Fuel Consumption at Idle

I have a 1996 4.0 AW4 XJ, which idles around 700-750RPMS warm.

It's pretty cold here in the morning before I leave for work.. It was 15 degrees this morning. Anyway, my commute is only about 10 minutes, and I'd rather be warm than freezing my ass off. I'm about 3/4 of the way there before the XJ is warm enough to put out a decent amount of heat.

In lue of this, I often start the Jeep up before I head to work. I usually leave it idling from anywhere between 5 and 20 minutes. The time usually works out to around 10 minutes though. There was a time where I forgot, and it was running for about an hour.

My gas mileage sucks!! How much gas am I using if I let it idle like this each morning, 7 days a week? Would it be using enough to make my mpg average 3 or 4 mpg below where it should be?

Is there any harm in letting my Jeep idle for extended periods of time? It does not overheat.

Since the cold mornings and evenings have set in here, I warm mine up for 5-15 min. before each leg of my commute.(I hate a cold ride too) My aged/stock 88 gets 18-20mpg in 'around town' and rural... but I don't drive aggressive usually. 10-15 of warm up 2x a day shouldn't add too much to the gas bill... especially with a 10 min commute.

I keep good track of my mileage and I always notice a drop off in winter when I let it idle to get warm.

Go to the dealer and buy a new thermostat. My 88 XJ 4.0 gets warmer much quicker than my Buick with a 3800 v-6. I am up to temperature in 2-3 miles on cold mornings.

Sounds like your thermostat is opening early, that why you don't get heat. You may of overheated over the summer and now you are seeing the damage to the thermostat.

You may want to install a new thermostat and a new radiator cap, that should get you heat faster than you do now. The drop in MPG indicated the engine may not be operating at temperature, that means it runs rich since it never makes 180 F or above for an extended period of time.

it warms up just fine... it's just I don't live far enough away for it to. Work is about 2 or 3 miles away, and it gets warm about 3/4 of the way there....

Lettting the engine idle for several minutes and then driving it only a short distance with the engine still cold will definitely give your gas mileage a hit so a 2-3mpg drop shouldn't come as any surprise.

I agree if the thermostat is working correctly then the MPG drop you see is normal at those temperatures. If you want to improve you MPG then go buy the engine block heater at the dealer and install it in the freeze plug indicated in the instructions.

With the heater installed you will need an exterior grade timer that will handle the current the heater says it uses. You are lucky with the Christmas lights being on sale there are a lot of exterior plug-in timers available. Just plug in your timer and set it to go on about 2 hours before you leave for work. See if you have heat, then play with the time the timer is on till you are happy. Try to get a timer with a 3 wire cord since I think my car's heater uses a 3 wire cord.

By using the timer the engine will get to temperature sooner and you will have heat sooner. My Dad plugged in his car to get heat at 6 am when he went to work.

Maybe you could try one of these?
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=36711

There are also 120v interior car heaters. I run both the block heater and interior heater on a timer. The jeep gets nice and warm on the timer, by the time it would cool down, the jeep's heater is warm enough to keep the temp up.

Note: I also see a good drop in mileage during the winter. I also do a lot more of the short 10 minute trips in the winter as well.