Carbed 460, duraspark ignition.

Had an ignition miss problem last fall that I thought was fixed with a new coil, but returning home from a long trip yesterday I noticed it again; loss of power and a little roughness under light to medium throttle around 2,000 RPM. And if you look close you can see the tach bounce 100 RPM in time with the miss.

Like before I popped off the horseshoe connector and swapped in an old junkyard coil and the problem seemed to go away, for awhile, but after driving it some more a subtle miss is still there under light throttle but not noticeable under acceleration.

Plugs, wires, cap, rotor, distributor, coil, and module were all replaced about 2,000 miles ago with quality parts so that suggests the problem is elsewhere. The old original horseshoe connector does not strike me as the most secure way to make an electrical connection and since fiddling with the horseshoe coil connector seems to have an immediate effect on the problem I wonder if the problem is dirty or loose coil contacts?

Thanks for reading, any thoughts you might have are very much appreciated!

 

I doubt it is the horseshoe connector if I understand the symptoms correctly. You are having a miss at part throttle but not full throttle. But, full throttle gives higher cylinder pressure, which is when you need the most from your ignition system.

To me this suggests something that changes under part throttle. The bounce of the tach may indicate it is ignition-related, but it could mean that your RPM actually is dropping due to the miss.

If it is related to the ignition then I'd suspect the wiring to the pickup in the distributor. It moves when you put vacuum to the advance, and it moves the most under light throttle. We had one at the GTG last year that would die if you added vacuum advance. Rogue_Wuulf diagnosed it as bad pickup wiring. Sure enough, swapping pickups did it.

But, perhaps the problem isn't ignition and the bouncing tach is showing that the R's are dropping. In that case, maybe you are running so lean at that point that with EGR in you aren't getting the mix to fire every time. Does it happen when the temps are up? After a long trip when the engine is warm? That leans the mix still further and might cause a line to be crossed. And maybe the cool down while you change out the coil is enough to go back across the line. To test the theory, maybe pull the vacuum to the EGR? Or, you might give the cruise mix just a bit more fuel by turning the idle screws out 1 turn - just for a test.

 

Thanks Gary!

Here's the full history.

4/18/15 @ 98,915 miles. Removed air pumps, replaced timing chain (straight up), water pump, and plugs.

At the same time it was discovered the distributor (that I'd put 40k miles on) housing shaft was broken. From the looks of it someone before me used a great deal of force, hammers, chisels, and pry-bars to break it loose, so for this reason the distributor was also replaced at this time.

6/29/15 @ 000,417 miles. The next day, after driving 100 miles in 100* heat, ignition started cutting out at 2,500 rpm. Replaced ignition module. Ran fine.

8/26/15 @ 001,300 miles. Last time out temps were over 100*. Ignition cut out, missing and backfiring under a load. Swapped module with my spare, no change. Replaced coil with a cheap one and problem seemed gone until...

8/28/15 @ 001,400 miles. Light throttle miss and bouncing tach. Finally quit on the freeway. Slapped in junkyard spare coil, fired right up and ran smooth home but tach bounced wildly. Sometimes reading zero!

9/5/2015 @ 001,460 miles. Replaced distributor, cap, rotor, wires, and coil with best parts I could source locally. Smoothest it's ever run.

11/11/2015 @ 002,254 miles. Vaccum advance can on new distributor failed. Replaced it with 28 year old original. *sigh*

6/23/2016 @ 003,555 miles. Trip into the mountains on the 18th was hot and driving back home several days later in the rain I noticed a miss, lack of power and slight bounce (down) on tach. Seemed to smooth out at higher rpm and heavy throttle.

Pulled over, set spare coil in convenient space between distributor, AC bracket and steel fuel line and switched horseshoe connector and coil wire over. Maybe 3 minutes. Fires right up, miss apparently gone and getting full power in the mid rpm range, tach solid. However, a little while later, noticed very slight miss under light throttle around 1,500 rpm but no miss under medium to heavy throttle.

This is where I am today.

Random thoughts:

Heat. Measured 230 at the t-stat housing at the top of the pass on the 18th and all the other problems seem to have happened days after running in the heat. I don't think my fan clutch is engaging. Been through 3 of them over the years since the original failed and haven't found one that works right yet! *sigh*.

Horizontal coil mounting. Factory coil mount is horizontal. Original coils were epoxy filled, today they are oil filled and horizontal mounting is not recommended.

Ignition problems all started with new distributor and have a second new one in there now.

