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w140 s500 1994,w123 230c, w123 280ce etc
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279 Posts
Discussion Starter #1
i need some help on this one please.the symptoms are as follows on my 94 model s500 125,000 miles.
problem started about 12 months ago . when accelerating hard shall we say i would get a slight misfire the car would hold back then would disappear if i backed of a little.no problems with normal driving at all. it has progresively got worse and now the problem arises as soon as the car gets up to temperature. no issues when the car is cold.
i have changed the following
spark plugs x 8
ignition leads
2 x coils
2 x distributer caps
2 x rotor arms

after reading some posts on here i have today changed a noisey fuel pump
and had a close inspection of the wiring harness which is in excellent condition and whilst im not ruling it out but the wiring all looks in very good condition.
if anyone can help i would appreciate it
 

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1989 190e 2.6 1993 500sel (sold) 1995 S500 coupe (sold) 1994 S500 blk/blk
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1,007 Posts
Could be many things, but one thing over-looked that can cause the symptom you describe is that your spark plugs are not fully seated. If you used an air torque device to seat or tighten the plugs at 15-22 ft.lb (20-30Nm) that will not be enough to seat the squash washers on the plugs. The torque device is fooled, so you will need to give each plug by hand one more nudge. You will feel the seating of the washer when you do this.

I know this because a Mercedes specialist told me and showed me how it was done. He said that if the washers are not properly seated that misfire could occur especially when the engine heats up

I'm not saying this will solve your problem, but it's worth a try.

Good luck mate.
 

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w140 s500 1994,w123 230c, w123 280ce etc
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279 Posts
Discussion Starter #4
i will check the plugs as you say drcharlesatlas a new set where fitted only on friday so will check,
having the car plugged into a star on tuesday so hopefully might throw up an answer.
thanks for the replies
 

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2003 Mercedes S320 CDi (W220)
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340 Posts
Hi,

I changed a whole load of other items including those you mentioned.

My car needed two fuel pumps and the problem was sorted.
Another item I changed but you havent mentioned is the ETA (under the Mass Airflow Sensor) . Looked ok from the outside but inside the insulation was shot.

Hope the STAR system identifies the fault for you - cheaper than swapping out parts!

Regards
Shane
 

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w140 s500 1994,w123 230c, w123 280ce etc
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279 Posts
Discussion Starter #6
thanks for that i think im gonna wait till its been on the star machine tuesday before i do anything else.
 

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w140 s500 1994,w123 230c, w123 280ce etc
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279 Posts
Discussion Starter #7
just out of interest i put a new fuel filter on it today as there not expensive and it cant hurt. to no avail it was as previous so its the star in the morning
 

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1989 560 SEL, 1993 400 SEL, 1986 420 SEL, 1977 450 SL, 1984 300D, 1981 380 SL, 1983 500 SL
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606 Posts
Did you check the fuel pumps? That was my problem, only one was good, the other was completly shot. If your pumps are not reaching their proper flow, this could be the problem. Maybe not completly shot, but not producing proper flow.
 

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w140 s500 1994,w123 230c, w123 280ce etc
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279 Posts
Discussion Starter #9
well i have an answer at last
firstly i did check the pumps and they both ran one was very smooth and quiet the other ran but was noisy,so i replaced it with a good used one(im a breaker for a living) but this had no effect.
had it plugged into the star and it came up with the camshaft sensor
the same camshaft sensor was used on the 104 series engines.
i have ordered a brand new one but have fitted a used on of an engine i have in stock and she runs like a dream.
im happy again and all is well with w140 ownership.
thanks to everyone who offered there help on here and i hope i can help others in the future.
 

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1989 560 SEL, 1993 400 SEL, 1986 420 SEL, 1977 450 SL, 1984 300D, 1981 380 SL, 1983 500 SL
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606 Posts
Glad to hear that everything worked out for you. I will keep this in mind incase I run into the same problem as you had.
 

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Merc.S 420-1995
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3 Posts
Hy, I have a S 420 1995 with a missfire and black smoke out of the exhaust,the right bank does not have spark on some of the cylinders,I replace plugs,ignition caps and rotors,plug wires and by the way were can I found the firing order and were the plug wires go on the distribuitor cap from the plugs,since the have many #s i couldn't figure it out.I am goin g crazy car its been sitting for 8 months can't use it .thanks regards.
 

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2003 Mercedes S320 CDi (W220)
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340 Posts
Great news!

I enjoyed swapping out the parts on my car but could probably have saved some money if I had hooked up to a STAR system first.

Happy motoring.
Shane
 

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Merc.S 420-1995
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3 Posts
I don't know if I can get the star system,but I went and checked the Air Mass sensor and found out it was dirty,I got some mass air clener from Autozone and clean it along with the Air intake sensor ,replace the spark plugs with a Bosh cooper ones and everything is working fine ,I thank you for taking your time and I hope that this will help someone else.Now there is a problem with the driver seat,if I move it foward it will move backwards.. and the SRS light on the inst. panel ,stays on for a while then goes off and then on...
 
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