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Bad plug wire effects?

 
Old 01-11-2019, 02:55 PM
  #16  
Hey_Allen
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I haven't looked at my plug wires closely (yet,) but are they possible to replace the wires and re-use the connectors and boots?
The MB cars of this vintage were re-buildable, by unscrewing the back of the plug end connectors, and pulling the threaded terminal out of the end of the wire. I think you were supposed to replace the distributor end, but some of the owners re-used those as well.

If this is the case, it could somewhat explain why a complete set of wires is so expensive on these engines.
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Old 01-11-2019, 03:04 PM
  #17  
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yes, you can reuse your ends. I felt that with 29 years of heat cycling and age, the connectors can't be in tip top shape,
so I bit the bullet and ponied up for the set. certainly the last set of wires I will ever buy. expensive, but the pain is over
and I will sleep well after installation.
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Old 01-11-2019, 03:45 PM
  #18  
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When you open the throttle, esp. quickly, the resistance across the plug gap increases to the point where the ignition pulse will take the path of least resistance, which can be in a spark plug cap or distributor cap, or directly from a wire with compromised insulation to the nearest good ground, often the engine, and the plug won't fire. It will just get worse as this alternate path inevitably gets a carbon trace built up, which decreases its resistance more and more.
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Old 01-11-2019, 03:58 PM
  #19  
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^^^^^yep. that's why when I put a wood spacer between the wires and the engine, the car ran better.

a few weeks ago I pulled a plug wire, and there was no difference, in sound, at idle. pulled one more, and got
the same result. believe its going to be a new ball game with the new wires. might be here on Monday...….

without any proper knowledge or basis in fact, I thought these bad wires would be causing a pull back in timing,
but it seems the consensus is no.
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Old 01-11-2019, 05:29 PM
  #20  
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Ive been chasing down a highway RPM rumble that increased under load from maybe there to clearly noticeable, it doesn't escalate much past that, but I now have a high RPM, WOT..screaming down the highway huge miss that comes and goes..

So, wires are next. Caps/Rotors already done.

Thanks Roger!
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Old 01-11-2019, 07:14 PM
  #21  
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Originally Posted by Hey_Allen View Post
I haven't looked at my plug wires closely (yet,) but are they possible to replace the wires and re-use the connectors and boots?
The MB cars of this vintage were re-buildable, by unscrewing the back of the plug end connectors, and pulling the threaded terminal out of the end of the wire. I think you were supposed to replace the distributor end, but some of the owners re-used those as well.

If this is the case, it could somewhat explain why a complete set of wires is so expensive on these engines.
You will need these: http://www.kingsbornewires.com/produ...01.300.003.htm . Fit the end of the wire which screws into the cap connector and the plug well extension.
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Old 01-11-2019, 07:22 PM
  #22  
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Thanks DocMirror, I thought I remembered seeing those, but wasn't sure if it was here or back on the MB forums a while back.

I do remember thinking that they probably needed replacing on the last set of MB wires I was looking at, as they had corroded in the plug boot housings, not instilling much confidence in their electrical conductivity.
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Old 01-11-2019, 08:10 PM
  #23  
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Originally Posted by merchauser View Post
^^^^^yep. that's why when I put a wood spacer between the wires and the engine, the car ran better.

Id call it a day..fixed!

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Old 01-11-2019, 08:18 PM
  #24  
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^^^^^^the wooden stir sticks were free at home depot!
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Old 01-12-2019, 03:01 PM
  #25  
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Just some clarification on the oxy sensor and the effects of poor ignition:

The sensor effectively reads CO% in the composite exhaust stream. CO is a product of combustion that's directly related to fuel/air ratio. Higher ratio = richer mixture. The CO is only generated from combustion, so a cylinder that's weak or misfiring doesn't generate any CO changes, as the mixture of what -did- burn is likely OK. The raw HC passed through unburned has no effect on what the oxy sensor sees, so there is no correction made when there's a misfire. Sampling the exhaust for high HC before the catalysts is a good way to find ignition and mechanical issues that interfere with combustion.
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Old 01-12-2019, 05:30 PM
  #26  
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Just some clarification on O2 sensor as it relates to mixture determination

The O2 sensor is the closed loop feedback circuit which primarily sets the injector pulse width to manage stoichiometric mixture of fuel and air. For a combustion event, the stoich ratio runs from ~12:1 up to 16:1 where combustion is no longer possible. The O2 sensor works on the aggregation of all combustion events in the engine, so a single event out of spec will have a 15.6% effect on overall combustion control setting(1 of 8 cyls). In context, where we are talking about 'weak spark' the affects will typically be felt on all or most cyl, given that the wires are typically not replaced individually. That means that ALL cylinders will experience the same poor combustion events, and that the stoichiometric ratio is tending toward very rich(12:1). Unburned fuel in meaningless except where the post combustion air injection is in play(928s do have this), which burns that unburnt fuel in the exhaust stream. This can lead to uncorrelated results of the O2 control because the fuel is not consumed in the combustion chamber, but is taken care of in the exhaust stream. As the spark weakens at the plug gap, the flame front moves slower, and weaker, and this causes both LOW mixture ratio, tending toward 12:1 which is a HIGH CO reading; And a certain amount of unburnt fuel which is partially burned in the exhaust stream. High "ratio"(16:1) is a "lean" mixture, not a rich mixture which would be about 12:1. As the mixture moves away from ideal stoichiometric of ~14.7:1 toward rich mixture of 12:1, the O2 sensor will report this, and the LH will shrink the pulse width. The converse is true as the mixture goes lean - 16:1 where the pulse width of the injector will widen and drive toward 14.7:1.

