91 Honda Civic erratic gas mileage

How often do you get gas? Every day or every two days? My experience is that the cut-off point on gas pumps varies a lot. The shorter you drive between fill-ups, the more of an effect this will have on your apparent gas mileage. Try using the same gas station and the same pump for several consecutive fill-ups and see if the gas mileage gets more consistent.

On a test run of 11 fill-ups picking random gas pumps (same station every time) my gas mileage varied from 28.5 mpg to 36.1 mpg with the same driving pattern. The mean was 32.4 mpg and the standard deviation

A later test run with 9 fillups using the same pump every time gave a range of 31.5 mpg to 33.9 mpg, average 32.8 mpg and a standard deviation of 0.86. Average amount of fuel filled was 7.88 gl.

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Two ot three guesses: Your reservoir isn't filled to the MAX mark, so your temp sensors aren't being emmersed, and your rad cap may be due for replacement. Your neighbor's kid may be stealing your gas too :-( Ever changed the PCV?

'Curly'

with that high of milage on that car you are blessed you haven't had to swap the whole engine out yet. you should maybe if not done for awhile flush out thoroughly and fill your radiator with prestone brand only antifreeze mixed with good water. have the thermostat replaced to.

Every other day I fillup. Intersting hypothesis you have on the pumps. Thanks for the reply.

PCV changed every 30k. I do randmly leak a little coolant but seems to seal with all of the stop-leak I have. Have a new radiator for it, will install when I replace the AC compressor. Was looking for any ideas of what to check since most of the front end will be opened up. Do you have an idea of exactly how the cooling fans shut down with the AC? Thanks for the reply.

Thermostat replaced. "Hose" flushing are all I've ever done because I use ONLY distilled water and for the last 105k Dexcool long life anti-freeze. Once I replace the radiator and cap will be interestinf to see if any affect on AC and cooling fan shut down. Thanks for the reply.

I am not sure what you mean by the "cooling fans", Your car should have one ventilation blower for the cabin, and under the hood most likely two fans, one radiator fan and one condenser fan. Which ones shut off with the A/C?

I don't have the wiring diagram for the '91 Civic, but I have one for the '91 CRX. On that car, the condenser fan and the radiator fan will either both be on or both be off unless there is a fault in the system.

There is a thermo switch on the evaporator, when the evaporator gets too cold, this thermo switch opens up and causes the compressor, condenser fan and radiator fan to turn off. Under normal operation, this thermo switch will cause the A/C to cycle on and off (except if it is hot enough that the A/C just barely can keep up). I doubt your problem is here, no reason this should act up only when idling.

There is a refrigerant pressure switch in the circuit that opens both if the pressure is too high or too low. In either case, the compressor will stop as will the radiator fan and the condenser fan (unless the coolant is hot enough that the coolant thermo switch turns on and causes both fans to run). My guess is that this is the switch that causes the A/C and fans to shut down when you are stopped. If there is insufficient air flow through the radiator and evaporator, the refrigerant pressure could become too high, and thus shutting down the system. Is your radiator and / or condenser getting packed with insects, dirt and debris?