1972 W108 280SE 4.5 Fouls Plugs 4 and 8 Help! - Mercedes-Benz Forum

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post #1 of 25 (permalink) Old 05-22-2015, 12:52 PM Thread Starter
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Date registered: Sep 2006
Vehicle: Current: 72 280SE 4.5 Previous rides include: 1984 230e, Three W123 Wagons, 78 450 SLC 5.0, 73 450se
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1972 W108 280SE 4.5 Fouls Plugs 4 and 8 Help!

Hi everyone!

My 72 4.5 W108 is driving me nuts.

I've replaced tons of stuff (plugs (bosch W7DC), Bosch points cap and rotor, all have 1000 miles on them), a shop cleaned the trigger points, fuel filter and tank strainer and cleaned the tank), set the timing, dwell is 33-34 degrees, new temp sensors (I and II), MAP sensor works well and checked out, new fuel filter, checked fuel pressure (right on at 30 psi).

I keep fouling plugs 4 and 8 (just soot, not oily or fuel wet). Should I second guess a good shop that did the trigger points cleaning and do this again?

I noticed yesterday that if I press down on the cap near the wire for plug #4 that the idle gets better instantly, so I put another wire in that spot but still idled like a fouled plug, even after cleaning plug 4. Could I have a defective CAP and ROTOR??

Could my dizzy shaft be THAT far off to foul plugs 4 and 8 ONLY? Could I have a bad new cap and rotor that I should replace? Wires are new too, by the way.

Don't the trigger points ONLY work fuel and NOT spark, correct?

When plugs 4 and 8 aren't fouled the car runs like a dream, but eventually they get sooty, like within a few miles.

Any ideas, MUCH APPRECIATED!
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post #2 of 25 (permalink) Old 05-22-2015, 01:11 PM
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[QUOTE=sixtow123;12104513]I noticed yesterday that if I press down on the cap near the wire for plug #4 that the idle gets better instantly. [QUOTE]

Don't go looking for a zebra in a field of horses. Brand new parts, especially recently, are known to fail. If you're able to change the idle characteristics by applying pressure to the distributor cap, something is not correct in that area. Be careful not to zap yourself while grabbing the distributor and its leads.
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post #3 of 25 (permalink) Old 05-22-2015, 01:21 PM Thread Starter
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Date registered: Sep 2006
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Right. It's strange, it's like the cap is out of round or somehow just off spec by a hair. Doesn't take much to improve idle. So little that I can't imagine the shaft having that much of a wobble to affect just two cylinders. I keep thinking those two cylinders ARE getting fuel, otherwise they wouldn't be sooty. Thanks much for your weigh in!
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post #4 of 25 (permalink) Old 05-22-2015, 09:37 PM
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number 4 and 8 injectors work out of same driver in brain [ ecu ] with a DVOM that also read mill second , see what injector on time reading you get bettw num 3 and 4 . also , the tranmission modulator vacuum line is connected to number 4 , if it leaks trans fluid , it will foul that plug . you could unplug the wire to the numb 4 and 8 injector with clean plugs and see if they stay clean .you will be running on 6 during that test .the trigger pointes should not cause this , but , with a stethoscope you should lesson to the injectors , you may have num 4 and 8 that give multiple shoots , this would double or triple the fuel mix on those 2 cyl . you can get a stethoscope at harbor freight , cheap and good .it is a good tool for early EFI diagnose .the injectors should make a nice crisp clicking noise .
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post #5 of 25 (permalink) Old 05-23-2015, 06:36 AM Thread Starter
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Ah! Didn't think of the transmission modulator. The trans is new but I guess the modulator could have been re-used? Where does that line come in? I see a line right behind #4, looks like a metal line covered with a plastic sleeve that goes into the manifold. Is that the modulator line?
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post #6 of 25 (permalink) Old 05-23-2015, 08:32 AM
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yes , but usually number 4 cyl is mostly affected . with this efi engine , get a stethoscope , and get used to the sound of the injectors , the answer could be right there . it is a great tool to test for proper operation of the trigger points / injectors .also to find noises .the next tool is a DVOM that can read mill second signals and also dwell .[ not pricey ]
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post #7 of 25 (permalink) Old 05-23-2015, 09:31 AM Thread Starter
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Thanks! I got new injectors and had the trigger points cleaned a month ago. Is it possible they're fouled again already after only 500 miles?
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post #8 of 25 (permalink) Old 05-25-2015, 10:00 AM
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are you talking about number 4 and 8 only , the other look fine ????
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post #9 of 25 (permalink) Old 05-26-2015, 07:57 AM
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One thing you could try which would help isolate the source is swap the connectors for the fuel injectors (swap #3 and #4 , and swap #7 and #8 ). If fowling switches with the swapping of connectors, then it is something in the ECU providing too much fuel. If the issue stays with #4 and #8 , then it could be injectors or ignition.
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post #10 of 25 (permalink) Old 06-03-2015, 06:57 PM Thread Starter
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Thanks all. Yes only 4 and 8 are fouling...
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