93 G20: mpg mysteriously LOUSY! why?

[chuckm]

Hi.
Am returning to a question I posted Nov.09--why is the fuel mileage on my 93 5 sp. G20 STILL so poor? Up to ca. 2 years ago, it was consistently ca. 30-38mpg in my normal driving cycle...then dropped radically to low 20's, from which NOTHING has worked to improve it. I still love this car and care for it scrupulously...but no attempted cure has worked. 3 local mechanics (none specialists for this type of car...there are none hereabouts) have tried as well, and throw up their hands.
Odo now at ca. 180K. Engine still tight & strong, and OEM stock except for Fram Airhog air filter (which I like). Have replaced in recent months: air/fuel filters, platinum plugs (@ .44), plug wires, cap/rotor button, O2 sensor (OEM stock), exhaust system from cat back, knock sensor, vacuum hoses (none of which were bad, but did it anyway), alterntor, battery, starter. Have followed FSM to check AFM, TPS, water temp sensor, timing and O2 sensor function--all within specs. Cannot get at idle control valve, but idle remains stable @ ca. 800 when it should be there. No discernable surging or stalling. Have had fuel lines, tank etc. checked for leaks--all clear. When exhaust system replaced, had cat checked, and mechanic said it looked healthy and clean.
The only remaining things not torn into and looked at inside: PAIR valve (which is only about 20K old) and injectors/seats. Mechanics all say these seem OK to them, judging from the way the engine runs.
So: anybody got any ideas? If I can't get my mpg back up to halfway normal, I have to give up and look for another car--which I don't want to do.
thanks in advance for any help at all,
chuckm

[turbonxsx]

If the your converter is plugged, it will create a restriction in your exhaust system. The buildup of backpressure will cause a drastic drop in engine performance and fuel economy, and may even cause the engine to stall after it starts if the blockage is severe.

The easiest test for converter plugging is done with a vacuum gauge. Connect the gauge to a source of intake vacuum on the intake manifold, carburetor or throttle body. Note the reading at idle, then raise and hold engine speed at 2,500. The needle will drop when you first open the throttle, but should then rise and stabilize. If the vacuum reading starts to drop, pressure may be backing up in the exhaust system.
Give this a try...

[chuckm]

Thanks turbonsx for the tip.
As soon as weather permits (currently buttcold, snowy, etc.) will give it a try. But...(a) 2 months ago, when exhaust system replaced, mechanic checked CAT and called it 'clean and OK looking'. Which may mean nothing. (b) Throughout my by now longthy efforts to figure out mg problems, have had no appreciable reduction in performance at all. Engine still starts fine, goes thru high idle till warm, settles back to ca. 800 rpm at idle, and accelerates pretty much as it ever did--and runs up to 90 or so with no problem that I can sense--just as it did years ago. To be sure: my 93 neve
r was exactly blistering--just pretty darned good for a bone stock 2 liter 4. I always kind of hoped to squeeze a couple more HP out one day...but the performance was acceptable, and till a while back, the mpg was super. Then it just fell off radically one day, and despite many efforts and parts replacements, has stayed pretty bad--about what you'd expect out of, say, a Crown Vic or Taurus.
I'd just give uip and get another car --but the G20 is still otherwise solid and satisfying. Besides: the mechanic in me says,
there absolutely has to be a definable cause for this drastic mpg drop--and it ought by God to be fixable!
I've also wondered what would happen if I bypassed the PAIR valve setup and EGR, neither of which seems to do any meaningful good (by 1995, PAIR setup was gone anyway). Perhaps someone has tried that and can say what results are.
As for PAIR: had to replace that assembly twice earlier on, since the reed valves go bad rather quickly. The present one is ca. 20K old and probably no longer up to snuff--but I can't find a new one for less that $200+ and see no point to that, since: the main reason a failed PAIR valve causes problems is if it 'backfeeds' back in the intake air box. I disconnected the air line into the air box, so that that line is simply 'open' in the engine bay: if the PAIR wants to pull in fresh air into the exhaust, it can--and if it wants to puff exhaust out the wrong way, those 'puffs' can't get into the intake flow. So far as I can determine, this way, even if the valve isn't working properly, it's also not adversely affecting the intake mixture.
Well, I will try checking the CAT for possible blockage. and will report results in a few days.
Thanks, chuckm

[PapaSmurf2k3]

Did you say you had your air fuel ratios detected? It might be worth it to plug in a wideband O2 sensor or something to see where you stand... the fuel has to be going somewhere!!

Only thing I can think of: Dirty/Bad MAF. Have you tried cleaning it or borrowing someone else's to swap in?

[chuckm]

Hmm. OK, I live in KY...where there is no sort of emissions inspection anywhere--which would be the only way I can think of to check the air/fuel ratio as an "output". Don't know anyone hereabouts with a similar car to "borrow" a MAF to provide crossreference. But it's a good idea, and I'll work on it. As for "wideband O2 sensor"--there you have me: I don't know what you mean. Sorry for my ignorance. The present O2 sensor is nearly new Bosch, alleged to be OEM and specific for this model--at any rate, it looks identical to the old one it replaced. But...you're absolutely right: the damned gas has to be going somewhere. Oyes--forgot to mention earlier that I did test the fuel pressure regulator (with vacuum pump) and the vacuum feed to it--both give every evidence of being still OK.
But--I now have a couple of possibilities to try, soon as weather improves. Maybe I can find a good (and cheap) used MAF somewhere just as a trial measure. I have wondered about the MAF, as some threads suggest that a bad one will sometimes not throw a code or cause poor running--tho I find that difficult to digest. Every bad MAF I ever had (on other cars, to be sure) either produced no running at all or really bad performance. I can state that, if you unplug the sucker, the car shuts down immediately and can't be made to start.
Don't you just love a good mystery?
Thanks for your tips, and I'll report results soon's I have any.
chuckm