Low heat/defrost is killing my MPG's

[qx4girl]

Hey all!

So here is my problem. My rig went in for it's 185,000 mile "service" I did a timing belt (it was original) thermostat, water pump, drive belts and I can't remember the other small thing they did. But since then (December ish) my heat kills my gas mileage. I go from non heat/old normal 17-18 highway to 12-13 highway when the heat is on!! Town went from about 15 to 10!!! Why would having my defrost/heat on do this now? It never did this before I got the major service done. I also never have my heat on past 68° nor past the low setting.

Now that it's getting close to summer. I tried the AC on a warm day and it only kills it the normal 1-2 MPG. D

I'm lost and since I'm a younger lady, the dealer just throws me around along with other local shops! I've never had this problem so have no clue where to start!

All help is greatly appreciated!!

[Towncivilian]

This is a strange issue indeed. I'm fairly clueless on this one. Crank up the heat all the way - is the heat sufficient? Are both of the heater hoses too hot to touch when the heat is cranked? How is the coolant level in the overflow reservoir?

[qx4girl]

Very sufficient heat! Wowza too hot! Hoses are very hot to the touch after its been running. Coolant and overflow are good also! I'm at a loss.

[Towncivilian]

What brand of parts were used for the service?

[qx4girl]

I took it to a Nissan Dealer so I would assume Nissan/OEM parts.

[Towncivilian]

I'd hope so, yes. I am stumped. There are no TSBs related to such an issue, and there is nothing in the FSM about it. I'm afraid it may be time for dealer diagnostics, especially since the problem started occurring after you had service there.

[qx4girl]

I'm afraid so too! Was just hoping someone might know. It even runs at a smidge lower temp than it used to and they told me because I got new parts. Too bad they have nothing to say about my MPG's going to crap.
Thanks for the help! Once I get the money, I'll take it in and hope for the best.

[Towncivilian]

If the thermostat previously used was an aftermarket one rated higher, or simply aged and slow at opening, new parts may very well have caused a small drop in operating temperature.