Originally Posted by RPatton76
Besides possibly old O2 sensors, and I think there is no MAF sensor for that year, I'd wonder if there are any vacuum leaks like loose intake manifold bolts or cracks in runners, etc. I saw you can buy an aftermarket intake manifold, but I don't know where they may "typically" fail, that the aftermarket decided to make a manifold for them. If you pulled all spark plugs after a normal drive, any one that looks different than the others might tell you if it's any one cylinder running lean, etc. I definitely second the idea of checking fuel pressure, but usually it's a problem of pressure dropping when the pump is hot, I'd guess, or maybe it is low all the time, but not compensated for in open loop.
Rob in AZ
I looked up parts, and it looks like that year IS a MAP (not MAF), oddly, I did not see an IAT (intake air temp) sensor, and usually if it's a MAP system (pressure), there's an IAT for air temp. Perhaps it relies on engine temp? In a MAF system, usually the MAF handles both.
Either way, at startup the engine runs "open loop" because the O2 sensors would not be warmed up to spec yet. Because of open loop, the engine has to use default parameters based on the volume and temperature of the air.
So if it's stalling/running rough at startup, it could be an old/faulty MAP and/or IAT (or engine temp sensor if it's the case) sending bad info to the ECU, which in turn feeds too little/much fuel.
The fact that it DOESN'T happen if only sitting a few hours, points LESS to this type of problem, since it would still start in open loop and he isn't experiencing the problem in such case.
That's why I suggested fuel pressure leak down at rest. If the FP regulator (I checked, and that year DOES use one - versus the later year's in-tank system), does not hold pressure properly, the pressure would draw down so when he tries to start their may be enough residual pressure to FIRE, but the fuel pressure immediately drops (stalling or inadequately fueling the car) until the pump can "catch up."
If you google "fuel pressure regulator" and "stalling," you'll likely see a lot of info on exactly what happens. I also think if you add "Trailblazer" to the search you'd likely see that it's a fairly common problem with the FP regulated Trailblazers.
IF it is the FP regulator, it's likely a VERY slow/small leak for now, given the amount of hours it takes to happen, but will get worse over time.
I THINK the older ones that use an external regulator make it easy to check the pressure - there should be a port to attach a gauge that looks like a tire pressure gauge at/near the rail.
But the problem is why you have to check with a gauge (not OBD), since as soon as you turn the ignition, the fuel pump starts going and you can't see if there was a pressure drop - unless it was so severe that you can see the pressure rise in the first few seconds. It doesn't seem like his is dropping THAT much - IF it's a fuel pressure problem.