What spark plugs for the M117 and M100

[Styria]

I'd be interested to know what plugs you guys use for your 350/450 and 6.9s and, not to forget, 6.3s - Waxenberger's and abl567's "baby".

For years, I have advocated and used Bosch platinum plugs with an 8 rating. That heat range seems a good compromise for suburban and highway driving, but for sustained highway cruising I have endeavoured to fit the Bosch 7 (one grade colder) range for better engine protection. This is for 6.9s, but I would venture to suggest that the M117 engine be treated in the same manner. I used to purchase those from Repco at about $10.00 each, but for at least the last four years I have not been able to obtain those Platinum plugs from them, nor have I changed them in between times, other than using spare units that I have always had on hand - bear in mind, I have owned my car for fourteen years.

So, just recently I wandered into the Supercheap store at Wentworthville, looking for this, and looking for that, with nothing particular in mind. Then I spotted their Spark Plug cabinet in one of the aisles. Ah hah, what's this ? Bosch Platinum plugs ! Beauty. How much ? $6.00 retail dollars each, and still made in the good, old Fatherland - Germany to the unknowing. They were no different to what I had been buying from Repco. As it turned out, they only had four of the 8 range in stock, but enough of the 7s to satisfy my Gleaming Beauty. So I struck the deal and decided to go for them.
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Of course, we also have a 1999 Holden Statesman with the Series 1 Gen.3 Chevrolet engine. Well, it 's due for its 170k. service, so new plugs would be high on the list, other than Oil and Filters. Naturally, after consultation with the salesperson, I spotted the plugs for the Statesman (very nice car, BTW, more on that later in another section of the board), but the price ? $96.00 smackeroos - wow. Needless to say, I got stopped in my tracks, and walked out with tail between my legs. So, off to the Holden Dealer at Baulkham Hills. I get on well with the Spare Parts guy, and discuss plugs with him.

Yes, of course, we have Platinum Plugs for your Statesman. Denso platinums, and of course, they are $6.00 each trade price. Very good, I'll have them. I then related the Supercheap experience and it turned out that they were 'Fusion" tipped plugs which the Holden dealer also has available, the manufacturer being, again, Denso (made in Japan). The price - from memory, $160.00. I collapsed on the spot, there and then. After a stiff drink and a couple of slaps across my cheeks, he asked whether I had the Series 1 or Series 2 Gen. 3 engine. I stutterd 'well, the former'. Ah alright, just as well, because the plugs for the Series 2 work out to, I think ( I am still dazed) about $360.00 (for eight). Just as an aside, I decided to buy a genuine GM Oil Filter, made in, you guessed right, good old Poland. Regards Styria

 

[BenzBoy]

Michael,
Please stop collapsing all over the place and get to the point!!
What plugs do you have in your Victa?
Regards,
Benz-Boy

 

[Styria]

Michael,
Please stop collapsing all over the place and get to the point!!
What plugs do you have in your Victa?
Regards,
Benz-Boy

Heh, fair go - I just have to tell how it is and how it felt at the time. Victa ? Wot's that ? I crank out the old super-duper GMC that cost me two bucks short of two hundred smackeroos - compliments of the Chinese and the "big" Big W. It's even self-propelled as long as you push hard enough ! Styria

 

[TJ 450]

NGK BP6ES or Bosch W7DC with NGK being the preferred option. Exactly the same as the 6.9, gapped to 0.8mm.
One must ensure that versions with a built in resistor are not used like BPR6ES, WR7DC, etc.

That seems way too cheap for Platinums, which are normally well into the two-figure mark each. I was also under the impression that they are not recommended for use on these engines.