In summary - a weak spark across all cyls will cause a movement toward rich mixture(12:1). The O2 sensor will report this to the LH, which will reduce the pulse width, and less fuel will be introduced. Power will drop off as the engine attempts to maintain running ratio within the parameters of the LH pulse width control. Unburnt fuel will be burned in the exhaust stream, falsely reporting a more normalized combustion event(s).
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Old 01-12-2019, 05:38 PM
  #27  
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One issue that I've seen is that a complete misfire (no ignition) will cause a cylinder's worth of un-burned fuel and air to be sent down the exhaust pipes, and skews the readings toward the lean side.
Enough of this can end up with fouled plugs, due to being soaked with fuel as the ECU tries to "correct" the mixture, and can cause further cylinders to start misfiring.
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Old 01-12-2019, 07:31 PM
  #28  
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Originally Posted by docmirror View Post
Just some clarification on O2 sensor as it relates to mixture determination

The O2 sensor is the closed loop feedback circuit which primarily sets the injector pulse width to manage stoichiometric mixture of fuel and air. For a combustion event, the stoich ratio runs from ~12:1 up to 16:1 where combustion is no longer possible. The O2 sensor works on the aggregation of all combustion events in the engine, so a single event out of spec will have a 15.6% effect on overall combustion control setting(1 of 8 cyls). In context, where we are talking about 'weak spark' the affects will typically be felt on all or most cyl, given that the wires are typically not replaced individually. That means that ALL cylinders will experience the same poor combustion events, and that the stoichiometric ratio is tending toward very rich(12:1). Unburned fuel in meaningless except where the post combustion air injection is in play(928s do have this), which burns that unburnt fuel in the exhaust stream. This can lead to uncorrelated results of the O2 control because the fuel is not consumed in the combustion chamber, but is taken care of in the exhaust stream. As the spark weakens at the plug gap, the flame front moves slower, and weaker, and this causes both LOW mixture ratio, tending toward 12:1 which is a HIGH CO reading; And a certain amount of unburnt fuel which is partially burned in the exhaust stream. High "ratio"(16:1) is a "lean" mixture, not a rich mixture which would be about 12:1. As the mixture moves away from ideal stoichiometric of ~14.7:1 toward rich mixture of 12:1, the O2 sensor will report this, and the LH will shrink the pulse width. The converse is true as the mixture goes lean - 16:1 where the pulse width of the injector will widen and drive toward 14.7:1.

In summary - a weak spark across all cyls will cause a movement toward rich mixture(12:1). The O2 sensor will report this to the LH, which will reduce the pulse width, and less fuel will be introduced. Power will drop off as the engine attempts to maintain running ratio within the parameters of the LH pulse width control. Unburnt fuel will be burned in the exhaust stream, falsely reporting a more normalized combustion event(s).
Doc, I should probably have called the fuel/air ratio in my post the more conventional air/fuel ratio that you use. That would help folks who don't pay close attention to the terminology.

I'm not current on the air injection reaction stuff on some years where air is injected into exhaust manifold ports to "complete" the oxidation of HC's that pass through the cylinders unburned.

Originally Posted by Hey_Allen View Post
One issue that I've seen is that a complete misfire (no ignition) will cause a cylinder's worth of un-burned fuel and air to be sent down the exhaust pipes, and skews the readings toward the lean side.
Enough of this can end up with fouled plugs, due to being soaked with fuel as the ECU tries to "correct" the mixture, and can cause further cylinders to start misfiring.
Again, this would show up only when there's partial combustion either in the chamber or, as Doc suggests, in the AIR ports in the exhaust manifold on cars so equipped. The oxy sensor does not respond directly to raw HC, only to the effects it has on combustion someplace upstream.
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Old 01-12-2019, 07:41 PM
  #29  
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^^^^^dr bob, docmirror, and hey allen: thanks for the detailed explanations of what is happening. informative and helpful. I enjoy learning more
about the cause and effect of various systems and failures.
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Old 01-12-2019, 08:50 PM
  #30  
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Originally Posted by dr bob View Post
Again, this would show up only when there's partial combustion either in the chamber or, as Doc suggests, in the AIR ports in the exhaust manifold on cars so equipped. The oxy sensor does not respond directly to raw HC, only to the effects it has on combustion someplace upstream.
The explanation I was given, and how I've read elsewhere, is that the Oxygen sensors were detecting O2, rather than the combustion byproducts of it.
When there is a misfire, A high O2 charge flows trough, since the fuel has not consumed the O2, and the sensor erroneously indicates a lean condition due to the high O2.

This is wandering into the realm of splitting hairs over the functional behavior on the sensors.
Here is the AllData FAQ on oxygen sensors, if it's of any interest.
http://repair.alldata.com/online2hel...ensor_work.htm

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