M103 Misfiring/fouled plugs issue.... - PeachParts Mercedes-Benz Forum

pawoSD · #1 01-09-2009, 07:26 PM

Our 300E seems to be having an issue with fouling the plugs.....

I pulled the spark plugs a few days ago and the plugs from cylinders 1 3 and 6 were pretty coked/fouled up with carbon and slightly oily. #6 was by far the worst, the entire electrode/gap was completely coked over... I attributed this to it running rich up until I solved its idle control issues. (had it running mechanically with stuff unplugged for about 700 miles or so) So I cleaned them, re-gapped them and put them back in....ran great. Now its misfiring again, only a couple hundred miles later. On top of the misfire, when you turn the car off, an oily smoke/mist emits from the exhaust pipe. It smells like burnt oil/gas. I'm assuming its oily gas burning off in the exhaust from the misfiring cylinder(s)? It really doesn't use all that much oil....we've driven it nearly 1800 miles since I did an oil change on it with M1 0w40....and I've only added maybe 1.2 qts to keep it near the top. Should I use a different (thicker) grade oil to combat oil burning/leaking?

Tomorrow I am going to pull and inspect the plugs again as well as swap the distributor rotor and cap with new ones....the previous owner(idiot) ran resistor plugs in it, who knows what that did to the distributor...

Any ideas? It doesn't seem to have a head gasket problem....coolant is clean and the oil looks fine..no overheating or pressure in the hoses....so there's no mixing going on or anything, and no coolant is lost at all.

Engine only has 135k on it....

pawoSD · #2 01-09-2009, 09:50 PM

Anyone?

slk230red · #3 01-09-2009, 09:54 PM

Make sure your Vacuum Modulator isn't leaking. Mine did once and burned trans. fluid through the intake manifold coming from the vacuum hose that connects to the VM. When my trans. fluid was low, I checked the VM vacuum hose at the intake and I could see fluid in the vacuum line. Replaced the VM and problem solved.

Just a thought.

pawoSD · #4 01-09-2009, 09:55 PM

Quote:

Originally Posted by slk230red

Make sure your Vacuum Modulator isn't leaking. Mine did once and burned trans. fluid through the intake manifold coming from the vacuum hose that connects to the VM.

Just a thought.

I'll double check it, but I don't think its leaking.....trans shifts great and doesn't lose any fluid. Plus the line was disconnected when I got the car but was not oily inside....

babymog · #5 01-09-2009, 10:12 PM

Oh you diesel guys, just helpless with the common cars ;~)

Sounds like you might have a valve seal issue. Not terminal, but it will require removal of the cam cover and rocker assemblies, valve springs and retainers, pressurize the cylinder through the spark-plug holes to keep the valves closed, or TDC on the cylinder you're working on, not sure how it's done on the 103 but I'm betting someone here will.

Funny thing about gas engines, they have this thing called "vacuum". It is greatest when the engine is running closed-throttle, watch your mirror when exiting the freeway or just downshift when coming to a stop. Tell-tale sign is to run the car up in RPM in 2nd gear, let off and let the car slow back down a bit, then give it gas again. If you get a quick puff of blue each time you push the gas pedal trying this, it's valve seals. With a stickshift it is easy to see, automatics hide it a bit.

1st gear is too jerky, and 3rd is probably too fast to get a good puff in the wind.

Good luck.

<> Also, leaking valve stem seals often cause a blue puff when starting the engine.

pawoSD · #6 01-09-2009, 10:31 PM

Quote:

Originally Posted by babymog

Oh you diesel guys, just helpless with the common cars ;~)

Sounds like you might have a valve seal issue. Not terminal, but it will require removal of the cam cover and rocker assemblies, valve springs and retainers, pressurize the cylinder through the spark-plug holes to keep the valves closed, or TDC on the cylinder you're working on, not sure how it's done on the 103 but I'm betting someone here will.

Funny thing about gas engines, they have this thing called "vacuum". It is greatest when the engine is running closed-throttle, watch your mirror when exiting the freeway or just downshift when coming to a stop. Tell-tale sign is to run the car up in RPM in 2nd gear, let off and let the car slow back down a bit, then give it gas again. If you get a quick puff of blue each time you push the gas pedal trying this, it's valve seals. With a stickshift it is easy to see, automatics hide it a bit.