Tim

 

[abl567]

I use W9DC's in Six 3, because the MFIS runs richer, 4-5% at idle, they tend to foul the cooler plugs after prolonged idle.
The non resister plugs are no longer made but I found some on ebay so I bought 14 sets of 8 for about $3.20 each

 

[Styria]

I use W9DC's in Six 3, because the MFIS runs richer, 4-5% at idle, they tend to foul the cooler plugs after prolonged idle.
The non resister plugs are no longer made but I found some on ebay so I bought 14 sets of 8 for about $3.20 each

That is a big grin on your face - no wonder as I would call that good buying. You will not do better than that. Regards Styria

 

[Styria]

NGK BP6ES or Bosch W7DC with NGK being the preferred option. Exactly the same as the 6.9, gapped to 0.8mm.
One must ensure that versions with a built in resistor are not used like BPR6ES, WR7DC, etc.

That seems way too cheap for Platinums, which are normally well into the two-figure mark each. I was also under the impression that they are not recommended for use on these engines.

Tim

Hi Tim, regarding price, you're quite correct as far as "usual" cost is concerned - as I said, some years ago I paid far more than the other day. I have indeed compared old and new side by side - they're exactly the same and made in Germany....and yes, they are Platinum plugs.

The plugs for the Statesman, whilst still Platinum, are completely different with a huge gap set as 'standard' - one would think that the plug would never fire, but that is the intended design.

Some years ago with the debate raging as far as leaded vs. unleaded petrol was concerned, I did ring Bosch Australia and they did in fact suggest that Platinums were not suitable when coupled to leaded petrol. I therefore took their advice and fitted conventional plugs - for one week. They did not burn nearly as cleanly and showed definite signs of coking up. I then refitted the Platinums, and have stuck with them ever since with no apparent ill effects.

I can tell you that even after about a thousand miles, the Platinums were still showing the wire brush marks when I cleaned them on the bench grinder prior to installation - that's how cleanly they burn. Regards Styria

 

[TJ 450]

Interesting. I think I might have to get a set for the 450, as it's due for a new set. I was having issues with no. 4 fouling, which seems to have just happened again.

Tim

 

[WGB]

This was my request for information here a year ago

http://www.topklasse.org/forums/showthread.php?t=263

There seem to be problems finding NGK plugs with screw off terminals but I agree BP6ES gapped at 0.8mm would be the sound choice.

My 6.9 has run cheerfully on the Bosch WR9DCX+ gapped at 0.8mm without missing a beat for the past year and the hospital gardener (a retired mechanic) remarked last week about how even and regular the idle was.

There have been lots of comments posted around the traps about problems associated with using resistor plugs in these cars but as that was all I could find when I last changed them I am pleased to report not a single problem over the past year.

In my experience the start is more immediate and it idles better with a gap of 0.8mm than WR9DCX+ plugs designed gap of 1.1mm.

Bill

 

[abl567]

I'm sure you are aware the non resister plugs are used because the required resistence is provided by the plug ends and the distributer cap ends, If the leads have no built in resistence the resister plugs will work well IF their resistence is the same as the leads should have, 9000ohms I think?

 

[John S]

My memory says 5K ohms. Can someone check?

 

[abl567]

The plug ends are either 5000 or 1000 ohms, the former for M100 the later M117, the balance comes from the wires and cap ends, the coil lead also has resistence. This is all from memory

 

[Styria]

Hi everyone, resistance, ohms etc. and how it affects the running of the car is a subject I have never understood.

6.9 engines have suppressors that show 5 ohms resistance - not sure about 6.3s, but I believe that the suppressors have 1 ohm resistance as well, and I am fairly certain this also applies to 117 and 117 engines. Speaking to people over the years, but never really coming to grips with the discussion and their points of view, some will tell you that it doesn't make one iota of difference whether you fit 1 or 5 ohms suppressors, yet others maintain it does make a difference. So, do we have anyone on our board that could really clarify this once and for all ?

Furthermore, high tension leads - some insist you must go genuine, others say that convential after market leads are okay to use. I have always gone genuine, but am I being pedantic in the process ? Regards Styria

 

[abl567]

Hi Styria,
There was plenty of discussion about this on the M-100 site.
As I recall the resistence provides a spark of longer duration, remaining active while the flame front moves through the compressed mixture.
Ron B explained it best with his description of clearing fouled plugs on worn engines by unpluging the lead at the cap and holding it close enough to arc to the terminal, this gave a longer arc at the fouled plug helping it to fire properly, the combustion then burning of deposits.

