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1986 560sl Black Pearl
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100 Posts
Discussion Starter #1
Hi, there everybody,

Looking for some advice; a few days ago I noticed that my 560sl was missing on a cylinder or two when I kicked down to a lower gear passing someone on the highway. After trying this a few times, it was when the revs go up towards
3000rpm, and it just would not catch on. It would not rev to more then 3500 - max 4000.

I’m sure this didn’t happen last season. (In Canada we don’t drive these years round) Anyhow, this was probably the first time this year the rpm’s hit 3000, as you guys know, these cars get a lot done below 2500.
Then, 2 nights later on the way home coasting to a stop at a red light, the engine cuts out and dies.
Would not restart! So I called CAA (the Canadian équivalent of AAA) and only 3 hours later got a flatbed ride to my house. I had a replacement fuel pump relay at home. (As to why it wasn’t in my car, I will plead the 5th or stupidity)
So after 30 min of swearing in Swedish and English the next morning, the car started right up again. Perfectly smooth idle and running on all 8, until of course I took it for a spin and the same misses again, up around 2500-3000rpm. Again and again.

So, what could be the issue, fuel pump(s)??

There is a new distributor, rotor and plugs less the 3000 miles ago.
Anyone had this happen?

Thanks in advance for suggestions
 

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1984 380SL
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2,284 Posts
Sounds like a nearly clogged catalytic.

Open up the exhaust before your cat and see if you can rev past 3k.

If this works, replace or straight pipe the catalytic converter section. I would also look into why the clog occurred (rich mixture) and aim at getting the car in tune.
 

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1986 560sl Black Pearl
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100 Posts
Discussion Starter #6
So, I tried Dugalds advice and the engine will rev cleanly and willingly to 5500rpm.

I guess that clears the catalytic converter of any wrong-doing.

And, Rowdie says it is not the vacuum advance.

Still misses while driving over 2500-3000rpm but not with transmission in park.

What could cause that?

Thanks again!
 

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Registered
C126.045 560 SEC 1988
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5 Posts
Hi, I am having a similar, possibly worse, problem on my 1988 560 SEC. In a similar fashion to yours mine has been laid-up for 7 months during an engine and gearbox re-build. On starting, it has a rough idle and it stalls when you stop in D at junctions, lights roundabouts etc.

I suspect it is the fuel distributor which is the mother of all evils on these engines when it comes to rough-running and stalling-out.

They gum up with stale fuel, they don't take kindly to standing for long periods etc. There is a rubber diaphragm inside that perishes and numerous o-rings that also perish etc. these units are now 30+ years old and need TLC.

There are threads on rebuilds elsewhere on the forum. Pelican want USD 1000+ for a rebuilt unit, they go for 400 on the bay of E, or you can buy a rebuild kit for 105 USD and do-it-yourself, there are videos on YouTube from the repair kit manufacturer on how to go about it. There is also a handy guide somewhere here, I have it on file but it is too big a file to upload, perhaps somebody can help with this?.

I would bet my last dollar that this is your problem.
 

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W107 W113
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1,310 Posts
I would try to rule out the easier items first.

It is definitely a fuel or ignition issue, most probably fuel.

Remove all spark plugs and compare them for any obvious differences in mixture/burn.

I would also see if your injectors have a good spray pattern and not leaking/dripping, this is especially an issue at idle.

Finally, make sure you are getting sufficient fuel pressure, at idle any irregularities in the combustion are more obvious.

Sent from my MAR-LX1M using Tapatalk
 

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1986 560sl Black Pearl
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100 Posts
Discussion Starter #10
Hmm, if it revs cleanly in N or P, how could it then be fuel starvation.

The idle is very good and even.

I am thinking maybe be vacuum advance as Jyuma suggested.

Being parked for 6 months is admittedly not the best for o-rings and rubber diaphragms.

We do have a very good Mercedes Indy in Ottawa. Ill play around with it for a bit,
and if I don’t get to the root of it I’ll bring it in.

I’ll post the results,

Thank you
 

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1986 560 SL
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7,300 Posts
Ahh, OK, thanks.
Canswede, I believe you have a fuel supply problem. Unlike @boatmaddaddy, I am not ready to bet my last dollar that the culprit is your FD. It's a likely suspect but other components in the fuel supply chain could be impeding proper flow: tank strainer, pumps, filter(s), injectors.
You mention taking it to your Indie mechanic. What he will probably do first is a pressure and volume test which will indicate any problems upstream of the FD. You can DIY with a pressure gauge and a measuring vessel - the procedure is described somewhere on this forum.
 

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Registered
C126.045 560 SEC 1988
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5 Posts
Your smooth idle would probably rule-out the FD that I mentioned earlier, if this was a problem then the stutter would also be noticeable at idle I would lean more towards fuel pressure issues or vacuum advance issues. Good luck. I am about to dive into my FD for a re-build, looks like lots of fun!!
 

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1986 560sl Black Pearl
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100 Posts
Discussion Starter #17
Hello,
I promised I would be back with an update. So, I didn’t get around to do anything before we left for a month-long European vacation. (Finally went to the Mercedes museum in Stuttgart. I mean Heaven in Stuttgart)
Anyway,
finally brought the car to the local Indy last week, and it turns out it was the fuel pump. One of the two pumps was dead. I didn’t know this but the second pump only kicks in at higher rpm’s. They also found that the EHA fuel regulator, was wet and replaced that too. It didn’t affect the rev issue, but it needed to be changed out.
So, new fuel pumps and fuel-filter! Good to go.
Thanks for your advise guys!

From the museum;
they have a new car showroom on the main floor of the place with a couple of Mercedes-renovated classics. I have always loved these, on BAT coupe’s like this usually reach maximum around $150.000 but there they are asking us$440.000. Wow! Like new!
 

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Registered
R107 300 SL, Year 1986 and C124 300CE-24 Year 1990
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99 Posts
Hello,

both pumps run at the same time all the time, there is no kick in at a special rpm.
Both pumps are connected in series.
 

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1986 560sl Black Pearl
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100 Posts
Discussion Starter #20
Marcel; i misunderstood my Indy, yes both pumps run in tandem. He said though that they speed up under full acceleration or under kick-down. The forward pump in my car was dead and the fuel filter was very dirty and full of gunk.
 
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