high speed missfire

[davids]

anyhelp had a high speed miss, changed plugs went away for one thousand miles now its back .running bosch wr7dc plugs

[Shvegel]

Try W8DC(one step hotter and no resistor) Also check resistance of all plug wires.

[Raymond]

I have spoken to 4 different parts retailers who tell me that Bosch has stopped making non-resistor plugs.  So if you find Bosch W7, 8 or 9 DC plugs, buy them all.  Failing that, NGK BP6ES are non-resistor with the same heat range as the Bosch W8DC. 

And, check the plug wires for age.

[Benz Dr.]

Any aplication book will tell you Bosch W7DC and I will tell you that they pretty much won't work right. They're too cold in all but engines with perfect compression and even then I still didn't use them. I say didn't because I can't get them any more. NGK BP5ES is what you really want and they change over to W9DC which what I was using.
Hotter pugs heat up faster and won't foul out as quickly. If you still have fouling problems lok for oil deposits on the plugs. Failing that, check your

[ja17]

Go to NGK BP6ES spark plugs and your miss shouldl go away for good.

If your plugs look very white or very light when removed, your engine may be running  borderline lean. Lean running can cause the Bosch Plugs to short out internally.  NGKs can handle the extra heat from lean running better.

[Shvegel]

I used to teach my student to be wary of three things. Champion resistor plugs, K and N air filters and Fram oil filters. I would spend about an hour on each taking them apart and analyzing why they were potentially trouble. With the champion plugs the connections internal to the plug are not good. The center electrode is inserted, a resistor "pill" is dropped in and the nut is pressed in after. Over time the "connections" between the resistor and electrodes begin to deteriorate and the resistance and firing voltages climb increasing the chance for a missfire. Joe's clue is interesting. Lean burning will cause higher firing voltages and stress the connections in the spark plugs. I wonder if Bosch has the same problems.

[m300cab]

I had this problem and fixed it by installing pertronix , instead of points

[thelews]

I used to teach my student to be wary of three things. Champion resistor plugs, K and N air filters and Fram oil filters. I would spend about an hour on each taking them apart and analyzing why they were potentially trouble. With the champion plugs the connections internal to the plug are not good. The center electrode is inserted, a resistor "pill" is dropped in and the nut is pressed in after. Over time the "connections" between the resistor and electrodes begin to deteriorate and the resistance and firing voltages climb increasing the chance for a missfire. Joe's clue is interesting. Lean burning will cause higher firing voltages and stress the connections in the spark plugs. I wonder if Bosch has the same problems.

What's the problem with the other two?

[philmas]

I had high speed misfiring issues with my former Bosch W7DC plugs.Replaced them with NGK BP6ES, which not only solved the problem, but also improved mysterious troubles I had for ages, like rough low-rpm running,hesitation in slow traffic when hot...

You have  to remember however that NGK BP5 ES is HOTTER than BP6ES, whereas  Bosch W7DC is COLDER than W8DC.

I also noticed that according to NGK's charts,  BP6ES is the right plug for W108, as BP7ES is for W113.

[J. Huber]

I've used the BP5's for years...many many happy miles.

[Benz Dr.]

The W7DC were too cold for modern fuels. The BP6ES crosses over to the same plug. The BP5ES is roughly the same as a W9DC. I found no high speed missing problems using these plugs and I used them for everything I worked on. If the engine is running even a bit too rich, which MB engines are prone to do, they will come out black upon inspection. They're hotter, but hot that much hotter.

I've known that Champion plugs were crap years ago - cheap junk. Fram oil filters are probably the poorest choice on the market. I have no experience with K&N filters - some people swear by them and others swear at them.

[J. Huber]

Somewhere here is a thread about the K&N Filter -- and how I switched from Mann to said K&N... I swore it improved things -- still in there couple years later.

I looked at it just the other day, in fact. Surprisingly clean.

[davids]

put in bp6es see what happens thanks

[Shvegel]

Jhuber,
Your K and N is clean because the bigger holes are allowing the dirt to go into the engine.

