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Changed out my OEM spark plugs two weeks ago. M112 had 65,000 miles on it. Plugs were surprisingly dirty and had gunk on them. Only 1 or 2 plug wells had any oil seepage from leaking valve cover gasket? But it was very, very minor.
I put in Bosch platinum-tipped plugs, all 12 of them. Job wasn't too difficult. And then I emptied an entire 16oz bottle of Seafoam into the tank when it was 1/4 full. I did this twice over the course of two weeks in preparation for a 400 mile road trip I was to take.
Results?
I didn't see any better fuel economy (surprise), but the engine pulled from down low noticeably harder than before. The off-the-line and rolling acceleration sprints were unbelievable. Engine felt like an E430.
60-100 mph sprints to pass trucks and cars on 2-lane farmland roads were pretty damn rapid. Faster than before, IMHO. I'm not saying my E320 is a Tesla Plaid now, but man, I've never before felt the improvements new spark plugs and cleaned fuel injectors had before. The car only had 65k miles on it, so how bad could they have been? Bad enough, I guess. During the first-half of 2021, I took the car to Denver and back 4 times, each way non-stop, and it performed as expected -- plus gave me 28 MPG to boot! I had no performance complaints. But I had a nagging thought of changing the plugs -- even if only because they were 20 years old and it wouldn't hurt.
Just wanted to share my experience. Now I imagine I ought to change plugs every 50k or so -- platinum or not. Your mileage will undoubtedly differ. Close-up of one spark plug. All were generally dirty, a little gunky, but no missing electrode pieces or anything. All six plugs from the passenger-side bank; top plug was closest to firewall, all plugs in order from back to front. All six plugs from the driver-side bank; top plug was closest to firewall, all plugs in order from back to front. Plug wells were essentially free from any leaking oil from valve cover gaskets. A teeny bit of oil appears to have collected in this well. Not enough to warrant a valve cover gasket replacement tho.
The placebo effect is quite powerful. My engine runs sooo much better when I wash the car. Your plugs were running a bit cool judging from the deposits on the insulator but I would not expect that to be noticeable, but it is possible. You would likely benefit from one notch higher in plug heat range.
"Placebo effect?" Yeah, sure... could be. But honestly, I wasn't expecting "any" noticeable improvement in any performance measure. Seriously. So the newfound low-end grunt was a very noticeable surprise and hard to explain away as something I was expecting and desiring to happen. Whatever.
Not sure what you mean by the plugs "running a bit cool." ? How is that possible? Their proximity to combustion means very little variation in temperatures once the engine reaches operating temp. In fact, I'd go as far as saying the variation in temperatures of spark plugs between all E320s is negligible. Even when comparing northern to southern locales. Please explain what you mean. I'm curious. Thx!
Every brand and design of plug is sold in several heat ranges. The number on a plug recommended by a car manufacturer not only tells you that the plug will fit the engine but also tells the recommended heat range for that specific engine. Plugs should be designed so the insulator runs hot enough that the carbon burns off, but not so hot that it causes preignition of the fuel charge. Carbon on an insulator can allow the spark to follow the insulator and never jump the gap. Old school mechanics "read the plugs" to see if the insulator is carbon free and it is also possible for those with skill to tell if the plug is running too hot. It is possible to go to a parts house with a plug and say, "I would like a plug that is one level hotter", or cooler. They will go to their chart and give you the number of the different heat range plug in that brand. They will also have a cross reference chart to get approximately the same heat range in a different brand of plug. Of course, the guy behind the counter will likely say, "Huh?" and you will have to find an old timer.
Some of your plugs have too much carbon on them and could benefit from a notch higher heat range.
@nelbur Thank you very much for the info. I never knew that. If my plugs look like the OEMs when I yank 'em out in 50k, I'm gonna go a step hotter per your advice. : )
Sorry for the last reply... I had a misfire in the passenger-side cylinder closest to the front of the car. Is that cylinder #6? It occurred last Fall, a good year or more after I replaced all the plugs.
I replaced the coil pack for that cylinder and it still occurred. So I swapped ignition wires and it went away. Weird. You'd think the error would still be present.
Anyway, the car has been motoring fine for months now. The last issue I dealt with was the passenger side front seat sensor that set off the SRS light on the dash. Taking everything apart, including removing the seat and its cover, inspecting the wires and re-assembling everything fixed it. Sometimes these older Mercs have such crazy quirks. But I'm glad I was able to work on it myself.
10-25-2021, 08:52 AM