 

[TJ 450]

I haven't really looked into the effects side of things until now, but you are effectively placing two resistors in series, when using resistor plugs.

A standard resistor plug already has a 5k ohm resistor built in, so you now have a combined resistance of 10k ohm rather than 5k ohm with an M117, or in the case of a 6.9, 6k ohm.

So, given that the gap is 0.8mm on both examples and the coil is capable of emitting the same values, the 6.9's setup allows a higher energy spark across a shorter duration. It is also less likely not to fire due to slightly fouled plugs, which increase resistance drastically, hence no spark.

As with most things, it is all a compromise between RF noise emissions and engine performance.

Basically, the resistor is there primarily to suppress RFI, which occurs as the spark is produced and emission levels are directly related to the speed at which the coil's energy is discharged.

A lot of aftermarket leads have a special spiral winding that is also a built in resistor, so that would then lead to a spark of lower intensity than specified and one that lasts for an even longer duration, if used with resistor plugs and MB suppressors as well!

There are also many diffent conductors used as well. Leads manufactured to the OEM specs are without doubt, the safest option.

If you wanted to use resistor plugs on an MB and retain the factory setup otherwise, you would have to remove the suppressor ends and replace them with conventional versions.

BTW, I don't claim to be an "expert" in the subject of spark plugs. I'm just passing on some acquired knowledge. The way spark plugs function in the system as a whole is rather complex.

Tim

 

[WGB]

The problem I faced when attempting to replace my plugs a year ago was finding any plugs that had screw off terminals so that they would fit the original caps.

I was aware that resistance plugs were not advised but as the car already had them fitted (by a Mercedes Mechanic) I replaced like with like.

Whatever the theory the old plugs were gapped at 1.1 mm and the 6.9 always took a few turns to start.

The new plugs were set for 0.8 and have run trouble free for 6,000 km with immediate start on first crank - hot or cold - even though they are resister plugs.

One of the more important factors in plug fouling and plug life must be the mixture control and how oil tight the combustion chambers are - specifically in these motors - the valve stem oil sealing.

Next time I change the plugs I will fit BP6ES so long as I can find them with screw off terminals and see if there is any difference.

Bill

 

[Styria]

Hi TJ450, I really appreciate your post and possible (!) explanations therein, but in all honesty I am still confused and none the wiser - this is not meant as a criticism, it's just that I don't understand too much of it. For instance, I don't understand the meaning of the abbreviations, are they RF and RFI ? (I think that's what they are). I'm just lost.

All I know is this - 6.9s have 5 ohms suppressors, 450s have 1 ohm suppressors - they are the metal sparg plug caps. When buying replacement metal spark plug suppressors, the after market people can normally only supply you with 1 ohm resistors. I presume you can buy 5s from the dealers, albeit at considerable cost. The leads, I suspect, are the same between 100 and 116/117 engines, except for the length. The 'genuine' high tension wire (lead) can be bought from Wurth Australia, and the little brass fittings can sometimes be bought after market. With the correct tool to crimp the brass fittings (I only have an imitation but it works of sorts), I can make up individual leads.

However, why should the various types of engines require suppressors with the different ohm characteristics ? Given that distributors, leads, plugs and coils and ignition units are essentially the same between 450 and 6.9, where is the answer to this variation in suppressors ? I mean, a genuine set of 6.9 leads used to set you back a cool grand at least - nowadays probably more. Regards Styria

 

[TJ 450]

The 6.9 has the 5k and the M117 has the 1k ohm suppressors (not the other way round, which I think I stated in the previous post). As for the reasons behind the difference... I really couldn't say, although I'm sure the engineers knew exactly what they were doing.

On the 6.9, they could allow a lower energy spark for a longer duration, On an M117, they required a higher energy spark for a shorter duration. This would be stated by the engine design, I'm sure.

RF stands for Radio Frequency, RFI Radio Frequency Interferance. Every time a spark plug fires, there is also RF energy emitted, just like when lightning strikes, but on a smaller scale.

