420SEL Rough Idle
 

My dad's 1987 (60,000 miles) 420 SEL developed a rough idle after I replaced the spark plugs the other day. It ran fine before I changed the plugs. I also replaced the cap and rotor. I have my suspisions that this problem is related to the Bosh Platinum plugs I put in it. (Bosh Tech support said that these plugs would work with this engine and that they have extensive testing to prove it, the tech support guy said my problem was due to a bad coil or wires.) So I tested the plug wires, they are Beru and say 1k on them. My results are as follows...

8 1.226
7 1.203
6 1.171
5 1.19
4 1.18
3 1.24
2 1.15
1 1.18

with the meter set at 2k

wire from coil to cap showed good continuity.

Should I replace the wires and or coil or just put the old plugs back in? They didn't look that bad, we just decided to replace them because I read 60k was a good time to do it...Or does anyone know of a better plug to use than the Bosh Platinum?

Thanks, Joe

go to Oreliieys auto parts and ask for a set of 8 NGK bp5es and put those in. you will thank yourself.

have you had the same problem?

I have. My 500SEL does not like platium plugs and I have a brand new set to prove it. I second the NGK BP5ES plugs. They are the best for the 116 and 117 engins.
Paul

My mechanic recommended them.

Cheap Bosch plugs work the best
 

Look in the owner's manual and use recommended plugs.

Did you take one plug wire off at a time so you didn't cross them?

Did you break one of the plug wires?

Remove them one at a time to see if you have a miss in one cylinder.

Good luck.

The BP5ES plugs fixed the problem, the car runs great...Thanks everyone