Engine won't start when hot...

[rwabdu]

For the last few months, my engine won't start after I recently shut down and the engine is still warm. The starter will crank and the prop will spin a few times. if I keep trying the starter it won't spin the prop at all and I need to turn the master switch off and wait a while.  I replaced the starter and the battery this year with no improvement. Also replaced the boost pump before this issue happened in the fall. It always took a little extra cranking when it was warm but I have almost been stranded twice now. If I give it an hour or two it will usually eventually start. I am kind of at a loss for ideas about what to do next.... Pretty annoying whenever making a fuel stop. 

Two videos:

https://drive.google.com/file/d/151idG2w5KEFizbbxkTloKfWZXjtDOx4w/view?usp=share_link

https://drive.google.com/file/d/102h7BNLJavxX8N1Hw0XMnbLMo8wWxuv0/view?usp=share_link

[gwav8or]

Not a mechanic but that sounds like a battery issue. Have you tested the battery to see what the cold cranking amps are? Not a mechanic so I am probably wrong but it’s where I’d start.


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[rwabdu]

Thanks for your thoughts. I suspected that as well and just replaced the battery last week (it was 5 years old). The plane started right up the at the hanger but I landed for fuel and could not get it started again.

[1967 427]

Without knowing if you have had a recent overhaul or new cylinders, I have to agree the battery sounds weak.  In the second video no Bendix or starter spinning sounds, again signs of a sick bat.

also not an AP mechanic, but a retired mechanical engineer who has live under the hood of may cars

[1967 427]

You may need to confirm your charging system is operating properly 

[47U]

With anything electrical, first check the grounds.  Where does the battery negative terminal cable go to? 

And then check all the other electrical connections in the starting circuit.  Looks for signs of corrosion on the wires/cables/terminals.

And don’t forget about the starter solenoid.  Without knowing what year C you have, it could be just inboard from the ship’s master solenoid on the firewall.  The heavy cable from the starter will be attached to the starter solenoid.  

Maybe a long shot… engine heat could be affecting the starter solenoid after shutdown?

[MikeOH]

I'm with @47U on this.  Smells like a temperature dependent bad connection.  If the starter solenoid is in the engine compartment, I'd start with it.  Could be grounds, but the temp dependence makes me suspect the solenoid first (since you've already replaced the starter and battery).

[skykrawler]

Perform an open-circuit voltage (OCV) test of the battery (master switch off).  It should have greater than 12.3v and if it doesn't you may have a problem with the charging system or battery.   Do this test after a flight.   What is the voltage on the system when the engine is in cruise with normal avionics on?  Should be > 13.4v.

If that checks out then look and the ground between the engine and the airframe.  All the starter current must go through that ground.

[Fly Boomer]

1 hour ago, skykrawler said:

Perform an open-circuit voltage (OCV) test of the battery

The Concorde CMM suggests that the battery be at rest for at least 4 hours:

Open Circuit Voltage (OCV) – The voltage of the battery at rest (no charging or discharging current present).  A stable OCV requires a rest of at least four hours.

https://batterymanagement.concordebattery.com/BatteryDocs/5-0171.pdf

 

[Kelpro999]

Might be difficult to diagnose given the short problematic period but measuring voltage at each positive connection from the starter back to the battery during cranking will identify the voltage drop point. Measure each point with the negative voltmeter terminal to the negative battery post.

 The problem may still be found cold yet not to the point of no crank if volt drop is one volt or more at any check point. Also measure voltage between the starter housing (gnd) and the neg battery post while cranking for overall ground path back. 

 Remember, you are only comparing battery cranking voltage vs system check point voltage during these tests.

[47U]

3 hours ago, skykrawler said:

If that checks out then look and the ground between the engine and the airframe.  All the starter current must go through that ground.

This would be true if the battery is located in the avionics bay in the tail.  I think (?) all the C models (unless modified) have the battery on the left side firewall above the pilot’s footwell.  My battery is grounded to the engine case, so the path to the starter is from the starter solenoid, to the starter, through the engine case, and back to the battery negative post through the ground wire on the engine case.  

My battery is grounded to the engine case just above the lower-left engine mount isolator.  

[MikeOH]

8 hours ago, skykrawler said:

Perform an open-circuit voltage (OCV) test of the battery (master switch off).  It should have greater than 12.3v and if it doesn't you may have a problem with the charging system or battery.   Do this test after a flight.   What is the voltage on the system when the engine is in cruise with normal avionics on?  Should be > 13.4v.

If that checks out then look and the ground between the engine and the airframe.  All the starter current must go through that ground.

Per the OP's description, his issue is ONLY when HOT.  That (and that he replaced the battery) tends to suggest the problem is NOT with the battery or charging system.  Otherwise, he'd have a problem with cold starts, too.

[outermarker]

What is the whining noise? I know it is not the SOS.

 

[rwabdu]

17 minutes ago, outermarker said:

What is the whining noise? I know it is not the SOS.

 

Fuel boost pump

[outermarker]

Should that be? I would think you would prime few seconds then turn off the fuel pump. An 1/8" "cracked" throttle yields about 1000 rpm when the engine is running. Anything more than an 1/8" open could cause a flooded condition and really make it difficult to start before the battery drains. What does the manual suggest for hot starts? Difficulty in starting could also be caused by the engine timing being off a touch.

[jeremyc209]

I don't even turn the boost pump on to start after refueling usually. Doubt that's the problem though.

I'd be inspecting/replacing cables in the starting system, possibly heat is causing the resistance to rise enough in a bad cable that it can no longer support starting amperage. Maybe the master relay/contactor? 

[Hank]

@rwabdu, I only run the fuel pump on my C for cold starts, and then only until the fuel pressure stabilizes, then i turn it off.

For a hot start, just Mixture Rich, pump the throttle a couple of times and turn the key. It takes a little longer than a cold start.

If the engine won't turn over, and you've replaced battery and starter, then something is either discharging the battery or keeping it from charging in flight.