I just received a new set of h9dc0's for my '90 300se (184k miles) and found that the plug in cylinder #1 was heavily fouled, while all of the others looked relatively normal. I decided to install the new set and drive around a little. The idle was so smooth that I thought the car had stalled while waiting for the green light at the intersection! I thought that it was incredible, as the passenger side engine mount is collapsed, and the driver's side is torn (I've already ordered them).

Anyhow, around 15 miles later (hey, that's pretty far here in Hawaii ), I pulled the plug in cylinder #1 and found that it had already started to gain a shiny black coating on the insulator. The plug itself was a little wet, and there was a clear liquid on the threads. I tried sniffing it but couldn't decide if it was fuel or motor oil.

A few days later, I pulled the same plug again and found that there were now a number of black "clumps" stuck to the insulator. The slightly rough idle that I experienced before changing all the plugs returned, so I think that the engine is misfiring in that cylinder.

I've done a lot of searching on this forum and it looks like I might have to get the head done, but what I'd like to make sure of first is that it is in fact oil fouling rather than gas fouling. Is there a sure-fire way to determine this? Maybe I could try and light it while it's wet?

Indeed. I've already done a lot of searching on these forums, as I had mentioned. I couldn't find anything regarding determining whether what I am experiencing is fuel fouling, oil fouling, or a combination of both. The exhaust smells rich, even with a fresh plug installed.

I already found via prior searching that on the m103, it is usually the valve seals that go bad. They were changed by the previous owner around 5000 miles ago, so that probably leaves the valve guides, as compression is good. New cap/rotor/wires have been installed with the same results. I have not checked for a leaking injector or bad fuel distributor yet.

You likely won't see oil fouling until consumption gets down to a few hundred miles per quart. When changing valve seals a competent mechanic can tell if the guides are excessively worn and would suggest replacement to the owner.

So what is your measured rate of oil consumption?

If it's not oil, the other culprit could be gasoline fouling due to misfiring.

Check the plug wire! Improper removal of the wires (without using a special pair of "pliers" to remove them by firmly grasping the metal RFI shield) can damage the internal connection between the RFI resistor, wire, and terminal.

Duke

Hi Duke,

I'm going to try and find out which mechanic at the shop had changed the valve seals (I have the receipts). If that doesn't work out, I'll have my indy check them out and see what he says about the guides.

I've read one of your previous posts regarding fouling, in which you mentioned that the plugs should be okay even at a quart of oil per 300 miles. I usually drive around 500 miles/month, so I haven't really driven the car around enough to figure out its oil consumption just yet. So far, the oil level has been relatively stable even with the head gasket leaking near cylinder #6, and the slight leakage at the oil level sender.

I've checked the resistance on the wires and they seem to be in spec (all around the same values, based on length of wires). But I will try swapping wires from the old set that I have to see if that is the problem.

Some additional info:
There is no smoke when I start the car, and very little of it on idle while warming up even with the old fouled plug installed. The amount is so little that I can only see it within inches of the tailpipe, so I can't really tell what color it is. Once the engine and exhaust are up to temperature, the smoke goes away but I still smell fuel. There is no smoke on acceleration, but I can't smell the exhaust then

My Cosworth Vega got down to a quart every 200 or less before I pulled the head for an overhaul - new guides and a modern seal design (the OE seals are junk) - and it never fouled a plug, which is the same type/heat range as M103 except they are the resistor type. Interestingly, even with the huge oil consumption it was squeaky clean on emission testing. The oil comsumption is now about a quart every 4-5000 miles - I drive it so little it's tough to measure - but the emissions actually went up a little on the first test after the head refresh. Go figure!

When testing wires you should twist/bend them around a bit and pull on them lightly as there can always be an intermittent high resistance. I also found that a couple of terminals on my OE cap had "flaky" resistance readings, so I replaced it and the rotor with a new Bosch OE equivalent parts - not cheap, but the idle quality and emission test performance improved, and at the rate I'm accumulating mileage they should outlive me!

It doesn't sound like your engine has excessively high oil consumption.

Duke

I have a 88 300ce that just had the seals replaced yesterday. The shop left the plugs in the car ( in a box - they replaced them). Anyhow the amount of deposits on the plug was incredible. But this car need the seals for a while. It finally had the work done when I was using about 1 quart of oil per 150 miles.

It seems you have it narrowed down. As a next step I might suggest a new spark plug wire on that cylinder, drive it and see what happens. My other thought: if the cylinder is fouling the spark plug in a short time frame as you described then you are experiencing an obvious mis-fire. You might want to have a compression and leak down test performed on that cylinder. This will help determine if the cylinder is mechanically capable of igniting the air/fuel mixture for maximum efficiency. Low compression and or high leak down will be a good indication that "you're going in!"

I'm not familiar with M103, but in general Mercedes wires don't have any measurable resistance. They use copper wire. Resistor (1k ??) is in the plug end connector. If 103 is the same you should not see any different resistances between the wires.

IIRC that was my observation. Each wire measured 2K, which I think is from the spark plug terminal resistor, and there is also a 1K RFI resistor in the rotor and each cap terminal.

Duke

Thanks to everyone for the great responses!

