[W126 Coupe] Rough idle

[Marcus Johnson]

Hello all,

After rebuilding the heads on my 1989 560 SEC, I am having trouble  
getting the idle right.

My mechanic has put in a lot of hours to find the source of the  
problem and it has improved somewhat. However, it's still not right,  
particularly when cold. It’s currently winter here and temperatures  
are getting quite cold overnight.

The issue seems to be a vacuum leak between the head(s) and manifold.  
I had the car there today and while in Drive, he squirted some solvent  
at the rear of the driver’s side. This caused the idle to fluctuate  
between rough and smooth. He also had a vacuum gauge hooked up and we  
could see when it was rising and falling as the solvent temporarily  
sealed the gap.

Driving the car around today, I could see the Economy gauge in the  
dash rise and fall as well. When it was idling rough, the gauge was  
almost up into the red. This suggests there’s a fair bit of air  
getting in at times.

Also, the link with temperature seems logical as the head/gasket/ 
manifold is contracting when cold then sealing a bit better when it’s  
heated up.

The mechanic's analysis is that either the head or the manifold is out  
of whack. He said there’s about .7mm clearance at the front left of  
the head. Could be from a previous repair or was like that from the  
factory.

The current fix is a thicker gasket and some Hylomar(?) to seal it up.  
But that doesn’t seem to be getting to the real problem – an  
inconsistent seal between the two mating surfaces.

He’s thinking of discussing this with an engineer he knows and maybe  
we need to machine the manifold.

He has had a smoke machine on the car, checked all the various bits  
and pieces associated with cold starting, he says the Lambda is spot  
on, we fitted new injector seals, regapped and checked the plugs and  
distributor, replaced the EHA and checked the timing.

In doing the heads, we also replaced any 20-year old rubber bits that  
may have been susceptible to leaking. The fuel distributor also got a  
good going over at the time.

Any clues as to what to do here would be helpful. After a thorough  
reconditioning of the upper end, I had hoped for a smooth running  
machine.

To be honest, it's getting to the point where I just want a car that  
runs good and is fun to drive. That's not taking into account the  
expense this is accumulating.

My mechanic is a good, honest guy but these cars seem to be real  
problems on wheels unless you really know the subtleties and have lots  
of experience.

If you would like to see where things are at, take a look at my blog:  
Rebuilding a legend.

It's at http://sec560.blogspot.com



Cheers,

marcus




marcus johnson

1989 560 SEC
melbourne, australia


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