Spark plug wires and related issues - 91 500SL - Mercedes-Benz Forum
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post #1 of 3 (permalink) Old 04-17-2019, 09:23 PM Thread Starter
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Vehicle: 1991 500SL
Location: So. California
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Spark plug wires and related issues - 91 500SL

Soooo, I inherited my parents 91 500SL and have done mostly cosmetic and minor cooling system repairs until now. I am fairly new to Benz myself but am OK with other brands. Not a mechanic but can maintain and fix some basic stuff. This is a play car not a DD.

Anyway, the r129 (150k miles) started running rough after warmup and hesitating to near stall. I did a night time spray test on spark plug wires and observed no sparks. I pulled the plugs and saw carbon but no scoring on insulators or rounded tips. I did notice that the threads were wet but my plug wells were clean. Not sure if someone lubed the threads or the anti-seize broke down. The plugs were Benz (not Bosch) brand F8DC4. Aside from the threads they looked pretty normal to me. I replaced and torqued to 20ft/lbs.

I pulled the distributor caps and noted they looked like they and the rotors had been replaced. No scoring or tracking, and pretty clean and shiny. However, behind the orange gasket under the caps was some trace of what looked like oil. Although for the life of me I cant figure out where any might come from around there when it was not evident anywhere else.

Finally, I used my multimeter to ohm test each wire in place. I was looking to make sure they were all working not for accurate measure per inch. I found that #4 and #5 were dead. As in no reading whatsoever. I checked them 4 or 5 times each. Also, I noticed that 4 of my wires were 2k ohm Bosch and the other 4 were 1k ohm Beru.

What gets me is that the engine starts and runs seemingly normal at first until it warms up, then has some hesitation and missing issues.

questions for you folks with more experience:
1. Do I seem to be on right track here?
2. Any concern with the wet plug threads or is this a Benz thing?
3. Concerns about the mismatched plug wires? I am considering replacing whole set and if this is a potential issue may complain to the dealer who used to service this car (although they are 7 hrs away).
4. How was this thing running if those two wires were dead? My meter seems to be fine, steady with probes on each other and shaken leads. I am not seeing something and it bugs me.

Hope you all can help a bit, I appreciate any thoughts, ideas and ridicule if I have royally botched something.

I have pics of most of these issues but didn't want to load the thread.
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post #2 of 3 (permalink) Old 05-03-2019, 01:30 PM Thread Starter
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Date registered: Jul 2016
Vehicle: 1991 500SL
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two weeks, 45 views and nothing but tumbleweeds for responses.

Well, without any community input I went ahead and played part roulette. The new spark plug wires and plugs arrived yesterday and I will be installing tomorrow and crossing fingers.
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post #3 of 3 (permalink) Old 05-03-2019, 02:34 PM
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IMHO you are giving yourself a baseline to work from for future diagnosis, even if this doesn't fix the problem. So this is a good thing. I'd think with two dead wires it would run like crap 24/7. But continue on with normal maintenance, like LOF, run some injector cleaner etc. The fact that it appears to be temp related would have me guessing it's more for a fuel issue than a spark. It sounds as though you haven't been involved with the care and feeding of the SL until this time. So you need your own baseline to begin as a starting point instead of guessing what's been done before you. Good luck and keep us posted !!!
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