intermittan stall at idle/decel and occasional lack of power

You have covered about every possibility that is sensor related and I am guessing that you maintain the car as needed.

If the spark plugs were ever replaced, is there any chance that the wrong heat range could be the culprit? This would be immediately obvious after changing the plugs.
I assume the air filter is clean or new.
Were the injectors checked and the fuel pressure tested?
Have the coils been tested? I can't recall if you would have a coil over plug setup. I am guessing that that would have been covered.

The only suggestion I would have is to use a good fuel injector cleaner. I have used BG44K ever since I have seen how well it works. It works so well that the oil needs to be changed after running a can or two. You would have to go to a dealership or gas station to get it. Dealerships sometimes offers the direct injection of BG44K that requires a special delivery system, but it only gets the fuel rail and injectors. There is something about running the gass additive that makes a big difference. If you have never used it, run two cans.
The second choice if you can't find BG44K is SEAFOAM. It works pretty well too.

I will keep an eye on this post for your reply and get back ASAP.

Hi Doc,
Thanks for the reply. Yes, it's very well maintained with regular oil and filter changes (oil, air, and fuel). I have not put a gauge on it myslef, but the dealership said they checked the fuel pressure when they had it. I don't know if the injectors were checked or not. It runs really good 95% of the time, but it seems to be worse at 1/2 tank (could be my imagination at this point). The Infinity has a computer controlled fuel module that adjusts fuel pressure a various times during startup, etc...to lower noise, but even that shouldn't cause the stall. You are correct, it is a coil on plug. I did change the plugs awhile back, but the problem came a few months later. I found a broken lead when I did the plugs and replaced one of the coils, but other than that I think the ignition is sound. It honestly seems like something goes off scale and confuses the computer, but the lack of a CEL doesn't support that. I'll try the Seafoam or BG44k just because it's cheap and couldn't hurt. It typically acts up at idle or when it was the freeway it was a low/part throttle position (idle and cruise or lite loads). With the sensitivity to throttle position when it stalls and the periodic loss of power, I'm beginning to think it may be computer related, but I'd like to try a few other things first. I'm not sure how much the computers are, but I'm sure they are not cheap. Do you know if th dealer has a way to check the computer?
Thanks in advance,
Bill

With the information you have and it being so well maintianed, it is getting down to something like the computer.
If you have to get a new computer, of course look online. I think Kingmotorsports.com does infiniti, (Toyota). I am sure you can get something that has been flashed for performance cheaper than an oem replacement. Also try The Swap Shop, tell them Mike from Maryland sent you. The owner is Brian and he usually answers the phone, great guy.
BG44K is near $25 a can on average, but worth it. SeaFoam around $15.
Good luck and reply to this post at any time down the road as I will get an email notification that you replied.

Good luck,
V/R

Dr. C

Hi Doc,
I tried the 2 cans of additive and it had no effect. Since then I noticed a few things:
1) there seems to be some black dirt and grime at the seam where the upper and lower intake manifolds meet. I thought it could have a vacuum leak, but when I sprayed it with carb cleaner there was no change in idle.
2) The car idles rougher than it used to. When it's not trying to stall, and the tach is stable, I can feel the engine running rough.
3) The car is most likly to act up right when you crack the throttle or at cruise under VERY light throttle condition (less than 1/8 throttle). Whatever it is it's very sensitive to throttle position and feels very similar to a bad accelerator pump on a carb'ed vehicle. When it acts up if you put your foot to the floor it's okay.
4) When its acting up on the freeway at crusing speed the tachometer remains absolutely rock steady, but the car bucks and doesn't want to go. When it's idleing and acts up the tach is all over the place between 0 and 1000 RPM due to wanting to die.
5)When it begins to stumble at idle, just before stalling, it starts emitting black smoke out the exhaust.

Any more suggestions?

Bill

Electrical problems are tough to get at. You have componenents that are part of a larger system, sensors, etc. that feed the car computer. Then these components can fail electrically and/or mechanically. They also do this intermittantly, which makes it a game of patience and logically eliminatiting the most likeley , ...and hopefully the cheapest, suspects one at a time.
Form what you describe I said the same words about a carberater as I read them. So, the next thing I would try is the Throttle Posistion Sensor. They are not hard to install, but not easy to calibrrate. As long as you have a multi-meter and the specs. you will be okay. Also, it would not hurt to replace the O2 sensor, (sorry if this has already been mentioned), as it may be dumping too much gas because it sees a lean condition. An exhaust leak near the sensor might do the same thing.

I will keep an eye on the post.