The only way to get less restriction is to have more open area(larger holes in this case). K and N claims that because the media is a labyrinth and the oil is sticky the particles will bounce off the oil and get stuck. Wrap some tape around your hand sticky side out and slap a dusty surface a few tiimes and you will get my point.

As for Fram. Cut one open sometime. Maybe a third the surface area of a Mann or other good European filter and the ends of the cartridge are cardboard glued to the filter paper. They usually omit the check valve that keeps the oil from draining out of your engine when you shut it off as well( not applicable to our cartridge filters).

[gimp]

I had that problem years ago and found that the plug gap was set at .o28 when it should have been set at .026

[ja17]

Hello gimp,

Most likely if your engine is that sensitive( .002" to difference in gap), something else is borderline. I would take an ohm meter and check your ignition wires and ends. Don't forget to check the coil wire. A closer gao means less resistance, but a cooler spark. As the gap increases, your ignition must be strong enough to overcome the resistance of a wider gap.

A lot of techs use up to .035". Myself, I have always used .028".  In the long run, it is what works best for your engine. Ignition coils, ballast resistors, spark plug makes, gaps and heat range, ignition wires, and resistor ends all make a difference in the ignition systems. Many of these items have been altered or changed over the years in many of these engines.

[graphic66]

I bought the Velleman K2543 Electronic ignition kit for my 230SL. It really made a big difference, My points were pretty bad, put this in and the car ran much better. I then put in new points and it ran about the same. It uses the points to trigger the coil, much lower voltage through your points, they last about 3 times as long and you can convert back in a few minutes to your stock system. They are really inexpensive, just found some for $13.50 plus shipping. You must be able to build a very simplecirquit board, soldering skills. It is very easy and fun. I have had mine in about 3 years now, no problems and way less maintanance. It also eliminates the condensor. I left mine in place, if I ever have a problem with the Velleman I can hook it back up to stock very quickly. Here's the link http://www.hobbytron.com/vk2543.html   Check it out.

[Benz Dr.]

Joe.
I've been using the stock .032'' forever and it seems to work well. A customer I had once used .024 and the engine missed badly. Thinking it was the FIP he pulled that and had it tested but it was working perfectly. Finally he called me because he had run out of ideas. I asked what plugs and gaps he was using and he told me. I suggested opening them up and the engine ran perfectly after that.
I've seen .024 in some manuals so it's very possible for someone to believe this and use that setting. Proof you shouldn't believe everything you read and there's plenty of examples of that around here

 In fact, some people take that good advice one step further and don't believe anything I tell them.

[66andBlue]

...   I've seen .024 in some manuals so it's very possible for someone to believe this and use that setting. Proof you shouldn't believe everything you read and there's plenty of examples of that around here   ...

Dan,
the problem is that it is not just "some manual" but it is the Owner's manual that tells you to replace the plugs when the gap has grown from 0.5mm = 0.0196" to 0.8mm = 0.0314"   (see photo below).
Could it be that the plugs made in the 1960s performed better with a narrower gap? Or difference in gasoline quality or composition (leaded) between then and now that favors the wider gap?
For sure your favorite plugs (W9DC) are pre-gapped at 0.9 mm = 0.035", see:
http://www.sl113.org/forums/index.php?action=dlattach;topic=11232.0;attach=3110;image

[Benz Dr.]

Well Alfred...... I was joking back there.

  Today's unleaded fuels run hotter than the older leaded fuels. The plugs will last far longer than they used to and cleaning or gapping is a thing of the past. If your engine isn't burning oil you could easily go 20,000K without having a look. I'm not recommending that but as long as your engine runs well you could do it.
Modern oils play a larger roll in this than is probably considered. However, I still use a 15 W 40 or synthetics in my engine to reduce valve train wear.

 Regardless of what plugs you use or what gapping makes your engine work properly, you still need a matched system. The plug wires have to match the spark plug ends,  which have to match the spark plugs. I concentrate on getting the lowest impedance system I can run because the lower voltage systems used on these cars only work best when all parts are matched.