Why is #3 Plug fouling?

[TheEngineer]

I found that plug in #3 indicates excessive oil in that cylinder: It is black with an oily residue as compared to plugs in other cylinders which are a nice white/grey colour. I thought that it may be the valve stem seal on the intake valve, but I just replaced it and the plug looks black like before. The compression is good: About 195 psi and there is very little leakage around the rings (I had to put 80 psi airpressure into that cylinder to keep the valve up) So why would #3 foul? My oil consumption is about 1 qt every 500 miles. I'm using Quaker State SAE HD-30. Help please!

[ja17]

Hello,
Oil burning will usually cause a crusty deposit after a time. Black sooty wet plugs are usually  fuel fouled, or coolant fouled.

However if this has happenend soudenly you could have a guide loose in the head or a seal out of place. These conditions may not effect compression for a time. They will cause oil smoke.
Try replacing that plug first and chedk it after a short test drive.

If this doesen't tell us anything, next remove the valve cover and take a flashlight and see if a valve seal is out of place.

Is the car idling smoothly? If you have a bad ignition wire, bad injector, you will have and uneven idle and you will fuel foul the spark plug.

Coolant on a spark plug can also leave it black and oily looking.

How about a little history?



Joe Alexander
Blacklick, Ohio

[TheEngineer]

A little history as requested: #3 has had that problem since I had the vehicle, about 5,000 miles ago. But I’ve had so many other oil leaks that #3 had to wait. It is a crusty, oily deposit all right. When I had the old seal out, I did not check for a loose valve guide. This check would be to try to drive the valveguide out using soft hammer blows. I’d have to have the head on a bench. I wonder, if I should have changed the seal on the exhaust valve also; but I figured it wouldn’t cause a fouled plug. The engine idles just fine at about 700 rpm but it smokes blue when I accelerate after the vehicle pushes the engine, like slowing down from 70 to 30. That’s when there is max. vacuum and I can see oil in the breather pipe. The puzzling thing is, that only #3 is shows a black, oily plug. There is evidence that the engine was worked on: It has a nice coat of black paint over red primer/sealer, but I also found studs not sealed, no sealing washer under the fwd RH Allen head bolt of the cylinder head, where that threaded hole is open on the bottom and some other indications that whoever worked on that engine was not the most experienced Mercedes engine mechanic. I also found the valve timing a little off because someone took the head down to 84 mm and didn’t install an offset key to adjust for it. All these things I corrected, but I have not mastered oily #3. I was so certain, it must have been the valve seal. It is easy to damage them when pushing over the valve stem without using the protective sleeve. So I installed a new one and expected an end to this oiled plug. I was wrong. I’m running Bosch Platinum plugs and Pertonix ignition in the 009 alu distributor, which I reworked to give me about 34º advance at 3,000 RPM but the fouling of #3 was with me when I ran the 051 distributor using points at factory setting. I also switched injectors between cylinders, just for giggles.

[graphic66]

Your other plugs running white grey would ussualy indicate a lean run condition, not helping your problem but maybee some future ones.  What plug are you running, resistor or regular. and what heat range?

[ja17]

Oil can get on your plugs from one of two places. The intake guide or through the piston rings. Oil passing through a exhaust guide will drop oil into the exhaust port of the cylinder head and out the exhaust without entering the cylinder. If you have an intake loose it is usually easy to spot expecially if the head was off.  

How many miles on the engine. Also Bosch platinum plugs may not handle oi as well as regular Bosch supper plugs (cooper core) or NGK.

You may have to unhook the spring on #3 intske valve and grip the guide to see if it is loose in the head. Take a good look at the exhaust guide, but as mentioned this is most likely not possible to be the culprit.



Joe Alexander
Blacklick, Ohio

[TheEngineer]

Well Joe, the problem solved itself: Yesterday, after I returned from a local car show (and there were ten Pagodas)I took #3 plug out to look at it: It still isn't good. When I put it back in, the tortured thread stripped out. (It had been damaged for a long time) I'll remove the head and inspect things. What do you prefer: HeliCoil or insert with flange?

[rwmastel]

quote:Originally posted by theengineer

....I returned from a local car show (and there were ten Pagodas)....

And you told all of the owners about our group?

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[TheEngineer]

Thank you Joe, for the instructions how to install an insert without removing the cylinder head. I did see that kit yesterday when I visited my friendly Mercedes dealer. I removed the intake manifold and looked at the valve guides. #3 has a ring of oil hanging at the bottom of the valve guide, around the va;ve stem. It is very noticable. The area is surprisingly clean: No carbon, just clean metal surfaces. Obviously, I didn't do a good job replacing the seal, or there is too much play. I found other things too: The throttle lingkage under the exhaust manifold is damaged and must be replaced and some fasteners are the wrong kind. Its a big job because the air conditioner bracket ties the block and the head together. Last time I installed inserts was just about 50 years ago on a Jaguar I owned. This Mercedes seems to be much more work. Thanks for your help.

[ja17]

Hello,

Install spark plugs with a section of rubber fuel hose. This makes installation easy and also will keep from cross threading the threads in the block. After the spark plugs are wound in with the hose final torquing can be done with a hard tool.

Allowing the spark plugs to remain in place many years can cause them to become stuck. Eventual removal may cause thread damage.

If you have a head with damaged or stripped spark plug threads you can repair them without head removal.
I use the kit which has the tool with the cutter and threader in one. I will
email a picture. It cuts the opening and then threads it. I
pack it with grease so the shavings stick to the grease. Clean and change
the grease once or twice.  Most of the shavings stick and get caught up in
the grease.  Afterward you can crank the engine without the plug in and blow
the shavings out of the cylinder. The shavings are soft aluminum and will
not score the hard cast iron cylinders. I have done this many times with no
problems. The most difficult thing is to start the new threads at the
correct angle, pay close attention to the angle of the other spark plugs.

