Troubleshooting ignition problem (need advice on where to look)

[XH29N0G]

I have a problem I am hoping someone will be able to help me diagnose.

I took the car out today and noticed that it was cutting out as I cruised along at 2500 rpm on the highway.  The there would be a miss that would coincide with a drop in the tach.  I have had similar things happen in the past and they have been wiring related (loose connection from the key).  I have also had this behavior.  I was able to make it home and on the way home I noticed that it seemed better (less misses) when I drove very gently (keeping the rpm below 2000 rpm).  If I drove at higher RPM 2500-3000 (did not try higher because I wanted to get home) the miss would return.  It would also be there for a while even at lower RPM.

My hypothesis is that it has something to do with the hot engine compartment since this is the first hot day I have driven the car. 

I have an MSF blaster III coil and a 6al ignition.  Have any of you experiences a similar thing and where would you look first.  If there is not a test, I am thinking of buying a replacement for one or the other (or both) to bring along in case I need to swap it out. But if it is clear that it is one and not the other then I will save some money.

Are there other possibilities or options about what to carry along as a precaution?


Thank you in advance.

[crj1968]

I know with the MSD the test is if it's sparking it working as far as MSD is concerned.

Are you sure it's a miss and not a fuel starving issue? Could have the same symptoms...

Maybe check out your cap and rotor for starters. Will it rev in nuetral?

[XH29N0G]

I appreciate the quick reply.

I don't think it is fuel because I just swapped the fuel pump and pump pushrod (less than a tank and a half ago and it had 6-7 psi).  Also I have an A/F and the A/f has been steady (which it wasn't when the fuel pump went).  I can rev it all the way to the rev limit I set (6000 RPM).  It pulls well. 


The issue today was that if I ran it steadily at 2500-3000 RPM, it had a random miss (maybe once every 5 to 6 seconds, sometimes more or less frequent).  I did not want to try it out more than that.  I also found that the miss would settle out if I ran at a lower RPM.  If I ran it up to the point where it was missing every 5 or 6 seconds, it would still  miss when I dropped RPM, but then the miss would go away.

If the line from MSD is it needs to give no spark whatsoever, then it runs according to their definition, but all I care about is being stranded.  I am trying to figure out if it is more likely to be the MSD unit or the coil or something else so I have along a spare if it decides to fail. 

[crj1968]

hmmmmmm. Not sure - hopefully someone more smarterer than I will chime in.

I dont know where I read this but maybe unhook your tach and see if that does anything...   

[firefighter3931]

Where do you have the coil mounted ? Is it on the engine or inner fender ? Is the coil mounted vertically or horizontally ? How old is the coil ? Did it do this from the beginning or only after the engine got hot ?

Often when coils overheat the ignition will behave as you described 

I would try another coil and see what happens. If it's an oil filled coil and it's mounted horizontally the windings can be exposed and overheat. Oil filled coils should be mounted vertically and away from the engine for best results. MSD even recommends this in their instructions. 

The 6AL also needs good voltage...12V minimum to work properly so check that as well. 

Other possibilities include ; pickup resistance out of spec or reluctor gap too wide, spark leak/arcing from ignition wires, excessive resistance in the ignition wires, cracked/defective distributor cap, burnt rotor etc....

Lots of (potential) Gremlins in there....



Ron

[XH29N0G]

Thanks.  I'll try to swap the coil. I have a MSD blaster 3 coil (oil filled with the pin connector (not recessed) on top).  I can move it.  Would you recommend mounting on the inner fender?  This coil or is there another coil you would recommend (like the ss coil)?

Some answers and some more background:  Coil is presently mounted vertically, it was horizontal for 3 years and then I switched it this winter.  It did not start doing this until the engine was hot.   The MSD is full 12 volts (no ballast resistor).  I think the plug wires (firecore) are good.  They are 3 years old and have about 4000-5000 miles on them. Your mention of the plugs brings back another issue which could be pertinent and suggest this is an issue that has been around longer than I thought.  I had a misfire a few months ago after putting in plugs gapped to 045 that I thought was the plugs.  I regapped them to 0.037 and the misfire went away.   Maybe it was just the start of a coil issue and now with the heat (or time) it is back simply because it is worsening.

