another issue:car miss at idle when warm

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cam.man67

cam.man67

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So...another issue. Now, the car starts up and runs fine cold, drives completely fine. It even idles pretty good now...right up until it doesn't. The symptoms now include a horrible miss and most often stalling at idle, but it only happens once the car is good and hot. Idle adjustment screws aren't making a difference at all. I mean, they can make the car idle better or worse, but they're not fixing the miss. The only way I can keep the car running is to pop it into neutral at stop signs....seems like keeping the idle up alleviates the miss.

Here are the car vitals:
Vacuum:19-20
Base timing:12 deg advanced
Idle rpm:around 1000, drops when in gear to about 800
Valve lash:12/22

I'm back to it being either a carb or spark issue. Would a failing fuel pump do this? It runs absolutely fine off idle.
 
Fuel pump probably wouldn't cause it because at idle the engine needs lees fuel than any other time.
If you are sure it only misses when warmed up I would highly suspect an idle circuit problem. (like lean)

If you choke it a little with your hand and the miss goes away, then you know that is the problem.

Vacuum leaks can cause what you describe also, so go over things real close in that regard.
 
When the motor misses, how many increments, or inches of vacuum does it fluctuate?
 
Is it a consistent miss on one cylinder or an erratic miss? What is 'good and hot'? Driven 5 minutes or 30 minutes or ??

Since the idle mixture screws don't directly efect it, I would be less inclined to suspect carb or fuel. When it gets hot, the ability of the ignition to fire the mixture can change.
- Distributor shaft wobble
- Spark plug heat range too hot: what type is being used?
- Ballast resistor wrong value; it needs to be less than 1 ohm cold; higher and the spark energy will drop too much when the ballast gets hot
- Coil overheating: check for ballast being in place and voltage to coil + being in the 6-9 volt range idling
- Valves sticking; have you done any cleaning of the valve guides or crankcase oil with Seafoam or Marvel Mystery Oil? The light valve springs of the /6 are prone to sticking.
 
Recheck the lash.Check it HOT. Check the rocker arm condition. The pads like to get beat up by the stems. The stems pound little divots into the arms making accurate adjustment nearly impossible..Loosen the intakes up, if you have to. Better a tic loose than a tic tight. I use .013minimum intake.

Consider a leakdown test

My slanty idles sweetly at 600(N), and the TC hardly drags it down at all.
One time I had this bright idea that a little tighter lash would make it more powerful.Well that was not so bright. Just a couple of thou, and that six seemed like a done motor. And the cover was back on and everything. I hate do-overs! I don't recall what I set it too. Ima thinkin 8 ish. At 12/13 it idles sweetly, and I don't care anymore how little power it puts down. Afterall the Odos rolled over at least 3 times since I rebuilt her. That would be about 220,000 miles. I might even have rolled over the 4th time already. Can't say for exactly sure.
 
I don't know what your previous issue was,, but if you haven't pulled the plugs ,, do so,, and scrutinize each plug,, keep them in order,.. I think one plug will look "different".. Find the cause of that plug looking diifferent,, will likely cure the miss...

Are you sure the plug wires are in the correct order,, while on choke and fast idle,, rpm's can hide a miss-fire, till chokes off,, will also hide a vaccum leak..

hope it helps
 
nm9stheham said:
Is it a consistent miss on one cylinder or an erratic miss? What is 'good and hot'? Driven 5 minutes or 30 minutes or ?? At least 20 minutes. I can run over to my rental farms and back without any issues, and that's about 15 minutes of driving. It's most often when I head into town and have a lot of errands to run, and am in stop and go traffic (obviously, I guess ;)).

Since the idle mixture screws don't directly efect it, I would be less inclined to suspect carb or fuel. When it gets hot, the ability of the ignition to fire the mixture can change. After thinking hard about it yesterday, I've ruled that mostly out, too. The carb has been rebuilt twice since I've owned it
- Distributor shaft wobble Had that, but no more since the distributor got new bushings
- Spark plug heat range too hot: what type is being used?I don't honestly remember, they were recommended on here but I don;t remember till I can get out to the garage this morning.
- Ballast resistor wrong value; it needs to be less than 1 ohm cold; higher and the spark energy will drop when the ballast gets hot Hmm...mine reads 1.5 ohm. Too high? I recall Allparts saying 1.5 was ok, but I could be wrong.
- Coil overheating: check for ballast being in place and voltage to coil + being in the 6-9 volt range idling Will do that. I'm leaning towards faulty coil--I have a spare that I may try today while running around to see if that helps, hurts, or changes nothing.
- Valves sticking; have you done any cleaning of the valve guides or crankcase oil with Seafoam or Marvel Mystery Oil? The light valve springs of the /6 are prone to sticking.Sure have! Marvel treatment last oil change.
Click to expand...

Thanks for the responses, guys.
 
I see that you show a 1.5 ohms ballast, If that resistance is when cold, then that is a problem for the ignition. The original ballast was .5-.6 ohms cold and < 2 ohms hot. A 1.5 ohm cold ballast will be around 5 ohms hot. That will cut coil current by more than half, and since the stored coil energy is proportional to the square of the current, then the spark energy will be dropped by a factor of 5 or so. Just like a weak coil..... (And what is posted on Allpar may be for hot ballast resistance but no matter; there is at least one other misleading bit of info there in the gauge circuit operation.)

So I would correct the ballast, regardless, if you are sticking with the original ignition system. You can sometimes find an MSD ballast that is .8 ohms cold at a lot of local box parts stores, or order from Summit/Jegs/etc. If not, then a BWD RU19 sold many places will be 1 ohm cold so is at bit better. The OEM ballast is PN 2095501 for the 2 prong part: I have bought them off of eBay.