1st gear is too jerky, and 3rd is probably too fast to get a good puff in the wind.

Good luck.

<> Also, leaking valve stem seals often cause a blue puff when starting the engine.

It doesn't put out any blue as far as I've been able to tell....if you drive gentle for a while then "punch" it, it actually lets out a slight stream of black smoke, then it clears up. Oil consumption seems to be too low to be valve seals....less than a qt per 1000 miles.

Hey now, I am gaining ever more love for diesels because of this car!

If this thing keeps being a pain to maintain (funny how that rhymes

) I'll be looking for another diesel!

babymog · #7 01-09-2009, 11:01 PM

Leaky valve seals don't really consume much oil. The probem is that when they leak the most is at high-vacuum, which is when the throttle is closed and there's not much other than oil going in the cylinders. The other time is when the engine is off and the oil runs down the valve, starting with a good shot of oil on the valve to hit the combustion chamber at once. Little oil but at bad times for the plugs. The other way to tell is to look in the intake at the intake valves which of course means removing the intake if you don't have a scope.

It's either than or someone's been giving it a steady diet of Slick-50 and the rings are now gummed up/stuck, ... your call. Have a compression tester?

Quote:

Originally Posted by pawoSD

It doesn't put out any blue as far as I've been able to tell....if you drive gentle for a while then "punch" it, it actually lets out a slight stream of black smoke, then it clears up. Oil consumption seems to be too low to be valve seals....less than a qt per 1000 miles.

Hey now, I am gaining ever more love for diesels because of this car!

If this thing keeps being a pain to maintain (funny how that rhymes

) I'll be looking for another diesel!

pawoSD · #8 01-09-2009, 11:18 PM

I suppose I could pick up a compression tester.....what should the values be for a healthy M103?

I am probably in denial....perhaps it does need a valve job.

Is it hard to do? It doesn't sound like something I'd be comfortable tackling....

Can I hire you to do the seals?

ps2cho · #9 01-09-2009, 10:15 PM

Valve Stem Seals.

Look like this?

pawoSD · #10 01-09-2009, 10:34 PM

Quote:

Originally Posted by ps2cho

Valve Stem Seals.

Look like this?

Actually not. They were black and sooty. With hardened black carbon deposits all over them. Didn't really look like that at all.

They looked a lot like this, just not as wet:

Tomorrow I am also going to check my spark plug wires....if tested with an ohm-meter, what should their resistance be if they are "to spec"?

babymog · #11 01-09-2009, 11:48 PM

I checked with the Department of Licensing and Regulation, ... I'm only licensed to work on my own cars ;~)

Besides, you'd have to haul away the rest of my 126 junk so there'd be room in the shop.

Not valves, just seals. Head and manifolds stay on for seals, usually in/out in a day but I can't say on the M103, have never done M-B valve seals (never owned an older gas one 'till now).

pawoSD · #12 01-09-2009, 11:58 PM

I'll do some further testing to see if its for sure the valve seals....I'll do the second gear accelerate and coast thing tomorrow to see if I get blue exhaust....

Tomorrow I am going to pull the plugs again to have a look and also I'm replacing the distributor cap/rotor/plastic shield....and I will test the wires for proper resistance.

If it does need valve seals then it may be headed to Betten!

babymog · #13 01-10-2009, 12:10 AM

Say Hello to the shop foreman, ... he's the guy with the 400E.

pawoSD · #14 01-10-2009, 12:30 AM

Quote:

Originally Posted by babymog

Say Hello to the shop foreman, ... he's the guy with the 400E.

Yeah, I think he's the one who did the inspection on it when I brought it in last week. Is his name Don? He seems to always be the guy that works on the older cars.....

Do you know by chance what the ohm reading should be on the wires for a M103?

And....what is the "shop time" estimate for valve seals. (and should I have the guides done too if I end up doing that) Any idea?

mak · #15 01-10-2009, 10:54 PM

Do you know by chance what the ohm reading should be on the wires for a M103?

the ignition wires are normally solid . the problem is at the plug connectors over a period of time the rubbber inside becomes conductive due to oil/water residues .The ignition will then develops a light miss and hesitation on acceleration.
when you remove the plugs look for a corona ring on the porcelin portion of the plug indicating a leaking plug connector. resistor type plugs compound the problem. use non resistor bosch HD 9c plugs

mak
300se 103