here is my problem: I have a 91 SE 5spd with 143k on it. I have changed the following recently:
-spark plugs
-plug wires
-distributor/cap
-fuel filter
-O2 sensor

My car is still crappy on gas mileage after changing out the O2 sensor ~ 18 mpg/200 miles before gas light comes on. The car will not have power at all until 2.5k RPMs, and just seems to take its time before the 2.5k mark. can anyone help me figure out whats going on here?

 

Read in the FAQ's about a knock sensor. If you have the original one in there it is most likely the problem. You can "test" to see if it is the problem by putting a 470k ohm resistor from radio shack in the harness instead of having the KS hooked up. This is only a test though and should only be done temporarily or else you could damage your motor. If the motor runs fine with the resistor in, the knock sensor is the culprit.

 

Well he has a VG, in which the knock sensors usually do not go bad and cause a loss of power.

Try pulling out one of your new plugs to see if its fouled out. I doubt that it would be an air filter, but you could also see if thats clogged. Check your ECU for any trouble codes. These would be good places to start. Its pretty hard to say for sure though, usually this kind of problem is due to running rich/bad O2 sensor.

 

VG's don't have bad knock sensors? That's kind of cool. Are VE's bad just because of their location in that pit or what? BTW a really bad air filter will dog down your car. I changed mine and it was much more responsive and better gas mileage.

 

Yeah the VE sensors go bad because of corrosion, which is due to location. The VG sensor is near the rear exhaust manifold, not too hard to get too either, I could reach it from under the car on my old max.

 

I am going to try a water temp sensor. This may solve the problem. What the hell, I'll order a new water temp and knock sensor.

This problem is not an air filter being dirty, I checked it and it seems to be fine. I am also thinking it could be the MAF sensor.

 

The MAF is a possibility, get a chiltons manual and check it with a voltmeter. As for the knock sensor, its more than likely good. Even if it was bad, I doubt that it would cause bad mpg/rich conditions.

 

I have another issue with my car. When I start the car, the electric fans automatically turn on. Anyone know what would cause this?

 

probably a failsafe from a malfunctiong radiator fan switch

 

Well, today after work, went to pep boys and bought some throttle body cleaner and electric parts cleaner.

Got home and took off the intake hose going into the manifold and followed the directions using up the whole can. Then I took off the MAF sensor and sprayed the MAF sensor on both sides. Looked like it was still intact.

Went for a test drive, not sure if I can tell a difference or not. When I take it out of gear, it will still drop down to 500 RPMs, but does not seem to surge during idle. I guess I will find out if cleaning the O2 sensor was a result in horrible fuel economy (200 miles per tank).

 

Changed out the PCV valve this morning, still having flat spots when accerating. Taking it to Firestone this afternoon for a diagnostic, $79 =(. I do not know what else to do, so I will get the definite answer to my problem.

 

Water Temperature sensor. I read and found this on the internet: Coolant Sensor Strategies: The coolant sensor is a pretty reliable sensor, but if it fails it can prevent the engine control system from going into closed loop. This will result in a rich fuel mixture, excessive fuel consumption and elevated carbon monoxide (CO) emissions - which may cause the vehicle to fail an emissions test.