[W126 Coupe] Rough idle

[Jonathan]

Marcus,

Was this car ever overheated??  That's about the only time I've heard of 
an intake manifold getting out of whack like that.  Also, hylomar may 
not IMHO be quite stout enough to hold everything together if the gap is 
that large.  He may want to look to Mercedes flange sealant goo if need 
be..some black stuff they use on Everything these days.

Popping the intake manifold off and giving it a check w/ a straight edge 
shouldn't take more then an hour or so...Even some 2000 grit wet paper 
on a flat surface would let him see any high/low spots w/o any ill effects..

Hope that helps Some!

Jonathan

Marcus Johnson wrote:

>

> 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

>

>

>

> ------------------------------------------------------------------------

>

> The MB Coupes Website!

> W126 SEC Mailing List

> Postings remain property of MB Coupes, L.L.C.



-- 
Blue Ridge Mercedes
Offering Enthusiast Service, Restoration and Modification.
Providing quality parts to discriminating customers in all parts of the world.
Located in the Greater Atlanta area.
http://www.blueridgemb.com
*Updates weekly*

1996 S600 Euro "Scharnhorst"
1991 560 SEC Euro ECE Conv. (for sale)
1991 560 SEC 226Hp/271ft/lbs (TT conversion IP)
1989 560 SEC Euro ECE (*parts*)
1989 560 SEC AMG (6800RPM Mech.lifter/radical cam conv.)
1988 300 TE (Mosselman TT awaiting install)
1988 560 SEC AMG Euro ECE
1987 AMG Hammer 6.0L 32V
1986 560 SEL AMG 6.0L 32V 
1980 280 SE Euro
2006 Z4 3.0Si M-Spec


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.pairlist.net/pipermail/mbcoupes/attachments/20080806/4f1a75e4/attachment.html>