91 Honda Civic erratic gas mileage

I figure anyone who can answer this is hard core Honda.

I have a 91 Civic LX, 1.5 liter, manual 5 speed, AC, power steering,
windows and door locks. It has an erratic gas mileage and idle problem.
Anytime it sits in the heat after being run, when it is first turned on,
engine already warm, will idle at 2k until it runs a distance. It seems
like something overheats and then cools down once air starts moving.
Strictly a warm weather problem. Also, after idling in the heat for a
while the AC just cuts off AND the cooling fans quit too. The only thing I
can do in traffic is turn the heater on wide open until the fans decide to
come on.

Next, maybe related, the gas mileage is all over the place. I commute 168
miles a day, same traffic patterns, same roads. Yet on Monday it may be 37
mpg and the next 30 mpg and on Wednesday back up to 35. I?d say the
average is around 34.

Background: Car owned since new, 309k miles, replaced alternator,
distributor, primary fuel injector and EACV. The O2 sensor replaced
several times and the ECU was swapped just to see if it makes a difference,
no it didn?t. If you need further details or history please let me know.

 

:

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
2.67 mpg. Average amount of fuel filled was 7.5 gl.

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?

 

Fill a Honda with Prestone? I know of no faster way to kill off the water
pump.

 

What do you use? Autozone brand? Prestone is the best.