I recently acquired my 89 300SEL...it seemed to run okay for the first couple of days...except for a slight hesitation when starting from a stop.

Then today, I had to sit through almost two hours of stop-and-go socal traffic... and it seems like the car didn't like it too much. The CEL came on, and the car started acting really strange. When coming to a stop, it would act like it was going to stall, it would hesitate when trying to accelerate, if I didn't ease into the throttle slowly, and the idle will surge slightly on it's own once in a while. (I had to mash the brake pedal several times to keep from running into the person in front O_O) Also, there is a lot of smoke coming from the exhaust under WOT. I got home and checked the codes, and it is throwing a "code #4", which has to do with the Air Mass Meter, correct?

I should also add that the car is almost impossible to start when cold...I had to crank for nearly 10 minutes off and on this morning to get the engine running. The whole time I was cranking, there were light wisps of smoke coming from the exhaust...the smoke also smelled strange, not like coolant or oil, but more of a caustic smell...like nail polish almost.

Anyone have any insight on this? It's really bothering me, and I can't seem to find much info about the W126 chassis on the net...

Thanks!

Replace Your Spark Plugs Or Remove And Clean. It Heat Range Of Plugs, Stop And Go, Needs A Hot Plugs To Burn The Carbon Off, Go To Dealer They Will Give You Right Heat Range

Ok, that's a start...but what do you think the Check engine light and code 4 mean?

check the ovp relay[with fuse in top] first sounds like limp mode especially with hard cold start.

__________________
David S Poole
European Performance
Dallas, TX
4696880422

"Fortune favors the prepared mind"
1987 Mercedes Benz 420SEL
1988 Mercedes Benz 300TE (With new evaporator)
2000 Mercedes Benz C280
http://www.w108.org/gallery/albums/A...1159.thumb.jpg

I checked that, and the fuse is ok...Also, my ABS light isn't illuminated. Is there some other way I should be checking it? thanks.

Nobody? :[

Hmm...this is interesting...I replaced my spark plugs with Champion Coppers and now the car runs perfectly...no hesitation, solid idle, and no smoke that I can see...the real test will be when I try to cold start it, tho.

The plugs that came out were a little dirty, and looked to be OLD Bosch plats...they still had the green stripes on them, and only one ground strap. The CEL also hasn't come on since...we'll see what happens...but I'm pretty happy!

Well, the CEL is back on...still flashing a code 4, and the only noticeable problems are a slightly rough idle at times, and the idle will raise and lower on its own slightly...Which part exactly does the code 4 refer to? Is it the actual Air Mass Meter itself...or is it the electronics within it...can they be replaced seperately?

John, your 300SEL doesn't have a MAS, so that ain't it!
Sorry, I don't know for sure what the code 4 is, but someone on here should reply.
On the spark plugs, it's prety important to use non-resistor plugs in the 103 engine. See the sticky at the top of the Tech column. Bad news is, these are virtually impossible to find these days.

DG

On 2nd thought, you don't have an electronic MAS like the later cars, but the big air valve in the intake horn does kinda the same thing, so the code 4 may refer to that.
You can do a couple of quick checks to see if the flap valve is functioning:
Pull the air filter:
1. With the engine shut off, the flap should be up near the narrow section of the intake horn, with just a crack around the flap to pass air. Pushing down, there will be a smooth resistance, and the flap will return to the original position. Should be able to slide a dollar bill between the flap and the intake horn, but not much more.
2. With engine running, if you push gently, just a bit, on the flap, the engine should stall a bit, then recover as the system compensates.
These checks will test the basic working of the fuel distriibution and the EHA.

May not be your problem, but maybe an easy place to start.

DG

Thanks for the response! What is the reasoning behind using non-resistor spark plugs? And where can I find them...best brand to get...etc?

And as far as the Air Mass Meter, I haven't tried the dollar bill test yet, but the metering plate seems to move freely with uniform resistance, and returns to its original position. If I would push on the plate with the engine running, the idle would get rough, and didn't seem to recover, and if I pushed it harder, the engine would stall.

Come on...I find it hard to believe that NOBODY knows anything about my code 4...

John, I've had my 91 model since new, and never set a code, so I haven't had to check all the code stuff....yet.
I have an '86 service manual, and I'll take a look at it, but I don't remember much about CEL codes, etc; in there.
Sounds like the air meter valve is not the guilty party, if it moves as described. (the dollar bill thing only serves to check that the valve is kinda centered in the bore, so don't worry too much about that if it moves freely).

Another cheap and dirty way to check fuel metering performance is to check the duty cycle of the EHA system, and the O2 sensor operation.
All you need to do is buy a cheap $30 Sears multimeter, with the duty cycle function. Model 82139 will work.
Read the instructions, set it on % duty cycle (requires pushing a button to toggle to the duty cycle %, not frequency). Then open the screw top on the diagnostic connector on the left fender. It's round with about 8 plugs.
Plug the red wire into pin #2, and black wire into #3. The meter will show you actual percent duty cycle of the fuel enrichment system, the reaction of the EHA valve to the O2 sensor inputs. When the engine is cold, you get 50%, when it goes closed loop, it will toggle, hopefully between 40-60 percent.
This is a really good indicator of the overall condition of the fuel system. If it rerads below 50% it's correcting for a rich mix, and above 50%, correcting for a lean condition. Check it at idle and 2500 RPM - should be within 10% on both checks. Lots of info here on this, might research it a bit.

The theory behind non-resistor plugs is that the wires/boots and rotor cap have proper resistance built in. If you add another 5k ohms or so with a resistor plug, the ignition system can overload and eventually be damaged.
There is a big "sticky" on the forum about this. Many, many guys have seen the 103 engines begin to run badly after a few thousand miles on resistor plugs.
The Bosch H9DC or H9DC0 is the preferred, but hard to find. I found a few sets in the Dallas Napa chain, and bought 30 awhile back, so I hope I'm set.
Check the "sticky" for possible alternatives, I think NGK may make a suitable on now.

i'll look for that code 4.

DG

Thank You!!! I'll look around and see if I can find those Bosch plugs!

Okay, John; here's what I gleaned out of this difficult-to-read 1986 manual.
Disclaimer: I have not tested this on my car, so I hope an expert chimes in.

The code 4 is apparantly an airflow sensor potentiometer issue.

This is the reostat the attaches to the big air valve we have been discussing, and measures the deflection of that valve.
The unit is mounted on the front of the injection unit , just to the left of the fuel pressure regulator line. Kinda where the fulcrum of the air valve would be. To test it:
Pull the plug partly out so you can get meter probes between pins.
1. With engine running, volts between pins 1-3 should be 4.6 - 5.1 volts.
Voltage on pins 1-2 should be 0.57 - 0.81.
2. With key on, engine off, slowly deflect the air meter valve, voltage should slowly rise to approx 4.6 to 5.1. (just like a modern throttle position sensor).

That's all I have for now, maybe a start.

DG

DG