There is noticeable difference in the performance open vs closed loop on my 96 150 4.9 5sp. It has been lightly modified with; head milled enough to require .060 shorter push rods, Crower 19212 cam & appropriate springs and retainers, some porting, 3" exhaust from the head pipe to the exit behind the RR wheel, included high flow cats and muffler. Running @ about 12° ign advance. all sensors, distributor and vacuum lines replaced with the rebuild.
A lot of this was done when I got it, so I don't have all details, but it seems to be pretty well done.
Anyhow when it is in open loop still cold, it runs like a raped ape, but when it goes into closed loop it still feels a little peppy, but a shadow of the open loop.

 

Sounds like the EEC wasn't recalibrated for all those changes, and those changes have made it flow too much air for the fuel trim. So when it's OL, and the EEC is trying to run it slightly rich, it's running closer to stoichiometry. And when it goes CL, it might be running a little lean. (I'm just guessing, of course.)

Does it run hot in the summer? What happened to the oxygen sensors when the Y-pipe was rebuilt?

 

It has shown no tendency to overheat even in the hot Texas summer. The y pipe was pretty much left intact, so the O2 sensors are in stock location. The 3" begins at the exit of the front converter (Magnaflow 99047HM.) I too suspect it is a bit lean in CL. It actually sounds like a lean flutter sometimes at highway speeds, like back in carburetor days. I'm temped to put bigger injectors in it, but that would most likely make the OL way rich.

 

Winging a few things - the 0.060 shorter rod suggests a result of 295 rather than 300 CID - don't think that's enough to matter, though.

Thinking the fuel trim is still within its control range if no codes are being thrown...? Eh, doesn't address the performance issue here, though...

Any chance of a vacuum leak anywhere, or duct leak between the MAF and throttle butterfly assembly, or a dirty MAF - any of which may allow more air into the engine than is being measured?

 

It is the push rods that are -060, not the connecting rods . I've put a vacuum gauge on it, and all seems normal, unsure of the MAF, I've sprayed a little MAF cleaner on it, but that's about all I've done. It never throws codes unless you start it up with a sensor unplugged, or something silly like that. Seems to do ok on gas, I don't have a hard number, but it isn't constantly empty, like my 460 was.