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Discussion Starter #1
Engine: 3.9L V8 with 92k miles bone stock
Issue: after the engine is at operating temp it is often hard to start, i.e it takes more cranking to get it started.
No issues with the starter
Battery is in good shape
Wires are a few years old, about 5k miles
Plugs have about 15k miles, champion
Rotor and cap, coil and ign mod have about 8k miles
Replaced fuel pump relay because I noticed the pump would not turn on occasionally it's now fixed, it turns on every time the key is turn to the second position
At operating temp, I shut off the engine and let is sit for about ten minutes to simulate the issue. I checked the ohms of the fuel temp sensor (800-1000) and the coolant temp sensor for the ecu (220-230)
Can someone check their ohm levels in the same situation so I can compare?
I'm wondering if the sensors are giving bad signals which would cause improper fueling when warm.
Can anyone help?

Cheers...
 

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02 sensors, low fuel pressure upon warm up, temp sensor and a shorting amplifier or coil can cause your concern.
 

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have you tried starting it with the gas pedal to the floor ... if that works it may be a sticky air idle control valve. In fact this is a very common issue with these engines.

I would also check the hoses for leaks ... an imbalance or air / fuel causing hard starting when warm could definitely be because too much air is getting into the system. When warm the ECU adds less fuel so you not be getting the right mix.
 

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Discussion Starter #5
Thanks or the replies. I'm wanting to find the smoking gun and not throw parts at it...
My o2 sensors have about 40k on them.
My fuel mileage has not changed since this issue started.
I've had o2 sensors fail on a Mustang 5.0...it ran terribley.
My D90 runs great, just needs more cranking when warm...
With a timing gun I checked for spark upon startup when warm...I've got spark.
I can check the ign mod with a voltmeter.

Cheers...
 

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Check the fuel temp sensor again & take an IR temp reading to compare. They've been known to cause hot start problems. If nothing else you'll be able to rule that one out.
 

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did you check the air idle control valve ... it's a 5 min job and a very common issue.
 

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Here is what I got from my test:
Fuel temp sensor @ 150f - 800-1000 ohms
Coolant temp sensor @190f - 220 -230 ohms

I have a spare fuel temp sensor (old with unknown mileage) and I installed and ran the same test with the same results/readings.
Fuel temp looks a bit off. 150º should be just under 500 ohms. Not sure if that is enough to effect the starting.
 

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Had a hot start issue on a GEMS...replaced coil, wires, O2 Sensors, and IACV and the problem disappeared. However, this was in conjunction with high mileage maintenance and not just for the hot start issue. Did not touch any fueling component at that time.

I have been known to throw part$ at problems though in lieu of proper diagnostics...
 
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