Negative Long Term Fuel trims while cruise speed

Hi Guys,

I've been looking for opinions on fuel trims.

Montero 2006 3.8l V6.
New Sparks, wires, coils, air filter, and both O2.

While at cruise speed around 2000 rpm 60 mph long term fuel trim for both banks average negative 3.9, short term in both banks oscillates from minus 7 to 10 and positive 7 to 10 (in that ballpark)

Long terms never go positive at idle or cruising , idle is king of rough at parking (no load) fuel efficiency has decreased.

+/- 10% is normal. A long term fuel trim of -3.9 is nothing to be concerned with, in my opinion.

Cruising fuel trims are well within bounds but for diagnostics they are more valuable when compared with the fuel trims at idle. If there is a big difference between the total trim at idle and when cruising then it can be pointer to a vacuum leak or MAF or fuel pump fault.
For example With a vacuum leak especially on a MAF engine the fuel trims add fuel at idle but go negative at higher RPM but that isn't the case for your vehicle.

Where you get a low speed roughness or misfire that goes away at higher RPM particularly coupled wth higher fuel consumption then a compression test is worth considering.

I thought of that and certainly I've not done a compression check since is kind a pain in this engine due to the plenum has to come out to do it, however there is no smoke at tail pipe and no blow-by is felt thru the PCV orifice on valve covers (pcv removed).

I know the tests I've described are no definitive but car is only 66k miles and very well maintained (bought it used).

Still kind of puzzled since those long term trims do not go positive...

LOL...

Now the vehicle has developed a slight hesitation, gas pedal lag!

Did you ever figure out what was wrong. This sounds just like my 2013 F150 5.0

What are the total trims at idle vs 2000 rpm in park? Total trims are combined short and long term.

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Mine are hard to calculate off idle they jump so much both long and short. 0 at idle and at 2000 in park they go to -20 then when you let off the gas they go to say 15 and eventually come back to 0. I think I figured mine out last night. I had an 02 fail and had a cat melt down on me a few months ago. Both cats replaced along with all 4 02 sensors just for good measure, she's got 125,000 miles on her. I got to thinking about whats happening with trims and figured it had to be an exhaust leak but can't find one, last night I put snoop leak detector on the manifold collectors and sure enough when my nephew hit the throttle it bubbled pretty good. I'll update after I change the gaskets this weekend and let you guys know whether this was it. BTW it was the Ford dealer who replaced the cats