How Often to Replace Cap & Rotor 300E? - PeachParts Mercedes-Benz Forum

gmercoleza · #1 01-24-2006, 10:54 AM

I did a search and found nothing - maybe I'm using the wrong words or am just blind. What is the recommended replacement interval? Mine have about 28K on them and the idle is starting to get rough, also have to give it gas when cold or else it will die. Wires have 28K also, and plugs have 10K. Cap & rotor have a bit of carbon and burn marks. Can they need replacement so soon? I ran my 560SEL on the same cap/rotor for about 65K and it idled just fine...

TIA

#2 01-24-2006, 11:08 AM

These are not scheduled preventive maintenance items. Replace them when inspection or testing indicates it.

Your rough idle could be due to any number of things. The most common source of rough idle is a vacuum leak. On the M103 engines, platinum plug can cause a rough idle. Use copper cores gapped at .035 for good idle quality. Platinum plugs have very thin electrodes which make for a very thin spark.

Good luck,

Duke2.6 · #3 01-24-2006, 11:28 AM

The spark plug issue is resistor versus non resistor rather than electrode design. The only readily available line of non-resistor plugs is Bosch Super, and the H9DC is the proper plug for M103 engines. They're easiest to find at Mercedes parts specialists for about two bucks each. Most commonly available platinum plugs are resistor type and the non-resistor Bosch Supers are not commonly available at "McParts" chain stores. You can try, but be sure you don't buy HR9DC plugs. These are resistor type Supers - note the "R" in the plug designation.

Resistor plugs add more resistance to the existing RFI resistors in the cap terminals, rotor, and plug end terminal on the wires, which alters the wave form and spark energy and is a probable cause of rough idle and high HC emissions due to misfires.

The OE cap and rotor are high quality - expensive and should last at least 60K miles, but you would be wise to remove them for inspection. You can dress the brass terminals with a sharp steel dental scrapper or Xacto knife to remove oxidation. DO NOT USE SAND PAPER.

At 60K when I inspected my cap and rotor I found a "flaky" resistance reading on one of the terminals. The RFI resistor should read about 1-2K ohm IIRC. One of my cap terminals had high resistance but dropped if I put some force on the tower.

I replaced both the cap and rotor with Bosch replacements - equivalent to OE, and idle quality and emissions test performance improved.

You should check the plug wires and inspect the spark plug connectors for damage. The wires should all measure about 2K, most of which is in the RFI resistor at the plug end. Many are damaged by ham fisted mechanics who pull the wires from the plugs without using a proper set of spark plug wire pliers.

Duke

ILUVMILS · #4 01-24-2006, 12:22 PM

I don't know much about after-market parts, but I've seen OE stuff go 100K with no problem. Just remember that neglecting spark plugs will shorten the life of ALL secondary ignition system parts. Inspecting and cleaning the cap and rotor is a good idea, but don't overlook the coil terminal inside the cap. The area around the electrode can crack with age. This can cause stalling, among other things.

lkchris · #5 01-24-2006, 01:55 PM

These are parts that affect emissions, and the manufacturer is required to replace them free if they don't last the car's first 50-80K miles.

You can count on it that Mercedes/Bosch designed them so they would.

#6 01-24-2006, 02:44 PM

Quote:

Originally Posted by Duke2.6

The spark plug issue is resistor versus non resistor rather than electrode design. The only readily available line of non-resistor plugs is Bosch Super, and the H9DC is the proper plug for M103 engines. They're easiest to find at Mercedes parts specialists for about two bucks each. Most commonly available platinum plugs are resistor type and the non-resistor Bosch Supers are not commonly available at "McParts" chain stores. You can try, but be sure you don't buy HR9DC plugs. These are resistor type Supers - note the "R" in the plug designation.

Resistor plugs add more resistance to the existing RFI resistors in the cap terminals, rotor, and plug end terminal on the wires, which alters the wave form and spark energy and is a probable cause of rough idle and high HC emissions due to misfires.

The OE cap and rotor are high quality - expensive and should last at least 60K miles, but you would be wise to remove them for inspection. You can dress the brass terminals with a sharp steel dental scrapper or Xacto knife to remove oxidation. DO NOT USE SAND PAPER.

At 60K when I inspected my cap and rotor I found a "flaky" resistance reading on one of the terminals. The RFI resistor should read about 1-2K ohm IIRC. One of my cap terminals had high resistance but dropped if I put some force on the tower.

I replaced both the cap and rotor with Bosch replacements - equivalent to OE, and idle quality and emissions test performance improved.

