ODBII codes P0171 & P0174

[MyBlackBeasts]

I know this is not about a MM but... my 97Tbird (3.8ltr v6 190,000 miles) started throwing codes a while back - P0171 (bank 1 lean condition) & P0174 (bank 2 lean condition). It 1st started on a long wind out such as a long on-ramp. Happens more frequently now. Engine runs fine - no drivability issues - just kicks the light on. Clear code and might go 3/4 weeks before light pops back on. Can't find any vacuum leaks and all seems ok. Any ideas? :confused:

[ctrlraven]

It's either a vacuum leak which you might have to search high and low for it, if everything is fine then it's the front O2 sensors.

[MyBlackBeasts]

It's either a vacuum leak which you might have to search high and low for it, if everything is fine then it's the front O2 sensors.

Thanks for the info. I was leaning to the O2 sensors myself. Thank you for confirming my instinct. :)

[EMAS]

A dirty MAF is a likely culprit if it shows both O2 sensors as "lean" and you can't find any un-metered air. It doesn't have to be on the vacuum side of the throttle it can be in the plumbing between the throttle and MAF too. Pick up some CRC MAF cleaner, clean the MAF clear the codes and see if they return before condemning and replacing the O2 sensors.

[RF Overlord]

^^^what EMAS said^^^

It's unusual to have both O2 sensors go bad at exactly the same time. Before you go replacing O2s, see this (http://www.mercurymarauder.net/forums/showthread.php?t=74767&page=2) thread. I'm not suggesting you have the same problem as the OP, just that it's usually some other problem when you get simultaneous Bank 1 & 2 codes.

[fastblackmerc]

^^^what EMAS said^^^

It's unusual to have both O2 sensors go bad at exactly the same time. Before you go replacing O2s, see this (http://www.mercurymarauder.net/forums/showthread.php?t=74767&page=2) thread. I'm not suggesting you have the same problem as the OP, just that it's usually some other problem when you get simultaneous Bank 1 & 2 codes.

I've never seen both O2 sensors go bad at the same time... never.

[EMAS]

Also don't overlook the crankcase when looking for un-metered air leaks. The PCV draws air through the crankcase and the make up air comes from between the throttle and MAF. So for example leaving the oil cap off creates a huge un-metered air leak.

[MyBlackBeasts]

A dirty MAF is a likely culprit if it shows both O2 sensors as "lean" and you can't find any un-metered air. It doesn't have to be on the vacuum side of the throttle it can be in the plumbing between the throttle and MAF too. Pick up some CRC MAF cleaner, clean the MAF clear the codes and see if they return before condemning and replacing the O2 sensors.

Cleaned it, still came back. :shake:

[MyBlackBeasts]

^^^what EMAS said^^^

It's unusual to have both O2 sensors go bad at exactly the same time. Before you go replacing O2s, see this (http://www.mercurymarauder.net/forums/showthread.php?t=74767&page=2) thread. I'm not suggesting you have the same problem as the OP, just that it's usually some other problem when you get simultaneous Bank 1 & 2 codes.

This started over a year ago. Originally was a bank 1 code. Then after about 6 or 7 months bank 2 popped up. Then would get either or with a very random both.

I stopped running 87 octane and started using premium 91 with no ethanol. Gained 5 mpg and 1/3 power increase and also the codes stopped popping up. Now after 4 or 5 months of no codes they have started again.

As the sensors have 190,000+ miles on them will replace both and see what happens.

Thanks! :)

[MyBlackBeasts]

Also don't overlook the crankcase when looking for un-metered air leaks. The PCV draws air through the crankcase and the make up air comes from between the throttle and MAF. So for example leaving the oil cap off creates a huge un-metered air leak.

It never acted like a vacuum leak. Runs the same as it did new, no problem with HVAC control, etc.

It has always had a weird continuous buzzing/rattle from PCV valve. Done it from new, replaced valve - still there. Always considered it normal for the car. :confused:

[RF Overlord]

This started over a year ago. Originally was a bank 1 code. Then after about 6 or 7 months bank 2 popped up. Then would get either or with a very random both.

I stopped running 87 octane and started using premium 91 with no ethanol. Gained 5 mpg and 1/3 power increase and also the codes stopped popping up. Now after 4 or 5 months of no codes they have started again.

As the sensors have 190,000+ miles on them will replace both and see what happens.

Thanks! :)If you had said all this up front, my answer would have been very different. ;) I agree with that many miles the O2s are most likely lazy or shot.

[rayjay]

My wife's LS8 was throwing the exact same codes. Turned out to be a leak in the intake due to a loose clamp.

[SpartaPerformance]

I say vacuum leak too. Those have a composite manifold if I'm not mistaken, it may be warped. What about fuel filter, what condition is that it?

[EMAS]

If you had said all this up front, my answer would have been very different. ;) I agree with that many miles the O2s are most likely lazy or shot.

I agree that info changes things and point to the O2 sensors giving up the ghost.

[MyBlackBeasts]

I say vacuum leak too. Those have a composite manifold if I'm not mistaken, it may be warped. What about fuel filter, what condition is that it?

Aluminum Intake. FF replaced many times.