[W126 Coupe] Intermittant Rough Idle

[Robert Cavazos]

My answers are inserted below. Thanks to all for the
suggestions. 

Bob
1990 560SEC
1987 560SL

--- Mister McGoo <eelploot at hotmail.com> wrote:


---------------------------------

ROBERT CAVAZOS and his rough idle.........

You say the rough idle occurs after 30 or 40 minutes
of running?

***Yes, I can drive around for a about 15-20 minutes
noting that the engine is well up to normal temp (80+)
after about 5 min and it runs and idles fine. Seems
around 30-40 minutes when it starts idling rough, but
still runs fine.***

Did this condition start suddenly or slowly develop
over time?  I gather that this has started recently,
long after you replaced the injectors.

***Slowly developed I think. I seldom drive the car
and depend on my wife to let me know if it's have any
problems. The injectors were replaced several months
ago and there were no problems until very recently.
And I'm certain that I didn't leave any old seals in
the head as I cleaned the holes down to the shiny
metal to ensure a good seal.***

When did you last replace the ignition components;
distributor cap, rotor and ignition wires?  You can
get a spark straying anywhere in old components.  High
tension sparks like to find the nearest ground it can.
 If it has been many miles or a couple of years since
you renewed them, replace all three first.  It won't
be money wasted because even if that doesn't solve
your problem, the engine will run noticably better
when you finally do.

***I was also thinking about the ignition system as no
no of those components have been replaced for about
since we bought the car 5+ years ago. Maybe I'll get
the water bottle out and mist the wires at night and
look for leakage.***

If you have already installed new stuff recently, look
to the vacuum system.

***Only change recently has been to switch to Mobil 1.
It doesn't smoke since I replaced the valve guide
seals
about a year or so ago, so I wouldn't think it's oil
fouling, plus the plugs showed no evidence of oil.
Still, I suppose it's a possibility.***


If it happened suddenly that makes a vacuum hose
breaking or connection coming off likely.  The
symptoms would be a very rough engine idle.  But the
engine runs smoothly when accelerating because vacuum
goes to zero at that time anyway.  At high speed the
roughness is not as apparent.  But at slow speed, like
idling along at 30MPH, it IS noticable.  You can
usually hear the leak, particularly by using a length
of hose to your ear (cut a two foot chunk from your
neighbour's garden hose, that will work fine).  Check
around any vacuum temp sensors and connections to the
manifold.

***Good idea--hadn't thought much about a vacuum leak
but I did visually inspect the hoses I could see with
the air cleaner off. Nothing suspect. I'll try the
hose and carb cleaner method.***

(This is particularly applicable in pre-'86 engines,
older than yours.)

If the problem developed slowly over a period of time
it could be a gasket leak.  Check (as Richard
suggested) with carb cleaner particularly around the
air intake at the manifold.  If you have a leak there,
the idle will suddenly smooth out when you spray the
fluid on the gasket leak.

Those are just a couple of suggestions to try first.  

The ignition module usually just craps out and leaves
you stranded; the idle control module usually leaves
you idling with the engine racing, which is really
annoying.  But it DOES idle smoothly at 1500RPM.  So
those components are less suspect.

***Yeah, I am hoping that it isn't one of the
electronic ($$$) components.***

-Bellamy   





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