How to replace Coils in early W140 - PeachParts Mercedes-Benz Forum

kustom81 · #1 10-18-2006, 10:23 AM

I have a 93' 500SEL with 140K miles on it. It has developed a bad misfire on a few cylinders, and has no power from the starting line. It takes forever to get up to speed, but runs Ok once there. It has been suggested by my mechanic (He knows I am a DIYer) and people on this forum to start with a tune up and maybe the 2 coils behind the drivers headlight. I replaced the D-caps, rotors and plugs. The plug wires looked Ok, and who wants to pay $250 for a set of wires. The caps had the green oxidation, and the plugs weren't terrible. This provided no change. Next is the coils. Is there an easy way to check them. I can't even seem to get the covers off. Is there a trick to it? I have weakly tried to pull up on them, but don't want to force anything. Also, is there an easy procedure in taking them out. The one in back seems to be right behind the headlight. I can't even get my hand back there. Do I have to take out the headlight? I am thinking of having my indy do it.

Thanks in advance

HarryM · #2 10-18-2006, 11:10 AM

Coils are unlikely to be the cause of your problem. Remove the air filter box, start the engine in total darkness and look for tiny sparks around ignition wires. Check the engine wiring harness and oxygen sensor too.

timroperco · #3 10-18-2006, 01:01 PM

My car is different from yours (1994 E320, W124) but I recently had similar problems at about 110,000 miles. I had an occasional misfire while idling. The car accelerated okay when gentle, but stumbled if I stepped on the pedal hard. Ran smoothly once at speed.

My local parts store didn't have Bosch plugs on hand, so I replaced the plugs (correct gap and torq) with Autolite plugs and that seemed to clear up the problem for about 2-3 months; then it reoccured. I then read in the DIY forum to how important the correct plug for my year/engine was, so I ordered MB-OEM plugs (I think F8 DC4 but don't hold me to that) and saw no improvement. Sigh. I noted this time one of my plug wire caps was rather worn and corroded. So I ordered a new plug set (about $100 I think) and everything has been hunky-dory ever since; I've had no need to replace coils. From what I've learned, these cars need a major tune at 100M+ miles so even though your plugs are new and wires may seem fair, it seems you're likely due for new wires -- especially if they are original. It was a fairly easy job and only took an hour or so. If the problem persists after new plugs and new plug wires are installed, then the coils would be the next logical step.

kustom81 · #4 10-18-2006, 01:01 PM

Thanks HarryM, I will try that. My engine harness looks pretty good, at first glance.

kustom81 · #5 10-18-2006, 01:13 PM

Thanks Tim, the wires on that thing aren't cheap, but I will replace them. I usually don't skimp on that stuff, but when you have to buy 2 caps, 2 rotors, etc. it got out of hand. I did put the Bosch OEM plugs in though. I don't understand, this thing was running great with no hiccups, and than bang. I thought you get a little misfire every once in awhile when a tuneup is needed. I got off the highway one time and when I pressed on the gas to take off at the next light, it barely crawled away. So I am not convinced it is a tuneup problem. The week before it happened, the big plastic cover under the front bumper finally rotted in some places and was dragging. I took it off. This exposes the under side of the engine in the front. I went to a car wash that day. The car wash sprayed the tires, etc. I wonder if something got wet.

Thanks

sixto · #6 10-18-2006, 01:23 PM

When my 95 S420 (sold) misfired under hard acceleration it was bad plug wires.

Replacing the coil plug wires is difficult enough. Replacing the coils is downright impossible!

I noticed that one coil wire was touching a bolt just below the driver side distributor. Routing the wire properly made an improvement but the problem didn't clear completely until the wire set was replaced.

I'm not sure watching plug wires in the dark is conclusive. M119 plug wires are routed in tight channels. Cross-firing might not show in the dark. Still worth doing, IMO.

Sometimes these engine react adversely to platinum plugs. Check that you have the correct copper plugs. Bosch Super F8DC4, I believe.

Sixto
93 300SD

timroperco · #7 10-18-2006, 01:27 PM

Well, that's a whole new kettle of fish. As described, your problem reminds me of a similar lack of performance in my old 1954 Packard when the exhaust manifold valve broke and got stuck closed -- and when warmed up, the car started acting as snarly and sluggish as if someone had shoved a banana up the tail pipe, hardly any power at all. Perhaps you do have a "breathing problem" of sorts via the air intake or some other calamity. I suppose you could have the coils checked for output before replacing them for it sounds like you're problem could be elsewhere.

kustom81 · #8 10-18-2006, 02:49 PM

Thanks guys, I was also reading about a clogged catalytic converter. But I thought that triggers a CEL on. I have no dash warning lights on at all. I got a strong smell of gas too.

Thanks

kustom81 · #9 10-30-2006, 10:39 AM

Turns out, it was the coil. It was the one tucked in behind the drivers side headlight. Had to take the headlight out to get to it. PITA!! But, the car runs like brand new.

Thanks for all of the help!