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Car Forum / Chrysler Cars / November 200798 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?? > Asking for advice > 1998 Caravan SE 3.3 163K [quoted text clipped - 13 lines] > > 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 > <jroma...@hotmail.com> wrote in message > [quoted text clipped - 24 lines] > > - Show quoted text - I've been having the problem since May not a gas grade problem > > <jroma...@hotmail.com> wrote in message > > [quoted text clipped - 26 lines] > > 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. > Asking for advice > 1998 Caravan SE 3.3 163K [quoted text clipped - 13 lines] > > Anyone have any ideas what may be wrong?? > Check the EGR valve to see if it is stuck closed. How would an EGR valve stuck closed cause a reducton in fuel mileage ??? >> Asking for advice >> 1998 Caravan SE 3.3 163K [quoted text clipped - 13 lines] >> >> Anyone have any ideas what may be wrong?? 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. >> Check the EGR valve to see if it is stuck closed. > [quoted text clipped - 17 lines] >>> >>> Anyone have any ideas what may be wrong??  SignatureGyz Everyone thinks they know how to drive. Everyone thinks they are good drivers. > Exhaust gas is inert. Correct Adding it to the intake stream increases the > charge density and thus the amount of gas that will be expanded by > ignition. 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 > systems. Works about the same on all of them. > One possibility is a drop in MPG while cruising. The OP stated that the EGR valve was stuck CLOSED. If it was stuck open it may cause a drop in mileage. >>> Check the EGR valve to see if it is stuck closed. >> [quoted text clipped - 18 lines] >>>> >>>> Anyone have any ideas what may be wrong?? > > Exhaust gas is inert. > [quoted text clipped - 51 lines] > > - Show quoted text - 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 > > > Exhaust gas is inert. > > [quoted text clipped - 57 lines] > 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 > [quoted text clipped - 66 lines] > > - Show quoted text - no tires 4 yrs. old On Oct 14, 12:32 pm, jroma...@hotmail.com wrote: > > did the tire size change?/ if it died was the pinion factor also changed > [quoted text clipped - 70 lines] > > - Show quoted text - installed new front O2 sensor this morning will let you'll know what the MPG is after I run thru a tank full On Oct 15, 7:11 pm, jroma...@hotmail.com wrote: > On Oct 14, 12:32 pm, jroma...@hotmail.com wrote: > [quoted text clipped - 77 lines] > > - Show quoted text - 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 >> Exhaust gas is inert. > [quoted text clipped - 7 lines] > dilutes the A/F mixture which reduces peak combustion chamber temps to > reduce the formation of NOx. 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. > The results will vary with different types of engine control >> systems. > > Works about the same on all of them. 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. >> One possibility is a drop in MPG while cruising. > > The OP stated that the EGR valve was stuck CLOSED. If it was stuck open it > may cause a drop in mileage. If the EGR valve is stuck open the engine will not idle, or idle very badly. >>> Exhaust gas is inert. >> [quoted text clipped - 11 lines] > the same amount of fuel/air to produce the same amount of energy. Adding > exhaust increases the amount of gas in the cylinder. 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 > 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 [quoted text clipped - 30 lines] > If the EGR valve is stuck open the engine will not idle, or idle very > badly. >>> Exhaust gas is inert. >> [quoted text clipped - 10 lines] >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. 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. >Adding exhaust increases the amount of gas in the cylinder. 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. >The more gas >there is in the cylinder the greater the expansion when the fuel/air is [quoted text clipped - 31 lines] > >If the EGR valve is stuck open the engine will not idle, or idle very badly. > >>> Exhaust gas is inert. > [quoted text clipped - 62 lines] > > - Show quoted text - current tank fillup 50/50 city highway 285 miles@14.58 gallons of gas =19.54 mpg |
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