[c00nhunterjoe]

Sounds like a plug wire or coil dropping out

[XH29N0G]

I just thought of a question.  During yesterday's trip, the tach would drop with each miss.  I assume the signal from the MSD 6AL to the tach was dropping out as the misfire occurred. 

Would/could the coil do this?  The tach is an inexpensive sun supertach that is wired to the grey wire coming off the 6AL.

[crj1968]

That's why I thought maybe unhook the tach and see if it changes. good luck!

[firefighter3931]

Thanks.  I'll try to swap the coil. I have a MSD blaster 3 coil (oil filled with the pin connector (not recessed) on top).  I can move it.  Would you recommend mounting on the inner fender?  This coil or is there another coil you would recommend (like the ss coil)?

Some answers and some more background:  Coil is presently mounted vertically, it was horizontal for 3 years and then I switched it this winter.  It did not start doing this until the engine was hot.   The MSD is full 12 volts (no ballast resistor).  I think the plug wires (firecore) are good.  They are 3 years old and have about 4000-5000 miles on them. Your mention of the plugs brings back another issue which could be pertinent and suggest this is an issue that has been around longer than I thought.  I had a misfire a few months ago after putting in plugs gapped to 045 that I thought was the plugs.  I regapped them to 0.037 and the misfire went away.   Maybe it was just the start of a coil issue and now with the heat (or time) it is back simply because it is worsening.

When you widen the plug gap it will put more stress on the ignition components. I run mine at .040 gap but use an e-core style coil which is mounted on the inner fender away from heat/vibration. 

For a 6AL I would recommend either a BlasterSS or better yet a FireCore CD mini-coil. 

It wouldn't hurt to run an ohms test on the ignition wires and especially the coilwire. With the 8.5mm wires you should see 50/ohms per ft resistance.

Quick story :

In the past I had a customer who fried his coilwire ; the terminal was not properly seated in the coil and created a lot of resistance which fried the coilwire at the terminal. This is why I prefer the HEI style terminals. The connection is much more secure.


Ron

[XH29N0G]

Ron,

Thanks.  for the suggestions.  I measured the resistance on the wires and the coil.

I get 1(178), 3(183), 5(203), 7(222), 2(91),4(99), 6(128), 8(118) for the firecore 50 plug wires.
The coil wire is an accel wire and has a resistance of 288 ohms.  It is 300+ ferrospiral (I think it is 2 foot long)
The coil has 0.8 ohm and 5.05 ohm (it is specified at 0.75 ohm and 4.70 ohm).

I don't know how to tell bad wires, but my guess is that all of this would check out.

I am inclined to try a coil swap to see if it makes a difference.   

 

[firefighter3931]

Everything looks to be in spec. The coil was probably tested after the engine bay cooled down... correct ?

If that's the case try testing it when the engine bay is hot and the motor starts to misfire. If the coil is the problem your readings will be different 


Ron

[XH29N0G]

Coil was cold at the time of the test.  I'll swap the coil to see if that changes things.  I suppose a missed fire must also feed back to the tach circuit in the MSD unit.

[firefighter3931]

Coil was cold at the time of the test.  I'll swap the coil to see if that changes things.  I suppose a missed fire must also feed back to the tach circuit in the MSD unit.

A coil swap is definitely the right place to start. 

A misfire will show up on the tach...as it should. 


Ron

[XH29N0G]

Update: I swapped the coil for a firecore minicoil (thanks Ron).  I only had a chance to take the car for a short spin today (about 20 - 30 minutes at 2500-2500 RPM after warm up) and so far so good.  I won't be able to do more until next weekend at the earliest and if weather permits will run it out to my fathers house which should test it out fully.

An observation, I put the new coil on the fenderwell and left the old one in its place on the intake.  There was a considerable difference in temperature between the two locations and the coil at the intake (that was not running) could even have been hotter than the one on the fenderwell. 

[firefighter3931]

That sounds much better. It appears that the old oil filled coil was the problem 

Conductive engine heat is a killer on electrical components. I bet of you had a lazer temp gun and shot readings on both coils the surface temp difference would be quite significant. 

The inner fender mounting is a better location for sure and will extend the service life of any coil, inmo 


Ron