I'm writing this in case anyone here has experienced this.
I own a 2008 RX350, relatively low mileage for its age at roughly 80,000 miles. It has 6 new spark plugs, new battery and new alternator, all replaced about a year ago. Voltage reading is good and when tested with my basic OBD scan tool it shows no fault codes.
Here is the problem:
It starts immediately in the morning, within a second of cranking (it's never very cold where I live).
When fully warmed up to temperature and parked for a short duration, say 15 minutes or so, it restarts instantly, even faster than a normal cold start.
When fully warmed up to temperature and parked a bit longer like 2 to 3 hours it needs 5 to 7 seconds of continuous cranking or two shorter cranks of 3-4 seconds before it splutters to a slow start and then needs another second or two to get to a smooth idle.
When parked even longer like 5 hours or more (meaning enough time for the engine bay to cool), it starts the same as a normal cold start.
I suspect some kind of evaporation going on in the fuel system, maybe in the fuel line due to heat soak in the engine bay but not sure. My mechanic suggested replacing the fuel pump but why does it only happen when parked 2-3 hours?
Not a big problem (yet) as it has never actually failed to start, but I'd like to catch it early before it gets worse or one day fail to start on me..
Anyone come across the same? Any suggestions or ideas would be welcome.

 

I've got the same 08 RX350, new battery and alternator at 170k miles and experiencing similar cranking issue when hot and still haven't found the cause yet.

Not sure if it's a sensor that's upset only when the engine is hot, a fuel pump or leaky fuel injectors.

Did you find a solution to the hard starting?

 

Interesting symptoms, I have no idea.

If it was the opposite and running rough/hard start only when cold I would say you have a cracked coil pack, that's what happened to me.

 

Yeah, recently checked changed the spark plugs and coils looked fine. While I was at it I replaced back three coils because they are hard to reach. The hard starting began before I did the tune up and happens when engine is hot. I'll run into a store only to return a few minutes later and will try 2-3 times to start the vehicle before it starts up.

Crank sound strong and vehicle drives smoothly once started.

Possible causes I've heard are either bad fuel pump, leaky fuel injectors or fuel pump resistor.

Not sure how to check for leaky fuel injectors.
MattRX was kind enough to explain how to check the fuel pump resistor and where to locate that. I'll check that.

 

If the injectors are leaking internally into your engine it will run very rough and possibly "Hydro-lock", if they are leaking outside the engine that is most often from the injector o-rings becoming dry and cracked... those are very cheap adn easy to replace but your symptoms sound nthing like the injectors (if it were the injector seals you'd see fuel leaking around the injectors) so I'd cross that off the problem list though it is a good idea as a maintenance item to replace those seals at some point.

Another possible point of issue could be vent lines, go through all your vent lines check for dry/cracked hoses, mine had a few. Again at these miles/age these should be replaced as a maintenance item they are also cheap and easy.

Bad gas could cause this but doesn't sound like you're issue.

 

Vent lines meaning the gas tank vent lines? Not familiar so just asking to clarify.

I only use top tier Citgo or Mobile gas. I used a Seafoam & Techron fuel system and injector cleaner a couple times this past year. Many I need to use a stronger product?
I bought this RX a year and 1/2 ago and didn't have hard starting issues until a few month ago. Since purchase, I've only used top tier gas so unless there was gunk sitting in the tank or somewhere else, the gas I've used should have only helped to keep the system clean.

Does it make any sense that the injector cleaners clogged an injector just enough to allow it to drip? But why only when hot would be my question?

 

Seafoam and Techron won't do anything for debris that could be possibly clogging your injectors (contrary to their marketing claims) but that is not your issue here. I sometimes use Seafoam as well more than likely it is nothing more than a "placebo effect" but taht is another story A stronger product won't help here.
Vent lines - I'm referring to all the smaller black vent lines winding through your engine compartment going to your air filter, intake manifold and various solenoids, they range from only a few inches long to severla feet long. They are made of rubber and they do crack and get much less elastic with both time and age hence why they need replacing. A vacuum leak could be a culprit even if they are not teh direct caus repalcing them is a good idea both very cheap and easy... Autozone charged me $2 per foot. Do a self test here, pull a few off, one at a time, so you know where it goes back to... if they pull off the nipple easily they are in definite need of replacement. I think I bought 10" did the ones I could easily access at least one was very dry/brittle allowing vacuum to escape (2004 RX330 172K miles).

Injectors don't just fail at high temps, if one is stuck open they flood the cylinder with fuel causing big problems, if they get debris in them it is constant not situational so cross that off your list. Remove the engine cover, watch the engine when running look at the injectors, do you see any gas escaping around the injectors? if so then you likely need new seals (very cheap and easy to access the front ones the rear ones suck to get access to)

 

Thanks for explaining.
I'm in the engine bay now, and its hot. Looking at the front fuel rail I can't see any small leaks or smell any gasoline there.

I see the small back vent tubes your referring to. Checking those now. So far the tubes feeding into the air box are snug.

I'll keep checking. If these aren't the cause, any ither ideas?

 

If it does not source your problem my best guesss, using the SWAG system, would be something from the fuel tank to the injector rails (likely fuel pump). Perhaps check the fuel filter, this issue is a very common occurance on airplanes usually cuased by fuel tank venting

 

So the hot crank but no start is still occurring randomly. Takes three tries of cranking to start the RX.

I purchased an upgraded scanner (xtool A30M) to see if it would pickup more complex system codes. It found code B2799, issue with the immobilizer, stored in the ECU. I cleared the codes and waited for the next hot crank no start occurrence. A week later it happened and the only code to appear is B2799.

I did have two new keys made by my auto locksmith in the past year. Keys had new blades cut and used OEM inner electronics, not aftermarket. However, I can't figure out why this B2799 code issue would only trigger when the engine is hot or was just driven. The issue doesn't always happen either even if the engine is hot.
Locksmith claims it's not the keys and that he remember another customer had a similar issue with his Toyota and therefore replaced the immobilizer.

Would resetting/reprogramming the immobilizer or keys help solve this issue?