70 Bronco Wont Start After Warm Up

[tonytony9]

Hi Everyone,

Took the Bronco on a few longer drives recently in hottest weather since I purchased; about mid 90's and around 100 miles each way. When I stopped for gas, could not get it to start back up. Not crank whatsoever, although, still had all auxiliary power. Opened the hood, checked my coil ground and positive, checked connection to the distributer, and let it cool for about 10 minutes. Then, fired right back up. This is now happening consistently the past four days when hot. On cold start, no issues.

I have limited knowledge on where to start in terms of diagnosis. Any advice would be greatly appreciated.

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[cldonley]

Have you been on longer drives and heated it up and had it start fine? Is your fuel line well clear of the exhaust? Could be vapor locking, but that usually takes more than 10 minutes to clear up. Do you have a spare coil? Maybe take if tor a drive with a known good coil and see if it happens again. Also, just to clarify, when I hear it 'won't crank' I think the starter is not turning the engine over. Sounds like you mean it is turning over (starter is cranking) but the engine isn't firing. Correct?

 

[Slowleak]

Next time it does that, look down in the carb and work the throttle lever. You should see a stream of gas shooting into the carb. If you don’t then you know it is a fuel problem. In hot weather you can get a vapor lock, or fuel can evaporate from the carburetor bowl.
Generally when a stock Bronco is hot like that you can hold the pedal halfway down and it will start after a few cranks….

 

[tonytony9]

Have you been on longer drives and heated it up and had it start fine? Is your fuel line well clear of the exhaust? Could be vapor locking, but that usually takes more than 10 minutes to clear up. Do you have a spare coil? Maybe take if tor a drive with a known good coil and see if it happens again. Also, just to clarify, when I hear it 'won't crank' I think the starter is not turning the engine over. Sounds like you mean it is turning over (starter is cranking) but the engine isn't firing. Correct?

This is a new problem, I bought the bronco back in March and it has started without problem up until 4 days ago. Its been between 80-90 degrees now for the last few weeks and I have been on more than a few long drives. I have to check the fuel line but I do believe it is well clear. Runs along the frame on the driver side, exhaust on passenger side. Just need to confirm distance from drive side exhaust manifold. I do not have a spare coil. But I could see if I can grab one from an autoparts store.

The best way I can describe it (I will try to grab a video next time), is when I turn the key to crank the motor, nothing happens. As if the ignition itself was not hooked up to anything. Then, after brief cooling period, it will fire right up as normal.

Next time it does that, look down in the carb and work the throttle lever. You should see a stream of gas shooting into the carb. If you don’t then you know it is a fuel problem. In hot weather you can get a vapor lock, or fuel can evaporate from the carburetor bowl.
Generally when a stock Bronco is hot like that you can hold the pedal halfway down and it will start after a few cranks….

Assuming there is no electrical issue this makes sense to me. But I am not getting a crank in this situation.

 

[Slowleak]

That helps clarify things. Check your cables and connections to the battery, starter, ground wire to the block. Heat increases resistance and bad connections fail. You could also have a heat soak issue. If the exhaust is close to the starter it can cause the starter to suffer from heat soak. A heat shield might resolve that.

 

[m_m70]

That helps clarify things. Check your cables and connections to the battery, starter, ground wire to the block. Heat increases resistance and bad connections fail. You could also have a heat soak issue. If the exhaust is close to the starter it can cause the starter to suffer from heat soak. A heat shield might resolve that.

This^^^ I installed a mini starter and it solved my heat soak issues that came up when I installed long tubes.

 

[DirtDonk]

Even with an overheated starter though, there should be a good healthy click from the fender mounted starter relay.
Assuming there is still one installed that is.
Some people install the new style starters and remove the old starter relay. So definitely check to see how yours is laid out.

 

[m_m70]

Check your neutral safety switch wiring as well if you're running an automatic.

 

[cldonley]

I would go straight to the parts store and get new battery cables and the cable that runs from the solenoid to the starter. People often replace batteries, starters and other parts when all they needed was battery cables. Ask me how I know...

 

[tonytony9]

Even with an overheated starter though, there should be a good healthy click from the fender mounted starter relay.
Assuming there is still one installed that is.
Some people install the new style starters and remove the old starter relay. So definitely check to see how yours is laid out.

I believe it is the starter solenoid that is causing the problem. It was moved by PO to right next to the passenger side exhaust manifold. The problem occurred again today. Let it cool, disconnected the starter and ignition wires, reattached, fired up.

