1987 mercedes 300E not firing on no3 and no6 cylinders.no spark replaced coil ,plugs ,dist cap rotor.battery is good ,charging is good,not sure whats up,thanks also new fuel pumps and filter 8months ago

Check wire resistance... should be about 2K ohms due to the RFI resister inside the plug boot. Wiggle them around while checking and verify that resistance reading is in spec and does not change. Also check that the wire terminal fits snuggly on the plug. I bought a special tool to remove the wires from the plug. It grabs the RFI shield with a plastic covered jaw. Removing by pulling on the wires will damage the internal connections.

You say a new cap for for reference I had a slight intermittent misfire at about 60K miles that was difficult to isolate until I removed the OE cap and found an intermittent open in a cap tower, especially if I put force on the side. That was the OE Bosch cap, and it was obviously a manufacturing defect. I replaced with a correct Bosch cap. The idle smoothed out and HC emissions went down, which were on the ragged edge of failing.

Duke

Are the wires good and do you have a timing light? Use a multimeter to see if the resistance is the same across all the wires. You should also check you plugs and grounds. If that's good you can use a timing light to see if its firing at the right time to rule out things like the crank sensor and ignition module.

EDIT: Looks like duke2.6 beat me to it. Though I would not suspect your cap to be an issue since you say you just replaced it.

You have done 99% of the ingnition system now do the last bit and replace the plug leads .Cam you get a spare leed to pop in the one thats not running.

It is probably just the suppressor ends on those two wires. The wires themselves rarely fail. Part #A000 156 54 10 or A000 156 57 10. These ends just unscrew off the end of the wire. If the threaded part comes off the end of the wire you will need part # A 000 159 14 38 also which gets crimped on the solid core wire.

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Mercedes Benz Master Tech - Retired

I purchased a tester from Ebay .You just touch the plug lead to see if it firing.,a bit like a pen Or just pick up new leads for nos 3 - 6 ..Other than that its the dustributor top or rotor arm .But if you have replaced them both ,then its must be the leads ..or the push on cover for the plug . I dont like the screw on type .Not sure if they are on a merc but not 100% on that

Hi all, optimusprime, the part you are calling the push on cover for the plug is the suppressor. It is screwed onto the end of the plug wire with what looks like machine threads. I have seen some screw on type that look like a sheet metal screw and don't like that version. The wire on these engines is solid core metal wire if they are original wires. The suppressor ends are usually what goes bad and can be replaced without replacing the whole wire.

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The pliers are the tool used to crimp the end onto the wire but it is possible to use other types of wire crimp pliers and accomplish the same thing.
depending on how many ends you need to replace sometimes it's more cost effective to replace the whole set of wires. Just be sure to buy a quality wire kit. Bosch and Beru were the oem suppliers.

I hope this helps, Robert

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Mercedes Benz Master Tech - Retired

Check the orange suppressor plate in the distributor. If it's old, has cracks or small holes - replace it.

Have a look at what I'm referring to0 - it's item number 8 : 1990 Mercedes-Benz 300E 2.6 Sedan - Ignition - Page 2

RPM 55 what i was taking about is the plug caps push on to the plug .And not the plug wire .Never seen the tool you have there to fix them on with . Bit of good kit .I will look for them over the pond . What would i ask for .And can you buy all the fittings to make your own wires up. Over here the bosch wires are £100 a set .


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