What would cause my truck not to start when the temp drops below 30 degrees out side on a 2001 Blazer

Ran into the same problem with a '98 lemon-law buyback that GM donated to our school. I built a lot of electrical "bugs" into our vehicles for the kids to practice their troubleshooting skills. One of those bugs was I cut the sensing element in the intake air temperature sensor and soldered in a regular resistor in its place that made it read 50 degrees all the time. In warmer weather the Engine Computer commanded too much fuel resulting in a little black smoke from the tail pipe until the engine warmed up. In winter, when it was well below 50 degrees, the engine would not start if that bug was switched in. If it was switched to use the original sensor, it started fine because the computer knew it had to inject an extra priming squirt of fuel, just like pumping the gas pedal on older carbureted cars.

Based on how that one acted, and since your description sounds the same, I'd be looking at the ambient air temperature sensor, often called the intake air temperature sensor. On our truck it was poked into the rubber fresh air tube near the right rear corner under the hood. It will have two wires, possibly a black one and a light brown one.

The only proper way to test it is to connect a scanner that can display live sensor data, and see what temperature it reads after the engine has been off for a few hours. It should read the same as the current temperature. The voltage on the sensor will be between 0.5 and 4.5 volts, (lower at higher temperatures). The two wires would have to be shorted together or the connector would have to be unplugged to make the voltage go outside that acceptable range. THAT'S what would turn on the Check Engine light and set a fault code. You can have a starting or running problem caused by that sensor reporting the wrong temperature even though it won't set a fault code as long as it stays within the acceptable limits.

Just to add to this. I would test the fuel pressure with an accurate gauge when the temp is in the area you describe. This is a high pressure system that is very sensitive to pressure. It requires a MINIMUM of 55 PSI to run and should have over 60 PSI

Thanks Wrenchtech. It never occurred to me that some cars are known to have rubber o-rings that shrink in colder weather and leak so it's no stretch to think there could be a similar problem allowing fuel pressure to bleed down overnight. You would think that would just cause an extended crank time rather than a no-start, but it seems like the combination of low system voltage during cranking and some of the injectors bleeding off pressure as soon as it begins to build up contribute to it remaining a no-start.

Since the pump was already replaced, would the clue be the engine would fire with a squirt of starting fluid, then run fine? Also, what about cycling the ignition switch two or three times before cranking the engine? If that doesn't get the pressure up for starting, would you suspect a problem with the new pump?

That's not what I meant at all. This is a poppet injector setup and they will just not operate if the pressure drops even a few pounds and they require the most fuel when cold. Many aftermarket pumps barely supply enough pressure when new. Fuel pressure is just a common problem on Vortec engines.

Oh. So you're saying it could just be the new pump is no better than the old one? Is that a dealer / name brand part vs. Inexpensive aftermarket thing, or is it just luck of the draw and a different aftermarket pump may solve the problem?

I try to stay with Delco which does sell aftermarket. Everybody seems to be selling Carters and I've used a few but had a few bad ones too. The big problem is that there is no margin for error in the pressure. At 55 PSI it will run but start hard when cold. Anything under that and you have a hard time even getting one to run. Fuel pump is the number one cause of no starts on Vortecs.