Spark Plugs: 3 Clean - 3 Jet Black in M129 - PeachParts Mercedes-Benz Forum

Shab · #1 11-12-2006, 10:39 AM

Ok. M129 in my 67 W111. Plugs 1,2 &3 clean as a whistle. Plugs 3,4 &5 black as coal. All are bone dry. With brand new NGK’s, the 3 rear plugs foul immediately. A respected vintage MB mechanic suggests leaking intake manifold, having only been appraised of the issue by me over the phone. Car also suffers a distinct lack of power. Haven’t done a compression test yet.

Picture of the plugs here: http://advanced-events.com/Plugs/plugs.jpg http://advanced-events.com/Plugs/plugs2.jpg

Any suggestions?

Thanks in advance.

250 Coupe · #2 11-12-2006, 01:02 PM

I would tend to discount the intake as it looks like the runners are paired 1&2, 3&4, 5&6 and not by "threes's".

A compression test might show a blown head gasket but I don't think that would lead to black plugs.

I know nothing about your fuel injection setup so I won't go there. Altho, having said that, I wonder if a leaking cold start injector, if there is one and it were mounted toward the rear, would be a suspect?

I don't think it's the cam as that would cause a problem across the board.

My first thought is mis-routed plug wires.

Michael

Arthur Dalton · #3 11-12-2006, 01:39 PM

First check wold be CSV.. take the Hex plug test port cap off at CSV and see if it is leaking when key is ON . Should not .

Next suspect is oil level in Injection Pump..check level with dip stick at rear of pump. Also see if it has gas diluted oil.

Shab · #4 11-12-2006, 02:25 PM

Would fuel weeping out at the air intake (right up front where the hose connects the intake to the air cleaner canister) suggest the CSV as well?

Also, I can't for the life of me find the dipstick on the MFI pump. It does sound like it needs lubrication as well.

Any photos of the suspects would be greatly appreciated.

Arthur Dalton · #5 11-12-2006, 03:11 PM

Dip Stick is a T handle screw head . It is at the very rear/lower section of the pumps rear casing,.... rear meaning towards firewall [ next to where the idle mixture knob is] .
The test port hex for CSV is 7mm hex and it is right on the side of the CSV..
..very simple. 30 sec test............

Shab · #6 11-12-2006, 07:03 PM

Thanks Arthur.

Did the CSV test. Removed the 7mm hex, turned the key to "ON", dash lit up, fuel pump whirring away, no fuel coming out of the port.

Shab · #7 11-12-2006, 07:14 PM

Ok, found some good MFI pics and now can see where the dipstick is. There is so much mechanical clutter around my pump it was impossible to see. I'll go see if I can give it a pull.

Arthur Dalton · #8 11-12-2006, 07:40 PM

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It does not pull..it is threaded in . That is why it has a T handle..so you can unscrew it.
It has to be threaded back in to read correct level.

Shab · #9 11-12-2006, 07:51 PM

Thanks Arthur, I figured as much re the "T" handle. Couldn't budge the little bugger. I'll have another go tomorrow.

Thanks for your patience and guidence.

Thanks to Micheal also. It's not out of the realm of possibility that the ingnition wires are slightly bolluxed up from the P.O.

300SDog · #10 11-15-2006, 02:58 AM

Carbon fouled plugs? I'd be thinkin secondary ignition components, wire ends, radio noise suppressors first. Noise suppressors are interchangable all the way up to w123's, 116's and probly beyond. And have you run a feeler guage to check valve clearances? Like if exhaust valves are tight or with zero clearance, then the plugs might carbon foul from incomplete combustion.

But yeah can be fuel air mix too. And if you suspect injector tips then rotating em to front 1,2,3 cylinders will bring the problem with them. And if you pull the injectors gotta replace the seals.

Good news is they look dry carbon fouled instead of wet oil fouled - so valve seals and guides oughta be tight.

(edit: And what kind of choke cold-start system does the 250SE Coupe have? My experience with the inline 6 cyl is that irregularities, compression and fuel/air mix variations most often occur in the last couple cylinders 5 and 6, also needing the most frequent valve adjust - i dunno why.

