351M Ignition System Problems - FSB Forums
 
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post #1 of 11 (permalink) Old 09-12-2005, 05:50 PM Thread Starter
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351M Ignition System Problems

Ok, first off, I sincerely want to apologize: I have a full-size Ford with a 351M, but it's a '77 Thunderbird, not a Bronco. Problem is, the T-bird forums for that age car are almost 100% dead. Not many answers to be had from them. The full size Bronco crowd has probably more 351M's than any other group, so I'm hoping that you guys can help me out.

This is my problem:

There is an intermittent fault in my ignition system. It seems to shop up after 20 minutes or so of driving. I can always start the car in the morning, but 20 minutes into the 35 minute drive to work, BAM - it's done. When I try to start it again, there is no spark. Well, more precisely, there is no spark while cranking, but as soon as the ignition is released to the "run" position, there is one spark ... but not, of course, enough to start the engine.

I've already replaced coil, ignition module (expensive red grommet, of course, not the cheap blue one! why is it that always happens?), cap, rotor, wires, and plugs. Only thing that I haven't done is replace internal components in the distributor. I'm getting power to both the ignition control module and the coil, even during the no-spark times. I'm pretty sure that the reluctor and the pickup coil are both good - the other day, when this happened, I could turn the rotor just barely a hair, but it was enough - the reluctor got close enough to the pickup coil and I could hear it click.

I'm completely at a loss as to what the problem with this is! :

Any suggestions that y'all could offer would be greatly appreciated!!!
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post #2 of 11 (permalink) Old 09-12-2005, 06:24 PM
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Sure sounds like the module again... Might have it tested just to be sure.. But try to test it while it's warm. Hell drive around the parking lot at the parts store till it dies and then have them test it... Other than that I'm out of ideas....Fog

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post #3 of 11 (permalink) Old 09-12-2005, 06:50 PM
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Sounds like either a ground, or something getting hot, most likely distributor at that point.

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post #4 of 11 (permalink) Old 09-12-2005, 08:10 PM
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Almost sure it's the ignition module. They're heat sensitive and even generate a lot of heat internally. Even had one melt out the "potting" plastic. I always carried a spare in my '79 Bronco's console. Don't go cheap on them with something made in China, get a Borg-Warner.
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post #5 of 11 (permalink) Old 09-12-2005, 09:49 PM Thread Starter
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I've tried three separate ignition modules. The junkyard one, the original one, and the brand new fresh outta the box one all exhibit the same behavior.

There's just no way that it can possibly be the ignition module. (And, unfortunately, there's no more ignition modules of this type in southern california!!!!)

The distributor thing ... I've been thinking about that, but then, -what- in the distributor could it be, if the pickup coil and the reluctor are triggering properly? I don't get it!

Where else are there wires that could come loose? Are there any other grounds besides the one on the valve cover bolt on the left-hand side?
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post #6 of 11 (permalink) Old 09-13-2005, 11:08 AM
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Quote:
Originally Posted by OctoberRust
I've tried three separate ignition modules. The junkyard one, the original one, and the brand new fresh outta the box one all exhibit the same behavior.

There's just no way that it can possibly be the ignition module. (And, unfortunately, there's no more ignition modules of this type in southern california!!!!)

The distributor thing ... I've been thinking about that, but then, -what- in the distributor could it be, if the pickup coil and the reluctor are triggering properly? I don't get it!

Where else are there wires that could come loose? Are there any other grounds besides the one on the valve cover bolt on the left-hand side?
For about 50 bucks get yourself a reman distributor,or replace the pick-up in the distributor,I had the same problem on an old 78 T-bird I had.I finally found the problem to be the pick-up.Wires going into the dist.looked ok but soon as a wiggled them it would stall,most noticeable when the engine was warm and the wires were able to flex around a little breaking contact.
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post #7 of 11 (permalink) Old 09-13-2005, 11:16 AM
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My bad. Didn't read where you had already replaced the ignition module. Sorry.

If it consistantly cuts out after 20 (+- a few) minutes, might be heat related. I'd thouroughly check EVERY wire in the ignition system, especially if they run close to heat sources like an exhaust manifold, the radiator and even the coolant return to the radiator.

If everything's okay, then the problem is probably internal to the distributer.
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post #8 of 11 (permalink) Old 09-14-2005, 03:07 AM Thread Starter
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So, inside the distributor itself, are there any other things with wires, aside from the pickup coil itself? If we rule out any mechanical issues, then is there anything else in the distributor that could conceivably be going wrong?
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post #9 of 11 (permalink) Old 09-14-2005, 09:04 AM
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Quote:
Originally Posted by OctoberRust
So, inside the distributor itself, are there any other things with wires, aside from the pickup coil itself? If we rule out any mechanical issues, then is there anything else in the distributor that could conceivably be going wrong?
Other than the pick-up itself going bad,or the wirs going into the pick-up,no.Well other than the vacum and mechanical advance,but those generally will not cause a stalling problem.
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post #10 of 11 (permalink) Old 09-14-2005, 06:55 PM
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I think the clue is in the +-20 minutes where it consistantly dies. Makes me think engine heat somewhere. If sometimes 20 minutes, sometimes an hour and 20 minutes, and sometimes never, I'd think mechanical.

But one question, are there any mods to the engine?
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post #11 of 11 (permalink) Old 03-24-2014, 04:40 AM
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F-100 w/351M

Hi
I know this is a very old post, but I have the exact same problem - just started this spring: "there is no spark while cranking, but as soon as the ignition is released to the "run" position, there is one spark"

I have check just about everything. I cold starts fine, then when is half warm it stops. And will not start for some time.

I suspect the distributor - or some internal parts there. I planned to try to change the pickup coil, thou I see i looks quite rusty inside. Maybe just as well to buy a complete distributor via rockauto..

@OctoberRust: I guess you found your problem..? (2005 post..)
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