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Strange worsening misfire at idle
https://slantsix.org/forum/viewtopic.php?t=34867		 Page 2 of 2
Author:  Aggressive Ted [ Mon Apr 20, 2009 10:32 am ]
Post subject:  MOPAR Resistor values
Quote:
I've now seen and read your thread asking essentially the same question about ballast resistor values. I saw that Eric W uses the MSD 0.8ohm ballast with the Blaster 2 coil too. Theoretically this would give around 6V to the coil which, from what I've read, seems about right. May I ask what you went for in the end and how well it worked?
Sure.....I tried several resistors with the Orange ECU and MSD Blaster 2 chrome coil. The last two resistors I really liked. One was the MOPAR resistor they sell in the MOPAR racing catalog which measures around .5 ohms and the other is the large MSD .85 ohm coil that comes with the Blaster 2 coil. It has been working well for the last 35,000 plus miles. It reads 10 volts on the output of the resistor and 10 volts at the coil (600 rpm) idle.

After the last 35,000 plus miles NGK UR4 plugs gaped at .045 are a very light beige to white. They are super clean with some very light powdery build up that just rubs off. With the lower voltage (3 ohm and 4.5 ohm resistors) they had been running a medium to dark cinnamon color. I can post a picture on the my "link" later tonight if that would help. Overall I am pleased with the idle quality and lack of build up.
Author:  otherroutes [ Mon Apr 20, 2009 2:54 pm ]
Post subject:  Re: MOPAR Resistor values
As you can see from my last post, I've pretty much committed myself to the 3ohm coil / no ballast route. I was especially drawn to this because it eliminates the ballast resistor which, having read a lot on this forum, seems to be a bit of an ignition Achilles heel. Of course I had to add one when I retro-graded from the "computer" to the EI, so was pleased to see a way of running without one.
Quote:
I can post a picture on the my "link" later tonight if that would help. Overall I am pleased with the idle quality and lack of build up.
I'd be very keen to see them, thank you yes - I have little experience before my /6 and enjoy learning anything I can, even if it's not directly related.

I started this thread with a specific problem which has gone down this EI / ballast path based on my theory. Whilst I'm sure I've found something worth dealing with, I'm nowhere near as sure that this is the root cause of my idle problem. I'm wondering whether my theory is more delusional wishful thinking over substance. Views, as always, welcome please.
Author:  SlantSixDan [ Mon Apr 20, 2009 7:03 pm ]
Post subject: 
I think you are probably right that your lumpy tickover problem is elsewhere. Excessive primary resistance can certainly affect how the engine runs, but tickover is the easiest operating condition as far as the ignition system is concerned.

More than once, I have seen carburettors mimic ignition-related tickover problems almost perfectly...have you tried blowing out the idle circuit with a spray can of carb cleaner? Is it possible some grit or water has got past the fuel filters?

(And you're definitely not alone on thought-trails to nowhere productive. It happens to me oftener than not when diagnosing tough-to-resolve problems; I'll get an idea of what's causing the probllem and come up with a complete logic chain to support it…but in the end it all comes to naught.)

All that said, tho, two questions: Are you still running the orange MP box, and are you still running the unmodified MP "light spring only" distributor advance curve?
Author:  otherroutes [ Tue Apr 21, 2009 12:49 am ]
Post subject: 
Quote:
I think you are probably right that your lumpy tickover problem is elsewhere.
Thanks Dan. Whilst it's nice to be right, I find it even more satisfying to work like an engineer should - methodically and with measurement. It's better, if sometimes more frustrating, to have your feet pushed back down on the ground.
Quote:
Excessive primary resistance can certainly affect how the engine runs ...
Actually, the Blaster2 provides too little primary resistance; nominally only 0.7ohms. So with the MP ballast I'm only getting around 4.5V to the coil. My (probably delusional) theory was that the consequential lower spark energy is not keeping the plugs clean at idle.

Another detail to the symptom (that I've managed to weave in to fit into my theory) is that it will "free idle" with only the occassional "pop" of a misfire. The worsening misfire followed by a stall is only when the transmission is engaged.
Quote:
...have you tried blowing out the idle circuit with a spray can of carb cleaner?
No, but I will.
Quote:
Is it possible some grit or water has got past the fuel filters?
I have no idea. Changing the fuel filter was high on my to-do list and has now been bumped up higher :)
Quote:
(And you're definitely not alone on thought-trails to nowhere productive. It happens to me oftener than not when diagnosing tough-to-resolve problems; I'll get an idea of what's causing the probllem and come up with a complete logic chain to support it…but in the end it all comes to naught.)
Some of the greatest scientists have made the facts fit the theory. It's human nature I think. All we can do is be aware of the tendency and do our best to stick to some sort of method.
Quote:
Are you still running the orange MP box ...
I'm still running the MP EI conversion, yes. Everyone calls it the "orange MP box", but mine's blue.
Quote:
... and are you still running the unmodified MP "light spring only" distributor advance curve?
I'm still using the distributor that came with the kit, yes. DusterIdiot has curved me a dizzy, but I want to get the engine to a stable baseline so that I can get some solid MPG figures before I (a) install that dizzy, and (b) start to find the right initial advance. Even with the issues around MPG as a measurement, I still think it's one of the best actual measures I have available to me of engine efficiency.