Cruise control is an example of a wonky electrical connection. On occasion the cruise control will malfunction and it will hunt back and forth. Wiggling the connector cures the problem, for a while. I cleaned and sprayed the connecter and that fixed it for 6 months but it's doing it again now. That got me thinking about the horseshoe connector.

Sure would like to figure this thing out!

 

So, two different distributors have given you the mis-fire? That being the case, it isn't the pickup - unless you swapped it from one dizzy to the other.

But, you had the miss while driving both in the heat as well as during a rain - when presumably it wasn't hot. So, it may not be heat.

What if you are getting too much advance at part throttle? Do you have any pinging? Could you hear it if you did?

And, back to the fuel mix. Any chance of disabling the EGR for a test?

 

I've had to replace mine.

It did fix my intermittent spark problem.

 

Jim - I'm assuming you mean your horseshoe connector? If so, I bow to experience.

 

The wires entering the connector had apparently broken inside the insulation.

The connector was inexpensive on Ebay so I soldered a new one on and it got rid of my hiccup.

 

Thanks Jim! Was your hiccup similar to mine? I wiggled the horseshoe wires while it was running but it didn't effect the idle in any way. At the very least I'll clean the horseshoe contacts. I looked at them as best I could and they certainly aren't shiney and bright.



Gary, thank you, I've thought about the EGR being the culprit and it's easy enough to disconnect and take it for a drive.

I still wonder if the 8/28/2015 ignition failure and crazy tach, to the point of reading zero while still running down the highway, is an important clue? How can that be except for a bad connection or bad tach?

 

If the tach is reading zero while the engine is running then you are right - it has to be a bad connection or a bad tach. And, since the tach is driven by the same signal that runs the ignition, there has to be a connection. (Pun intended. )

Any chance you have a bare wire on the lead between the coil and the tach? That would be the dark green/yellow wire.

 

The easiest way to tell is to listen. On my '84, when the fan engages fully, it sounds like a hovercraft under the hood. I can hear it over the engine noise. It's not super loud, but it does make a distinctive sound.

I wonder if you've got two faults at play.

One could be a fuel-related problem causing the minor RPM fluctuations you've noticed. An automatic transmission could easily allow such fluctuations at the engine with little or no effect on road speed. (Not so with a stick shift, as that is a rigid connection.)

The second issue could electrical, causing the major fluctuations on the tachometer that obviously don't match actual engine speed.

Just something to consider while troubleshooting. Could be you fix one issue but don't realize it because the second is still present. Or swapping in old parts you thought were good fixes one problem but reintroduces another.

 

I've also had an intermittent electrical problem that was definitley the horseshoe connector.

It was difficult to pin-point, & left me on the roadside several times while troubleshooting. I swapped various parts over a two-day period & each time appeared to have solved the problem......only to have it return in an hour or so.

Finally, after the truck had cut out in the the dark, my son saw arcing at the connector, as I cranked the engine.

Easy fix after two lost days

 

How hard would it be to disassemble the connector? Can you release the individual contacts with a small screwdriver (or special tool) like on most plastic connectors?

If so, you can give the individual contacts a good cleaning. You could also bend them slightly to increase tension against the contacts on the coil. I think I'd lean towards a cleaning and tweaking of the contacts versus splicing in a complete connector.

 

I briefly considered the idea of repairing the original connector, but Rockauto had new ones for around $4 so I ordered one. While I waited for it, I just crimped terminals onto the wires & slipped them directly onto the coil posts.

 

Thanks for the replies guys, very much appreciated!

Thinking and googling on the situation some more I'm beginning to think a bad connection, or short on the tach wire, would pretty much result in a complete ignition shut down rather than a miss. But a bad coil, wires, plugs, dizzy pick-up, or ignition module, might.

Please correct me if I'm wrong or there is something else to add the the suspect list.

I want to avoid a bias confirmation situation (where you've already made up your mind and are seeking confirmation) so as not to overlook anything, but the fact that switching coils immediately improves the situation just keeps leading me back there.

But 3 bad coils in 5,000 miles? Really?

As far as I know the ignition wiring is original. I believe the resistor wire is in place because I read reduced voltage at the bat + coil terminal with the key on and the motor not running. I've had the truck for 15 years and did not have this problem until the timing chain and distributor replacement about 5,000 miles ago and since that time I've replaced the distributor (again) along with cap, rotor, wires and plugs in an effort to solve this ignition miss problem.

IF these coils are all going bad in such a short time what could be the cause? Otherwise, what else is there?

 

To test the resistor, ohm from the red terminal of the horseshoe to the power in run pin of the DSII module connector.

You do this test with the key out of the ignition.