Around the begining of last winter I noticed a huge dip in my mileage. My 1997 4-litre Jeep Grand Cherokee has an 80-litre tank and was getting around 550KM, since the winter 80-litres gets me 480KM. I just noticed black soot around the end of my tailpipe and assume that is a result of the bad gas mileage. I put in new plugs, cap and rotor about 20 000 KM's ago and don't know what else to do.

Black soot typically means you are running rich. Is your check engine light on? How long has it been since you inspected / replaced your air filter?

Sean

Black soot also means short trips in the winter time.. The cold engine is requiring a richer mixture until it warms up. Your computer knows the outside temp and the coolant temp and leans the mixture as much as it can. Make a tank trip and see what the milage is.. Also as summer comes around see if the milage increases..
As a general rule if the computerized engine is running smoothly, without lights showing on the dash, you are getting the best it can give.
Oh yeah, about the air cleaner, I think there is even a message for that.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

I put in a new air filter a few weeks ago and my mileage increased about 15KM per tank, before the new filter I was getting around 465KM-470KM. I also have no warning lights on the dash. Any other suggestions? would running a tank of premium clean things up in the engine?

Big Jim, you are correct about taking short trips. Instead of driving 50 minutes to work, I started driving 10 minutes to the bus station last summer and got to work that way.

Like Jim said, I'd take a long trip and see what your mileage is. One other thought, are you letting the jeep warm up before driving it in the winter when its cold out, or do you just drive it like you would in the summer?

Sean

I'm always driving the jeep and measuring the mileage. Or do you guys mean take a very long drive to clean out the engine. I don't warm up my Jeep in the winter.

The engine is already clean... Using a different test fuel won't help anything.. I never have "warmed" an engine.. Unless it simply wouldn't go for a while. Short trips mean worse milage... Long trips keep the engine at it's best ecomony settings for a longer time so the milage is greater than short trips.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

X2 what Jim said. Take it for a longer trip and see what you get for mileage. If you had been warming up your jeep, that would be wasted fuel too. It sounds like since you cut your commute from 50 minutes to 10 minutes, you decreased your fuel mileage due to driving style. I bet your drive to work previously was a fair bit of highway driving or just cruising a long, and now your 10 minute commute is more stop and go to the park and ride. So while over all you are saving money and using less fuel by taking mass transit, you are getting worse mileage because you donÆt have that nice long commute where you and the jeep can run in more of a steady state type mode. Another thing to consider too is a lot of places will oxygenate the fuel and/ or add more ethanol to the gasoline in the winter months, both of which are an attempt to reduce emissions which result in decreased fuel mileage.

For the record too, I also do not ôwarm upö my vehicles before driving. If itÆs cold enough (teens) I will put the tx case in neutral and let the trans and tx case spin over for a few seconds to get the lube all around, but for the most part I just hop in and hit the road. I donÆt drive anything too hard until things are up to operating temperature though.

Sean

Everything you guys have said makes sense and is very true. I was just worried my engine was headed to the "shitter". Thanks for all the input.