Cold=Bad MPG?

I've recently noticed a decrease in mileage since the weather got cold. '92 4.0 auto,mostly interstate commute, 65-70 mph, Dynaflow, hollow cat. But, a few years ago, when I emptied the cat, I installed a 150 degree t-stat, thinking that without the need for high heat to light off the cat, why bake all the soft stuff under the hood? Is the engine not warming up enough to get out of the rich start up mixture? Last summer it ran fine, giving good MPG. It still runs great, but seems thirsty. O2 sensor? The check engine light does not indicate any problem, although I ignored the 'maintenance required' light a few years ago- the parts man at the dealer said it is a timer for the 02 sensor, and if it runs fine, just pull the bulb! Does it need to see a certain temp to lean the mix? On a cold day, the temp needle barely moves. Yeah, the heater is not as warm as it could be, but a cool engine is a happy one? Any thoughts? TIA..

1. A cool engine is NOT a happy engine. The Cherokee 4.0L engine was designed to run at 210 degrees, using a 195 degree thermostat. The 150 degree stat is not warm enough to get it out of open loop mode.

2. Your O2 sensor is probably way past trashed if the light went on years ago, but if you're not operating warm enough to get into closed loop mode it doesn't matter because your ECU isn't operating on O2 data feedback anyway. The O2 sensor is supposed to be replaced every 75,000 to 80,000 miles. Do it.

3. Winter fuel has added oxydizers to supposedly help it burn clean in cold weather. These additives also reduce the efficiency. Winter gas produces in general about 10% to 20% worse economy than "summer" gas.

4. If you have been running for a period of years with a faulty O2 sensor and a cold thermostat, your catalytic converter is probably trashed and plugged up. That also reduces fuel economy.

In summery, it sounds to me like you need a thermostat, an oxygen sensor, and a catalytic converter.

hollow cat.

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That lowers you gas milage due to lack of back pressure.

150 degree t-stat

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Kills MPG. Should be 195 t-stat!!

Is the engine not warming up enough to get out of the rich start up mixture?
Does it need to see a certain temp to lean the mix?

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YES!

but a cool engine is a happy one?

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No. Too cold is bad and overheating is bad. 200-210 is right.

If you wan't to have good MPG, fix all the stuff that you messed with. :doh:

yeah, ive noticed that my MPG is bad too....but mines really bad....9 on my last fill up. it could be b/c i was on the skinny pedal a good bit in all the snow we've had. or it could also be the O2 sensor...ive never replaced it. ill have to check into all that stuff if it keeps staying low. im runnin a 180 thermostat. and it runs right around 200.

my "normal-for-me" mileage is around 17.... cold weather i drop down to 13 to 15 mpg.... the low point was about 9mpg when it was sub zero and i would idle it 30 mins before we left on the morning...
could be worse.... could be out in a boat in this cold..
mike

same Prob

Funny I was here this morning to post about my MPG Problem. I just bought a brand spanking used 99Sport. First tank of gas got 218 miles before refueling!! A FULL 20 Gallons. My old 88 got better, but here in North Chicago it has beeb very cold lately and I idle to warm up for longer than the drive to work. It has 51K on it, might be time for a real tune up anyway?

My MPG jumped up almost 5 MPG by gutting out my cat and picked up 3 WHP dyno verified. My experience has been that cold air makes more power and I drive with the windows up in the winter giving me better mileage.

Just my .02...............