Arching on distributor cap cause idle drop? - Page 2 - Mercedes-Benz Forum

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post #11 of 22 (permalink) Old 04-09-2012, 10:02 AM Thread Starter
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Fuel checked out ok last week, replaced filter (which was new anyways) and it was clean but as a precaution. Replacing screen in tank this time and will have injectors cleaned. Just seems very odd it is the same symptoms as last time that were immediately cured by new coil, as in it ran poorly for a few weeks, shut down, replaced coil and resistor and restarted 1/2 hour later and ran perfect for one week.

-Marrs

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post #12 of 22 (permalink) Old 04-09-2012, 03:10 PM
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im having a similar issue however my 250 when cold idles ok and when first warm, after driving it say 100 miles and i get off the highway and come to a stop it misses like crazy and I have to keep pumping the gas pedal to keep it alive and accelerate past the misses and then all the cylinders kick in, I suspect its the plugs but this thread has me wondering.

Im gonna replace the plugs anyway since we never changed them after buying it. I have a coil somewhere too.
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post #13 of 22 (permalink) Old 04-09-2012, 04:04 PM Thread Starter
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Please do keep me updated on this thread on what you find, maybe between us we will figure it out. I know my plugs aren't to blame, I've had three different sets in over the last 5-months, to troubleshoot not because they needed changing. I am having them check the wires again tomorrow. I won't get my car back for another week I'm sure so probably no info in the meantime unless there is an "Ah Ha!" moment tomorrow.

-Marrs

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post #14 of 22 (permalink) Old 04-09-2012, 05:46 PM
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ok so i changed the plugs with ngk's, the engine has bosch platinums in it, however I think thats overkill on a car that doesnt really need them.


3 of the spark plugs were good. the other 3 had black sut on them and oil was on the threads and leaking out from the plug holes and was sitting near the plug holes on the valve cover. obviously they arent firing at idle.... I labeled them for reference.

the car started and seemed to be idling better and definately has more pep however its started missing at idle right after i left the auto parts store after I changed the plugs...

I'm going to clean the distributor cap and rotor they look dirty. Im also going to look at the points however they seeemed clean..

two of the sparkplug wires are old wires that seem ok but they are different formt he other 4.
I have a set of sparkplug wires somewhere from my ford they are 9mm ford motorsport racing wires... I know they are good so im gonna swap them out if it still misses after i clean the distributor cap and rotor.

just so you know I have the blue bosch coil with external resistor/ballast whatever.
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post #15 of 22 (permalink) Old 04-09-2012, 07:00 PM
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You might want to check out the plug wires to make sure that you're using the correct type plug + wire combination. On the later ones like mine, the design is for non-resistor plugs and resistor wires (actually, the resistor is in the plug connector). If you have both resistor plugs and resistor wires you may be cutting down the juice to the spark enough that it would cause problems.

See attached photos courtesy of another user on the 107 forum.

You need to have some resitance in the circuit, to prevent RF noise and to get a good current to make a nice hot spark. It's just that putting two reisitors in there causes problems, as you can see. The photos are taken with OEM type resisitor plug wires, and show the difference between resistor and non-resistor plugs.

Just an idea of something to check out.
Scott
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post #16 of 22 (permalink) Old 04-09-2012, 07:47 PM
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nice! funny enough, when I took the wires off the distributor I saw the coil input on the distributor was GREEN (eureka!)and corroded but all the other inputs for the cylinders on the dist cap where nice and clean copper.... after I cleaned the distributor and rotor I reinstalled..... SAME THING

So with the motor running I started taking off the wires.... the two old ones with the metal cover(I think they have resistors in them) I took one off and the engine lowered its rpm's... put it back on, took the other off(one of three with the oil around the plug area and Nothing changed...

I took all the other wires off one at a time and the two other plug wires I took off were the ones that had oil by the sparkplug hole and SAME THING no change in rpm....

i just ran outside and checked the wires

4 are bwc hi temp silicone with a mag core
2 are bosch optilayer with a copper core...

tomorrow im going to switch the wires around to see if there is any difference and after that if there is no change in cylinders not firing its time for a compression test... unless I need to do something before that?

I really hope its not the carbs screwing everything up but I dont think it is.. and if its low compression then I am done working on it for now, I dont have the loot to fix this... I borrowed it form my dad so I can save gas money and expenses so I can save up to fix my audi
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post #17 of 22 (permalink) Old 04-10-2012, 06:10 PM
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OK finally PROGRESS! FIXED!!! Running beautiful!!

So, I changed the plug wires because radiotek brought up the resistor and non resistor wires... NO CHANGE in misfiring... However I wanted all the wires to be non resistor since the NGK plugs I used had resistors in them.

I also chalked up the plugs to needing a tune up any way and it def had more pep OFF idle(2nd stage on the carbs)

So I kept looking at the points on the distributor but they looked fine...... The auto parts store guy, Bryan had said you have to take them out to really look at them... so I did... I cleaned them with a dremel with a wire brush attachment... and I noticed the points were touching each other on a angle and were not mating perfectly flat. So i decided to bend the arm Just a tad to get the mating surfaces to be even.... I knew that even on angle it wouldnt matter much as long as they made contact....

However, when i put them back in I decided to adjust the points so they were touching the cam on the distributor... I noticed when I took the points out the screw marking was in the furthest out position.. when I adjusted it the screw wear marks were about 1-2mm off.

I went to start the car and PERFECTO! Finally running on all 6....

I adjusted the idle air screw adjustments (1.5 turns out) and took it for a ride.... NOW it was detonating if I was anywhere past 1/4 throttle... I went back tot he auto parts store and bought the only set of points they had in the store... FUNNY THING, the points were $7 the wires were $20 and the plugs were $20.. like i said though I chalked that up to running better and all the same wires... I bought the points but did not install them I have them as a spare in case I run into this issue again.

So now im detonating... What did I do? I adjusted the timing, first time detonation was worse(i think i advanced it too much) and more detonation. second time I retarded it and sure enough all detonation is gone and the car is running beautiful....


SO My summation about the idle issues with these cars is check the points adjustment first, bend the arm if they dont contact flat and make sure they are close to touching the distributor otherwise the cam will not interupt the coil from firing....

I know people claim the carbs are nightmares however I could see how the points could really screw someone up...

my car would run on 3 cylinders then after cleaning up the corrosion would run on 5 and It was driving me nuts... I think to even a mechanic with experience, checking the spark on all cylinders multiple times, it is of major importance before tinkering anywhere else.... I thought it was the coil at one point but I could not understand how it would spark for some and not others.. and then spark on the ones that weren't firing intermitentl
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post #18 of 22 (permalink) Old 04-10-2012, 10:35 PM Thread Starter
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Glad you got yours fixed, congrats! Now go out for a celebratory cruise!
Mine is back at the shop now and I gave them a list of things to retest, check wires, etc. My car has a mechanical fuel injection and a Pertronix system, so the other details you shared do not apply to mine. We will see how things turn out...

-Marrs

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post #19 of 22 (permalink) Old 04-10-2012, 11:12 PM
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Thank you! Already did the cruise I am sure your shop wil have your baby running again!
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post #20 of 22 (permalink) Old 04-17-2012, 12:38 AM Thread Starter
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So the screen in the fuel pump was filled with lots of sediment, the other filter/screens checked out ok so we didn't check it straight away, but apparently this was deferred maintenance and somehow overlooked by the PO. We don't believe it has a very rusty tank, but time will tell. In the meantime I'm just glad to have my baby back and rolling!

-Marrs

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