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1994 Geo prizm poor fuel economy.

7K views 16 replies 8 participants last post by  Bitter 
#1 ·
My Geo Prizm (toyota corolla in all but name) with a 4a-fe (1.6L) and a 5 speed manual gets only 30 miles per gallon at best. I replaced the air filter 3000 miles ago, the oil (10w30) and filter the same (it has lost maybe 4 oz at most, and that was when I checked the oil right after running it.)
I upshift at 2200-2500 rpm and have 75w90 full synthedic oil in the gearbox.
My tires have 35 PSI in them.
With the air intake as it is from the factory, I only get 26 mpg. With the last part of the air intake off, I get 30. I have larger then stock tires because the seller had them and they have at leask 20k miles left in them (if memory serves, 195/70r14)
My drive to college is 7 miles highway driving at 35 mph, 7 miles at 55 mph, and then 2 miles city traffic at 35 mph.
Am I missing anything that would hurt fuel economy?
 
#7 ·
I second. It is indeed the thermostat that causes poor fuel economy. You stated that you've replaced the air filter and had tires with the right PSI, it should be running smoothly. I believe Corollas have good fuel efficiency so given the things you've done to your auto so far, you should be enjoying that now. Go and have your thermo replaced.
 
#8 ·
A rotor, thermostat, and distributor cap are at most $20, replace them all and see where you end up. I get 26 with my Corolla driving in the city, and I got 42 on a trip to TN from OH. It's all in how and where you drive. Take off your main intake hose at the throttle body and have someone spray 2 or 3 spurts of some carb or throttle body cleaner directly into the intake as it's running, giving it some gas as they spray so it doesn't stall. While you're there, you should take the throttle body off and clean your Idle Air Control Valve with the carb/t body cleaner. It's 4 bolts and 4 screws to get to it. Easy, preventive, potentially gas saving maintenance.
 
#9 ·
Turns out that the previous owner gutted the thermostat so it was just a metal disk. Replaced the thermostat and the hoses (even the bypass hose and heater hoses.)
Replaced cap, rotor and wires too. I filled up last night so now we will see what fuel economy I get.
 
#11 ·
On my normal route, in rainy weather, after an ECU reset, I got 31.5 mpg. Chances are that it will go up a bit more before the year is out.
Normal fuel economy in this weather is 28 mpg.
 
#15 ·
Turns out that my odometer and speedometer are about 10% slow.
With the CAI defeated, I'm getting an average of 37 MPG with the same mix of highway and city traffic as before.
When I do 70 MPH on the highway, I get 36 MPG with the CAI setup.

I also replaced my PCV valve, sprayed some carb cleaner into the throttle body, and changed the oil afterwards, this rewarded me with the engine burning practically no oil.
 
#16 · (Edited)
Necroing this thread for an update: I found that I was staying in cool fast idle (made it run at 1000 RPM idle most of time) outside of hot part of summer due to my heater valve being slightly open due to a poorly adjusted cable. I guess that this had my idle fuel consumption 33% higher than what it should be.

Time will tell if I get better FE, but after adjusting the cable I can't run an upper grill block in as warm of temperatures without issue so I know the engine was being cooled by the heater core and defeating the thermostat a bit. I found software that can read the data stream the pre OBDII ECUs put out to VF1 when TE2 and E1 are jumpered so I'll try to see how hot the engine actually runs and if it isn't running hot I'll enough block the upper grill a bit.

I get better FE on my commute compared to highway trips due to high rate of speed on long trips.
 
#17 ·
A little offtopic but the heater core does have a pretty big heat load on the engine as you've found out. I used to leave my heat off totally on the coldest mornings until about a mile down the road, then I'd turn it on. I found that warmed the engine faster and got me heat faster even though I had to freeze for a mile. Rather than slowly trying to warm the cabin and engine, I would concentrate on getting the engine to temp the fastest by leaving OD off and the heat off. After a mile the gauge would be starting to come up off C and I could turn the heat on and have some heat, but you could see the cold coolant dump into the engine, the gauge would tick down to C again for a minute as the coolant passed by the sensor! Heater on but OD still off for another couple miles until it was at at least half between C and the mid mark. Even on -30F days this worked pretty well to get the engine warmed up. In the winter you may find that a butt warmer (seat heater) can keep you quiet warm until the cabin temp gets up so you can keep the heat off and work on getting the engine warm which will help out the MPG situation when the temperature outside plummets.
 
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