[W126 Coupe] rough idle

[Tim Graney]

Jonathan your insights are very much appreciated. I agree with you that
it will take some time but it is worth figuring out. When I did the
heads the EGR passages were completely clogged and I cleaned them out of
course. I did replace the EGR valve. The euro fuel distributor is not
available new anymore only rebuilt. I keep thinking vacuum leak but
those are a bugger to track down. When the AC is on it seems to smooth
out and when I turn it off it gets real rough and then smooths out. I
pulled the plugs several times and they look real good...no carbon nice
and clean. 

-----Original Message-----
From: mbcoupes-bounces at mbcoupes.com
[mailto:mbcoupes-bounces at mbcoupes.com] On Behalf Of Jonathan
Sent: Friday, August 10, 2007 7:23 AM
To: Mercedes Coupes Mailing Lists
Subject: Re: [W126 Coupe] rough idle


Doug Gavin wrote:

> Gerry Van Zandt wrote:

>> Reman fuel heads (fuel distributors) in general are, I hate to say

>> it, problematic -- even the rebuilds that Bosch themselves do. Your 

>> best solution -- and I know it is not cheap -- is to get a factory 

>> new unit from MB. Last time I checked they were over $2K from MB, but



>> at least they have a warranty on them, and you know what you are 

>> getting.

>>

>> This definitely sounds to me like a fuel distributor or related

>> issue. My second guess is a vacuum leak -- similar deduction to you. 

>> Have you thoroughly checked out the ENTIRE vacuum system for leaks?

>>

>> Cheers,

>> Gerry

>>

>>

>>

>>

>>

>> On Aug 9, 2007, at 4:28 PM, mbcoupes-request at mbcoupes.com wrote:

>>

>>> Message: 8

>>> Date: Thu, 9 Aug 2007 11:43:45 -0700

>>> From: "Tim Graney" <graneyt at americancylinderhead.com>

>>> Subject: Re: [W126 Coupe] rough idle

>>> To: "'Mercedes Coupes Mailing Lists'" <mbcoupes at mbcoupes.com>

>>> Message-ID: <000001c7dab5$37d06b10$7901a8c0 at timpc>

>>> Content-Type: text/plain; charset="us-ascii"

>>>

>>> I bought the FD from German Star...? I did replace all the rubber 

>>> gaskets in the plenum. vacuum stays nice and steady but does hiccup 

>>> every once in a while. I have looked till the cows come home for any



>>> vacuum leaks. where does one get the right spark plugs...these are 

>>> Bosch plugs. what gap should they be? Thanks again

>>>

>>

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>>

> I priced a FD at CarQuest Auto Parts. New Bosch they would sell for

> $1700 although the MSRP was $1950.  Gee, thanks! Ouch.

>

> Just my two cents. I had a slightly rough idle, in my '90 SEC not

> quite a skip but an annoyance more than anything. My indy MB mechanic 

> replaced 3 injectors and the car ran as smooth as silk for about 3 

> weeks and the same mild roughness returned. I guess we could replace 

> all of the injectors but at the cost of injectors and labor, I'll live



> with the slight roughness. He told me that he sees this  often in the 

> 560s and it's difficult to solve and that he knows we might have to 

> mortgage the house to get it to my satisfaction.

>

> Prior to having the injectors changed. (They were the original with

> 78K miles) I stopped at Star Motor Cars in Houston and got to talk to 

> a mechanic that has been there for 18 years and has worked on many 

> 560s. He told me that condition was very common and he  had addressed 

> this more times than he could remember. His diagnostics was that it is



> the EGR tubes within the engine (my technical term may be off , bad 

> memory, but some will know the proper term) There is one on each side 

> of the engine internally.  He went to his tool box and pulled out a a 

> drill bit about 15 " long X 5/16 " and said this is what I use this 

> for. He went on to tell me that the carbon build up restricts the 

> passages of these tubes and causes the roughness. The most bizarre 

> thing he told me was this was consistent in the 560s and it would 

> begin  at just over 60,000 miles.  His recommendation was to get used 

> to it like everyone else did until I had the heads done then go ahead 

> an have them drilled out. My comment to him was was that these folks 

> in the late '80s and early '90s  paid around $90K for these cars and 

> tolerated this? He just shrugged and said, yup. Oh well.

>

> Doug

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>

Interesting...I block off the EGR ports on 560's  (as they are from the 
factory in ECE Euro trim) when I do the heads and they idle 
beautifully.  Many times when I go in to do said heads the ports are 
utterly clogged with carbon thus it's not as though much is changing.

I find that hard to understand, I don't tolerate it, and granted it can 
be frustrating to track down but you Can get these cars to idle 
beautifully should you really want too.

Jonathan


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