Help Me Diagnose My Miss

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mopowers

mopowers

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I warmed up my 340 Dart earlier this morning and started driving it down the street. It fired up like normal and was running great. A couple lights down the street, it suddenly dropped rpm and tried to die. I was able to nurse it over to the side of the road and nothing was out of place under the hood. I ended up nursing it home and it sputtered and popped the whole way.

The engine is a 340 w/ mild hydraulic flat tappet cam, 750 Holley DP. Ignition is MP electronic distributor w/ HEI module, MSD blaster SS coil, generic plug wires and RC12YC plugs gapped at .040".

It's running like it's got a mean miss. It doesn't want to idle and back fires out the exhaust when under load. The AFR gauge is reading as normal. Because I've had issues with the carb in the past, I swapped in a different carb and it ran the same. I swapped in my spare HEI and it ran the same. I checked the spark w/ a timing light and all 8 wires are receiving spark. The plugs all look the same as well. I've also checked a few other items as well. Coil seems to be receiving normal voltage.

Any ideas what I should check next? I'm assuming it's ignition, but am pretty perplexed at this point considering it happened so quickly. I appreciate your ideas!
 
"Swapped in the spare hei"
Cap and rotor stay with the dizzy or the car??
 
Do a static compression test. Could be a valve train issue. I know you don't want to hear that....
 
Have you checked for a vacuum leak? If you can't find a leak I agree with Mopar Tim, Check the compression.
 
4spdragtop said:
Chk distributor shaft for wobble? Elec ign? Maybe ECU?
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I can pull the distributor and check. I already replaced the HEI module with another one and it acted the same

Tooljunkie said:
"Swapped in the spare hei"
Cap and rotor stay with the dizzy or the car??
Click to expand...
Not sure what you're asking? The cap and rotor look great.

Mopar Tim said:
Do a static compression test. Could be a valve train issue. I know you don't want to hear that....
Click to expand...
I'm afraid of that, but doing a compression check is on my list. Hopefully it doesn't come to that. Nothing is rattling, so it doesn't seem like a valve train issue.

SGBARRACUDA said:
Have you checked for a vacuum leak? If you can't find a leak I agree with Mopar Tim, Check the compression.
Click to expand...
I checked for a vacuum leak, but couldn't find anything. It happened randomly all of a sudden. I'll do a compression test tomorrow.

dartman1965 said:
Could be the coil breaking down or leaking oil out of it.......
Click to expand...
It's an MSD blaster ss e-coil. I suppose it could be bad, but it's pretty new. Is there an easy way to test it?

Thanks for all the input guys.
 
If the backfires are out of the exhaust only, and not up through the carb, then it sounds like an ignition misfire, letting unburned fuel go down the hot exhaust and then exploding there. But that would not happen after a cold start...

How did you check for the vacuum leak? Put a gauge on it? Numbers?

What exactly is that 'normal voltage' to the coil? In numbers....

You can check resistances in the coil windings: First, put your meter leads together and read just the lead resistance; do that several times and average those lead resistance readings. Then measure resistance though the coil primary; check + to - terminals with at least one of them having the wires disconnected. Then subtract the lead resistance from before; primary resistance should be around 0.4 ohms. (It is not easy to measure this low resistance accurately with normal meters...)

Secondary resistance from coil - to the spark terminal should be up around 4000 ohms.

Note that resistances can be good and the coil can still be breaking down under high voltage. There often is no visible clue to this.

Check the lead resistance through the spark wire from the coil; it ought to be no higher than a few thousand ohms.

Reinstall the spark lead on the coil and put the other end 3/8" from metal; it ought to jump that size of gap easily with a snappy blue spark, and do it steadily while cranking.

Recheck under cap for any carbon tracks on the inside surface.
 
Bad rotor, or bad cap will cause a backfire.
 
I didn't get a chance to mess with this today much. Hopefully I can spend more time on it tomorrow. I appreciate everyone's help.

nm9stheham said:
If the backfires are out of the exhaust only, and not up through the carb, then it sounds like an ignition misfire, letting unburned fuel go down the hot exhaust and then exploding there. But that would not happen after a cold start...

How did you check for the vacuum leak? Put a gauge on it? Numbers?
Click to expand...

The vacuum numbers are the same as they always have been- ~11" at 750 rpm

nm9stheham said:
What exactly is that 'normal voltage' to the coil? In numbers....
Click to expand...

I measured 12.2V at the coil + terminal.

nm9stheham said:
You can check resistances in the coil windings: First, put your meter leads together and read just the lead resistance; do that several times and average those lead resistance readings. Then measure resistance though the coil primary; check + to - terminals with at least one of them having the wires disconnected. Then subtract the lead resistance from before; primary resistance should be around 0.4 ohms. (It is not easy to measure this low resistance accurately with normal meters...)

