Here's the deal:

Start the car for the first time in the morning, and it does the usual high idle warmup. All is good.
However, after driving to work and shutting the car off, if I were to only let it sit for maybe an hour or two, then started it back up, on first firing, the motor really struggles to stay running for a few seconds... idle is extremely low, and the car shakes quite a bit as a result.
It corrects itself after a few seconds, but it's annoying none the less.

Any thoughts on what the cause might be?

 

I think this was a programming error in the ECM by Honda. Alot of people complain of this, and it happens to me for the first few minutes too. No big deal unless it gets to the point where the car stalls.

 

I can envision one of several things and I'll mention a couple here.
First is the often reported MAP sensor "brain fart". You can do the "MAP whack" on it and this may help (firmly tap the MAP sensor that is on top near the throttle body - use the rubber end of a screwdriver to thump it several times).
You may have idle air control solenoid issues but this is remote due to the circumstances that you described.
Sometimes, this ECU is slow to learn when atmospheric conditions alter slightly. Give it time, maybe several driving cycles of "near death" experiences and it may very well fix itself.
Now, the last thing that "might" be a cause is driver technique in starting. You would be amazed at how many people who used to drive carbed cars still press on the gas pedal when they start a car. You don't do this, do you? One should never touch the gas pedal when starting a fuel injected car (unless for some reason it has been flooded). Doing it for a cold start, you can get away with it but usually not for a "hot" start.

 

First off, I'll give the MAP sensor whack a try.
Second, the car has been doing this for some time, so giving it "several driving cycles" isn't really a consideration.
Third, yes, I know how to start the car, and no, I don't touch the gas on start, cold or otherwise.

I'm curious though why everyone is so adamant about whacking the MAP sensor to fix it... you do realize a new one only costs about $40, right? I don't really want to throw money away, but "whacking" something to get it working right isn't usually a long term solution.

 

Glad to hear it's not your starting technique. I had to throw that one out there just in case.
You're right, the "MAP whack" is a really odd thing to do but it does work. Ultimate Lurker once did a little experiment with several "new" MAP sensors and discovered that some of the new ones were not any better than the old one. It's almost "pot luck" to get one that is truly "better".
If this situation has persisted for some time, you might need to look further. Possibly the type and brand of fuel you are using. Is your air filter in need of replacement? Fuel filter? How old is the car - injectors maybe getting a bit clogged? All these things may be more prevalent during "hot" starts.
And again, don't discount the idle air control thing. This has also been reported in a couple of cases of being faulty over time.
Oh, and lastly, consider re-setting the ECU by pulling the "BACK UP" fuse for 30 seconds.

 

All typical maintenance items are maintained regularly (i.e. plugs, air filter, valves have all been replaced/adjusted recently). The motor is in good running order with this exception.
I'd replace the fuel filter... but it's located inside the gas tank So I don't think I'll be replacing it any time soon.

Fuel isn't an issue. I use the same octane/station for all my cars. They're fine... Mobil, 93 octane. I ran a bottle of techron through it for kicks, and I noticed no change in behavoir, better or worse. There are only 62,000 miles on the car.

 

Also take a look in through the throttle body. At your mileage, this area could be quite dirty due to the way our PCV and breather tubes are set up. Many have reported the intake getting quite oily. You might try finding some fuel injector safe carb cleaner and giving that whole area a good spray down.
If it is idle air control valve/solenoid, do you still have warranty?

 

At 60k miles? No, no warranty...

I'll take a look at the throttle body.

 

If it were a software issue, you'd think that the problem would have shown itself the day the car rolled out of the factory. Software is usually pretty consistent and repeatable. If this problem is slowly getting worse as the car is putting on miles, than I'd say something is wearing out or getting stuck.

Having experienced many un-even idle problems with 240SX's in the past, some of the more complicated problems usually boiled down to either a bad ground on an electrical connector (and the ECU either was getting a bad signal, or some solenoid wasn't getting enough current or voltage to operate), or there's an air leak or sticking valve in the intake track somewhere. A lot of the values and senors rely on a vacuum to work, and if there's a leak closer to the intake valves than the device, that can kill the required vacuum pretty quick.

With the electrical connectors, check the major grounds running from the battery to the car chassis and the engine to the car chassis. It's amazing how bad they can corrode. Those are the first to go.

With the air leak, if the problem goes away after the car is running for a little bit, I would suspect a valve that is sticking somewhere. Especially valves that recirculate exhaust gasses anywhere in the car. Exhaust gas loves to gum those up and make them stick either closed or open.

Just some more ideas.

Chris.

 

UPDATE:

So, I was perusing a copy of the Helms manual I got ahold of, and worked my way to the Idle Control section.

Lo-and-behold, there's an interesting note about performing what they call the "Idle Learn Procedure", which should basically be done any time the ECU is reset.

So, what I proceeded to do was pull the Backup fuse (which, by the way, caused the radio to lose power... be warned, in case you don't have your code) for 30 seconds, then re-inserted it, effectively resetting the ECU.

I then proceeded to carry out the Idle Learn Procedure:
1)Start car, and hold RPMs at 3000 until the fan kicks on
2)Let the car idle for 5 minutes, ensuring that the throttle is completely closed and that there are no electrical accessories (fan, AC, etc) on.
3)Turn off car.

After this rather peculiar exercise, I restarted the car and found that it idled completely fine. The RPMs also didn't drop too low when revving the motor and letting the RPMs fall as they did prior to this.

I'll report back after I've had a chance to drive the car some more, but I'd say it's worthwhile to try if you've been having problems (especially if you've had your ECU reset, either on purpose or just by disconnecting the battery, and DIDN'T do this procedure afterwards).

Oh... I also whacked the MAP sensor for good luck