You should check the plug wires and inspect the spark plug connectors for damage. The wires should all measure about 2K, most of which is in the RFI resistor at the plug end. Many are damaged by ham fisted mechanics who pull the wires from the plugs without using a proper set of spark plug wire pliers.

Duke

Okay, maybe it's because their resistor, or maybe it's because of the thin electrode. Either way, stay away from anything but regular Bosch copper core plugs OR I have even had good service from Champion copper core, non resistor plugs. Anything else will result in poor idle quality and, at least I perceive, a little worse fuel economy.

Have a great day,

t walgamuth · #7 01-24-2006, 10:40 PM

you recommend similar plugs for the 500 sec engine? 84.

tom w

anziani · #8 01-25-2006, 12:20 PM

I bought my '87 300E at 90K. I do not know if the cap/rotor had ever been replaced. I drove it to 150K and replaced them. They showed wear but there were no drivabilty problems.
Neil M.

jhodg5ck · #9 01-25-2006, 09:36 PM

I've found the M103's are more sensative to ignition bits being in tip top shape then 117's/116's, 110's etc.. Usually by around 30-35K the rotor on my wagon was showing it's age (wear beyond what a good clean would tend to).

A note on the Bosch plugs, they will no longer be manufacturing the non reistor type plugs, so you'll want to stick w/ NGK or I've heard the Beru plugs are decent as well.

For an M117 I like to run W9DC's gapped a 42...

Jonathan

Duke2.6 · #10 01-25-2006, 11:33 PM

Quote:

Originally Posted by jhodg5ck

A note on the Bosch plugs, they will no longer be manufacturing the non reistor type plugs, so you'll want to stick w/ NGK or I've heard the Beru plugs are decent as well.

Jonathan

What's your source for this statement?

I'm not aware that NGK makes a correct non-resistor plug. The TR5 is the correct geometry and heat range, but it's a resistor type.

Duke

anthonyb · #11 01-25-2006, 11:39 PM

Like the others said, I would just check them at every spark plug change, and possibly clean them up. I'd expect them to last at least 100k, although you may find performance related issues as they wear.

For data points, the original distributor cap and rotor in my 1987 300E lasted 185k miles. I replaced it again at 260k miles, but only because I had a spare on hand and had the whole thing off the car to reseal the front timing cover. There was some wear, but no significant driveability problems.

The original on my parents' 1991 300E went 170k miles before it was replaced. That was a preventative replacement, and not the result of a part failure.

jhodg5ck · #12 01-26-2006, 09:46 AM

The BP7/8/9's are all the correct non-resistor type and are the correct heat range for a number of MB's. I spoke w/ my suppliers and they all confirmed that yes, Bosch is ceasing manufacture of their non-resistor style plug.

In regards to cap's/rotors perhaps I drive the Hell out of my cars as my ignitnion bits never last that long, but then again it's not a proper day if redline isn't hit @ least a Few times;-)

Jonathan

Smokiesman · #13 01-26-2006, 11:11 AM

When I replaced the plugs in my E420 V8 the local parts house did not have the Bosch plugs. But did have the right ones in Delco! Well I needed plugs so I went with them figuiring if they did not work I would just replace them. Well they have been in for 6 mo. and seem to be working fine. They are non res.

I was also getting a problem with missing and total power lose . Found out if there is a misfiring the computer will shut down 4 cyls to save the cats. I put on 2 new caps and rotors and the above plugs and Bigblue has been purrin like a kitten since!! I am not trying to plug Delco just my experience with them.

Smokie

Fastlane Phil · #14 01-26-2006, 02:06 PM

As far as I know, the resistor plugs are and will be made for a long time. We sell hundreds of these under part number 0021597603 which is the Bosch H9DC's

Duke2.6 · #15 01-26-2006, 02:13 PM

Quote:

Originally Posted by jhodg5ck

. I spoke w/ my suppliers and they all confirmed that yes, Bosch is ceasing manufacture of their non-resistor style plug.

Jonathan

I just swapped a couple of e-mails with Phil at Fastlane. He reports he has heard NOTHING about non-resistor plugs being discontinued by Bosch, and went on to say that (obviously) he sells a lot of them.

Mercedes has been specifying non-resistor plugs for over 20 years, so there is a big market, world wide.

A supplier like Bosch is not going to discontinue a part that continues to have significant demand.

Unless someone comes up with a named, unimpeachable source on this "rumor", I would consider it to be just that - a rumor that is unfounded.

Duke