This is all very relevant now, as my 450 has fouled two plugs and was running on seven cylinders yesterday. The plugs were dealer fitted and were Bosch WR8DCs, resistor plugs. I went to get a new set of plugs, although the only ones I could get that were remotely suitable were WR7DCs, as they were out of BP6ESs. The only Bosch plugs available were resistor versions.

Anyway, I had the mixture adjusted, changed the plugs and all was OK for a while. Then, the idle started to get very lumpy again. I started it this morning and I was on seven cylinders again.

Inspection of the plugs reveals extreme dry carbon fouling on #4 and #5.

Now, for a bit of background on this car...

It has:

New fuel pump, filter, accumulator.
New fuel injectors.
Alloy CIS fuel distributor and matching WUR.
Properly functioning Aux. Air Valve.
New leads (1k ohm), Cap, Rotor, tested OK.
Compression 165 - 170PSI.
Chain stretch 3 degrees ATDC, renewed 5000km ago.
Ignition timing, TDC with vacuum.
Completely reasealed intake manifold.

The idle has never been smooth since day one, but now I need to get to the bottom of it as it is the daily driver. Surely a 450 shouldn't idle like a diesel truck?

Tim

 

[Styria]

Hi Tim, this is a pretty curly one - especially if one needs to come up with some form of diagnosis some four thousand kilometers away. First of all, is your 450 D or K - Jet. If D, I haven't much of a clue, if K, we might be able to come up with some sort of solution.

First of all, you don't mention the leads - condition and age wise. My first and foremost suggestion would be to trek to someone with a Bosch or Sun tune-up machine. If however, you'd like to 'tinker' in the comforts of your garage (carport on busy road), there are some basic checks. Start off by checking your leads AND suppressors for impedance - in other words, are they working properly.

Secondly, dry (?) plugs on cylinders 4 and 5 would indicate lack of petrol and the car not firing properly. Am I right regarding "dry plugs" ? If that is correct, you'd need to look at your fuel distributor. You say it is an alloy unit with matching warm-up regulator - has the fuel distributor been tested ? Is it okay ? Who checked the operation of the WUR ? A guy with SUITABLE equipment ? Start the engine, and crack open each fuel line connection one at a time. If you get no change in engine revs with any that have been opened up, you have a fuel problem. Also crack open the fittings to each injector just to make sure fuel is getting to them.

At this stage, I'd reckon that's about as far as you could go. You also say that it sounds like a Diesel ? Why ? Are the lifters okay ? Why would they be noisy ? Anyway, please keep in touch. We all might learn from your experience and investigations. Regards Styria

 

[TJ 450]

Hi Styria,

It is a K Jet, 1979.

The leads are about a year old and I checked them with a multimeter as well as trying a known good set.

Those two plugs were dry and completely black.

As for the Fuel Distributor and WUR, they came straight off my 380SE, which was running prior to removal, so that was the test.

I'm fairly certain that the previous fuel distributor had a blockage leading to #5, which is why I installed the alloy unit yesterday. I'm aware that The fuel Distributor and WUR need to be adjusted as a pair, which is why I changed them both.

I might add, that it is now firing on all cylinders with the change of those parts, but the idle is how it was before all this and it has exactly the same slight missfires on the same cylinders.

Also, when I bought the car a couple of years back, it was running on
seven cylinders because of #4 having a wet fouled plug. I'm wondering if the original issue is leaking valve guides/stem seals and this recent problem was the blocked fuel distributor.
Another possibility is the modulator vacuum unit on the transmission allowing too much ATF into the mixture... it's vacuum input is closest to #4!

I was referring to the lack of smoothness when I mentioned the diesel truck. There's nothing wrong with the lifters, although some rockers eppear to have worn through the hardened surface. Camshafts are excellent though.

Anyway, I'll investigate further and report my findings. At least I'm narrowing it down... there's not much else it could be. I need to get to the source of the problem though.

The car was also plugged into a Bosch diagnostic machine last week and there was nothing wrong with the ignition system from the coil at least.

Tim