I have a 1986 560sl with 100K the engine has become rough running at
low speed. It shakes at idel and becomes smooth as you start to drive.
It sounds like a solid miss at idle but any plug wire pulled off will
I have ohmed out all wires and found them to range 1.1K to 1.2 k the
distributor looks great too. After pulling each plug I have learned
that my dry compression is 145 to 150 each cylinder. I found four
plugs to be quite dry black and the other four to be lean looking
,showing white at the porcelain and ash gray at the tip. I put new
plugs in and ran the engine for say ten minutes, the cylinders that had
the black plugs now have new black plugs. I think I have a rich fuel
problem. Would this be problems with injectors? Can some one tell me
what to check next.
Thanks in advance Bruce Buchanan.
It's probably a problem with a clogged injection ditributor. DO NOT
EVER run injector cleaner through this engine as it can cause a lot of
crud from the injection system to be sent down stream and in a worste
case scenario foul your intake valves.
The problem is that this car runs a Constant Injection Stream
semi-mechanical injection system as opposed to the Electronic Fuel
Injection in almost all other mercedeses.
My guess is that you need a rebuilt injection head installed on the
car, which is not cheap. Anyway, don't let anyone put injector cleaner
in the tank, and remember to keep the fuel filter clean. Best of luck.
Well yes, I have put injector cleaner into the tank but after this
symptom had developed. In an attempt to try and solve the problem.
So I am rich because crud has jammed open something in the injection
head? Is this something that can be cleaned? I am pretty good if I
know the goal and have a trusted diagnosis.
Thanks Bruce Buchanan
Where are you located?
I'm in Atlanta (have a '78 450SL) and I (although I do work on the car
myself) get my engine work done by Mat Cama at http://oldworldauto.com /
- he's in Lawrenceville on the NE side of Atlanta - very knowledgeable
and meticulous - highly recommend his services.
Mat's a good friend of mine and has worked on my cars. He's good alright.
Sounds like you need a new fuel distributor. Not a cheap part Im afraid,
the only other car that uses that part is a Ferarri 308. But it's 20
yrs old and they don't last a heckuva lot longer than that.
Need Mercedes parts ? - http://parts.mbz.org
Richard Sexton | Mercedes stuff: http://mbz.org
Hey, Since you already used the injection cleaner, it sounds as if you
are still alright because your compression test was OK. Like I said,
it's a worse case scenario, but injection cleaner is a bad idea with
this injection system and specifically not reccomended by Mercedes
Yes, the uneven richness does tell us something, but I'm blanking on
what. I haven't worked on one of these in about three years, but if
memory serves it points to an uneven pressure between the two heads and
might point to a cheaper fix than a whole injector head???? but like I
said, I can't remember. You'll have to wait for Tom to repost and see
what he was getting at. Sorry that I can't be of any more help. All the
Thanks Hazey , I will wait and see. I suspect that the fuel
distributor is the culprit. I can believe that crud or what ever would
allow it to give extra dosage to some cylinders. This problem has been
with this car since I bought it a year ago but has worsened recently
which is why I did this investigation Sunday. The car had been garaged
for a year without running until I bought it. This may explain crud in
I am not so much looking for the cheapest repair as I am wanting to
understand things about my car. I would enjoy changing the distributer
but don't know much about it yet. I know how my diesel Benz works in
all ways but this fuel injection system on this car is a little Greek,
well German to me. So as well as fixing it I hope to also understand
more when I am done. Will wait and see if Tom has an angle to the
plugs being black on one side. Thats got my attention.
Just thought of something, maybe unrelated but food for thought. The cold
start injector is on the 5,6,7,8 side of the engine. If it were leaking,
maybe it could be causing the running rich problem. Remove it, disconnect
the electrical connection, leave the fuel line hooked to it. Block the hole
where it was removed from with the corner of a shop towel. Put the injector
in a small cup or something. Start the engine and look for fuel coming out
of the injector. If it is spraying even a small amount or just dripping,
replace it. If there are no leaks, reconnect the electrical plug and
repeat the test. If it now leaks or sprays, you have an electrical problem.
It should only spray when the engine is cold and the starter is operating.
No, I was NOT onto that great idea!
I know very little detail about these V-8s and was looking for further
symptoms that might illuminate a possible a cause.
One or two overly rich cylinders would point to some fuel injectors but
so many..... I thought which ones and whether they were all on one bank
.... thinking ignition or valve stem seals. But a leaking cold start
valve is a much more elegant and logical idea.
Looks like you have a strong possible cause to check out.
First consider the simple stuff, there's always time later for the
complicated and expensive stuff.
This theory sound right to me also...
I don't know this engine, but the one side only things makes the fuel
distrib. theory seem bad...
Consider the air intake also. If this is dirty/plugged on one side it
could be the issue.
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