Won't start at first, yet runs fine
 

Hopefully the collective wisdom here can help solve this one:

2001 RX300 AWD w/120k miles.
Spark plugs have been replaced at about 115k
New Battery around 115K
New air filter around 115K
O2 senors replaced when CELs occured and appropriate code thrown.

Cars been running fine, fuel milage is good at around 19Mpg mixed driving. No hesitation, pinging, other problems. Accelarates fine. Runs smooth.

Within the last couple of weeks, an unusual problem has arisen. When you go try to start the car for the first time of the day, it won't. Engine cranks over with no problem (ie, battery is fine). Detonation is not happening. What's the wierd part is that it will eventaully start if I sit there and crank it over every few minutes for about half an hour. It's a slow process where eventually it will start to detonate and the engine will idle for a second and then die. Maybe over the next few cranks it will idle for 2 seconds. Then 3 secs. Then longer, it tries to adjust it's own idle but dies. If I try to give it gas on purpose, dies. Eventually I'll crank it and it starts right up. I can then turn it off and restart with no problem. Drive somewhere, park it, come back, starts with no problem. Never cuts out while driving, standing still, etc. The problem doesn't reoccur until it sits at least 12 hours (ie, overnight).

There is no CEL.
I've inspected the IACV and it turns freely (I cleaned the throttle body at about 115k and again today). Cleaned the PCV valve today. Looked for loose hoses or wiring. The car idles perfectly once it starts, and about 500 - 700 rpm when warm and at a traffic light.

This has got me stumped, for the fact that once the car is running, it runs perfect.

Anyone have an idea before I give up and take it to the mechanic?

Sounds like a fuel issue. Have you tried running some fuel injector cleaner through it? It could be a fuel pump, but I don't understand why it would only happen after 12 hours though...lol
Does the car have full power throughout all rpm's?

That's what I thought. Ran two bottles of Techron through it, once with half a tank of gas (to concentrate it) and again with a full tank. Did not solve it. Out of desperation I bought a bottle of B-12 ChemTool today since I heard it's stronger. Poured that it with 3/4 of a tank today. I'm hoping it's just a stubborn clog that this will clear up.

With regard to the powerband question, I'm pretty sure it's fine, but I will make note of it tomorrow (try some passing accelaration on the highway) and report back.

Update
 

Ok, so the car started on the first try the past couple of days. What is common is that the ambient outside temperature (according to the RX's outside temp monitor) was around >60 deg F. This morning it once again would not start for at least a half hour of trying and the temp was <50 deg F. The gas (and additive) in the tank has been the same for the past few days, not near the bottom of the tank, so no other variables involved.

So somehow this is a temperature related problem. Any ideas?

Just bumping it up before I have to give in and drop it off at the service shop so they can reproduce the problem, which means leaving it for at least 2 days.

Anybody ever seen a problem like this before??

Funny.......... Same thing happened to me about a week ago....... I though it was that security light on the left side of the steering wheel.

I already went down that path by using our other keyfob just to make sure it wasn't a bad transponder. Unfortunately the problem happens with both keys.

Thanks for the suggestion though, this one has got me stumped.

It still could be the IACV. I cleaned mine and it seemed to be working freely and I did not fix the problem until I replaced the IACV. Everything has been fine now for over a year.

Ok, the IACV could be a possibility since from what I've researched is that if it is going bad, but still functioning, it won't throw a code.

Here's a question for the mechanics here: if the IACV remained shut while trying to start the vehicle, what would happen? Would the vehicle still not start even if I depress the gas pedal (which theoretically opens the throttle plates and allows air in)?

I had the same starting issue caused by bad IACV, consistent with outside temp.. but mine was stuck when i inspected it, weird that yours spins freely and still has that issue.

I think it's temperature related due to the coil on the IACV actuator motor. I think the actuator motor is going bad and hence why it won't work at cold temperature but will at warmer ones. It's the only thing that makes sense, hence why I ordered a new one from RockAuto ($130, a bargain). Should be here by the end of week and I'll dismantle the throttle body and replace it.

Also ordered a RX300 Haynes Repair manual for about $15. Some of the prices at RockAuto are really good.

Problem solved
 

I'm following up here in hopes the experience will help someone else in the future.

It wasn't the IACV, and I should have realized that. If it was, then holding open the throttle while starting should have allowed the engine to detontate a little and catch. I was always pretty sure it wasn't a fuel system related problem since I could hear the fuel pump kicking in when you first turn the key to build up fuel line pressure, like it's suppose to. Plus, I could smell gas under the hood.

I also ohm'ed out the engine block thermistor (this controls fuel supply when engine is cold) and it measured within range. At this point I gave up and got it to my local service guy.

Took them over 3 hours to track it down since there was no CEL. Crankshaft Position Sensor. I guess the fact it wasn't fully bad is why there were no codes. The fact the engine would backfire every now and then while trying to start it should have been a clue to me. It all makes sense in hindsight.

Starts without a problem now. Hopefully all this info will help someone else down the road.