

Also, time and poor gas quality will take its toll on 02 sensors.

Another item might be the engine temp sensor. I have a 300E and it's sensitive to both of these items.

Not sure if your car has fault codes but it's worth a check to see if you are getting any codes.

X/11 port on the left front fender on my car.

Good luck,
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Old 03-23-2003, 10:30 PM
Registered User
Join Date: Mar 2003
Location: Southern California
Posts: 2,010
Thanks for the response. I checked the O2 sensor output a couple of years ago, and it appeared normal. Since my car is a CA model it has some limited OBD including O2 sensor health, and I have no check engine light or codes.

I've been reviewing the archives and cat temp appears to be an issue, as it is on some other cars. For my test two years ago I had it purposely done on a rainy day as CA has a "tire dry test" where they run on the dyno at moderate speed with no load for about 30-60 seconds before the test. I specifically asked the tech to run the tire drying protocol, and he did. This might have helped. The first test is at 15 MPH and this is where the HC is marginal. No problem on the 25 MPH test, and the 15 MPH test probably heats up the converter as it is done at a higher load than the 25 MPH test. This year I went on a dry day, but the car had been operating for about 30 minutes including a brief freeway jaunt, and the engine was not shut down prior to running the test. The coolant temp was at 100C when he began the test after doing the visual with the engine idling.

Car is delivering normal mileage - 20-22 in urban/suburban driving and about 27-28 at 75-80 MPH sustained freeway trips.

My archive search supports the suppostion that the M103 engine family seems to be marginal on emissions performance, but I couldn't find a silver bullet. Anyone else have any ideas?

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