I was watching a few videos of the new style starter. Thinking about doing this. More than likely will just replace the current solenoid and remount higher on the fender away from heat.

Check your neutral safety switch wiring as well if you're running an automatic.

Manual, good tip tho

I would go straight to the parts store and get new battery cables and the cable that runs from the solenoid to the starter. People often replace batteries, starters and other parts when all they needed was battery cables. Ask me how I know...

Also going to do this^

 

[tonytony9]

This^^^ I installed a mini starter and it solved my heat soak issues that came up when I installed long tubes.

If i understand correctly, the mini starters bypass the solenoid? I did some brief reading in the parking lot letting it cool off today lol

 

[DirtDonk]

No, they don’t.
Some people do it that way, because Chevy did it. But most of us here prefer to leave the original starter relay on the fender, and simply wire it to the new starter.
Win-win and the benefits of both schools of thought.

Your 70 model would probably have originally had the starter relay there on the inner edge of the wheel well, facing the exhaust manifold.
Somewhere between there and 71 or so, they moved it to the front facing surface, just behind the battery. Probably stay a little cooler that way. My 71 is on the front, and I’m pretty sure all the 69s I ever saw were facing the exhaust manifold. Pretty sure too, that I’ve seen ‘70 models there as well.
So yours may in fact be original.

 

[DirtDonk]

When it won’t click or crank, do you still have power to everything else? Headlights, radio, heater!

 

[gr8scott]

If i understand correctly, the mini starters bypass the solenoid? I did some brief reading in the parking lot letting it cool off today lol

Courtesy of our friends at Wild Horses.

https://www.wildhorses4x4.com/mm5/graphics/00000001/instructions/WH/6038%2009-16-19.pdf

 

[m_m70]

By no means an expert on mini (gear reduction) starters and how they should be hooked up. I bought mine from Wild Horses years ago and used their instructions which said to use the relay so that's what I did.
So I still have the starter relay. I moved my relay to the front of the wheel well facing the battery as it just looks better there. I went with this starter when I went with long tube headers. It's cooler for the starter, easier on the battery and a lot easier to install or remove.

No issues since.

 

[tonytony9]

When it won’t click or crank, do you still have power to everything else? Headlights, radio, heater!

Yes, power to everything else

 

[tonytony9]

No, they don’t.
Some people do it that way, because Chevy did it. But most of us here prefer to leave the original starter relay on the fender, and simply wire it to the new starter.
Win-win and the benefits of both schools of thought.

Your 70 model would probably have originally had the starter relay there on the inner edge of the wheel well, facing the exhaust manifold.
Somewhere between there and 71 or so, they moved it to the front facing surface, just behind the battery. Probably stay a little cooler that way. My 71 is on the front, and I’m pretty sure all the 69s I ever saw were facing the exhaust manifold. Pretty sure too, that I’ve seen ‘70 models there as well.
So yours may in fact be original.

Makes sense, I am planning to move it to the front side.

 

[DirtDonk]

OK, then it doesn’t sound like the typical bad battery cable issue.
A lot of us have run into that over the years, but the tell is that once it won’t click the starter, nothing else works either.

 

[DirtDonk]

That’s a good call to move the relay. And doing so will most likely necessitate at least some new battery cables.
Never hurts to renew, and upsize the battery and starter cables.

Some tests you can run, just in case it is still doing this when you get back on the road, driving it in the heat.
Carry some cool water with you and the next time it won’t start, dowse the starter relay. Give it a moment to cool, then try to start again. If that fixes the problem, then perhaps you need a more robust relay.
If that doesn’t fix it, then move on up the chain to see what else might be causing the issue.
Something between the ignition switch and relay. Perhaps one of the wires is compromised.
Follow the red with blue stripe wire as far as you can and see if it has any kinks, tears, cuts, or otherwise compromised points.

 

[DirtDonk]

Oh, and another thing…
Since you have a 70, that presumably has not been rewired(?) you need to go through and do some general maintenance that is not normally required on modern vehicles with modern connections and fuse panels.
Separate the two rectangular firewall connectors with all the wires coming and going, and make sure that all the connections are clean, tight and rust free.
Apply some dielectric grease and reinstall them.
Next go to the fuse panel and remove all five fuses. Clean them up, and reinstall them.
If the fuse contact points need cleaning, do that too as well.
Glass fuses build a crust of oxidation that, over time will reduce the ability of the electrons to pass through.