Thought I'd post my most recent finding in case it helps someone else in the future. I was trying to fix a problem I was having starting my 01 QX4. After sitting for extended periods (a few hours or overnight) it would take a few extra cranks and then there would be an uncomfortable hesitation and it would reluctantly start. During the hesitation, there would sometimes be a knock or two, like when an engine is running out of gas (you know the sound if you've ever blown or pulled the fuel pump fuse and tried starting it). I did some research on this forum and elsewhere, and kept hearing the hint that it may be the fuel pressure regulator. The problem is that when they go bad, the regulators allow too much fuel in and leaks back when the engine's at rest (caused by a bad seal or it hanging open). I found a list of symptoms, and I had most of them. List is courtesy of http://www.doityourself.com/stry/7-symp ... 4plWS81zFw:
1. Blackened spark plugs - This happens when the engine is running too rich, which is what happens if the regulator is giving the engine too much fuel. I changed the plugs when I got the car about a year and a half ago and saw the carbon, but didn't think much of it. I figured they had never just been replaced.
2. Engine doesn't run smoothly - This wasn't a problem for me at all except at a warm idle, when it would shake and quiver randomly ever so slightly, like a cold rabbit. Other than that, takeoff, acceleration, and deceleration had no noticeable problem.
3. Black smoke out tailpipe - I never noticed this, so it wasn't severe if it was the case. I did notice on cold days that the exhaust steam did seem to be much more plentiful than other cars, though.
4. Dipstick smells like gas - This and #7 were the clinchers. There was a definite smell of gas in the oil vs. fresh oil.
5. Gasoline dripping out of tailpipe - Never noticed this, but never really looked.
6. Stall on acceleration - See #2; not a problem here.
7. Gasoline in the vacuum hose - I didn't see any dripping out of the hose, but it reeked of gas.
I also had to have the catalytic converters replaced when I bought it too, and it was unsettling to not know why I needed to. If the car's well maintained, they generally don't ever go bad (especially with only 150k miles on the odometer). Idling for long times like people love to do in the winter can also kill them, so I had figured that was it.
The good news was that it was crazy easy to replace! If you've replaced the fuel filter, you'll be familiar with most of these steps. All you need is a 10mm wrench to remove the plastic cover on the top of the engine, a philips screwdriver, and a towel.
1. Pull the fuel pump fuse and start the engine a few times until it doesn't start at all.
2. Remove the plastic engine cover.
3. Unscrew the two screws holding it down and the loosen the fuel hose clamp.
4. Wrap the towel around the regulator. Fuel will spray everywhere for a second like it always does when you do this sort of thing.
5. I highly recommend donning goggles and ear plugs. You're not nearly as likely to get gas sprayed on your head since you're topside, but I still highly recommend it. Gas in the ears SUCKS!
6. You can either try to pull the hose off or pull it straight up. I pulled it straight up and it geysered a foot or two in the air but settled down quickly. Any gas that isn't caught by the towel should dry up pretty quickly. Pulling the hose off at this point is much simpler.
7. Swap in the new part. Connect the hose, plug it into the engine, tighten the screws and tighten the hose clamp.
8. Replace the fuel pump fuse, put the key in the On position for about 10 seconds to let it prime the system. Go check the regulator to make sure there aren't any leaks already. If you're good to go, start up the car and check for leaks again.
9. Install the plastic engine cover.
The regulator is the silver bit in the middle. It's located just to the right of the battery (the red plastic piece in the photo). There's another gold-ish piece that looks almost exactly like it just to the right. The way to tell them apart is the other one has the fuel hose crimped on it and the vacuum hose goes off to the side, instead of straight up.
The procedure only took about 15 minutes and was really straightforward. After doing this, though, you should consider replacing your oil and spark plugs. Gasoline breaks down engine oil, and the engine running rich means carbon builds up on places like the spark plugs. If the spark plugs are new, you can probably just get away with cleaning the carbon off of them.
I bought the replacement part at Napa - the Echlin CRB 219529. Man it was hard coughing up the $70 for a part that small!
What a difference the new part makes! The idle is creamy smooth, and acceleration is smoother and slightly more powerful too. The engine used to have a few slightly rough spots while taking corners and such, but they're long gone now. Starting isn't an issue any more, but we'll see what happens over the next few days.
Its successor has been working great though!