terrible mileage

[action jackson]

I have a 97 jetta GL that gets terrible gas mileage. It has been like that ever since I bought it last year, it only gets about 18 to 20 miles per gallon! does anyone have any sugestions to try? I just changed the cap and rotor(which were pretty bad)...didn't seem to help though.
thanks in advance!!

[90cIvIcSiR]

check to see if you have an intake or exhaust leak....could need a fuel filter too

[boschmann]

Is it a 2.0? Is the check engine light on? Is the tail pipe sooty or do you see black smoke when you rev the engine?

[action jackson]

yes it is a 2.0 and the check engine light isn't on. There is also no smoke or soot in the exhaust. The only other problem i can notice is a kind of rough idle for a few seconds when i first start it up in the morning, it seems to do this more when its cold out too. hope this gives some more help. Thanks!!!!

[boschmann]

I'd start by checking the coolant temp sensor. Do you have access to a multimeter?

[action jackson]

ya i do have a multimeter, but what would the coolant temp sensor have to do with gas mileage?
I was thinking it might be the o2 sensor but really have no clue!!!
Thanks again!!!!

[boschmann]

The ECM uses it to determine the proper fuel/air mixture. Usuallly when the sensor fails it gives a reading which makes the ECM give too much fuel (runs rich). The sensor is located on the water flange where the upper radiator hose connects to the cylinder head. I think it is a blue sensor. The wire colors should be blue & brown/blue. The sensor should read 2000-3000 Ohms cold and 250-300 Ohms @ operating temp. It's a simple check, the O2 can be a little more difficult & would normally turn on a check engine light.

[jacee74]

Dude, 18 to 20 is great. Shit I get around 15 or so, I have some mods but nothing fancy. Try 89 fuel instead of 87, new air filter and probably needs a new fuel filter, 18 to 20 is good for a heavy car with a 4cyl, such as Jetta.

[action jackson]

I checked the coolant temp sensor and the top set of prongs was 258 and the bottom set was 123 at operating temp. So a little low i guess but would that be enough to cause a problem?

[boschmann]

Which was the reading from the two pins that had the blue and brown w/blue trace wires going to them? You also need to check with engine cold.

[Samurai75007]

in my 2001 1.8T i get about 280-290mpt and its got a 14.5 galon tank so... thats like 19.4mpg

[jett-n-along]

I had the same problem. there is a hose in the front of the engine. It runs from the block to the resinator( a spaceship looking thingy). The hose is a hard hose and may be cracked or filled with a coffee ground substance. This problem is pretty common. The hose is called a return breather hose. After I replace it i got double the milleage. good luck!! jett-n