420's bad plugs - PeachParts Mercedes-Benz Forum

Dave Bunker · #1 11-12-2003, 11:16 PM

has anyone had any problems with the porcelain breaking off the spark plugs. I have discovered after 15,000 miles this has happened. As a result I lost compression in two cylingers and need to do a valve job. What's the chances my cylinders are damaged because of this? It will be the end of a fabulous 420sel if that is the case.......Any feedback will be greatly welcomed....

#2 11-12-2003, 11:35 PM

It could be carbon tracking so you need to replace both the plug and wire.

Also, we need to know what brand/model of plug being used.

Dave Bunker · #3 11-13-2003, 01:38 AM

plugs were autolite, unfortunately, unknow by me.....I should have checked them, car was running good, until problems popped up....

BobK · #4 11-13-2003, 06:53 AM

Never a good thing to have strange bits bouncing around in the cylinder. If you are deciding to go to expense of pulling head, maybe find a shop that has the scope to look into the cylinder and examine the walls/valves/pistons first. I replaced the plugs on my 560 recently and part of the tip insulator was MIA. Plug looked like the devil so I don't know if I had a wire failing or just spark going every which way with missing insulator area. Compression seemed ok on all cylinders, though not like new (345k miles). I believe detonation can cause damage to insulator.

Dave Bunker · #5 11-15-2003, 01:20 AM

Heads are off and cylinders look good. Heads go to the shop on monday for repair....new valves and rocker arms. Two of the cylinders exhaust valves were worn so bad they didnt close all the way....hence, a quart oil per week was being pushed through the cylingers......leaving lots of carbon build up but thank god thats all.......I'll keep updating the progress...

psfred · #6 11-15-2003, 11:04 AM

Broken porcelain insulators is usually a result of impact damage (unlikely unless you dropped something like a small nut in there) or detonation. With the valve guides as badly worn as they were, you were probably running over lean from air sucking down the guides, causing serious detonation on the highway. You won't hear this, unlike spark knock at slow speeds. Overheats the insulators, so they shatter when you suddenly open the throttle and dump a bunch of cold air on them! Check the unbroken ones for signs of blistering. If they are OK, it was a vac leak at the affected cylinders.

Check the fuel mixture when you get the heads back on,and check for vac leaks.

Peter

Dave Bunker · #7 11-15-2003, 11:25 AM

Peter, there was a noticable ping when departing from a stop. Not all the time but once and a while....what you said makes since. My only worry was cylinder damage....you know, rings and scoring....but looks good. If you think of anything else while the heads are in the machine shop, drop me an email.....and never use autolite.......

dave bunker

Dave Bunker · #8 11-15-2003, 11:27 AM

psfred, As I was typing from memory, I saw the name Peter in my mind....the mind is a funny thing.....I was out to late last night....sorry about that......

dave bunker

psfred · #9 11-15-2003, 08:00 PM

Dave:

Everyone else calls me Peter, except a pool playing buddy of mine who persists in calling me Jeff, for reasons I don't want to look into....

Autolite plugs aren't first choice for a Benz. Use Bosch or NGK. I've put Champions in for testing, but didn't leave them long. Platinums can cause problems unless you change the plug wires to solid wire non-resistor rather than the fine coiled wire resistor type the factory puts on.

Check your ignition timing, too -- early timing will cause the same problem.

Check the head bolts for evidence of thread pulling in the block -- those steel bolts tend to seize, and can remove the threads. Cured by steel inserts or Helicoils (I prefer the TimeSerts), but it requries a special jig to install them on the canted bolts.

Peter

Dave Bunker · #10 11-15-2003, 11:24 PM

Peter, head bolts are in good shape in cluding the thread holes. I look at all of the holes for that reason and examined the bolts as well. all are in great shape. Heads are at the machine shop as we speak. It will take a week for the work to be done. meanwhile I'm on my motorcycle rain or shine. Only have a couple miles to work so won't be so bad.....

Later,
Dave