98 caravan 3.3 poor gas mileage

Asking for advice
1998 Caravan SE 3.3 163K
Starting back in May my mileage dropped from 18.5mpg in city driving
since new to 16.
Just thought it was the hot summer and using the air conditioner all
the time.
Now with cooler weather the mpg has stayed at 16mpg.
I'm not getting any check engine/service engine so lights.
I've replaced the air filter ,plugs and wires with dealer parts no
change.
Brakes are not dragging. Tires@32lbs.
Transmission is not slipping engine revs 2000rpm@60mph since new.
Had it to the local service station and they checked for obdII codes
and none found told me that its going into closed loop operation and
the O2 sensor is switching.

Anyone have any ideas what may be wrong??

 

your gas station might have switched to a winter blend with higher alcohol
content that supposedly reduces smog (oxygenated fuel)

Ted

 

I've been having the problem since May not a gas grade problem

 

Start with the usual culprits then. Check all vacuum lines for leaks.
Clean
the throttle body. Check tires for proper inflation. Check suspension for
misalignment/
damaged bearings. Check for plugged air filter. Check exhaust for partial
restrictions or leaks. Spray
raw propane from a torch around the intake manifold and plenum connections
to see if there are any leaks. (idle will change if there are)

Have you pulled a plug or two and inspected the insulator now that
you have changed the plugs? What is manifold vacuum at idle? Use a
vacuum guage.

Note also that older o2 sensors still work but not as well and will impact
mileage. If it's the original o2 sensor(s) it's way overdue for
replacement.

What oil are you running and what is your change interval?

Ted

 

Check the EGR valve to see if it is stuck closed.

 

How would an EGR valve stuck closed cause a reducton in fuel mileage ???

 

Exhaust gas is inert. Adding it to the intake stream increases the
charge density and thus the amount of gas that will be expanded by
ignition. The results will vary with different types of engine control
systems. One possibility is a drop in MPG while cruising.

 

Correct

Adding it to the intake stream increases the

Wrong. It decreases charge density. Remember that it is an inert gas so it
dilutes the A/F mixture which reduces peak combustion chamber temps to
reduce the formation of NOx.


The results will vary with different types of engine control

Works about the same on all of them.

The OP stated that the EGR valve was stuck CLOSED. If it was stuck open it
may cause a drop in mileage.

 

I have you hesitation/stalling of engine at idle and no pinging of
engine at speed both good indications of the egr working correctly.
I am not getting any check engine lights.
I will be replacing the front O2 sensor this week.
will let you know if that fixes it.

 

did the tire size change?/ if it died was the pinion factor also changed

 

no tires 4 yrs. old

 

installed new front O2 sensor this morning
will let you'll know what the MPG is after I run thru a tank full

 

couldn't wait to use a full tank of gas
170.4 miles and 9.759 gallons of gas = 17.46 MPG
up about 1.5 mpg but still lower then past average of 18.5
I'll continue to monitor the mpg and see if it increases

 

It does not decrease the charge density, but rather increases it. It
takes the same amount of fuel/air to produce the same amount of energy.
Adding exhaust increases the amount of gas in the cylinder. The more gas
there is in the cylinder the greater the expansion when the fuel/air is
ignited. This is one reason why diesels get better gas mileage than
gasoline engines, because they have a higher charge density. The other
is the higher compression ratio. Eventually you will see cooled EGR
systems on gasoline engines which manufacturers will use to raise gas
mileage to meet federal requirements.

There are two different metering systems that I am aware of. One method
is based upon air flow, the other is based upon manifold pressure.

The ones that measure air flow will correctly meter fuel regardless of
EGR function as the exhaust gas is introduced downstream of the air flow
sensor.

The ones that measure manifold pressure make an assumption about EGR
operation under cruise conditions, expecting an increase in manifold
pressure when the EGR valve is active. The fuel map is leaner because
the intake charge is assumed to contain inert exhaust gas. These systems
will be affected by an inoperative EGR valve. Initially the fuel metered
will be too lean, but they will correct themselves using O2 sensor
feedback. The symptoms I would expect would be hesitation and lower gas
mileage.

If the EGR valve is stuck open the engine will not idle, or idle very badly.

 

What are you calling "gas" ??? The exhaust gas is an inert gas, meaning
it contains no oxygen or combustible gases. It takes the place of a portion
of the air/fuel mixture which is combustible. If the exhaust gas were not
introduced in the combustion chamber there would be more room for additional
air/fuel mixture which is combustible.


The more gas

 

Wrong. The closer the combustible elements are packed together, the
denser the charge will be. The INERT gas takes up space in the
combustion chamber between the combustible elements REDUCING charge
density.

Nope. The amount of gas in the cylinder is the same. The MAKEUP of the
gas is what changes. The INERT gas is not forced into the cylinder.
It's mixed with the fuel and air IN THE INTAKE MANIFOLD and is drawn
in as usual.

 

current tank fillup
50/50 city highway
285 [email protected] gallons of gas =19.54 mpg