Although my '88 Formula automatic is running well, the fuel economy seems very low. I used 5.9 gallons over the last 83 miles. That's just over 14 mpg in combined city/highway driving. There are no trouble codes. The only mechanical issue I know of is the catalytic converter. The internal parts are loose and it rattles over bumps, and a small exhaust leak.
Question: What kind of fuel economy should an '88 Formula automatic get that is driven normally?
When i had the 2.8/3sp auto in the 85, I got 14-17 city, 20-23 highway. In city with many short trips, the cold start injector causes it to use a lot. With the 3800 and 4sp auto, I got 14-18 city and 26-29 highway. But I couldn't help myself from getting into the SC. Did get close to 32 on a long trip and that was during cooler weather. I could only imagine if I had a 2.8/3.1 with the 4sp/overdrive transmission, I could get close to 36-38.
The following has been replaced on my engine: 02 sensor MAP sensor Coolant Temp Sensor IAC Cap, rotor, wires ICM plugs And yes, it does have a T-Stat. but...the gage barely budges off cold. I may need to look into that.
Check thermostat first. If it's bent or stick open, the engine will never warm up and more fuel will keep being injected into the motor than normal. (Assuming your cooling temp sender is accurate ha ha) The second thing, since you had a cat fall apart, is to hook a screw in type compression tester in where the o2 sensor is and look for high pressure in the exhaust. I think your looking for over 4 lbs at 2800 rpms? (Somebody correct me if I'm wrong). Anything above that would indicate a blockage.
I've got an '86 SE, 187,000 miles (original engine and 4-speed), 185* thermostat, that consistently gives 31-32 on the Interstate, and 23-24 in town. Completely stock, don't run the A/C, and occasionally hot-dog it . It was originally engineered to use the 185* thermostat, so that's what I use.
I have the dreaded exhaust leak at the inner or window side exhaust manifold. I think it is just a blow gasket, so my plan is get in there fix that leak and while the exhaust system is partially apart, either install a new converter or else the bypass pipe the Fiero Factory sells. And while that is going on, replace the thermostat. The exhaust leak is a small one, just a tick-tick-tick you can hear under acceleration, but I understand that could also be affecting things.
A few thoughts, I'm not good at diagnosing but I can give you some ideas.
The fuel pump isn't working correctly The fuel filter is gunked up There might be debris in the fuel lines which might cause a lot more problems Small vacuum leaks (spray around the engine bay with carb cleaner or ether starter fluid and listen) gunked up engine internals. I personally use seafoam in the brake booster line in the front hood of the car. This might get some of the gunk out clogged cat. The gases cant go anywhere and are creating backpressure.
You might just be running too rich. Have you pulled the spark plugs and diagnosed those? White/grey clouded tips = too rich. Black/ grimey = dirty internals or very old plugs. Also, the plug gaps may have stretched from use causing a weak spark and forcing your car to pump in more gas. I've have the best luck with Bosche +4 iridium plugs.