Secondary resistance from coil - to the spark terminal should be up around 4000 ohms.

Note that resistances can be good and the coil can still be breaking down under high voltage. There often is no visible clue to this.

Check the lead resistance through the spark wire from the coil; it ought to be no higher than a few thousand ohms.

Reinstall the spark lead on the coil and put the other end 3/8" from metal; it ought to jump that size of gap easily with a snappy blue spark, and do it steadily while cranking.

Recheck under cap for any carbon tracks on the inside surface.
Click to expand...

I will check the coil more thoroughly tomorrow. Thanks for your help!

Tooljunkie said:
Bad rotor, or bad cap will cause a backfire.
Click to expand...
Cap and rotor look near new.

AJ/FormS said:
As will lost lobes,

Does the miss affect power? or just idle?
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The miss is present under idle and heavy load, although at around 2,000 rpm when nursing it home, it seemed to smooth out a bit.
 
OK, re-check your coil + voltage when running rough, to make sure it is steady. Just looking for any indication of an erratic connection from the ignition switch and on through the wiring.
 
nm9stheham said:
OK, re-check your coil + voltage when running rough, to make sure it is steady. Just looking for any indication of an erratic connection from the ignition switch and on through the wiring.
Click to expand...

It may be difficult since it probably won't want to idle much, but I'll give it a shot this evening. Thanks!

BTW- The power for the ignition comes from a relay that is wired in. It's pulling power directly from the battery lug on the starter relay. The ignition switch is used to trigger the relay.
 
Are you running a vacuum advance? And how is it plumbed? I have seen the Vcan pull the plate around and cause the spark to jump to the wrong tower. This happened to me. The cure for me was to re-index the reluctor and the rotor. You may wish to try running it with the can disconnected. You may have to rotate the distributor with the engine running,until it begins to run properly.
Or it could just be cracked insulators on a couple of plugs, due to detonation...........
 
AJ/FormS said:
Are you running a vacuum advance? And how is it plumbed? I have seen the Vcan pull the plate around and cause the spark to jump to the wrong tower. This happened to me. The cure for me was to re-index the reluctor and the rotor. You may wish to try running it with the can disconnected. You may have to rotate the distributor with the engine running,until it begins to run properly.
Or it could just be cracked insulators on a couple of plugs, due to detonation...........
Click to expand...

Yes, I'm running vacuum advance from a ported vacuum source on the carb.
 
I was finally able to check a few things today before having to finish up.

1. Coil resistance was normal. Primary resistance was .36 ohms and secondary resistance was 4,400 ohms.
2. Cap and rotor look near new. I examined the hell out of them- No cracks or burns marks detected.
3. I checked all of the plug wires. Though I found one minor crack in one of them, the resistance of all of them was between 5280 and 7300 ohms. None of the ends were corroded or appeared suspect otherwise.
4. I double checked the vacuum advance it is working properly.

I did fire it up with hopes to check the timing, but got called to pick up the girlfriend from work. I'll have to start there tomorrow. Either way, it still runs very rough.

Tomorrow I intend to check the timing, vacuum, and voltage at the coil + while running (if I can get it to run long enough). I'll also run it at night to see if I can visibly see any arcing in the engine compartment.

I guess if I still don't find anything, it's on to a compression test and pulling the valve covers.

Any other thoughts?
 
Pull plugs, check compression and if it has vacum brake booster pull hose and plug off the vacum.

Had a fiso (f150) giving me fits today over a bad plug.

And last week an escape with bad gas. Didnt figure it out until i pulled a few plugs.
 
Yeah that bad gas is a pain. I had a case where bad gas lead to detonation which cracked a plug insulator.The EFI engine seemed to idle ok, but wouldn't take a load. I isolated the cylinder swapped in a new plug and away it went. A couple of weeks went by and the car showed up again. This time a different plug. I swapped in 4 new ones this time and told him to find a new gas station. I never saw it again.
 
AJ/FormS said:
Yeah that bad gas is a pain. I had a case where bad gas lead to detonation which cracked a plug insulator.The EFI engine seemed to idle ok, but wouldn't take a load. I isolated the cylinder swapped in a new plug and away it went. A couple of weeks went by and the car showed up again. This time a different plug. I swapped in 4 new ones this time and told him to find a new gas station. I never saw it again.
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Gas station. It stunk like heavy ethanol, co-op gas here is really bad. Ethanol sensor equipped car couldnt compensate. 19% ethanol. Havent bought a drop there in years.
 
