Truck won't start when sitting for 8+ hours in cold, but will with gas pedal

Question

I've had this problem on my truck (1996 Chevy S10) for about two years and it's steadily gotten worse. Whenever I leave the car sitting for over eight hours in a "cold" environment (the truck is located in Southern California so it's not super cold), it refuses to start (the starter works but the spark plugs won't ignite.) This is not an issue in hotter, dryer areas, where the car will start just fine.

I can sometimes fix the problem by sitting there for 30-60 min and pumping the gas while cranking the engine. When I crank and the engine refuses to start, sometimes it sputters a little after it dies ("diesels") which seems to indicate the gasoline/air mixture isn't burning. I'm not sure what the deciding factor to starting it is but after I've cranked it for a while while either slowly pressing down on the gas or pumping the gas, it will start, but this often requires me to crank it 15-30 times, with a minute or so wait in between, which I'm afraid damages the starter motor.

When I first had the problem, I got the spark plugs/cables/distributor replaced, and that seemed to temporarily solve it, but that's no longer the case. My current "fix" is starting the car every six hours but that's been messing with my sleep cycle and is starting to tire me out. If the car's been running, it runs fine - the issue is only when it's been sitting in the cold. I (believe) I've also had the fuel pump/filter replaced but can't remember for sure.

Does anyone have any idea what the issue may be?

Car won't start after sitting in cold weather for more than 8 hours (located in SoCal, so nowhere near freezing. Will start after sitting for 24 hours in Woodland Hills/Reseda/hotter areas, but won't start after sitting in Santa Monica/Long Beach/Santa Ana/cooler areas.)
Car will start when gas is pressed, but only after being cranked a lot
Car "diesels" after I stop cranking (unburnt gas in cylinders) on some of the no-starts
Starter fluid does not help.

zipzit · Answer

My guess is the check valve in the electric fuel pump is locked open via fuel deposits (varnish). Its not a major big deal as long as you understand what’s happening.

Fuel pump in tank pushes fuel up to the top of the engine. The check valve normally shuts when you turn the vehicle off, so fuel remains in the eight feet of fuel line. That’s to enable quick startup next time. On your vehicle that line is empty. It takes time to fill that empty line up again.

The fix?  Turn key to the run position but do NOT turn the key to start. Listen for the fuel pump. You may have to do this twice. After you hear the fuel pump stop, then turn the key to START.

Let us know how this works out.

SAG · Answer

Some vehicles have a coolant temp sensor that adjusts how rich the fuel mixture is depending on temperature.

I have heard that disconnecting the sensor simulates a cold start. You might as well give this a shot. If it starts with the sensor unplugged, then you probably have a bad sensor.

Truck won't start when sitting for 8+ hours in cold, but will with gas pedal

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