Mark Rayner

5.4 V8. 2006

LTFT1/STFT1 gets +% on higher rpm. (Bank 2 is fine)

I just recently replaced the Exhaust manifold on the passenger side Bank 1. The old was badly cracked and clearly taking in air on high revs. (Was pressure tested and was leaking badly) So it would seem logical that upon replacing a cracked manifold that my STFT and LTFT should stop running high +% on that side.
But NOTHING changed???

The LTFT pushes up to +16.2% @ 1500 rpm and 22%-25% at 2500 rpm.

I'm at a loss.. as after going through the PIG of a job of putting a new Exhaust manifold in to replace a badly cracked and obvious leaking one, I thought the trims would be closer to normal. I thought that was the fault..but no change in trims???

I'm wondering if the O2 sensor has been "poisoned" (if that is possible) by running in a high O2 environment for so long?

Both O2 sensors appear to be functional. I have charted the voltage using my scan tool and it is fluctuating up and down between 0.1 and 0.9 of course there could be a bias I cant see in the charts, not sure how to, or if you even can test for a functional O2 sensor which is potentially skewed?

This is clearly (?) Not a vacuum leak as Fuel trims are within +- 3% on idle. They only shoot up at higher RPM and only bank 1. A vacuum leak would do the opposite (high on idle and lower at high rpm).

Any help? Suggestions?

F150Torqued

After you have just finished a 'PIG' of a job on the exhaust - I hate to even suggest it. But sounds exactly like you have an exhaust leak ahead of the upstream O2 snesor.

CAM retard of exhaust valves operates as (E)xhaust (G)as (R)ecirculation. ie: that effect re-ingests exhaust gases and creates negative pressure in the exhaust manifold. Thus, any (even small) leak will suck fresh air in which gets to the upstream O2 sensor. The PCM *only* calls for retard at > 800 RPMs and > 25 to 30% engine load. Those are the conditions under which you are describing out of whack short term fuel trims.

Do a smoke test on your exhaust system.

Mark Rayner

Upstream=before Cat right?
I dont see anything between the manifold and the O2 sensor closest to the engine. This is the configuration; Manifold joint to flange. O2 sensor is only about 3 inches down from the joint with the manifold. Then the cat is about 12" down from that. Then it travels back and meets with the driver side pipe. I see no EGR unless its after the cat? I will climb under and check tomorrow.

I'm wondering now about the idea that the huge crack caused the trims to run rich for such a long time that the extra gas damaged the O2 sensor by coating it in carbon. I'm going to swap the O2 sensor from the other side tomorrow and see if the problem switches sides.

F150Torqued

O2 sensor play such a huge role in everything on these modern engines, and there is NO reasonable way to check their 'calibration', I'm never opposed to replacing or swapping them. The ones on our trucks just smell oxygen - or don't - but who knows how much oxygen does it take to be 'smelled'. The assumption is that that is at 14.7 to 1 perfect fuel/air ratio. They can get 'lazy' (don't react as quickly as they once did). I supposed a good coating of carbon deposits could be the culprit. I don't see why they would work better at low rpms though.

Don't go look for an EGR valve if that is what I lead you to believe. There ain't one. That is the natural 'result' of retarding exhaust valves opening/closing. The primary reason I suspected fresh air might be entering the exhaust stream post combustion.

I still agree with your logic re: Vacuum leak will / should effect fuel trims more at idle and less at higher RPMs. Other areas to investigate might be fuel pressure and performance of the fuel pump driver module

Mark Rayner

Thanks for suggestions. I'm thinking that if I had a fuel pressure issue that I would have fuel trim issues on both banks. But bank 2 is almost perfect, and bank 1 is way off. I tested injectors resistance and they were fine. But one could be blocked I suppose. If the O2 sensor is not the problem, I guess its check for blocked injector next??

F150Torqued

I would think that is a good call also. It is definitely one thing that would be sorta' linear with RPM. Seems like there should be some misfires associated with either the over lean cylinder AT the partially blocked injector or over rich misfires at the other three - but maybe just below the threshold for that.

I had a very similar issue on my wife's Buick V6 that resulted in random misfires on all three cylinders on that bank _AND_ a catalyst efficiency code on that bank. One 'chalky' plug and two 'black' ones. I took TORQUE PRO dashboard and log printouts to the Dealership Shop (For warranty injector replacement) and those IDIOTS wouldn't even look at them. Said they 'had' to run their own diagnostics - which they did and replaced the CAT on that bank and sent it back to me. Week later the codes were back and they replaced injectors. Yea for my "I told you so moment".

Mark Rayner

I hear you on the "I told you so" moment. The funny thing is I have swapped engines, done most jobs but not an official tech. In fact I'm a programmer and web designer. So when I started to learn about scan tools and fuel trims etc, it was a moment the lights went on. Debugging code is a similar mindset to running the scantool. But I'm shocked by the lack of logic I find in a lot of tech's. They make more money being part-swappers I guess and everyone expects the diagnosis for free. I have not swapped the O2 sensor yet, but will report back for the record.