[W126 Coupe] Missing 560SEC - possible solution

[Gerry Van Zandt]

Last week I had a condition whereby my 1989 560SEC developed a pretty 
pronounced "miss", which came about very rapidly, specifically after 
the car warmed up. Startup and initial idle were just fine -- no miss.  
This miss manifested itself as a hesitation when starting up from a 
stop; a rough idle; and sort of a feeling of "fighting" the car up to a 
running speed of 30 MPH when the miss was still occasionally apparent 
but not so pronounced.

I figured it was one of two things (obviously):  spark or fuel.  
Decided to start with spark first and put it on a scope. One look at 
the scope told the story:  the #3 cylinder (on the passenger side) was 
shorting out -- essentially not receiving any spark at all.  The lines 
for that cylinder were jumping off the scale!  So it was immediately 
apparent that there was either a problem with the plug, the wire, or 
the suppressor (connector between the end of the plug wire and the plug 
itself).  Pulled the plug wire off the plug, and pulled the plug 
itself.  Plug was like new (it had been replaced with a new one a scant 
5,000 miles ago at my 150K service, along with new cap and rotor).

So I sliced the rubber top of the suppressor with a razor blade, 
smashed the ceramic insulator with a pair of pliers, and unscrewed the 
suppressor from the end of the plug wire.  Sure enough, a green carbon 
track was present.  So the spark was essentially shorting out likely 
against the head -- finding the easiest path out, which electricity 
always does.

Replaced the suppressor (a $25 item) with a new one and all was 100% 
again.  Likely that others are going bad as well -- probably worth the 
money just to replace the entire set of plug wires than hassling with 
individual suppressors.

Anyway, a word to the wise.  Scopes can tell a lot about the electrical 
characteristics of the engine. And I had bad visions of $2,000-plus 
fuel heads (fuel distributors) going out or $40 apiece fuel injectors 
going bad as well.  Luckily all was an easy fix.  I'm going to order up 
a set of plug wires ASAP.

Also, another short maintenance tip for everyone, for the new year:  
open your fuse box, and rotate every fuse you have in there 1-2 turns 
to ensure that its contact is optimal. Takes 5 minutes and could save 
some heartburn (and burned fuses) down the line.  Make sure you have 
extra fuses of every amperage level in the little round holders inside 
the fuse box, too.  If you don't -- get some.

Cheers & Happy New Year!

Gerry