Dodge Grand Caravan 2003 ES model 3.8L engine - All wheel drive,current mileage 103,000.
I bought this van in June-09, everything was fine with it, the gas mileage was 15-16 mile per galon city, 21-22 mile per galon highway.
This is calculation with a pencil and a piece of paper, and the overhead display matched.
6 weeks ago I found out the mileage per galon suddenly dropped down to 10 mile per galon city, 14 or 15 mile per galon highway.
This is calculation with a pencil and a piece of paper, not the overhead display.
I though, may be the Oxygen sensor was dead, then I tried to replace the Oxygen sensor, I removed the heat seal to get litle more room to fit the socket into the Oxygen sensor,
I saw the exhaust manifold was cracked very very bad, I got so exciting and told myself "problem solved". I odered the exhaust manifold from dealer
($120+. not bad) then replaced it, after put evething back together.
I started the engine, and drove 20 miles highway to test the gas mileage, the overhead gas mileage/galon shows 13.6 mile per galon.
I let the engine cool down for 3 hours then drove 12 miles city road, the overhead gas mileage/galon shows 9.9 mile per galon.
I waited for the fuel level to drop down till the fuel warning light's up, then filled up 10 galons, I wrote down the odometer mileage and waited till the fuel warning light's up again.
After 5 days all I could have was 105 miles, so the average was 10 miles per galon.(Replacing the cracked exhaust manifold didn't fix the bad gas mileage)
I even tried to drive it with several different modes( grand mom mode, 18 years old boy/girl mode, 81 years old man mode,
drug cartel mode, road rage mode, casino royal mode) but still about 10 miles city per galon.
I've checked the break pads to see if they're stuck, nothing's with the break, no Check engine light what so ever.

Now I've replaced the following parts:
New sparplugs, sparplugs wires. Still about 10 miles city per galon.
New O2 sensor (before CAT). Still about 10 miles city per galon.
New MAP sensor. Still about 10 miles city per galon.
New Throttle position sensor. Still about 10 miles city per galon.

I've read several car forums to see if anyone got the vans(cars) with same problem, there are some Grand caravans got same problem but never solved, and of course the thread abanndoned.
Do you have any sugestions to fix it? (including replace the whole engine and the whole eletronic system).
Thank you.

 

Never heard of that problem. It should be getting 18 city and 24 Hwy. Mines 2002 Grand caravan 3.3 gets 20 city and 25 Hwy.

 

try your cooling temp sensor, if it is bad the motor could think that it is always cold causing it to use more gas, kinda like a carburator being constantly choked

 

Have you checked other parts of the exhaust past the manifold? A clogged or broken catalytic converter can lower mileage considerably. This can also cause higher pressure and temperature, which may have caused the manifold failure.

 

The temperature sensor is new, the themostat is new, the temperature gauge shows a litle bit below half way.

I bought a back pressure test gauge to test if the CAT was plugged, clogged, nope, the CAT is not plugged, clogged.
Let say if the CAT is plugged, clogged, I think the max speed I can drive may be 50MPH, 55MPH, or 60MPH, I did drive as fast as 70MPH, 80MPH, 90MPH.
I even replaced the O2 after CAT, but didn't help, now I intend to replace the O2 before CAT with a genuin one's instead of the after market as it is now.
I posted same question to some other forums to see if any new ideals, but people think that I didn't know how to canculate the MPG, that's why I got the MPG so low, and the speed odometer is no good, and the overhead display is no good,
the tire pressure is too low, the brake pads are jamped, etc...etc...

 

Some cheap, easy things that can't hurt.

Air filter. Also check to make sure the air intake is put together correctly. Sometimes a disruption in airflow can mess with the computer.

Look for a loose or broken vacuum line.

Run some STP through it the next couple of times you get gas. Don't use a higher octane fuel than what the manufacturer calls for.

This won't cause a severe mpg drop like you have, but the wrong viscosity of motor oil will cause a drop in mpg.

 

you've already mentioned O2 sensor. I've got a 1987 Shadow/CSX that has had a couple of problems when I tried to save some $$$ by not going to the Dodge dealership for replacement sensors.

 

The air filter is new, I've checked to see if vacuum leaked, even the exhaust manifold area, but no leaks.
I have put the original O2 back on the van and test it for few more days, if nothing change then I'll replace it with a genuine O2.
I'll be back and let you know.

 

my van also get 10-12 mpg in town driving like i'm 90 years old,, drive it until it on E then i'll put 10 gallons reset the miles i get 100-120 mile per ten gallons,, i do this all the time still get the same MPG,,, i have an 01 grand caravan sport with a 3.3L with 140k that why it is for sale now,,, i did order the front 02 sencor and will put it in when i get it ,,,just passed smog 2 weeks ago,, no ENG light on, if i'm only going to get only 10-12 MPG i'll do it in a way better looking car,, maybe mine van thinks it is a Hummer