Shab · #11 12-10-2006, 12:24 PM

Ok, did a compression test this morning. 1967 M129 250SE W111. Do these results look encouraging?
1- 150
2- 145
3- 155
4- 150
5- 145 +
6- 145 +

Plugs 1,2 & 3 again clean as a whistle, 4,5 & 6 dry fouled simply black as coal.

Per Arthur I am next going to check the oil level in the MFI pump. I can see that the line feeding #5 cyl is leaking fuel at the pump. I thinking valve adjustment as well?

Also FYI the engine does not smoke at all, no blue, no white, nothing. Revs eagerly standing still, can barley get out of it's own way when put in gear. PO said the car sat for about a year before he sold it. Got moved around the yard on it's own power as often as weekly but that's about it. No real driving.

FYI for Dog, new cap, rotor, OEM plug wires, NGK plugs, condenser. Voltage is right all around the engine bay, did Arthur's CSV test as well with good result.

*****Following up....MFI pump had NO oil. Took about 1/2 quart (painstakingly added a little at a time) before I got a reading. Certainly sounds better! Nice and quiet and ticking along nicely.********

Couldn't help noticing taking the vacume line on and off the distributor makes zero difference in idle. Absolutely nothing, no advance, no retard. Not sure where to go next. Valve adjustment? Fuel pressure?

300SDog · #12 12-10-2006, 08:58 PM

Quote:

MFI pump had NO oil. Took about 1/2 quart (painstakingly added a little at a time) before I got a reading. Certainly sounds better! Nice and quiet and ticking along nicely.

Dont be fooled by smooth idle, says me. Jeeze running the IP bone dry of oil would scare me to death!! Something about the bearings in 1960s bosch pumps made em a weak link - learned this mostly from diesel lore where the typical IP on 200Dc wont last more then a piddling 180k miles or so, assuming the oil gets changed at half assed intervals and never runs dry. Hate to say it, but you might want to get a recent bosch rebuild anyhow now as matter of course. Probly a bargain considering california condition of the car.

And that was Art Dalton's first guess after the CSV. Right now if it was me I'd start from scratch with fresh plugs and monitor IP oil level too. And test drive the car to know it better, like run at least 1/4 tank of fuel through it under guarded highway conditions like a loop to the fishing grounds. Then see if by some *miracle* the next batch of 4,5,6 plugs dont foul and the pump can hold onto its oil, yep thats what I'd do. Otherwise i think you gotta assume the IP aint out of the woods.

Meanwhile those 145/50/55 compression figures sound darned good, at least to me. Again pointing to low mileage as actual. Most of the 111's I've owned have ranged at 120-135, even as low as 115-25 if memory serves me well. Thats the M180 engine after cylinder head work at/above the first 150,000 miles. Brand new and not even broken in, i believe 175lbs is about par for the course.

Arthur Dalton · #13 12-10-2006, 09:40 PM

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That system does not have vac advance.. it uses vac retard at idle.
As soon as you take the hose off , the rpms should increase. If not, then pull cap on dist and see if the plate the points are mounted on can be turned by hand. If that is free , you may have a bad vac diaphragm.

Shab · #14 12-10-2006, 10:13 PM

Thanks for the input fellas. Fortunately, I haven't run the engine much on the dry pump, and certainly haven't had it out on the highway yet, just poodled around the block a few times, but I have a feeling it's shot. I'll get her on the highway this week and see what happens. Also going to run a can of BG 44K through the fuel system which worked wonders on my 108 M130. Might have go at the valves as well and see about that dist diaphragm.

All suggestions welcomed.

luvthe111 · #15 12-14-2006, 02:46 AM

did you try switching the injectors yet? I have the same problem plug fouling problem with two of my cylinders on my M130 powered 1970 cab. I have 155 +/- 6% compression, new plugs and points. It idles a bit rough when in gear, but runs great once I'm on the road.

My plan is to take the injectors out this winter and take them to a diesel shop to get them tested. Or I may just order a couple of new injectors and seals and hope that fixes the problem.

I had a similar problem with my early Porsche 911 w/MFI. It had horrible idle, one consistently fouled plug, and the smell of unburned hydrocarbons when taking off from idle. Turns out the one injector was tired and was just letting fuel rush in to the one cylinder, thus being overly rich at low speeds.