Odd Idle at Startup

[b13dw]

Some not-so-quick background....

My 2001 I30T would not start about 4 months ago. It would turnover, but never fireup. Had it diagnosed as: bad battery (which I knew was on it's way out), old spark plugs (had about 150k and NEEDED to be replaced), and some coolant leaking into the cylinders. All of the aforementioned items were fixed and the car started & ran great.

Fast forward about a month and a half, and I'm starting to have issues idling at startup in Park, as well as while waiting at a stoplight or in traffic, while in gear. The idle was erratic, jumping up to 1500 RPM, and dropping to around 350-400 RPM. A little gas would keep the engine from stalling (or so I thought - I never actually let it stall). Once I was driving, no problems. I scoured the boards here and thought perhaps it was a bad MAF. Bought a new one and replaced it (non-OEM; about $125). Seemed to have little impact.

Pulled codes and only received P0153 and P0430 which I figured were both related to a failing or bad S1B2 O2 sensor. So I ordered a replacement Bosch OEM sensor. While I was waiting for the sensor to arrive, idle became even more erratic, which led me to fear the dreaded IACV. Knowing this could, if it completely failed, ruin my ECM, I brought it in. They ohmed it out and the resistence was indeed out of spec, though it had not completely shorted out (which I was relived about). They replaced the IACV and did an idle relearn, also informed me of the P0153 + P0430 codes which were known.

When I got the car home, it started & ran great. Got the new O2 sensor that night, installed it and test drove the car. Everything seemed to work great. That is, until the next morning. As the car started, cold, idle started up around 1200 RPM which is normal, and slowly dropped down to around 750-800. Then it was like the engine didnt know what to do, and idle would erraticly dip down to what seemed like 300-400 RPM until I gave it gas. However, after giving it gas and driving the car, I never experienced another idle problem, even if I restarted the car while the engine temp was warm. I began thinking perhaps it was some kind of vaccum leak, though I didnt really hear any obvious sucking indicating a leak.

That night, I disconnected the battery and let it sit for a couple hours. The next morning, wouldnt you know, no idle problems whatsoever! Not when I started the car cold, not at a stoplight, nothing. However, over the course of the next week, idle problems returned. Now, what happens consistently is....

- Turn the car on, cold start (car has sat, off overnight). Engine idles completely fine in Park, on it's own. If I let it sit until the engine temp heats to normal operating temp, no problems.
- If I attempt to put the car in reverse while the engine temp is still below operating temp, the idle will dip to it's normal 650 RPM, before dropping down to about 400-450 RPM. Has stalled once.
- If I move the car during this low, rough idle, into either Park or Neutral, it's idle will return to normal (roughly 1000 RPM whil the engine is still warming up). I can then put the car right back in to Reverse with no ill-effects for the remaining drive.
- Any subsequent start with the engine even remotely warm still (i.e. within the next 4 hours after my drive to work), and I have ZERO problems. The issues only occur when the engine is completely cool. No codes are being thrown either.

This is a problem I'm willing to live with (and have for the last few weeks), but would love to try and figure out. Somehow I'm wondering if this problem is related to any of the O2 sensors (even though I thought during the idle warmup, O2 sensors were not used?), as when I reset the ECM by disconnecting the battery, my problems go away for a short time (which I assume is related to the computer's emissions sensors resetting over the next couple of days)?

Any ideas are welcome. Thanks everyone!

[NutriaforBreakfast]

Did you open up the IACV ? Mine had a lot of carbon it which made it run crappy even
though it would ohm out on the multimeter ok

[b13dw]

I had the IACV replaced with a brand new OEM unit.

[loystock]

Problem is with Bank 2 Pre-Cat (P0430). You have corrected P0153 by replacing Bank 2 Sensor 1(front O2 sensor). However, looks like you've had problem with the MAF sensor for an extended period without you knowing it since a failing MAF sensor usually will not throw a fault code. Unfortunately, your engine may have been running rich for a while due to failing MAF sensor and 'raw' fuel can prematurely degrade the Precat/Main Cat and O2 sensors. Bank 2 Precat can no longer hold O2 so the rear O2 sensor oscillation is approaching the front O2 sensor, and thus the code.

[b13dw]

Loystock - thanks for the reply and info! A couple of questions:

- I'm no longer receiving the P0430 code after replacing the bad O2 sensor. Would a failing bank 2 pre-cat necessarily throw that code, or is it possible it could be going bad/already bad, without the code? I had read other places that sometimes the combo of a B2S1 O2 sensor code in conjuction with a P0430 may be more coincidental than anything.

- If the bank 2 pre-cat is in fact toast and in need of replacement....I assume there are both OEM and non-OEM replacement cats to choose from (and I would guess the non-OEM would not have near the lifespan of an OEM model, given the levels of precious metals in each)?

In MN we do not have emissions testing, so I'm wondering if one possible route to go, would be an O2 sensor simulator, to replace the downstream O2 sensor? Perhaps that would trick the ECM into thinking the Bank 2 pre-cat is not bad? Or maybe there are federal emissions laws that would be violated with that?

[NutriaforBreakfast]

I think the computer will throw a code when it sees a significant difference between
the reading of the front o2 sensor and the rear 02 sensor. Your replacement reduced
the numerical variance and the computer has accepted that variance (and not thrown a code).
I think the spark plug change helped a lot too.

[loystock]

If the P0430 code is gone, then good for you. It's possible that the front sensor caused both fault codes. The failed front O2 sensor was probably slow to react to changing O2 level causing the ECM to generate the code. OEM cat is really expensive because of the level of precious metals - palladium, platinum and rhodium. Aftermarked cats have lower levels of these metals hence have lower price and but do not last that long.