I went through all this because my ballast was 2 ohm cold and 7 ohms hot! And the coil was weak too, and behaved somewhat like yours, except it took less than 20 minutes to falter. I replaced the ballast and the coil with a Pertronix Flamethower and the spark was all much better and consistent. Might not be your issue, but it helps to make these old ignition system as hot as they are supposed to be, at the very least. Or do the HEI conversion if you are not hep on originality.

BTW a more direct treatment of sticking intake valve guides is to hold the throttle open by hand and keep the engine revved while slooowly pouring 1/2 can of Rislone or Seafoam down the carb. Then put the rest in the crankcase. A lot of older engines had pretty weak valve springs and they ran cool and sludged in the guides more easily.
 
Ok, thanks! So, next question: what should the ohm reading be on the coil? I replaced the original with a flamethrower several year ago, reads 3.0 ohms. Too much? Not enough?
 
3 ohms is too much to run with a ballast, and with the wrong (higher resistance) ballast, that combo is really lowering spark energy. The coil that works with the Mopar ballast should be around 1.5 ohms. You can run the 3 ohm coil with the ballast bypassed. It would be worth a try to see if that helps.

The Flamethrower (and a lot of other coils) are available in both 1.5 and 3 ohm primary resistances. The advantage of running the ballast with the 1.5 ohm coil is that when cold, you get a bit hotter spark in the first 30-60 seconds while the car is warming up. Once warmed up, both combinations are about the same (assuming you have the right ballast.) And it can compensate at very high RPM's by increasing spark energy.. but that is not usual for a /6! But some folks just don't want to mess with the ballast so the 3 ohm coil works for that.

BTW, check the voltage at the coil + when warmed up and running, and also check the voltage at the ballast input while running to see if it is close to the battery voltage volts. The last test will make sure you do not have excess voltage drop through the ignition and wiring; that is a common cause for spark issues. If you don't have a voltmeter, you will be doing yourself a big favor to get a cheap one; they are quite useful on these cars.
 
nm9stheham said:
3 ohms is too much to run with a ballast, and with the wrong (higher resistance) ballast, that combo is really lowering spark energy. The coil that works with the Mopar ballast should be around 1.5 ohms. You can run the 3 ohm coil with the ballast bypassed. It would be worth a try to see if that helps.
Click to expand...

WOW! What a difference that made...totally fixed the spark issue! Thank you. :) Cold, the coil is testing at 9.6v...a lot different than the 6.2 it had when the ballast was in place. Cold startup is much easier now, too. Once warm, the spoil is staying at the same voltage, so there's no more miss. Win!
 
Good deal! You might want to keep an eye on the coil temps and maybe check coil resistance when good and hot. I'm not sure what the resistance should be on a hot 3 ohm coil, but just make sure it is not too hot, and the resistance is not too high. It seems like this setup changed on you recently and so I wonder if the coil is perhaps going away, and this just 'patched' it for a while. At least you know the cause of the symptoms. BTW, do you have the model number off of your coil?

I have to wonder how many Mopar (and other) systems have been pulled for something as simple as the wrong coil or ballast. I like to keep the (correct) ballast plus 1.5 ohm coil for the better warm-up running in colder weather, but you are much better off now.
 
Follow up: bought the msd real last but haven't installed it yet. I think I need a new cool. The current flamethrower is 45011, and warm, its testing at only 2.8 ohm. Voltage readings are around 10.1, which I think is too high for points. Am I right about that?
 
2.8 ohms is OK; the coil will have some tolerance and variation around 3 ohms. So, unless it is mis-firing, it would not worry about that part.

The points will handle 10V fine. As long as the condenser is there and good.

I have no idea if the MSD is designed to work with that coil. Check the instructions!
 
Dang...back to the drawing board. Last night the car ar saw some mixed highway and city. it was fine at speed, but idle roug and started to miss at stops. not nearly as bad as it was, but still not good. i guess since ive gor this new resistor, im goingto put it on and see about getting a 1.5 ohm coil. this is starting to get frustrating.
 
Almost sounds like the coil is misfiring since it acted up warm.... that is one possibility.

It this a consistent miss on 1-2 cylinders or a random miss?

You can try the coil; but once everything is good and hot, the combined resistance is about the same for the 3 ohm coil alone versus the ballast + 1.5 om coil.

When good and hot I would, back up and check:
- Vacuum levels
- Fuel pressure
- Adjust idle mixture screw to see if it has an effect (it should have a strong effect)
- Fast idle voltage at battery and at the input to the coil (or ballast for a ballasted coil)

You may have fixed the ignition and helped the main culprit, but need to look into another underlying problem. Nothing unusual in that on these old cars.....
 
Ok, we are sorta in business now! The coil definitely was the culprit. Replaced it with a 1.5 ohm flamethrower, added the msd ballast, and it runs/idles without any sort of miss. I haven't had a chance to run it hard for a long distance yet (other than hauling some chickens this morning.;)), but so far I think we are good. Thanks!
 
cam.man67 said:
Ok, we are sorta in business now! The coil definitely was the culprit. Replaced it with a 1.5 ohm flamethrower, added the msd ballast, and it runs/idles without any sort of miss. I haven't had a chance to run it hard for a long distance yet (other than hauling some chickens this morning.;)), but so far I think we are good. Thanks!
Click to expand...

Should we BAWK at that reference to Thunderbolt and Lightfoot? :)
 
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