Erratic idle, high fuel trims, and lean codes


1st Gear Poster
Mar 7, 2024
Ee arth
Vehicle Details
96 Tbird Lx, PI 4.6
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Car is a 96, have a PI engine from an explorer, and a chip from SCP for the PI swap, done 10-15k ago with new plugs and wires.

Car slowly started developing some minor oddness. Would drop from high idle fast when cold, rpm drop too low coasting to stop, fluctuating idle, sometimes almost stalling when in gear, especially coasting to a stop. Very intermittent and slight at first, but has continually gotten worse, all of the issues i just mentioned all the time.

Scanner showed a failing o2, high short and long term trims, high idle load / tps 27-30%. Replaced o2s with bosch, ran poorly still but differently, another drop in mpgs. Figured maybe it was the mafs as it has 275k now and the PO had, had a k&n in there, bought a cardone whole unit as it was "hearted" on rockauto.

Between the bosch o2s and new mafs the car ran terrible, would barely idle, usually would die right after starting when cold, mpgs down even further to 17-18 (usually gets 23-24 when running well), missing at idle in gear, and down on power until you gave it enough throttle to go closed loop. At this point i put the original o2s back in, things improved a bit, but still definitely not right.

Was put to me that there has been some knockoff bosch parts floating around so i bought some ngk o2s as i understand they built the oem pieces. Small improvement over the old originals but still not running right. Currently it has the new ngks and the cardone mafs. Going to swap the original mafs back in this weekend and see what changes. Hobo freight scanner isnt very intuitive.

Have checked for vacuum leaks with ether with no hits, i can create a very small leak at the pcv valve and get a hit spraying the opposite valve cover so i am confident that it isnt an external vacuum leak.

I dont know where to look next. Idle air control valve seems to work just fine, rpms drop drastically when unhooked. Im not sure if the egr is sticking or how to test. The TPS has always been kinda sus to me with it being so high but it coasts when you let off and it kicks down when you get after it, not exactly sure what all it controls.

I might have a small exhaust leak at the down pipe either side, i can hear it a little sometimes but it doesnt seem to be affecting trims as both sides are equal. I just picked up a fuel pressure tester to check the pressure regulator, has a new walbro pump with around 10k on it, fuel filter done not long after. Also have a new pcv valve to put in because its cheap and one of the only parts i could still find a motorcraft.

I apologize for the long post, im beyond tired of dealing with this thing running like this and im not thrilled with the prospect of just replacing shit, cost for one, but the new stuff seems to be worse off than the original as often as not.
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I'd bet you have a vacuum leak. give the intake the cigar test, and see what you see. The egr is commonly leaking. 8 pieces of 1/4" line fixes it.
Are your fuel trims high all the time or just at idle. Do they come down if you rev the engine up? What codes is it giving you? Have you tested the fuel pressure at the rail? Any idea when the fuel filter was last changed?
15-25 is what ive seen, havent driven it with the scanner hooked up. Like i said in the original post, fuel filter is pretty fresh, going to check the fuel pressure this weekend.
Well im not crazy at least. PCV valve had disemboweled itself, finally got around to putting in the new one i bought 6 months ago and its fixed lol.
Fuck me. I missed the part about Bosch sensors. Get some ntk sensors from ra, and that will likely fix your problem.You can move the bosch to the rear, but if they give you any errors toss them.
Yea, NTK or something like that in there now and its much happier.

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