Duke: I didn't think of checking the resistance on the cap, so I will try to do that this weekend. I've also swapped a wire from a different cylinder from my old set to see if I have a bad wire.

qdessa: Thanks for the info, the original plug that I pulled had so much crud on it that it almost covered the whole tip of the plug! The originals were h9dc, so I was hoping that running h9dc0's would foul a little less.

MrCjames: You're right, there is mis-firing when the plug gets fouled up. My mechanic did a compression check and said that everything looks normal, so I'll ask about doing a leak down test if the wire swapping and cap checking doesn't work out.

myarmar: Sorry, I should have made myself more clear - I tested the wires with the resistors and connectors in place (I didn't check the wires themselves).

Have a good weekend!

One of our forum members said MB uses copper wire (I assume well-shielded), so my following comment is just a general comment on non-copper plug wires. A bad plug wire can sometime's be not noticeable on idle or on the mechanic's oscilloscope, but mis-fire under load. I've experienced this a number of time's with my previous Detroit Iron vehicles. They originally, though I'm not sure if my memory is correct, dubbed the carbon composite wires that replaced the copper wires "radio resistance" plug wires. You could hear each plug fire with a click on the old tube-style radio's used in those days, hence, the switch to composite plug wires. Another problem with the old copper wires in Detroit Iron cars was that if they weren't pushed in the brackets correctly and one got too near the exhasust manifold, or even the valve cover, the wire heats, thus increasing the resistance of the wire in a curvi-linear fashion.

Over the weekend, I discovered that the plugs in the other cylinders had started to gain some black deposits. I'm not sure if they were fuel or oil. Late at night, I warmed up the car and headed out to a nearby intersection which leads directly onto a freeway that has about a mile or so of straight road (basically, a nice place to do a little "drag racing"). I gave the gas pedal a good push and saw a nice cloud of smoke come out the back. I kept my foot down until I reached the legal speed limit - a neck breaking 10 seconds or so and then drove around at that speed for around 20 miles (I did not do the full "italian tune-up").

I pulled the plugs again after the engine had cooled down and found that the plugs from the back 5 cylinders had burned off most of the deposits and were now tan in color, and that the plug from cylinder #1 had burned some off as well, leaving it cleaner than how it was before I started the run.

The only great outcome of this is that I no longer have to get out the dremel with the wire brush to clean the plugs . I'm going to do more driving around with the known good wire from the old set in cylinder #1 and see if that affects anything.

Since the other plugs also started to foul a little, could it be possible that I have a coil that is starting to go? I'm willing to try out that accel coil that was mentioned sometime back on the forums if it would make any sense to do so.

Given that all the plugs are showing some buildup in a short time your thinking that it might be the coil is sound thinking, thouogh cylinder #1 has some particular problem the other's don't. I'm assuming your mechanic checked the timing. I'm totally in the dark regarding an m103 engine, but if the timing has initial distributor advance, vacuum advance, and mechanical advance (spinning weights), it would be a good thing to know the entire timing advance curve from idle to all advance in. You mentioned checking the resistance across the distributor; also make a visual inspection to see if there is any corrosion in the spark plug wire end as well as the contact inside of the distributor that might be missed by just sticking your multi-meter sharp probes in. Pardon my ignorance of your engine, as perhaps none of this applies. Is your distributor vented? If you're in a wet or humid climate check for any moisture droplets within the distributor cap. If the combustion chamber is heavily fouled, hot carbon spots might be causing that cylinder to pre-ignite, thus fouling the plug. Since you seem to be having a problem, though lesser, on the other cylinders, I'd agree with replacing the coil and having the whole timing advance curve analyzed. Also, make sure the #1 plug wire is not sitting abnormally close to any hot engine parts; actually, make sure all the wires are kept as distant as possible from hot engine components. Heating a copper wire causes the resistance to increase.

Bingo.

I still bet valve seals and/or guides. If the plugs are fouling on one side only, it's a sure sign that oil is coming in through the intake valves.

__________________
1989 300 SEL that mostly works, but needs TLC

Just an update on the situation with my car. I had the valve job done a couple weeks ago and so far, no fouling . However, I'm now dealing with the poor fuel economy issue (I get around 10mpg in the city!). As a recap, here is a list of possibly related things that I have changed out so far:

- Cap + Rotor (Bosch)
- Ignition wires (Bremi)
- Spark plugs (H9DC0)
- Ignition coil (Accel 140008, borrowed from friend, didn't help so I removed it)
- Oxygen sensor (Bosch 13953)
- Air filter (Mann)
- All vacuum hoses and rubber fittings, including the two hoses for the ICV
- The two breather hoses (valve cover to air filter housing, valve cover to intake manifold)
- Transmission vacuum modulator

There appears to be no vacuum leaks in the system that I could find with a mityvac and some flammable liquid Lambda has been checked/adjusted and here is what I ended up with:

32-39% at 2500rpm
43-51% at idle

The thing that is confusing me is that searches regarding duty cycle in the forums generally indicate that the duty cycle to be leaner at 2500rpm, whereas my car is actually running richer then. However, according to the landiss page, the main goal is to adjust the idle duty cycle to within 10% of that at 2500rpm so that the system can "self-regulate" or be within its control limits. Am I understanding it correctly, or are my results indicative of something wrong somewhere in the fuel system as I've changed out most of the ignition system? Thanks for your input, and have a Happy New Year