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Discussion Starter #1
Hi All...Was wondering if anyone can help.

I have an 86SL with 79K miles on it. Recently, idle became rough with a small misfire that leads to the normal shaking in the engine.

1. We replaced the plugs with new bosch plugs. The original bosch plugs were in excellent shape with no differences in burn pattern.

2. Distributor points looked clean for the most part.

3. Issue resolves itself on accelleration and is still like a bat out of hell past 2000 RPM.

Our initial thought was this is was a vacuum leak but not sure why this would cause a single plug misfire. Maybe fuel injector issue?

Any thoughts?

Thanks!
jeff
 

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My best quick guess would be injector of fuel distributor issues. Going from a quick guess to a thoughtful diagnosis is a bit more difficult.
 

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2. Distributor points looked clean for the most part.

I assume by this you mean the cap and rotor contacts?


Our initial thought was this is was a vacuum leak but not sure why this would cause a single plug misfire.

Pull the plug wires one at a time to see if the miss can be isolated. If it is, swap a couple of wires and see if the problem moves with the wire. If not, and the miss is random, double check the cap and rotor.

If that's all good, consider the fuel injection.
 

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Discussion Starter #4
I assume by this you mean the cap and rotor contacts?

Pull the plug wires one at a time to see if the miss can be isolated. If it is, swap a couple of wires and see if the problem moves with the wire. If not, and the miss is random, double check the cap and rotor.

If that's all good, consider the fuel injection.

Thanks you..Yes rotor contacts... Thought about the wires....will attempt it tomorrow. It would at least rule the easy stuff out.

I appreciate the feedback.

J
 

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I've seen this before repairing Mercedes - have you checked for any vacuum leaks? Do you have access to a smoke machine? If so, you can hook it up to engine vacuum, fill it with smoke, and see if smoke leaks out anywhere. Please let us know what fixes this!!
 

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I've seen this before repairing Mercedes - have you checked for any vacuum leaks? Do you have access to a smoke machine? If so, you can hook it up to engine vacuum, fill it with smoke, and see if smoke leaks out anywhere. Please let us know what fixes this!!
I dont have a smoke machine but you may be correct. I brought it to a local garage and his first guess was vacuum, second was fuel injector.

Thx!
 

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Mine will get that way after driving just around town for a while. Taking it out on the open road and running it in 3rd gear 50-60 mph for 20 minutes clears it right up. Occasional heavy braking also seems good for that system's ills.

I'm slowly coming around to the point of view of those of us who say these cars are meant to be driven hard.
 

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+1 on vacuum leaks. Two bottles of Techron, a fresh tank of premium gas and an extended drive at highway speeds. Then see if the problem is still there.

I'm slowly coming around to the point of view of those of us who say these cars are meant to be driven hard.
hard and often!
 

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I did some more thinking on this problem too - I worked on a 500SL that drove like a dream but had a misfire at idle. Turns out intake gaskets were the issue. They were letting extra air in at idle, but when the car was driving down the road, it really didn't matter. Being that these cars are 30-40 years old, it's a great idea to swap out the gaskets! Also, leaking intake gaskets are small vacuum leaks too!
 

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Discussion Starter #12
I did some more thinking on this problem too - I worked on a 500SL that drove like a dream but had a misfire at idle. Turns out intake gaskets were the issue. They were letting extra air in at idle, but when the car was driving down the road, it really didn't matter. Being that these cars are 30-40 years old, it's a great idea to swap out the gaskets! Also, leaking intake gaskets are small vacuum leaks too!
THANKS! I did the techron to no avail. I'm actually pulling out the injectors and cleaning them just to rule it out. I'm also going to replace the injector seals since they're a cheap fix and easy enough to do. When you say intake gaskets what do you mean by that?- Sorry....still learning here.
 

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I fast forwarded to the bottom to post something for you to observe and possibly see if the wires are sound. I had a two year old Corvette LT1 in 96 with around 20,000+ miles but not much more, It started running slightly askew, nothing too noticeable, yet I could feel it at idle, probably no one else in the car could, but I could. A member of the local club suggested I open the hood when it got dark and observe the plug wires. I did that evening, and much to my surprise, I had two or three wires glowing and sparking thus getting far less than the specified conductivity to the plugs. That was the source of my problem. The removal and repair was incredibly difficult, but once installed, it was like night and day, at least to me.I'd of never guessed with such low miles, that such a problem would or could exist. But in the dark with the hood open, you will see every annomaly, plug wire related.
 

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Not yet. Is there a test for this?
Yes. Remove the air cleaner assembly. You will see the idle control valve at the front of the intake manifold. It has a skinny wire with a plastic, two prong connector . Unplug it. Start the car. It should rev high. Reconnect it with the motor running. If the car stalls or sputters, the valve is bad. I just went through this with my car. It's a 5 minute fix. Good luck.
Jim.
 

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Discussion Starter #18
Yes. Remove the air cleaner assembly. You will see the idle control valve at the front of the intake manifold. It has a skinny wire with a plastic, two prong connector . Unplug it. Start the car. It should rev high. Reconnect it with the motor running. If the car stalls or sputters, the valve is bad. I just went through this with my car. It's a 5 minute fix. Good luck.
Jim.
Sounds good! Will try this week!
 
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