[W126 Coupe] Vacuum problem?

[Dick Spellman]

Hi David:

I am assuming your rough idle and so on are with the engine at operating
temperature.  Could be a few things.  Vacuum as you suggest which can be
located on an idling engine using either propane seeping from a torch nozzle
(not lit) carefully laid around the intake manifold or spray carb cleaner
looking for the engine to smooth out.

Could be bad ignition wires made worse from the power wash.  Easy to check
after dark to see the sparks jump.  First one to check is the coil wire to
see if it's arcing to the chassis.  Then observe each spark plug wire run
for arcing to chassis or to one another where they are bundled or cross.
Could also be moisture under the distributor cap (less likely) but, easy to
check.  Could also be a loose spark plug or plug wire to one of the
cylinders.  Engine off, push pull each plug wire cap to see if any are loose
and with plug wire removed just check with a socket over the spark plug and
your fingers to see if any are not snugged down.

A more exact test would be to pull one plug wire from each spark plug at a
time with engine running until you identify which wire removal does not make
the engine run worse.  You'll need to be careful when pulling the plug wires
with engine running not to get jolted.  I use a pair of insulated lineman
pliers and grab the connector not the wire when pulling so as to not damage
a good wire in the process.  You'll feel the engine worsen when a good wire
is pulled.   Just replace it knowing it's good and go on to the next one.
Eventually you'll find the one that makes no difference in performance.
That's the wire or plug you need to inspect/replace.

Is there a chance you disturbed material in the air intake under the air
metering plate by spraying this area hard with water?  This would foul a
valve or two until it clears out.  One can of BG44K ($18 or so) in the fuel
would clear this up.

I wash my engines all the time using Degreaser and concentrated spray from a
garden hose.  I avoid the electronics in between the fire wall with direct
spray. I avoid hard spray on the alternator although I clear any signs of
oil build up on that side of the engine near the tensioner and front seals
so I can see if there is an oil leak needing attention after a few miles of
driving on the cleaned engine.  I cover the air plate with plastic wrap and
a rubber band.  I set the spray on the hose for a strong tight stream and
then simply kink the hose for on off while holding the nozzle just where I
need it.  Any tough stuff I spray a second time with degreaser and loosen up
with an old toothbrush or two then hose it down again.

I did a power wash on the V-8 just one time and too many gremlins popped up
as a result of the harsh pressure including the rough idle you are now
experiencing.  All were related to the ignition and remedied with new wires.
Pressure washing can create breaks in old brittle wire insulation or tears
you can see in the silicon plug wires, etc.  I think you would be much
better off in the future with just the garden hose and a more accurate
controlled spray, engine degreaser and a toothbrush.  You could follow up
any wash with your wet/dry vac set up to blow it's hot exhaust through your
vac hose to puddles or damp areas on the engine wiring harness etc before
putting things back together again.  Compressed air, if you are equipped,
works well too.

Good luck.

Dick




----- Original Message ----- 
From: "David Fatovic" <coloradocroat at yahoo.com>
To: "MBCOUPES" <mbcoupes at mbcoupes.com>
Sent: Wednesday, June 29, 2005 8:11 AM
Subject: [W126 Coupe] Vacuum problem?


> I was doing some work on my dads 85 500 SEL last
> night, and one of the first things I did was power
> wash the engine.  The valve cover gaskets were so old
> and hard they were letting oil seap out on to the
> exhaust.  So, steam cleaned the engine, then replaced
> the valve cover gaskets.  Now (actually had happened a
> bit before) under load from about 1400-1900 RPM the
> engine spits and sputters, shimmys and shakes.  I have
> a feeling it is a vacuum problem but not sure where to
> start.  Once you get past about 2K RPM, the engine
> seems smoother.
>
> Any ideas?
>
> Also the car in Drive used to idle around 1K-1200 RPM
> now it idles at about 650-700 RPM and shakes doing so.
>
> David
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