1988 is a transition year, so I am assuming 2.3l with fuel injection and TFI ignition system with EEC-IV computer
On the air filter box is probably the IAT(intake air temp) sensor, it plays a minor roll in calculating best air:fuel mix.
'88 won't have a MAF sensor, it uses a speed density system so would use a MAP(manifold absolute pressure) sensor as the main air:fuel mix sensor.
It will usually be located on the firewall behind engine, it isn't very big 2"x3" or so, it will have a Vacuum hose and electrical connector.
Check that the vacuum hose is in good condition, it runs direct to intake manifold.
MAP sensors rarely fail, not never, but certainly would be way down on the list.
Vacuum leaks don't generally effect MPG, but can in a Speed density system, simple test is to unplug the IAC Valve after engine is warmed up, idle should drop to 500rpm or engine may even stall, either is good, it means no vacuum leaks, if idle stays above 700rpm then there is a problem.
If you have messed with what "looks like" an idle screw that could be the problem, but if not then you have a vacuum leak.
On the fuel rail there will be a Fuel Pressure Regulator(FPR), it will have the Return Fuel line attached and a Vacuum hose, remove the vacuum hose and check it for Fuel, leaking FPR diaphragm will show up there.
This would cause engine to run rough because extra fuel is being sucked into the engine, and a drop in MPG for the same reason.
Check a spark plug tip, if it is blackish instead of light brown then you are running Rich, so extra fuel is being burned in the engine, not leaked out of a fuel line.
'88 2.3l only has the one O2 sensor, these generally last 100k miles, so if it is older, I would just change it on spec, these pay for themselves in fuel savings over that 100k miles.
And O2's tend to fail in a way that causes engine to run Rich, this is a "fail safe" since failing lean can cause significant engine damage, melted pistons and valves.
Do the Clear Flooded Engine test
Press gas pedal to the floor, hold it down all the way
Try to start engine
It shouldn't start or even fire, fuel injectors are shut off.
You replaced TPS(throttle position sensor), when throttle is closed computer gets under 1volt(.7-.99v) from this sensor, when throttle is wide open computer gets above 4.5volts from this sensor.
With key on, engine at 0 RPMs and TPS showing above 4.5v(gas pedal to the floor) computer enters "Clear Flooded Engine" mode, fuel injection is shut off but spark stays on.
So engine shouldn't start, if you release gas pedal even while cranking injectors will start up.
What this test can tell you is whether or not you have fuel leaking into the intake, either from FPR or a leaky injector.
Engine usually wouldn't start but would fire a bit if there is a leak, but if RPMs reach 400 then "Clear Flooded Engine" mode would end.
Last edited by RonD; 01-31-2016 at 10:06 AM.