Whether the current dizzy is "light spring only" or not, I'm afraid I don't know. I didn't examine it before fitting. I have measured the advance curve though. As you can see, there seems to be knee in the curve at around 1250rpm. My assumption is that this is where a second, heavier spring kicks in.
Author:  emsvitil [ Tue Apr 21, 2009 1:40 am ]
Post subject: 
Here, play with this to figure out the energy getting to the spark plug:

http://www.bgsoflex.com/igncoil.html


Note: It's based on a V8, so you need to do a little math for a 6 cylinder
(8000rpm V8= 6000rpm S6)


Drop voltage to simulate ballast resistor
Author:  SlantSixDan [ Tue Apr 21, 2009 8:04 am ]
Post subject: 
Quote:
I'm only getting around 4.5V to the coil.
Measured, or calculated? If that's actual (measured) voltage, it's definitely low. A quick check is to rig a temporary wire from line voltage (battery + or alternator B+) to coil +. Use a wire with insulated crocodile clips. When the engine begins idling poorly, attach the test wire and see if the idle clears up.
Quote:
Another detail to the symptom (that I've managed to weave in to fit into my theory) is that it will "free idle" with only the occassional "pop" of a misfire. The worsening misfire followed by a stall is only when the transmission is engaged.
This does not point towards insufficient ignition primary voltage.
Quote:
I'm still running the MP EI conversion, yes. Everyone calls it the "orange MP box", but mine's blue. I'm still using the distributor that came with the kit, yes. Whether the current dizzy is "light spring only" or not, I'm afraid I don't know. I didn't examine it before fitting. I have measured the advance curve though. As you can see, there seems to be knee in the curve at around 1250rpm. My assumption is that this is where a second, heavier spring kicks in.
All this suggests you aren't actually running the MP kit, but rather one of the "NOS Mopar electronic ignition kits" that have been offered on eBay by the likes of Central Jersey Motor Parts for the past few years. These use a stock distributor and a stock (or aftermarket) ignition module. Do you recall where you got the kit, and what sort of packaging it came in?
Author:  otherroutes [ Tue Apr 21, 2009 8:21 am ]
Post subject: 
Quote:
Measured, or calculated?
Measured. The voltage drop is across the ballast. It's running on all new, relay switched, wiring.
Quote:
This does not point towards insufficient ignition primary voltage.
Hence my concern about self-delusion :)
Quote:
Do you recall where you got the kit, and what sort of packaging it came in?
Yes, it's the NOS ebay kit. I assumed it was the MP kit on the basis that it came with MP documentation. Silly me!
Author:  Aggressive Ted [ Tue Apr 21, 2009 9:33 am ]
Post subject: 
Quote:
Quote:
Aggressive Ted wrote:
I can post a picture on the my "link" later tonight if that would help. Overall I am pleased with the idle quality and lack of build up.
I'd be very keen to see them, thank you yes - I have little experience before my /6 and enjoy learning anything I can, even if it's not directly related.
Reference:
I am running the large MSD .85 ohm coil that comes with the Blaster 2 coil. It has been working well for the last 35,000 plus miles. It reads 10 volts on the output of the resistor and 10 volts at the coil (600 rpm) idle.

After the last 35,000 plus miles NGK UR4 plugs gaped at .045 are a very light beige to white.

I posted some pictures of the plugs I took last weekend. Click on the red link by my signature.
Author:  otherroutes [ Tue Apr 21, 2009 10:09 am ]
Post subject: 
Quote:
A quick check is to rig a temporary wire from line voltage (battery + or alternator B+) to coil +. Use a wire with insulated crocodile clips. When the engine begins idling poorly, attach the test wire and see if the idle clears up.
Good plan. Although the next time I'll be able to work on the old bus will be at least a week away. But I'll try this before changing anything. Thanks.
Author:  otherroutes [ Mon May 11, 2009 2:07 am ]
Post subject: 
Update

Well, I know that proper scientists and engineers only change one thing at a time, then measure the effect. And, of course, that was my plan. But nobody seems to have taught the slant that.

Saturday morning I went to collect it. It usually starts with a crank or two. It did in fact fire almost immediately, then just as quickly died. A few more cranks - nothing. To save the battery we connected up a car on booster cables and then cranked it until, eventually, with a cough and a splutter it burst into high revs with everything disappearing in the cloud of fumes.

But then we realised that it was idling much better. Although not yet hot we just left it idling to see if it would do its usual build-up of misfires leading to stalling. No, it kept idling away. I started to wonder whether this event had cleared out some debris in the idle circuit as suggested by SlantSixDan.

Perhaps what I should have done was leave it at that and monitor over time. But I had my Pertronix coil, and the time I have available to work on the van is limited. So I decided to break the one-at-a-time rule and fit the new coil. I also checked the timing (since I'd noticed some pinking) and found it more advanced than my notes said I'd left it. So I think it had been advanced by the mechanic who got it through the emissions check for me.

So, the changes made were:
- 3ohm Pertronix Flame Thrower replacing the 0.7ohm MSD Blaster2,
- Ballast resistor removed from circuit,
- Initial advance reduced to 10*,
- A good "smoker's cough" first thing in the morning (the van, not me).

The net result of this is a very happy /6. It's idling beautifully with no hint of misfire. In driving it's pulling hard and smoothly.

At last I now feel I can baseline the fuel consumption before installing the DI's recurved distributor and finding the right initial advance.
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