PO171 and 174 after transmission swap.

[Tuxedo]

Hello. I've got a 99 ranger 4.0l with an automatic. When I put the new transmission into the truck I had to take the exhaust off after the exhaust manifolds. When I put it all back together it worked perfectly and all was well. But once I started driving it, every 20 miles or so I get a check engine light and codes 171 and 174. I undid the exhaust and put it back on and it was only a temporary fix. Nothing other than the exhaust has changed since the transmission swap. I drove the truck 4000 miles before the transmission swap and never got a light, so it must be ehxuast. Anyone have any ideas on how to stop the exhaust from sucking air and giving me these codes? I can't stand stand getting 10 mpg because of it. Any help would be greatly appreciated.

 

[Haywire6000]

What I do is have some one plug the tail pipe( just put there hand over the pipe) and crawl under the truck and listen for any leaks, when you find them, fix em!

 

[RonD]

P0171, Lean bank 1(passenger side)
P0174, Lean bank 2(drivers side)

O2 sensors can only "see"/read Oxygen levels in the exhaust not fuel

Lean to the computer means it is adding more fuel, opening injectors longer, than it has calculated for, based on MAF(mass air flow) sensor data.

If your MPG is down then you are correct, exhaust system is sucking in air BEFORE O2 sensors, after O2 sensors won't matter.
And the leaking is on BOTH banks of the engine.
So the down pipe where it connects to the exhaust manifolds on both sides would be the most likely problems, disconnect and check each one.
Or take it to exhaust shop.

Also a loose O2 sensor will suck in air via the threads, same issue as leaky exhaust

MAF sensor issue or vacuum leak can cause the same code but MPG wouldn't change, because air:fuel mix is correct.
Exhaust leak is a false Lean so computer is running engine Richer than it should, so MPG drops