Mercedes 300SE emissions?

[Finn]

Hello.


1991 Mercedes 300SE, 3.2 I6 with 195k miles on the clock.

I'm having trouble with Merc's emissions: CO is 0.45% (limit(0.30%) and HC around 200 (limit 100).

Basic parts have been changed ie. plugs, wires, ECM temperature sensor, O2 sensor, catalytic converters, distributor cap and rotor etc.

Now the car started to not react to gas pedal ie. it is reluctant to rev until you press pedal past halfway. Then it revs very freely. Also in freeway speeds ie. 3000 rpm it works like a dream.

Also, when I start it cold idle jumps up and down by say 300 rpm.

It was at the dealer and they could not see anything with Mercedes factory program.

Their opinion now is MAF - symptoms would point to that.

There is no coolant loss and oil remains clean. MPG is reasonable. There is no white, black or blue smoke cold or warm.


Any ideas?

 

[Audios]

Have you cleaned the throttle body? This used to cause all types of emissions issues with the older fuel distributor systems, its starving the engine of air at idle and raises the pollutants. I would also check the tightness of the accelerator cable and make sure theres no slack.

 

[Finn]

Yes, I cleaned that two weeks ago - it was dirty but not hopelessly so.

I pulled the plug and ordered a new OEM MAF sensor as well as intake manifold to cylinder head and throttle body to manifold gaskets just in case if there is a leak.

 

[CT8]

BHigh HC is unburned petrol, does the engine burn oil?

 

[Finn]

This uses very little oil for engine of this era and age.

 

[Audios]

How old is the coil? Is the spark bright?

 

[Finn]

Coil could be original. Spark is apparently good as above 2500 rpm engine runs really nice.

 

[Rolla07]

How about trying a bottle or two of fuel system cleaner just to see if any improvement? It wont hurt.

 

[mk378]

You have to resolve the driveability issues before expecting it to pass a smog test. I would look at the throttle position sensor next. Most of them can be checked with a voltmeter. If the spark timing is adjustable, check it.

CO means that the engine is firing, but running rich. Remove the spark plugs to see if they are all the same color. That will help tell if it's a problem with one cylinder, or generalized.

 

[zorobabel]

I guess that's why the previous owner sold it.

 

[Finn]

Could be although it passed last year the annual inspection including emissions.

We finally found the basic reason for the problems: engine wiring harness.

I was under impression that this problem existed from 1993 and onwards until -97 but apparently it exists on -91 models, too.

FWIW, we will remove the engine wiring harness from the car and rebuild it one wire at the time, perhaps 40 to 50 wires total. New OEM wiring harness costs about 1500 Euros.