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Old 10-23-2000, 12:09 PM
David Pare'
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I bought this 1986 420 with 129K on the odo. The maintenance records were not well kept. I am basically fixing one thing at a time and have been happy so far. I put my timing light on the car and it reads perfect, is there any value to changing the timing chain just in case? What's the approx cost?

ps.I put the +4 Platnum plugs in yesterday and noticed a boost right away.
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Old 10-23-2000, 01:56 PM
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Join Date: Jul 2000
Location: Toronto, CANADA
Posts: 1,193
I think there is a way to check for chain stretch. I think the $$$ to replace a chain and tensioner is aroun 800-1000 dollars US, from what others have posted. I have the same year and model as you. My mechanic said as preventative matenance it would be a good idea to replace the chain rails (or guides). I guess the biggest problems aren't with chains breaking, but with a rail breaking loose. This causes the chain to skip some of the teeth on the gear or else the rails get caught in the chain and cause major damage. Other than that get some kind of rust proofing done and you should be fine.

Jason Priest
1986 420SEL
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Old 10-24-2000, 10:49 AM
Mr. BILL's Avatar
Ghoulardi Rules!
Join Date: Aug 2000
Location: Austin, TX
Posts: 576

If you read previous posts, you will find the "Pro's" don't recommend platinum plugs for a car of your vintage. The answer given is, these plugs were not around then, and emit a different spark than the copper ones.

Do a search on spark plugs and you will find alot on this topic.


90 300SE
85 300D
94 300SD (sold)
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Old 10-24-2000, 11:25 AM
David Pare'
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I read that the Platnium plugs were not good due to the thin spark, from that info I got rid of the regular Platnium plugs and replaced them with the +4 plugs, which I think, have a wider spark. Hope I'm ok.
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Old 10-25-2000, 11:25 AM
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You are playing with fire to not at a MINIMUM:
1) Replace the guide rails to the timing chain
2) Replace the timing chain tensioner
3) Have the timing chain checked for stretch

If this were my own car, I would definitely replace the timing chain immediately.

It also would be a good idea to replace those new plugs, as mentioned in the previous posts.

I would check the plugs for oil residue as well. If your valve guide seals have not been replaced by now, they will need to be soon. You don't want your engine burning oil and dumping residue all over the valves and carboning up the rings in the bottom end of your engine.

I would also make sure that your trans fluid is cherry red. If it is not, I would have the torque converter emptied and refilled as well as the fluid in the pan.

Good luck. These are durable cars.

We have several clients who have kept up on the maintenance on their late 1980's 420 SEL's who are approaching 300,000 miles. We also have several whose timing chain and rails have gone at mileages far less than yours, who have had to spend BIG $$$ to have the upper ends of their engines rebuilt.


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