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#1
04-25-2026, 04:22 PM
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1991 420SEL just purchased!
In spite of my username on this site, I just picked up my first ever gasoline-powered Mercedes! A very nice 420 SEL.
I was wondering, being more familiar with the Mercedes Diesel engines, and not at all with any gasoline engine, what should I check out besides fluid changes and filter changes? I want to keep this car around a long time and take care of preventative maintenance! Thank you all in advance!
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1979 300D, sold 1984 300D, totaled 1984 300D, sold 1982 300TD, sold to Dad 1976 240D automatic unfortunately 1974 240D manual, sold 1981 300D, sold 1983 300TD, no rust, sold 1979 240D manual, rear-ended 1965 200D, four-on-the-tree 1982 300SD 1985 300D 1984 300CD „Rusty,“ gone but not forgotten 1980 300SD 1982 240D 4-spd, sold 1991 420SEL 
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#2
04-25-2026, 08:43 PM
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Check or better yet replace the timing chain guides and the plastic tees under the camshaft oiling tubes/rails. If the chain guides are beer bottle brown they're done, if they fail the chain will jump and valves will bend.
420 SEL is a nice ride, post pics if you can.
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"Rudeness is a weak man's imitation of strength" - Eric Hoffer
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#3
04-25-2026, 08:59 PM
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Thanks Sugar Bear!! I will definitely be replacing the chain and the guides! And I‘ll check those tees too. I‘ll post some pics when I can take some!!
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1979 300D, sold 1984 300D, totaled 1984 300D, sold 1982 300TD, sold to Dad 1976 240D automatic unfortunately 1974 240D manual, sold 1981 300D, sold 1983 300TD, no rust, sold 1979 240D manual, rear-ended 1965 200D, four-on-the-tree 1982 300SD 1985 300D 1984 300CD „Rusty,“ gone but not forgotten 1980 300SD 1982 240D 4-spd, sold 1991 420SEL
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#4
04-26-2026, 11:56 AM
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When replacing spark plugs I'd use only use basic copper Bosch or NGK.
For engine oil, that is a flat tappet camshaft that likes ZDDP, I'd use a 15W-40 like Rotella or Delo most of the time. In NY winter I'd use Rotella 10w-30. Fuel TopTier premium only, Shell, Chevron etc. Interestingly BP recently was not on the TopTier list??? Run it frequently, the fuel distributors are a nightmare if they get varnished or gummed.
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"Rudeness is a weak man's imitation of strength" - Eric Hoffer
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#5
04-26-2026, 03:56 PM
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Hey, thanks again! The previous owner said he replaced the plugs about 3 years ago when he got the car… Is it time to do them again? Forgive my ignorance, I know more about the Diesels!!
__________________
1979 300D, sold 1984 300D, totaled 1984 300D, sold 1982 300TD, sold to Dad 1976 240D automatic unfortunately 1974 240D manual, sold 1981 300D, sold 1983 300TD, no rust, sold 1979 240D manual, rear-ended 1965 200D, four-on-the-tree 1982 300SD 1985 300D 1984 300CD „Rusty,“ gone but not forgotten 1980 300SD 1982 240D 4-spd, sold 1991 420SEL
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#6
04-26-2026, 05:03 PM
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Quote:
Plugs that do not have precious metal electrodes will have service lives of 40-50K miles, provided that they are not oil or fuel fouled.
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#7
04-26-2026, 08:06 PM
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That makes sense, thank you! He only put about 12,000 miles on since he had it, so they‘ll be okay… Anything these need in the way of tune-ups besides spark plugs? I was thinking about things like valve adjustments, but I heard these don‘t need valve adjustments? Or am I wrong on that? Thank you all for your help, I‘m trying to figure out what these gas engines require… I‘m also going to try to get my hands on some sort of service manual…
__________________
1979 300D, sold 1984 300D, totaled 1984 300D, sold 1982 300TD, sold to Dad 1976 240D automatic unfortunately 1974 240D manual, sold 1981 300D, sold 1983 300TD, no rust, sold 1979 240D manual, rear-ended 1965 200D, four-on-the-tree 1982 300SD 1985 300D 1984 300CD „Rusty,“ gone but not forgotten 1980 300SD 1982 240D 4-spd, sold 1991 420SEL
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#8
04-26-2026, 08:39 PM
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No manual valve adjustment, self adjusting hydraulic tappets. Honestly if I was doing a timing chain I'd replace the plugs unless they are the exact correct plugs and look excellent. If it's yes to both then I'd reuse them.