Clean the new threads with some solvent afterwards so no grease remains. I
like to use the hat shaped thread inserts so I can use locktite to bond the
insert in place. Let everything set overnight before removing its
spark plug. Be sure to use an insert long enough!

Joe


Joe Alexander
Blacklick, Ohio

[TheEngineer]

Took the head off: It was "surfaced" down to 82.9 mm at one corner. It bottomed out on the water pump housing. Why do machine shops surface heads wether it needs it or not? And why does someone install 6-32 screws in an M3 thread? I'm going to sell this head on eBay. Description will be: Often surfaced without care. Found another head: This one has been surfaced too by a local machine shop: He was given the head just to grind the valves. No new guides, they were still within tolerance. So he surfaced the head anyway. Now the bottom is flat and the top is curved. The cam shaft binds. If he would have left it alone, the head was only warped .23 m/m It flexes that much when I bolt it down and I would not have to scrape-in the cam shaft towers. At least it has good spark plug threads. Because it's higher, I probably have to shim up the hood. Oh - they also painted it. Oh God: Please save us from shops who don't know what they are doing, can't be talked out of surfacing, and paint it to make it look good.

'69 280SL,Signal Red,007537,tired engineer, West-Seattle, WA

[mdsalemi]

Well Pete, this is why I swear by places like Noel's and Metric.  When all you do is work on MB engines, you are bound to be "more right" then a local machine shop.  You pay for expertise.

Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored

[Benz Dr.]

If the head has ben cut down that much ( below minimum spec ) my bet is that the valves sit a lot deeper in the head than they should. For some reason, when you grind the valve seats that much it changes the valve rocker geometry. Instead of the valve being pushed straight down it now gts pushed sideways and oil flows down the slight crack on one side of the valve stem seal.
Even though the vlave guides and seals are new the engine can still use oil and smoke a bit. I tend towards using new intake valves as stem and valve seat wear can really make this condition worse. Having seen this on worn out heads with heavily ground seats several times I'm pretty sure there's something to this.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[TheEngineer]

Thank you Dan - Dr. Benz - for your help and comments. I came to the same conclusion and I was pretty disgusted that anyone would cut down a head so much. One corner is actually at 82.9 mm! Fortunately, I found a head which had been molested only once, when in January of this year a local machine shop ground the valves. They did not replace the guides, claiming it was not needed. I borrowed the go - no go gages and checked the valve guides: Indeed they are within tolerance. I did the leak test on the valves using ATF like <ja 17> suggested and they are acceptable. They surfaced the head though: It is now 84.77 mm at the lowest corner and 84.97 in the middle. Naturally the camshaft binds. I inspected for that after you mentioned it. I have not corrected that situation yet, so far I have used die maker's blue to see where it binds. I am contemplating removing about 0.15 mm from the bottom of the two center pedestals using my Bridgeport. The car has airconditioning and the bracket spans the block and the cylinder head. It does not align just right anymore and I found that it was installed improperly. It just takes time. Thank you!

[TheEngineer]

And here is the rest of the story: I removed .005 inches from the base of the two center cam shaft supports. With that, the cam shaft rotated without binding: I could turn it using two fingers (well, one finger and my thumb) but as soon as I installed it, using the old head gasket, it was binding again. Without the headgasket, it rotated freely. So I measured the old headgasket: It was .005 thicker in the middle. It appears, that when the head is torqued down, it compresses the headgasket by .005 at the ends. I verified that with a dial indicator and by inserting a .002 in. feelergage in the cam shaft bearings. It helps if I torque the center six head bolts to 90 ftlbs. The engine runs fine now, but at first, I got insufficient fuel and had to shim the barometric pot of the injection pump by .01 inches. #3 plug still shows wetness and it looks like fuel. Is it possible, the injection pump delivers excess fuel just to one cylinder? I have switched injectors and the wetness does not follow the injector. Maybe I have to make a fixture to drive the injection pump on the bench and measure fuel delivery of the individual ports. I had the pump out to verify proper timing. Compression in #1,#2,#3 and #4 cylinders is nice and equal at about 185 PSI, but #5 and #6 are still low at 130 PSI and the leakage is all past the rings. It appears that the block was rebored to first oversize: 87 m/m. I measured 3.4262 at the top and 3.4245 at the center of hole #6. That translates to about 0,04 m/m of wear. The records I received show that the engine had low compression in #5 and #6 back in 1991. Looks like those rings never seated. I removed the glazing in those two cylinders using Ajax and 220 grit sandpaper, but it didn't help. The plugs look fine in both cylinders. The car runs fine and does not leak a single drop of oil onto the garage floor. I'd accept a few drops after running it hard and letting it sit for a couple of days: But it's almost spooky: No leaks at all. The rear axle leaked a little past the drain plug, like one drop every four days. I cleaned the area around the plug using a toothbrush and soap. The I smeared silicone sealant there. I didn't want to drain all the oil, just to seal the plug. And that did it. So now what? Do I want to pull the engine and rebore? Do I want to send the pump to Hans? My shoulder hurts and I crawled around on the floor in positions I would never assume voluntarily. Today we'll drive the car to Group Health because Hilda has an appointment with the cardiologist. I have to help her out of the car, because it is so low.When I was young, I used to make jokes about these old folks who drove a sports car: Now It's me.