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Registered
1990 420 SEC
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405 Posts
I'd try to find out what duty cycle is showing when it acts up and if there is a diagnostic socket. There might be some info available there.
This sounds like vacuum leak to me. Have you had a smoke test done?
 

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SuperModerator
1986/1990 W126
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14,481 Posts
Bugger. It sounded so hopeful.
 

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26 Posts
Discussion Starter #46
I'm also siding with a vacuum leak. I'm bringing it up to a shop tonight to have the exhaust patched up and put back on and see if he can see that anything has gone bust while it's on the lift. I don't have enough data to fully support this, but I put on 500 miles on incredibly hot, humid days and it was anemic before she was fully warmed up with occasional misfires under load. I'm thinking the rubber expands as the engine heats it and closes off, at least partially, whatever leak there is. It acted up after it got cold out, after approx 70 trouble free miles earlier in the day. I did do a backyard smoke test with a cigar through the brake booster line and found the air distribution hoses were all cracked and split, so I replaced those. I do have injector cups and seals also. I replaced a few breathers and im positive there are other leaks, I just haven't been able to track them down.

Do we think this could be contributing to my high idle? Being that its not all that high in drive?
Could a vacuum leak not related to the transmission make the car break up at higher rpm?
Also, what are symptoms of a failed coolant temp sensor?

IS THERE a chance this could be timing-related? I still haven't checked the condition of the chain and guides because I am both stupid and nervous to answer my own question.

I apologize for seemingly "winging it", i'd certainly do more diagnostic testing if need-be, but general consensus amongst members here is what got me this far in the first place. If it's simply vacuum leaks, under the knife she'll go, but donuts in the intake manifold sounds like a can of worms that I really have no interest in opening.
 

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1986 560SL 1986 420 SEL
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134 Posts
I think you might still have a vacuum leak that your cigar isn't showing. There is a YouTube out there showing how to make a smoke tester using a jar, solder iron, BIC pen parts and a hand air pump. That is simple and pretty cheap to make. It will give you more smoke and pressure to locate more leaks or prove you don't have any. The transmission has a vacuum operated modulator. Under heavy acceleration there is little vacuum and a leak could be affecting your shifting. Just my two cents and I am not a good mechanic by any means. But I did this and found a leak in two of the I ejector holders.

Would like to know the results if you try it.
 
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