1999 Honda Passport Rough Idle, Trouble Staying Started, Co

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Engine Performance problem
1999 Honda Passport 6 cyl Four Wheel Drive Automatic 146xxx miles

After spending the last 3 hours reading random posts, I must say, great site. Anyway, on to my issue: My girlfriend's 1999 Honda Passport has had numerous troubles since I've known her, I've had to replace Oxygen Sensors, Y-Pipes, Wiper Arms, etc. Over the course of our relationship, but the latest problem has really baffled me.
When the air temperature gets around the freezing mark and below, the car becomes hard to keep running. It will start just fine, idle up to around 2700rpm, then fall down to around 800rpm within a few seconds. From there, it will hold in that area for the first minute or so, but then will start to dip down into the 200 - 300 range, catch itself and ramp back up to 2000rpm and then fall back to normal and start the process over again. After a spell like that, it will "idle" at the point of near death (100-300rpm) range in which I've actually heard knocks from the heavily altered computer timing just to keep it running.

Now for what I've done: I have replaced the fuel pressure regulator, as putting a pressure gauge on it showed significant bleed off prior to starting (like 10 psi/sec, ) checked vacuum lines for any leaks, and replaced any lines/fittings that seemed suspect. I had found that there was a rather sizeable leak on the intake plenum gasket, which I "repaired" with some RTV silicone until it's not 3 degrees outside (its supposed to be almost 25 this weekend, and I'll probably replace the intake gaskets at that point, ) and I have tested my seal the same way I tested the leak, by spraying carb/choke cleaner near the seam and listening for the engine to respond.

I've also hooked into the computer with my Actron Scanner, and I keep receiving the following code:
P0121 TPS/Pedal Position Sensor A CKT Range/Perf

Now, I've looked up testing in the Haynes manual I have for the TPS, and it says that OBD-II vehicles require the use of a scan tool and is out of the scope of the home mechanic. Can I use my scanner to test this, and if so, how do I go about doing so?

Also, could this code just be the result of the sizeable intake leak that is leaning out the fuel trim?

I appreciate any responses I can get. Thank you in advance.

Do you
have the same problem?
Sunday, January 10th, 2010 AT 10:10 PM

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