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Old 02-07-2007, 11:46 PM
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Location: Mountain View CA.
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420 SEL Poor Acceleration

Hi All,

I just had the timing chain and exhaust replaced as well as a tuneup on my 1988 420 SEL. The mechanic is having a hard time getting the car to run well. When the accelerator is depressed the car barely goes. It seems as though the harder the pedal is pressed the slower the car goes. The car sat for years and was running poorly when I brought it in. I had been told that the catalytic converter was clogged. The car runs worse than when it was brought in but it idles well now. Your help would be greatly appreciated.

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Old 02-08-2007, 12:26 PM
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i have a 560sel with similar problem.once i changed the timming chain i had to adjust my timing since the other chain was slipping. i just turned the distributor some and the power returned.not the most high tech way but it worked.
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Old 02-08-2007, 12:37 PM
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A clogged cat will give the symptoms you describe. Try driving the car with the cat loose and see if the power comes back.
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Old 02-08-2007, 01:17 PM
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If the chain had a lot of stretch, you may need to retime it as johnlo suggested. The distributor is driven by the timing chain. When you say it goes slower does it sputter or just not speed up? And is this in park or drive? A complete lack of power usually means a very, very retarded ignition timing.
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Old 02-08-2007, 01:17 PM
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The timing was set and the entire exhaust and cat were replaced
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Old 02-08-2007, 01:56 PM
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Low fuel pressure or fuel not being delivered by CIS-e distribution head.
Have the O2 WOT it should read 0.980vdc to 1.02vdc,
any lower indicates very lean .

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Old 02-08-2007, 02:21 PM
david s poole
Join Date: Mar 2005
Location: dallas
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are you sure that the mechanic set the timing up right[valve timing]?it is easy to make a mistake if you are not familiar with these engines and their quirks.drivers side cam sprocket has a timing washer that has the mark on it it has two keyway slots one top and one bottom if washer falls off and you didn't see it it can be put on upside down and will change the firing order on the left me if you need to.
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Old 02-08-2007, 04:07 PM
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Thanks for your responses and David, thank you for your kind offer. I will relay the information to my mechanic and see if it helps. I also can describe the behavior in a little more detail; The car idles smoothly but acceleration is rough. Light depression of the accelerator yields better (still bad) response. Depressing the pedal significantly has an opposite effect from normal. It seems as though the engine pulls less the more the pedal is depressed.
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Old 02-08-2007, 06:05 PM
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Your eng. is out of time on that one bank,OR you are so dry roted from age and have massive air leeks at the intake system! but I think Dave hit it hard.

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