Fouled Sparkplugs...what is the cause? - PeachParts Mercedes-Benz Forum

sptt · #1 08-28-2010, 11:39 PM

OK...geez. It seems like I have a new issue with this car every day (probably self inflicted from trying to fix everythine myself). '91 300E again...car died on me today at work (drove there super good...great idle..everything). After battling with different ideas and almost draining the battery, I pulled the plugs. They were all black. I have noticed that around the base of the engine top cover there is recently something leaking but not sure if it's oil or coolant. Is this a rich gas mixture setting issue? If not, what might it be? I like working on this car but not every day

Could the wrong plugs cause this? I went and bought 1.99 copper plugs...started right up and came all the way home (50 miles). When I check them tomorrow, what should I be looking for?

Ferdman · #2 08-29-2010, 04:08 AM

sptt, what's the condition of the distributor cap, distributor rotor and ignition wires? All the ignition system components need to be maintained for the engine to perform properly. Copper, non-resistor spark plugs are best for that engine. Check your Owner's Manual for the proper spark plug.

Check that the EHA valve isn't leaking gas on top of the engine. You will smell gas if it is. A hairline crack can develop in the black plastic EHA valve body, if so you need to replace it.

david s poole · #3 08-29-2010, 10:27 AM

those engines were infamous for needing new valve stem seals so look at those plugs you pulled out and see if they are just black or if there is build up on the centre electrode.just black means running rich so that will be another issue.

sptt · #4 08-29-2010, 01:21 PM

Well, here's what I did...the car wasn't starting well like it had before so I did the 1/4 turn on the EHA. It did not appear to have any cracks in it and no fuel was leaking. Then, after I did that, it started better but was bogging down on acceleration so I cranked it back 1/8th of a turn. I had to mess with the air mixture to get it to idle right again. Still wasn't idling great after that. I can't remember when I started to notice the wetness around the plug area towards the front (going from front to back it gets drier on the plug side) so I removed the plugs I had in there and there was blackness around the thread end of the plug but not on the elctrode ceramic piece. I replaced with Bosch +2 irridium plugs and idle improved drastically. Two days later is when the car died. I took the plugs out and EVERYTHING on them was black this time. So, I went back on bought 1.99 copper plugs non-resistor. Started up and drove it home. Seems to be running fine now but that was only for 50 miles.
Now I know the starting issue may be my fuel accumulator because if I pressurize the system by turning the key 4-5 times but not crank until the 5th, it starts and idles beautifully! However, could I have done something to cause the fuel wetness? It probably does need the valve seals but not sure about EHA based on no leaks where it connects to the distributor. I do smell fuel though so it is getting too much somehow.
Oh, and all ignition components are brand new, including wires which I just replaced. I also may have had the lambda adjusted too rich. Could that cause the issue?

Ferdman · #5 08-29-2010, 05:13 PM

sptt, a leaking EHA valve is difficult to detect because the under hood heat evaporates the small amount of leaking gas quickly. It's easier to see it if you remove the air cleaner housing and start the engine when it's cold (say first thing in the morning after the engine has not run for hours). If there is a hairline crack the outside of the EHA valve will get damp/wet. Don't expect to see a stream of gas spraying from the EHA valve.

sptt · #6 08-29-2010, 05:30 PM

Ferdman....ok. I will check it. Will that cause the engine on the spark plug side between the cover and bottom to get wet as well (cause gas and oil to leak out)? So far today, the thing is running and idling fantastic. I leaned up the Air Mixture last night if that means anything. Thanks for the help and check tips!

sptt · #7 08-29-2010, 10:00 PM

Here is a picture of where the leak is occuring that I can see. I pulled the new plug and it looks fine (it's actually made of a black metal casing vs. silver but nothing rubbed off like carbon deposits or anything).

Ferdman · #8 08-30-2010, 11:53 AM

sptt, you may need a new valve cover gasket. Typically the gasket leaks in the back (by the firewall) because the gasket gets misaligned when setting the valve cover in place. Clean the valve cover and inspect the rear gasket area with a mirror and a flashlight.

sptt · #9 08-30-2010, 10:56 PM

That's may be it. I did take the valve cover off a while ago to put the water temp sensor in and probably need to get a new one and clean everything off. Today, it just leaked a little bit after about 90 miles driving. That thing is running and idling so good right now that I want to put new struts in and paint it! LOL. Do these cars also have shocks? I looked up a parts search and they offer both. BTW...I bought some 1.99 copper plugs at Auto Zone and they engine acts like it's grateful...

Ferdman · #10 08-31-2010, 06:44 AM

sptt, your 300E has struts in the front and shock absorbers in the rear. Unless you prefer an extremely stiff ride recommend staying away from heavy duty stuts. Regular struts will improve the ride dramatically.

Exercise extreme caution during strut installation. Keep the spring compressed so it doesn't fly out and injure you, or use a spring compressor.

pawoSD · #11 08-31-2010, 07:39 AM

Replace the valve stem seals. I had the dealer do it for about $390 1.5 years ago on my 300E. Went from burning a quart every 400 miles to using less than 1/3 of a quart per 5000!

Spark plugs are perfect and clean on mine after 15k miles. They used to foul after only a few hundred.

sptt · #12 08-31-2010, 11:02 PM

Alright! Thanks again! Great to know on the struts and shocks! Thank you! I want the smoother ride so no heavy duty struts here. Also, I will work on getting those valve seals done. $390 isn't bad and I have a mechanic friend that may want to do it for free. I can tell this car handled very well newer just by how it rides now with worn down parts, Can't wait to feel the difference.