Crazy low mileage, no gas leak detected

My '99 Plymouth Voyager is getting about 3 miles per gallon; used 1/2 tank
to drive to Detroit and back to Ann Arbor (about 60 minutes of driving).
Notice needle going way down on short trips as well.

No smell of gas, tank doesn't empty when car's not running, exhaust doesn't
smell overly gassy. Replaced thermostat, air filter, and O2 sensor about 2
months ago. Engine sounds fine, seems to run exactly as it has for 6 years,
only 70,000 miles on it.

Does anyone have any idea of what could cause this tremendous use of gas,
with all of the above? My only guess was that the O2 sensor was put in
wrong, or was the wrong type, but a mechanic told me over the phone that I'd
get trouble codes or something if the O2 sensor was the wrong type, or not
working correctly.

This is a really serious issue, and I was hoping to get some advice before
paying $100 to have the thing hooked up to the computer. I mean, w/ no
Service Engine light, that means there's no trouble codes, right? So
wouldn't the big computer say the same thing?

Any advice whatsoever would be greatly appreciated, and thanks for reading.

 

Could be any of a number of things. No way to tell without seeing the
vehicle.

That will likely be the best $100 you will spend. No, no light does NOT
mean no codes. There are several codes that won't illuminate the light.

Matt

 

I'll tell you what... if you are getting 3 miles to a gallon you need to
find out who pulled the check engine lite out of the dash because one of the
OBD2 monitors would have picked it up and set the lite.
And if you say you smell no obvious raw fuel out the tail pipe or under the
hood/vehicle then you need to find out what is wrong with your fuel gauge.

Glenn Beasley
Chrysler Tech

 

I appreciate the response. The Service Engine light is still there, the
dash runs through the cycles per normal when starting up. I was hoping it
was the fuel gauge too, but when it hits empty, it's really empty.

 

so when it runs empty and you fill it up how many gallons does it take to
fill it up?

Glenn Beasley
Chrysler Tech

 

Hi...

Or perhaps who the thief is that siphoned your tank while you
were stopped for coffee...

Ken

 

On the serious side, You might have a clogged catalatic converter. This
could cause the engine to work slightly harder to make the car go since
the exhaust is being restricted. That would also cause the conveter to
literally burn any excess fuel. Another thing I came across was a
baffle like thing after the muffler that was collapsed inside. It was
causing car to almost not reach highway speeds. So check your exhaust
system it may be where your problem is. Steve

 

I remember once on the freeway at night I saw a car driving very slow,
and the whole exhuast system was glowing like a hot poker, one of the
weirdest sights I've ever seen, must have been the cat convertor really
roasting on raw fuel or something?