Hi all, I've never had to use the 911 forum before, and I sure hope someone here can help ASAP. The title says it all - starting a few weeks before Christmas, my truck has been unable to start (or very hard starting) when the temperature dips into the single digits (Fahrenheit). My truck is a 2000 4.0 SOHC. On turning the key, the engine turns over normally. If I crank it for a few seconds, it will fire once or twice, but will not start. Once the temperature rises to the upper teens, it starts just fine. There has been no problem idling or running once started. Holding the accelerator doesn't help (ruling out an IAC valve problem). After the first incident in December, I thought it was an isolated incident - either dirty gas or water in the tank. I tossed in a can of HEET, and let it sit overnight. The next day it was a bit warmer, and my Ex started without incident. Several days later, weather was colder and the problem reappeared. I suspected the fuel system, so checked fuel pressure - it tested at ~60 PSI after priming, and ~55 PSI running. Despite the normal result, I replaced the fuel filter for kicks since it;s been a while. There was evidence of minor bits of crud in the filter, but nothing serious. I also cleaned my K&N filter, MAF sensor, and IAC valve for good measure. There was a good amount of carbon buildup in the throttle body and upper intake, so I removed and cleaned them as well. Before reinstalling the intake, I also swapped in a new set of upper intake gaskets I had laying around and replaced the PCV valve. For the next several weeks, it was in the 20's, and there was no issue starting. However, this morning it was once again 8 degrees and just as I feared, my truck wouldn't start. I checked the fuel pressure again and it was 60 after priming and 45-55 while cranking. The engine is turning over just fine, but as before it is only sporadically firing. One difference this time is that after several extended cranking sessions, the engine backfired once. I know the truck is getting fuel and air. I pulled a plug wire and there is spark. Next up for testing: Coil pack: Even though there is spark, it looked weak to me. However, I've never heard of a coil pack having problems in the cold (they usually fail with heat in my experience). My multimeter died, so my unscientific test is to bring the coil pack inside tonight, let it warm up and then reattach and see what happens. Spark plugs: Somehow, I managed to misplace my socket. Will pick one up tomorrow and look at the plugs. and... I don't know what else. ECT sensor crossed my mind, but that shouldn't cause a problem like this. And input/ideas on what the problem could be would be appreciated. I really can't afford to wait until Tuesday when it warms up a bit.