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Hi All,

Hope this is the right forum/thread to post this, as there isn't really a great fit out there and this forum seems to be quite active with knowledgeable folk, so here it goes.

Its a Navara, in oz, 2008 Spanish build 4.0 V6 petrol auto...its been around, with a healthy 355,000 odd km on the clock. Its still strong, but it was alot crisper up until a few months back.

It all started with a cracked exhaust, since I had to replace it I (maybe stupidly) replaced the second cats to high flow, a new "sports" muffler over a nice 3" pipe all through with a resonator to tone it down.

Coincidence or not, a few weeks later I started getting o2 sensor errors on bank 2 sensor 1, resetting those made no difference and the fun started...

I've replaced both bank o2 sensors, new MAF, PCV valve, all vacuum hoses (thinking vac leak), had the exhaust checked for leaks, had intake plenum smoke tested for leaks, thorough throttle body clean...everything seems ok.

I reset the codes, and it appears that when warming up, when it switches from open loop to close loop it trips o2 sensor 1 in bank 2 every time. I also notice when warm and running the o2 sensor readings aren't even across both banks, ie, this is an snapshot of what I see every day;
Bank 1 Sensor 1 - 0.31v
Bank 2 Sensor 1 - 0.55v
Bank 1 Sensor 2 - 0.78v
Bank 2 Sensor 2 - 0.73v

Its not a great ODB2 tool but the wave patters on sensor 1's on both banks are quite different.

So why am I not just ignoring the codes? Mainly the fuel consumption has gone from about 450kms on a tank to about has a rough idle, and it lags more on acceleration than what it used to do.

My next steps is to pull the plenum and intake manifold, check for damage/cracks and give them a thorough clean. Just waiting on gaskets to arrive and a free weekend to do this....

So....I gather we have people here that have pulled the VQ40DE apart and done more than change plugs :) ...So I'm hoping this post will be able to give me two things;

1. Advice on these symptoms could be and how to check for issues that I may have missed.
2. Since I'm pulling the plenum and intake out for cleaning, what else can I "service" whilst the engine's top bits are bare?

Its been close to 20 years since I've done any major engine rebuilds, and it was on simple naturaly aspirated beasts...these engines require a little more know how.

Thank you in advance for all advice.
Aren't your vehicle system monitored by a "referee" ? Some your potential mods to fix this might void your ability to drive it On Highway. The above ideas are bad you weren't in THe States. Here in Florida there is no inspection on vehicles. I hope you get it all sorted out...looks like you've had for a long time. Good Luck and the guys on this forum are good.
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