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Diagnosis of a bad coil 1951 Hornet

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(@autotran)
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Hi all - this is really an ongoing story for the past few years. Every once in a while, especially in hot weather, I'll turn the car off, run into a store, or do something for 5 - 10 minutes, come out, and then my car, a 1951 Hornet, will not start. Then, if I let the car cool down with the hood open for 10 - 15 minutes or so, the car starts and usually runs normally. In some cases, it runs rough for a while and then smooths out. I had been attributing this to a fuel problem, but now I'm wondering if it's a bad coil. I have an NOS coil, so I could swap it out, but this problem is so intermittent, I'd like to be more certain that this is the cause. I'd hate to get stuck somewhere.

My hypothesis is that when the coil gets hot, from sitting in a hot, but motionless car (with no air flowing by), the primary circuit either opens, due to a break, or shorts out. Once the car cools down, it runs again. Also, the car runs fine while driving around. Maybe because there is sufficient air flowing over the coil from the cooling fan and/or motion of the car to keep the coil cool.

Does anyone know how to check a coil? For example, I assume the resistance of the primary circuit should jump up when hot? If so, how many ohms should the primary exhibit when at room temp, and how hot would I need to get it to fail? Would a hair drier work, or do i need to put the coil in the oven?

Thanks everyone,

Mike

PS - could a bad coil cause a loss in fuel economy (I can only get around 11 MPG) without a noticeable deficiency in performance? Also, I notice that when I'm going down a hill with my foot off the gas pedal I hear a fair amount of rumbling and very small/short backfires from the exhaust system. I know the carb is set for lean best idle, so I'm wondering if a bad coil could cause this too.


 
Posted : 16/09/2015 4:03 am
(@holden)
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It certainly could be bad. I know I wouldn't trust a 60+ year old coil. I'm a big NOS fan myself, except when it comes to structural or integral components directly related to the operation of the vehicle. An NOS window regulator is one thing, a coil, something else.

If you do replace it, keep in mind that nearly all components these days are made "somewhere else", meaning some items have a 1 in 10 chance of being any good at all. Back when I was running points, I would buy condensers a dozen at a time and planned on returning 9 or 10 of them.


 
Posted : 16/09/2015 6:10 am
(@m-patterson56)
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On the subject of "made somewhere else", I had a nearly identical issue with our '51 Hornet and had also perceived it to be fuel related based on symptoms. It, too, would start and run fine until the engine bay got hot, and would refuse to start after a shut down/restart attempt. I had previously changed out the coil with a Chinese made 6V unit from NAPA while chasing an earlier problem (it made no difference in that case) but left it installed because it just seemed like the right thing to do, i.e., replace a 60-year-old coil. Welllllll. Guess what was causing my hot restart problem? I changed back to the original coil and my problems disappeared. Too bad a big outfit like NAPA can't see fit to sell US made products or at least make them available. Twice the price would have been a good deal.
Like you, I hate to just start replacing components until I hit on the one that's bad......just not my way of troubleshooting, but at some point, you just have to give it a try if all arrows point to that part.
Frank


 
Posted : 16/09/2015 12:05 pm
(@autotran)
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Hi Russ and Frank - thanks for the very interesting and relevant stories. I'm going to hold off on swapping out anything until I've done some more diagnostic work. Call me crazy, but i want to see the smoking gun. I'm thinking about doing a before-after test with my coil; that is resistance of each the primary ans secondary at room temp and then after a short stint in the oven at 175 (the lowest my oven goes). Maybe something will jump out at me.

There's a lot of really good stuff coming out of China, but some that's not so hot. I wish there were a way of paying extra for USA-made parts. It'd be well worth it.


 
Posted : 16/09/2015 12:38 pm
(@holden)
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[quote="Michael Cohen" post=13363] . . . but some that's not so hot. :woohoo:

Not so hot. I love puns.

Seriously, though. I agree with both of you. One good thing about NAPA, they'll always take it back. Of course, as y'all mentioned, it's never a good idea to just start replacing at random. That said, keep in mind, especially when it comes to our old cars, it's always a good idea to have good, known spares on hand. Coils, points, condensers, spark plugs and wires and cap included.


 
Posted : 16/09/2015 12:50 pm
(@jomoali)
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Mike,

It never hurts to have a spare coil in the trunk, so it seems worthwhile to buy another coil. Another option is to borrow a known good coil and see if the problem stays the same or not.

Back in "the day", a frequent cause of non-starting when a car had been off for just a few minutes was the piston rings shrinking when warm, instead of continuing to press normally on the cylinder walls. After 20 minutes or so, the car would usually start. A sign of this problem is the starter turning a bit faster than normal (due to lost compression past the rings). I saw this problem with several '46 to '48 Plymouths and Dodges. Also with a '41 Ford. These cars ran very well, only the starting was an issue.

I have not experienced this with a Hornet, but I did have a '47 Hudson six which had been an instant starting car, but towards the end of my using the car, when the smoke from the breather was about the same as from the exhaust pipe, (a lot from both!) it was beginning to have a problem starting when warm.

Per


 
Posted : 16/09/2015 12:56 pm
(@dlm31)
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Per, this sounds like a story from my teenage days. " Ran good at first, after I got done with it, it didn't run so good" LOL
I believe there is a test in the mechanical procedures book for this.

[quote="Per Christiansen" post=13365]Mike,

It never hurts to have a spare coil in the trunk, so it seems worthwhile to buy another coil. Another option is to borrow a known good coil and see if the problem stays the same or not.

