Low part-throttle power and awful mileage

[jamyers]

'92 Park Avenue with just under 300K well-maintained and mostly highway miles.

At part-throttle driving, it just doesn't have the power it used to - it'll take forever to get up to 60mph unless you really put your foot into it (and then it'll take off like a scalded dog). It doesn't feel like the ignition is missing or like it's stumbling, it's just smoothly underpowered. The engine starts right off, idles great, and has good power at WOT.

In the last year it's gas mileage has dropped from a previous average of 27-30 to a current average of 18, best of 20 mpg.

Within the last year: new air and fuel filters, fuel pump, spark plugs, plug wires, alternator, water pump, thermostat, radiator, and main + rod bearings (not that it needed them, but...). Compression is still within OEM specs, and it doesn't use or burn oil. Fuel pressure shows (iirc) 41psi at idle and checks out ok according to the chassis manual. The oxygen sensor has roughly 50K miles on it. Transmission was rebuilt about a year and 10K miles ago by a reputable shop.

We've taken it to a reputable shop, and they can't find a thing wrong with it. No "Check Engine" lights or stored trouble codes.

Suggestions?

[HotZ28]

We've taken it to a reputable shop, and they can't find a thing wrong with it. No "Check Engine" lights or stored trouble codes.

You need to monitor live data on the scanner to see what is happening. Too many possibilities here without the results of a data scan. The fact that you have a "NO check engine light", does not tell you what you need to know! BTW, have you checked exhaust back pressure? Sorta sounds like a cogged cat!

[jamyers]

Thanks for the reply!

OK, we'll look for an OBD1 scanner that'll give us live data. Any offhand ideas what we should be looking for?

Haven't checked the cat / exhaust for plugging. I'd guess the easy way to do that would be to pull the O2 sensor and screw in a pressure gauge? My bro-in-law (it's his car) doesn't think that's the issue since the engine will go right to the redline at wot. Would a clogged cat do that and still kill off the mid-throttle power and economy?