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'85 380SL Tune up

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Old 12-04-2008, 03:56 PM   #1
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'85 380SL Tune up

I bought my 1985 380SL 2 years ago and started working on it. I's in really good shape and starts well. I decided to start replacing engine tune up parts because it was idling rough. I changed the plugs, rotor and cap. I have not yet replaced the wires or fuel filter and have not re-timed it. I did adjust the idel speed from that hole on top of the air cleaner.

My problem is this. The car starts fine and seems to idle pretty smooth for a while. Once warmed up, I can hear what sounds like some missing. There are puffing sounds at the tailpipe, but no smoke. I took the car on a long 60mph run today. At high speed it runs smooth but idles a little rough once I stop. I feel flat spots on acceleration at a standstill and if I accelerate under load at high speeds (eg uphill), the engine doesn't have the pick up it should.

I am planning to replace the wires and fuel filter soon, but I wonder of it could be some other problem, like fuel injectors or fuel pump. I hate to take it to Mercedes at this point, because it seems like it may be a minor problem. Should I check the timing? It's not backfiring. Also, is there a diagnostic plug in the engine compartment that Mercedes can hook up to and maybe diagnose a problem? Any help or suggestions are appreciated.

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Old 09-08-2012, 11:29 AM   #2
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I just bought an 85 380 sl ... it had the same problem ... puffing on idle ... missing at speed ... bad performance.
New NGK wire set(guaranteed for life) ... new NGK plugs ( perfectly gapped) ...mercedes cap and rotor and dust cover... air filter ... fuel filter.
I then added Barhdal fuel system and injector cleaner and filled the tank with 89 octane.
After 50 miles It ran like it had an airplane engine in it ...
The tranny works perfect now and the car sounds great too.
These cars are Awesome!
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Old 10-20-2012, 12:24 AM   #3
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I just found out that we are supposed to use premium gas in these ...
They even warn of engine damage over 3000 rpm using regular unleaded.
Make sure you give her the good stuff ... I know i'm going to now that I know.
91 octane unleaded min. 15% alcohol ok

Last edited by roadster Kev; 10-20-2012 at 12:26 AM. Reason: omission
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Old 10-20-2012, 05:26 PM   #4
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Owners manual with my '82 calls for 89 octane (regular) fuel. I'm thinking your '85 does as well.
Models before and after the 380sl, 450 & 560 SLs, call for the 91 octane fuel.
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1985, 380, 380sl, 85, diagnostic, high, idle, mercedes, problems, schedule, sl, smoke, tune, tuneup, tuning

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