__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer
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#9
04-27-2026, 05:36 AM
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Sounds good, thank you!! I‘ll check the plugs! What should the gap on these be?
__________________
1979 300D, sold 1984 300D, totaled 1984 300D, sold 1982 300TD, sold to Dad 1976 240D automatic unfortunately 1974 240D manual, sold 1981 300D, sold 1983 300TD, no rust, sold 1979 240D manual, rear-ended 1965 200D, four-on-the-tree 1982 300SD 1985 300D 1984 300CD „Rusty,“ gone but not forgotten 1980 300SD 1982 240D 4-spd, sold 1991 420SEL
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#10
04-27-2026, 06:29 PM
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Unfortunately, I just found that water has entered the car on the passenger side… I felt wetness under the back of the passenger seat (where the rear passenger on the right side would put their feet), and traced that wetness forward. I removed the air duct that curves under the seat and pulled up all the insulation on the floor, it was saturated and came readily away from the floor… The firewall behind the dash and the plastic plate with the warning about the airbag system was also wet. Is this indicative of a bad windshield seal, or could this wetness be coming from somewhere else?
Thanks all again in advance
__________________
1979 300D, sold 1984 300D, totaled 1984 300D, sold 1982 300TD, sold to Dad 1976 240D automatic unfortunately 1974 240D manual, sold 1981 300D, sold 1983 300TD, no rust, sold 1979 240D manual, rear-ended 1965 200D, four-on-the-tree 1982 300SD 1985 300D 1984 300CD „Rusty,“ gone but not forgotten 1980 300SD 1982 240D 4-spd, sold 1991 420SEL
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#11
04-27-2026, 07:19 PM
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First look for plugged sunroof drains and plugged rocker drains it's common. Also look for plugged drains in the hood hinge pockets.
__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer
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#12
04-27-2026, 08:04 PM
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Okay, sounds good, I will check all those places tomorrow when it‘s light out again. There‘s a good puddle of water in there. I think I‘m going to have to pull the passenger seat to dry it all up. It‘s all under the carpeting. Thank you for all the insight!
__________________
1979 300D, sold 1984 300D, totaled 1984 300D, sold 1982 300TD, sold to Dad 1976 240D automatic unfortunately 1974 240D manual, sold 1981 300D, sold 1983 300TD, no rust, sold 1979 240D manual, rear-ended 1965 200D, four-on-the-tree 1982 300SD 1985 300D 1984 300CD „Rusty,“ gone but not forgotten 1980 300SD 1982 240D 4-spd, sold 1991 420SEL
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#13
04-28-2026, 09:02 PM
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Found a pile of dirt under the battery tray. Not sure if this might be related to my leaking problem or not. The sunroof drain seemed to be free and clear, I fished it through and poured water down the drain, and it leaked out from under the car. I did not check the rocker panel drains yet, I have to get the car up on ramps and do that tomorrow
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1979 300D, sold 1984 300D, totaled 1984 300D, sold 1982 300TD, sold to Dad 1976 240D automatic unfortunately 1974 240D manual, sold 1981 300D, sold 1983 300TD, no rust, sold 1979 240D manual, rear-ended 1965 200D, four-on-the-tree 1982 300SD 1985 300D 1984 300CD „Rusty,“ gone but not forgotten 1980 300SD 1982 240D 4-spd, sold 1991 420SEL
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#14
04-28-2026, 09:10 PM
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I think there are 4 sunroof drains, one in each corner of the sunroof cartridge. Be gentle to not damage the old drain tubes.
How about the hood hinge pocket drains?
__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer
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#15
04-29-2026, 03:00 PM
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Okay, I just checked the ones at the front of the sunroof, I‘ll check the back too. Around the hinges was clean surprisingly besides the pile of crud under the battery tray
__________________
1979 300D, sold 1984 300D, totaled 1984 300D, sold 1982 300TD, sold to Dad 1976 240D automatic unfortunately 1974 240D manual, sold 1981 300D, sold 1983 300TD, no rust, sold 1979 240D manual, rear-ended 1965 200D, four-on-the-tree 1982 300SD 1985 300D 1984 300CD „Rusty,“ gone but not forgotten 1980 300SD 1982 240D 4-spd, sold 1991 420SEL
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