Warm no-start

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Oct 22, 2018
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I'm reaching my wits' end here, so I'm tossing this out to the big guns. Maybe something will be obvious to somebody out there.

The rig:
- 1978 FJ40
- Original 2F, rebuilt, de-smog'd
- Chevy TBI (AFI)
- OEM dizzy

The issue(s):
Starts up mostly fine when cold - it runs rough for the first block or so, but gets up and going just fine after that. Once warm, it idles rough. But if you give it the gas, it smooths out nicely. Then, once I've been driving it around a while, and it's nice and warm, if I turn it off and come back to it somewhere between 10-60 minutes later (and usually nowhere close to my house), it won't start. Or if it does start, it will stumble a little as it catches. But mostly it doesn't start. It acts like it's carb'd and flooded or has no spark - just cranks and cranks but never starts.

Things I know/have tried:
- No obvious vacuum leaks (though it's entirely possible I just haven't found it yet)
- Spark is good (coil is delivering spark to the dizzy, and it's making it to the plugs)
- Fuel pressure is good (12 PSI, new fuel pump and new pressure regulator diaphragm)
- Injectors are spraying fuel (under all conditions where they should)
- Fresh ECT
- Fresh IAC
- No codes from the ECM
- Fresh coil
- Fresh distributor cap
- Fresh plugs and wires
 

spotcruiser

Geezer
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It sounds like the way my stock 78 ran when hot and my carb cooling fan wasn't operating. Have you got a functioning carb cooling fan? Additionally, look for cracks in the bottom of the intake manifold directly below the throttle body. Additionally, verify that your heat riser valve in the exhaust manifold operates properly (changes position from cold to hot).
 
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Just a guess, dodgy igniter / ignition module or the like playing up when it gets hot.

Are you using a ballast with the coil? I think the 78 has the ballast wire in the harness.
One thing to try to eliminate a low coil volts possible ballast problem is next time it won't start when warm, run a wire from bat+ to coil+ temporarily and see if it starts.
[once it fires the extra volts to the coil might keep it running]
 
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Joined
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It sounds like the way my stock 78 ran when hot and my carb cooling fan wasn't operating. Have you got a functioning carb cooling fan? Additionally, look for cracks in the bottom of the intake manifold directly below the throttle body. Additionally, verify that your heat riser valve in the exhaust manifold operates properly (changes position from cold to hot).
It sounds like the way my stock 78 ran when hot and my carb cooling fan wasn't operating. Have you got a functioning carb cooling fan? Additionally, look for cracks in the bottom of the intake manifold directly below the throttle body. Additionally, verify that your heat riser valve in the exhaust manifold operates properly (changes position from cold to hot).
Totally acting like it did when it was carb’d and would vapor lock. I have a heat riser block off plate. No cooling fan. But I know it’s not vapor locking by the fuel coming out of the injectors.
 
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Just a guess, dodgy igniter / ignition module or the like playing up when it gets hot.

Are you using a ballast with the coil? I think the 78 has the ballast wire in the harness.
One thing to try to eliminate a low coil volts possible ballast problem is next time it won't start when warm, run a wire from bat+ to coil+ temporarily and see if it starts.
[once it fires the extra volts to the coil might keep it running]
It does have a ballast resistor. I’ll give the direct wire to the coil a shot.

I did just pop in a new coil in this process to eliminate it as a possibility. The reason I did was because I noticed when I was cranking that my tach would show ~1200 rpm and then occasionally drop to ~200 rpm. It seemed like the new coil fixed that symptom, but maybe not?
 
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I think the US 78-80 40's have the ballast wire in the harness. If you have a separate ballast resistor and your wiring is original it might have a ballast in series with another ballast if you get my drift.
That would lower the volts too much .
 

Green Bean

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Also, just throwing this out: A fidgety ground for the engine can do weird similar things. Might want to double all of your grounds are solid.
 
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I think the US 78-80 40's have the ballast wire in the harness. If you have a separate ballast resistor and your wiring is original it might have a ballast in series with another ballast if you get my drift.
That would lower the volts too much .
So there might be a resistor somewhere else the igniter? Here’s what my igniter+coil situation looks like…

17EBD49A-858A-44EF-B3D7-DC534F675903.jpeg
0A0783DD-B01F-4369-8B80-5EF9462045AB.jpeg
 
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Could be.
If you turn on the ignition and take a voltage reading from the feed wire to the resistor [not the coil connected side] and it reads 12v it means no extra ballast wire. If it reads 8 to 10v the original ballast wire is still wired up.
Well this seems promising. Voltage TO the resistor is 10.2V. Voltage on the coil+ side of the resister is 6.9V.
 
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Well this seems promising. Voltage TO the resistor is 10.2V. Voltage on the coil+ side of the resister is 6.9V.
Was that running or just with the ignition on?
More of a curiosity than anything else.


Any day of the week, the power going to the ballast resistor should be very close to battery voltage (as a guess max 0.5v less). Say 12v parked and 13.5v while running.

Either you have a resistor wire or bad connections. If you bypass the stock wiring with a jumper does it change how it runs?
 
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Was that running or just with the ignition on?
More of a curiosity than anything else.


Any day of the week, the power going to the ballast resistor should be very close to battery voltage (as a guess max 0.5v less). Say 12v parked and 13.5v while running.

Either you have a resistor wire or bad connections. If you bypass the stock wiring with a jumper does it change how it runs?
The measurement I made was just with the key on / not running. I verified the voltage across the battery terminals was >12V. I'll be running a jumper to the coil+ this afternoon. (Unfortunately I'm stuck in meetings earning the money to throw at my old car.)
 
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Well this seems promising. Voltage TO the resistor is 10.2V. Voltage on the coil+ side of the resister is 6.9V.
Welp, not as promising as I thought. The voltage on the wire that was going to the ballast resistor was 13V once the vehicle was running and 10.6V to the coil.

Here's the interesting part that may or may not be pertinent:
With the jumper to coil+ with 13V the engine ran at really low RPM, but really smoothly. When I switched it back, the engine ran at the normal warm-up RPM around 1k rpm, but it kind of stumbles. Then I realized that I'm running what should be my 12V "ignition one" wire for my fuel injection to the coil+. I'm wondering if supplying low voltage to my ECM's power relay is causing some heartburn here.
 
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low voltage to my ECM's power relay is causing some heartburn here.
The ecm needs full 12v for sure. At this stage it might be worth ditching the the ballast resistor and wire the original wire straight to coil plus. I'll take a guess the original ballast wire is still in your truck.
Even if it isn't , full 12v to the coil won.t damage the coil for a short period of time. But take a volt reading from coil+ when running,
 
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The ecm needs full 12v for sure. At this stage it might be worth ditching the the ballast resistor and wire the original wire straight to coil plus. I'll take a guess the original ballast wire is still in your truck.
To be clear, it's just the 12v that switches the main power relay. The wire from the coil+ is not supplying the main 12V power to the ECM and injectors.

Re: ballast wire...
How would I find it? Just start testing for the ~10V coming out of the igniter while it's running?
 
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To be clear, it's just the 12v that switches the main power relay. The wire from the coil+ is not supplying the main 12V power to the ECM and injectors.

Re: ballast wire...
How would I find it? Just start testing for the ~10V coming out of the igniter while it's running?
Actually, it looks like the wiring diagram shows the ballast resistor much like I have it wired.
 

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