Have a problem that'* got me stumped. Working on a 2004 Impala with a 3.4L, that will start sometimes, usually when cold. Then may or may not start otherwise. When it won't start, it'll usually fire and die, fire and die, and maybe fire and die once more, then just turn over without firing. If I put the throttle down about 1/4, the chances of it starting the first or second time are greatly increased. And if I pull the nipple in the air intake duct (the hard line that goes to the rear valve cover) and spray a second of starting fluid in it, then put the throttle down 1/4 or so, it'll start just about every time. When it does, it runs perfect. No power loss, no misses.
I have no idea what could be causing this. Fuel pressure is good, swapped in a borrowed ignition switch, verified spark is present always, security system is not tripped, maf sensor swapped with good known one, as well as IAC, TPS reads correct with pressing of gas pedal, map sensor passes electrical tests, owner swapped a new Fuel pressure regulator (which I don't think was bad). I am wondering if for some reason the fuel injector pulse is getting cut off when it'* cranking for some reason? There are no BCM codes related (that I know of, only for interior dimming high and low, and driver'* side door lock issue). It does throw 2 engine codes, though. P0327B, Knock Sensor frequency, and P0520C, Low Oil level (If I remember right). This car doesn't have an oil level sensor that I can find. I can see where someone polished off some what I'm guessing was junkyard paint on the upper intake, so it'* certainly possible that the engine has been swapped and they forgot to change pans, but I didn't see a pigtail for it either. I'm assuming the knock sensor can't cause a no start, but maybe I'm wrong. Anyone have any ideas?

it may be pulling enough timing to make it hard to start with a knock sensor issue

Well, it needs to be replaced anyway, right? So I'll grab one and throw it in.

If you crack the throttle open while trying to start it, this seems like the typical IAC stuck closed/gummed closed.

Yeah, I thought the same. Swapped the IAC to my Malibu, works fine in that car. I also checked the front 3 plugs this morning when it wouldn't start. They were dry. I did get a new code today, P0336B. Which pertains to the 24X crank sensor, but all the research I did on it said it wouldn't cause a no start.

Ended up being one of the wires to the cam sensor in the intake valley being broken. I've repaired them before while doing intake or head gaskets, but never seen one that caused these symptoms. It'* too bad the computer didn't give me the idea to look there.

thanks for updating your fix

Confused about the addition of starter fluid, and it runs fine, and yet we have a bad wire from the cam sensor....

Yeah, so am I. I thought the cam sensor only affected spark/fuel timing, and there was a default program that ran it under 400 rpms. There had been a junkyard engine installed in this before they bought it, and I've seen plenty of the cam sensor wires frayed at the sensor in the intake valley, so I can certainly believe they were bad, but I have no idea why it would cause the injectors not to fire, or maybe not to fire enough to start, yet run fine once started. I didn't go talk to the dealer tech after the owner told me they figured it out, mainly because I was extremely miffed that I didn't figure it out.

I'm thinking it was a coincidence it started with the fluid, if the cam sensor is bad the ECM doesn't know when the number 1 piston is TDC on the compression stroke so it will have a extended crank and kind of guess when to fire the injectors but once its running the engine seems to run fine.