In order to rule out a fuel delivery problem, try squirting carb cleaner into the carb throat when it is running and see whether it runs better or worse when you do.
 
Tooljunkie said:
Gas station. It stunk like heavy ethanol, co-op gas here is really bad. Ethanol sensor equipped car couldnt compensate. 19% ethanol. Havent bought a drop there in years.
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19%! ho-lee!
Hey where did you get the tool to measure that, I want one. Can I afford it? Probably not,lol.
 
AJ/FormS said:
Are you running a vacuum advance? And how is it plumbed? I have seen the Vcan pull the plate around and cause the spark to jump to the wrong tower. This happened to me. The cure for me was to re-index the reluctor and the rotor. You may wish to try running it with the can disconnected. You may have to rotate the distributor with the engine running,until it begins to run properly.
Or it could just be cracked insulators on a couple of plugs, due to detonation...........
Click to expand...

So I figured this out, kinda. I think a similar thing happened as described above. Turns out the ignition timing was advanced a LOT. I could barely see my 40* mark from the balancer on the timing cover, so there was probably around 50* advance at idle, causing it to run like ****. My only guess is that the reluctor phasing was off so much that the spark was jumping to the previous plug terminal, like what AJ was saying. How does this happen suddenly? And how do I ensure that doesn't happen again? Maybe the reluctor wheel slipped.
 
mopowers said:
So I figured this out, kinda. I think a similar thing happened as described above. Turns out the ignition timing was advanced a LOT. I could barely see my 40* mark from the balancer on the timing cover, so there was probably around 50* advance at idle, causing it to run like ****. My only guess is that the reluctor phasing was off so much that the spark was jumping to the previous plug terminal, like what AJ was saying. How does this happen suddenly? And how do I ensure that doesn't happen again? Maybe the reluctor wheel slipped.
Click to expand...
Well the reluctor is pinned to the shaft, so that's doubtful.
Give the rotor a little twist and let it snap back on it's own. It does snap back right?All the way to it's parked position,right? Every time right? If not,find out why and fix it.
If it behaves properly, prove the TDC mark is in fact TDC using the piston stop method. Then
put the crank at TDC#1, on the compression stroke. Then back it up to 20degrees advanced. Next pop the cap. And the rotor. Then check the reluctor vanes. One of them should be more or less directly lined up with the metal spike on the pick up (called the pole-piece).If it isn't, make it so, by loosening the D and rotating it. But first, make sure the Vcan rod is properly installed, not jammed and that the Vcan is properly working; use a vacuum pump.Next,sight a line down from the firing tip of the rotor to the aluminum housing, and with a Sharpie mark that position on the housing where you will be able to see it with the cap back on. Put the cap/rotor back on.Make sure the rotor is properly located and holds it's position. Now; That Sharpie mark better be right under whatever tower sends spark to #1 cylinder.

But if it's not, Then the reluctor may be improperly installed. Pop the cap and rotor again. Look at the top of the reluctor. You should see two slots down by the driveshaft; one with the roll-pin stuck in it. Pop that reluctor up off there, and put it back on using the other groove. I use a couple of screwdrivers to do this, whatever works, they're rarely very tight. Ok now align a reluctor vane (Any one) again with the pole-piece on the pick-up, by rotating the D. Erase the old Sharpie mark, and make a new one. Replace the rotor and cap.Again, that sharpie mark should be right under a tower. If it is, make sure the #1 wire goes to that tower and relocate the rest accordingly. Fire it up and set the base timing. Lock the D down and prove it's locked down by attempting to rotate the housing, by pulling on the Vcan.
So if it didn't move, then the base timing should theoretically never move again, except the tiny bit it does when the timing chain stretches.
If it starts running funny again, the first thing you can do is check the rotor alignment to the Sharpie mark you made. If it goes off by itself, that is not normal and should never happen. The possibilities are; jumped timing chain, a slipping intermediate driveshaft, faulty mechanical advance system, faulty reluctor plate location,slipping rotor. Of these, the first two usually spell disaster for the engine. The last three are the most likely. The very last one is the easiest to check.

BTW, the rotor does not have to be exactly in the center of the tower. But it has to be less than 1/2 the distance to the next nearest CCW tower. And since the advance mechanism rotates the rotor clockwise, the rotor can only be slightly CW of the tower. Else the advance mechanism will drive the rotor to the next tower. If it gets to the halfway mark, and the coil has enough power, it will rather spark there. So you do have a fairly wide target window, maxed out at about 15/16 degrees; Maybe 10CCW and 5 or 6 CW.
Ok so have at her,lol.
 
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