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1986 560 SL
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7,448 Posts
Discussion Starter #1
560 SL idles and runs rough. Feels like it's missing a cylinder or two...

- Pulled the plugs: 7 nice toasty beige, 1 fouled
- Put in new plugs, runs about the same.
- Disconnected wire to suspect cylinder, runs about the same.
- Wire tested good, reconnected, runs about the same.
- Pulled new plug on suspect cylinder, looks ok so far (10 miles)

Any ideas what's going on here? Do I maybe have a fuel delivery problem?
 

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Always Remembered RIP
1973 450 SLC AMG, 1995 Chevy Tahoe
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3,381 Posts
yup Its what I have happen every 4 months ..

It's your POINTS AND ROTOR in the cap. get new one Buy extra for later.:D :D :D
 

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1986 560 SL
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Discussion Starter #3
Thx for the speedy reply, ussun.
When I say the wire tested good, I mean I got spark at the plug end.
Are you saying the spark delivery is intermittent?
 

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1980 450SL & 1988 560SL
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1,667 Posts
I would look at the plug wire with the engine running at night in the dark and see if you can see any fire jumping. If not, it might be time for a compression test.
 

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2001 SLK320, 2001 C320
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2,267 Posts
ussun said:
It's your POINTS AND ROTOR in the cap. get new one Buy extra for later.:D :D :D
A 560SL ain't got no points. Even my '79 ain't got none of them stone-age pieces.

I'd check plug wires first, but the head gasket is a potential culprit (eek!).
 

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1986 560 SL
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Discussion Starter #6
Actually, unlike the 450, the 560 has no points. There is a rotor.
 

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1986 560 SL
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Discussion Starter #7
page62 said:
A 560SL ain't got no points. Even my '79 ain't got none of them stone-age pieces.

I'd check plug wires first, but the head gasket is a potential culprit (eek!).
What leads you to head gasket?
 

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1986 560 SL
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Discussion Starter #8
Nascar Fan said:
I would look at the plug wire with the engine running at night in the dark and see if you can see any fire jumping. If not, it might be time for a compression test.
Had a look in the dark. No obvious electrical leakage.
 

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2001 SLK320, 2001 C320
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560SL's are known for head gasket issues. But only a worst-case scenario...the cylinder in question could be sucking oil past the gasket into the combustion chamber.

Mostly, though, they tend to leak oil out of the head gasket into the intake valley.

Troubleshooting ignition problems (i.e., wires) would still be first on my list. :)

Note to USSUN: If your car is eating points and rotor once every few months, there is some other problem. You are treating the symptom and not curing the problem.
 

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1986 560 SL
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Discussion Starter #10
I suppose I could throw a set of new wires at it, but that goes against my logic and
parsimony. I did after all get a spark at the plug end of the wire. Weak maybe?
 

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1986 560 SL
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Discussion Starter #11
I reviewed the service records for the car.
They show a head gasket replacement on that side about 8000 miles ago, so I am hoping I can rule that problem out.
They also show replacement ignition wires, dist cap, rotor about the same time.
So why the fouled plug??? And the persistent rough running after its replacement??
 

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dugald said:
I reviewed the service records for the car.
They show a head gasket replacement on that side about 8000 miles ago, so I am hoping I can rule that problem out.
They also show replacement ignition wires, dist cap, rotor about the same time.
So why the fouled plug??? And the persistent rough running after its replacement??
Anything about valve work while the head was off? Maybe a compression check is in order?
 

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Always Remembered RIP
1973 450 SLC AMG, 1995 Chevy Tahoe
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3,381 Posts
page62 said:
A 560SL ain't got no points. Even my '79 ain't got none of them stone-age pieces.

I'd check plug wires first, but the head gasket is a potential culprit (eek!).
UUUUUUUOOOOPPPS again I forgot you youngsters dont have them:D :D :D
Listen to the other guys. To bad though it fixes mine great.:D :D :D When it's running ruff. or as my machanic says, running like shit.
 

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88 560SL, 06 Prius, 07 Prius
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61 Posts
In my experience a rough idle can be caused by many things. Below is a copy-n-paste from a previous thread where I described what my mechanic did to resolve it:

My car had been having a rough idle for a while. After a lot of detective work by my mechanic ruling out the other (cheaper) potential issues, he came to the conclusion it was the fuel distributor. He temporarily replaced the fuel distributor w/ a known good one and the rough idle (a misfire on cylinder 7) went away. He tried opening up the bad fuel distributor seeing if there was anything he could do for it, but he said one of the seals (i think) was totally corroded and you can't buy it separately.

Here is a list of other potential causes he looked at before determining it was the fuel distributor, if I remember correctly:

1) examined and cleaned fuel injectors and replaced two of them
2) checked whether the (electrical) distribtor and all cables were good
3) eliminated any vacuum leaks the engine might have. I think this required replacing a valve gasket or two. Once this work was done, this actually had the effect of pronouncing the misfire on cylinder 7 even more!
4) cleaning out heavy carbon deposits in the exhaust manifold. I believe this may have just been one of those "while I'm in there" things.
 

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1986 560 SL
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Discussion Starter #16
isthisdave said:
Anything about valve work while the head was off? Maybe a compression check is in order?
Compression test shows good all round. Cylinder with previously fouled plug is highest of 8.
Where next?
 

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88 560SL, 06 Prius, 07 Prius
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61 Posts
Unfortunately, its an expensive part to replace. We had a spare one to test to make sure it was faulty before I purchased a new one. Without a spare, I'm not sure how u would conclude it was the fuel distributor for certain.

If u do end having to replace it, the dealer price is ridiculous. Like over $1k. I found a good quality refurb (by CIS Flowtech) with germanstar.net for around 500 I think.

-- J.
 
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