Hey guys, maybe someone here has an idea on what to look for next?

When it is 25-degrees F or lower out, our 2006 Nissan Sentra with a 1.8l and 83k miles is a PITA to get started. If you start it within 4-5 hours of it having already been up to temp it starts without issue. Only when it sits overnight is it a problem.

Basically when you crank it (it cranks perfectly fine, brand new battery) it sputters a bit. Do that enough and it gets warm enough and it will act like it actually wants to run, but you have to feather the throttle to kick the RPMs up far enough and then it will settle down and idle. When it is coming down from high revs it stumbles a little around 1800-1400 rpm and then drops into idle without a problem.

A little background, about 2 months ago it would start but the idle was erratic. Scanned it for codes and got a camshaft sensor error. Replaced the camshaft sensor (supposedly a common failure) and cleared the code and no more code, but every now and then the idle would do the same thing (basically upon startup rev to 1800-2000 rpm, start settling down to normal but stumble once or twice while getting there).

Now, here is what was done since it started flaking out:

- New spark plugs, OEM double platinum, gapped @ .044" factory spec
- MAF cleaned
- Throttlebody cleaned
- Throttle position set via procedure
- Throttle close position set via procedure
- Idle air set procedure run through
- Fuel pressure verified good
- Coolant levels are constant

Once it is running it runs great. No stumbles, no hesitations, no codes, great throttle response, etc...

I don't think it is a head gasket, since I have full heat and I haven't noticed a drop in coolant levels, with no smoking out the exhaust. Not to say it couldn't be a problem with the head gasket (another known issue on the 1.8l), but it has no usual "signs" of a head gasket failure. With the fuel pressure good, new plugs and no codes I am kinda at a loss. I don't have a jumper lead to test for spark because of the coil-on-plug design, but I can only assume it isn't a coil problem since it runs fine once it gets started (ie, the coils wouldn't get warm enough quick enough to recover from a bad internal connection).

If anyone has any other hints I would be more than interested in checking things out. It is a throttle-by-wire setup and I would have to assume that entire system is overly double and triple self-checking and would throw codes if it was a problem there. If the crank position sensor was at fault, shouldn't it throw a code?

I have done a ton of reading online and I hear of everything from O2 sensors (doesn't make sense on cold-start) being replaced to fuel pumps (fuel pressure checks out), to crank position and cam position sensors being replaced all the way to head gaskets/warped heads and the PCM itself being the problem... Sad part is that most of these are $600-1800+ solutions with most people being screwed by the dealerships (2-4 visits with them just throwing parts at it, which is complete BS).

Any and all help would be greatly appreciated.