Back in "the day", a frequent cause of non-starting when a car had been off for just a few minutes was the piston rings shrinking when warm, instead of continuing to press normally on the cylinder walls. After 20 minutes or so, the car would usually start. A sign of this problem is the starter turning a bit faster than normal (due to lost compression past the rings). I saw this problem with several '46 to '48 Plymouths and Dodges. Also with a '41 Ford. These cars ran very well, only the starting was an issue.

I have not experienced this with a Hornet, but I did have a '47 Hudson six which had been an instant starting car, but towards the end of my using the car, when the smoke from the breather was about the same as from the exhaust pipe, (a lot from both!) it was beginning to have a problem starting when warm.

Per


 
Posted : 16/09/2015 3:20 pm
(@m-patterson56)
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I had a near carbon-copy situation with our '47 six, where it would start/run great 'til it got up to temp, then it was almost impossible to get a hot restart. This was in the heat of Summer (85 for us Western Wa. people) and it had all the earmarks of a fuel/carb problem. I hooked up a mobile oscilloscope and drove it with my brother (an electrical engineer who knows what those squiggly lines mean) watching the o'scope. The problem became instantly obvious as the engine bay temp rose. Again......a bad coil that got progressively worse with elevated temp, so bad near the end that we barely made it home. An old coil off of a Jet cured it immediately but I'll put a new one on eventually..........probably the next time it quits.
F


 
Posted : 16/09/2015 5:10 pm
(@autotran)
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Hi all - thanks for the great suggestions. The scientist in me is about to kick in. I've been working on cleaning up the leads on my volt-ohm-ammeter so that I can measure small resistances; from what I've been reading on the internet, the resistance of the primary winding in an ignition coil is in the range of 0.5 to 2 ohms. So, here's my experiment - take two coils, one off the car and the other NOS unit that I bought last year, and measure the resistances of both the primary and secondary windings on both. Then set the oven for 175 F, get them both hot and then measure again. If my hypothesis is correct, the resistance on the assumed bad coil will jump up to infinite once the coil gets hot, whereas the "good" coil will have only a marginal change. I'll post the results, probably on Sunday.

The other thing I'm thinking is to bring a can of Freon, or whatever we use nowadays that won't hurt the atmosphere, in the trunk. When this phenomenon occurs again whilst I'm on the road, I can spray the coil with the Freon to cool it down, and then try to start the car.

Also, a spare spare coil makes sense. I bought the NOS one because it came with the bracket that i knew would fit. Is there a NAPA part number that someone can recommend?

Thanks.


 
Posted : 17/09/2015 12:12 am
 ok54
(@ok54)
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Does the engine turn over normally when it doesn't start or does it turn slowly? If it's slow, make sure you have the right battery cables, and for that matter make sure your battery is good. If it turns over OK then you might have vapor lock.


 
Posted : 17/09/2015 1:09 am
(@autotran)
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Hi Eddie - The engine turns over just fine, so it's not the cranking system. As for vapor lock, that has always been in the back of my mind. But, the last time this phenomenon occurred (this past Sunday), I opened the hood, took off the air cleaner and slowly worked the throttle whilst looking down the throat of the carburetor. The two streams of gas squirting out of the accelerator pump jets were clearly visible, so i don't think it's a (lack of) fuel problem anymore. Also, it's not likely to be flooded because 1) the choke valve was standing straight up - indicating that the choke is working properly and 2) when I originally went to start the car after it had stalled (in the middle of the road!!) I didn't use the gas pedal at all.


 
Posted : 17/09/2015 2:29 am
(@m-patterson56)
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Micheal,
Ditto on all (including "in the middle of the road"). It had me convinced it was a fuel related failure for a very long time......[i]both[/i] cars.
Frank


 
Posted : 17/09/2015 4:34 am
(@Josh Keller)
Posts: 0
 

Try changing the condensor. I had a similar problem with no hot start or even warm start this summer; even though when started, it ran fine cold and hot. It wasn't the coil. It wasn't vapor lock. Nothing made sense, but when I replaced the condensor, the problem was solved. Not sure why, but why argue with success?


 
Posted : 17/09/2015 8:06 am
(@autotran)
Posts: 107
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Hi Josh - thanks for the suggestion. I should have stated earlier in this thread that, in fact, I did have a bad condenser back in the Spring. That was the subject of another long thread called "Three fuel problems - maybe all related?". The problem I faced was an occasional miss that just got worse and worse until eventually the car wouldn't even idle anymore. After exhausting all things fuel related, I finally heeded the advice i had received earlier to check the coil and condenser. Swapping out the coil didn't seem to help - that problem, but swapping out the condenser did. So, that problem was fixed, and i thought i was on my merry way.

The way I'm thinking about it is that I had had an acute failure of my condenser superimposed on a years-long background of this intermittent hard starting problem.

As I say, I'll do some diagnostics soon and let everyone in on the findings. BTW - I may find out that there's a problem with the new condenser too, just as you suggest. Just because it fixed one problem doesn't mean that it doesn't have other faults. :unsure:


 
Posted : 17/09/2015 8:58 am
(@Josh Keller)
Posts: 0
 

Michael - I changed the condensor in the spring too, and that one went bad fast...they are just a wild card. Been spending time on the Pertronix website lately, might eliminate the points and condensor.


 
Posted : 17/09/2015 3:25 pm
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