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Discussion Starter #1
Hi all,

Put new plugs and leads in the 86' 420SEL last night.
Noted. the plugs that came out were black and sooty, ie. rich.

I've noticed the car has had a rich condition and really fumey (since I bought it in may).
I swapped my failed ECU a few nights ago which might help (lol) - but, either way it still had a fair "fuel rich exhaust smell to it".

Set my duty cycle (about 15-20% fluctation which seems like alot) it was around 70% (rich) initially, now 45% (still with the big fluctuation though).

Even with the new ECU, I've had the check engine light come on and off qucikly... too, thoughts?

Do you guys often find the V8's run rich in general and soot up plugs?
Or should I be looking at a fitting the new O2 sensor I have in the box? (or something else?)
 

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Did your ECU match the year/engine/transmission of the previous ECU? I'm not sure if there were some changes over the years for the 420 (a new transmission was introduced in '90-'91 for 560).
 

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Discussion Starter #3
Did your ECU match the year/engine/transmission of the previous ECU? I'm not sure if there were some changes over the years for the 420 (a new transmission was introduced in '90-'91 for 560).
Have the exact same part number on it, so should be fine.

Noting, it did apear to run slightly leaner after swapping the ECU, so it actually improved.

Might have to do a couple of hundred kms on the new plugs now since doing the duty cycle and then pull one and check.
 

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Hi all,

Put new plugs and leads in the 86' 420SEL last night.
Noted. the plugs that came out were black and sooty, ie. rich.

I've noticed the car has had a rich condition and really fumey (since I bought it in may).
I swapped my failed ECU a few nights ago which might help (lol) - but, either way it still had a fair "fuel rich exhaust smell to it".

Set my duty cycle (about 15-20% fluctation which seems like alot) it was around 70% (rich) initially, now 45% (still with the big fluctuation though).

Even with the new ECU, I've had the check engine light come on and off qucikly... too, thoughts?

Do you guys often find the V8's run rich in general and soot up plugs?
Or should I be looking at a fitting the new O2 sensor I have in the box? (or something else?)
Hi there,

In answer to one part of your query, over the last three or four times I have done plug changes on my 560 & now the 500, my plugs have consistently looked similar to this example taken from my 500SEC at about 40K or so miles....

Drivability issues have not been a problem, save the rear crank sensor/fuel pump relay
'hiccup' earlier this year..

M
 

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Discussion Starter #5
Hi there,

In answer to one part of your query, over the last three or four times I have done plug changes on my 560 & now the 500, my plugs have consistently looked similar to this example taken from my 500SEC at about 40K or so miles....

Drivability issues have not been a problem, save the rear crank sensor/fuel pump relay
'hiccup' earlier this year..

M
Thanks mate!

Mine are definitely more sooty than that... see attached. (also note, not sure why there is oil on the threads, thinking rocker cover gaskets were done while these plugs were in.

No idea how old, since this is the first change, either way... hoping the new plugs, leads, ecu and duty cycle adjustment make a difference - will be sure to swap out that O2 sensor soon too.
 

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Replacing bad O2 sensor with good one should change duty cycle ratio. Other things making engine run rich could be air leaks at exhaust side before O2 sensor or misfires.
 
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