Lost gas mileage

I have a 2007 Titan that just turned 100,000 miles. Over the last 6 weeks or so I’ve noticed a huge drop in gas mileage. Used to get close to 400 miles a tank and now as of last night I got 248 miles on a tank. Average of 10 miles a gallon. Nothing has changed as far as driving habits or anything like that. Any suggestions? I’m taking it to Nissan dealer Monday and wanted some opinions prior to taking it. Thanks.

To drop that much you probably have a leak somewhere, assuming you measured the gas mileage correctly. If you had a leaking injector there would be a check engine light and probably smoke from the exhaust.

Other causes could be a dragging handbrake, or a transmission that won’t upshift.

…and/or a thermostat that is stuck in the open position, thus not allowing the engine to get to full operating temperature quickly enough.

…and/or tires that are seriously underinflated as a result of plummeting temperatures.

Where do you live? Low temperatures/weather have a tremendous effect on fuel economy.

100,000 miles…due for a major service. Worn spark plugs, perhaps a lazy oxygen sensor, low tires, any number of things can contribute. Be sure all the maintenance is up to date first.

… and the plummeting temperatures themselves.

Where I live the temperature dropped to well below freezing a week or so ago and stayed there. Everybody is getting terrible mileage. I’m not suggesting that it’s the sole cause, but it could certainly be a contributor.

my gas mileage has gone down a lot since the weather got cold.

Great. Thanks for input. I am in East Texas so cold is not a huge issue however it does get there a few times a year. I think it could be brakes dragging and/or the thermostat. I guess I will maybe find out a bit on Monday when they hook it up to their computers. Planning on a tune up as well. I’ll let ya know.

If a brake is dragging, you should be able to feel (carefully, of course) that one wheel is much hotter than the others after a drive.

Yeah. Meant to check that yesterday. Will definitely do it today. Feels as if the truck is pulling ever so slightly to the left and that when taking foot off of accelerator that is may be slowing down quicker than normal. Doesn’t quite feel like a normal coast. May just be me looking/feeling for things as well.


Is the engine reaching proper operating temperature?

Is it reaching it within a reasonable amount of time?

Even though this isn’t very accurate . . . is the coolant temperature gauge in the middle?

Been watching it as of late. It is staying on the cold side of the dial. Not sure where it normally is though

If it’s staying on the cold side of the dial, the thermostat is probably stuck open. Fortunately, T-stats are cheap. In most vehicles they’re also easy to replace, but I don’t know about the Titan.

If the thermostat is stuck open you’re operating in the open loop mode. That’ll really eat up the gas!

It’s hard on the engine, too.


Sounds like you’re thermostat is stuck open, or at the very least, it’s sluggish

Replacing the thermostat and coolant is the very first thing I would do

Are you checking mileage the proper way or really going by how many miles you drive between fillups? I am sure your mileage has dropped but by how much actually.

Good comments above. Most of the tough to diagnose problems (injectors, fuel pump, fuel pressure regulator etc) would flag the check engine light. With no CEL, the most likely cause is the engine temperature, or an error in what the computer thinks the engine temperature is.

Ask the shop to check for

  • Faulty thermostat.

  • Faulty engine coolant temperature sensor

It’s a good idea to make a small mark on the dash temp gauge where the needle normally points after the engine is warmed up, then watch for any changes on subsequent trips. Can forewarn that the thermostat is slowly degrading.

Thanks. This is all great info. Yall are awesome. I’ve already called dealership and changed my appointment to have the lookout t stat and another issue in the dash. As far as checking mileage every time I fill up I reset trip odometer. When I know I’m checking it out I will run on tank till light comes on before filling up. 28 gallon tank and It normally takes 24-25 when I fill up so I know it’s close to empty. Not scientific but it shows how many miles I’ve driven on that tank. Anyway, will keep ya posted after appt. thanks again for input.

Anyone replace a thermostat on a Titan before? After googling it it doesn’t look like it would be that hard. Should I go for it or not? Rather try and save some coin if I can and not have them do it Monday at the dealership.

Go for it!
The absolute worse possible outcome is that you’d have to have it towed in instead of driving it. If you can’t get it done, it cannot cause any other damage, so the risk is minimal. Get a manual, which is always good to have handy, and give it a try. Let us know how you make out.

Oh, and be sure you have mixed coolant and buy the gasket along with the T-stat. And don’t try to tighten the bolts to record levels; the torque isn’t critical, but be reasonable. If you’re unsure, the manual should suggest a torque value and a torque wrench is cheap.