I was happy with 8-10 but all of a sudden I smell coolant out of the engine bay after she warms up and my gas mileage is down to 5-7? It looks like I have a leak around the thermostat so I am set to replace that tomorrow. The exhaust also is super strong - cant have windows down at a stop sign or I might pass out.

Could a thermostat issue cause a drop in MPG?

Ordered new oem plugs and aftermarket wires which have yet to be done at 178k.

Anything else I should check?

Fuel pump, filter and o2 sensor replaced 1/12 at 168k

radiator at 156k in 2009.

 

O2(s) could be bad. What brand did you use. And maybe the pcv is no good and the cat(s) are going bad . Does it run at the correct temp even with the thermo area leaking? Not sure with a V-10. Don't they get like 8 or 9 anyway?

 

Temp barely moves above ice cold. I was getting a solid 8-9 around town or with my 1400 pound camper on the back and up to 11-12 on the highway when she's empy.

Now, empty if get 7, somethings up.

02 was done by someone else so most likely aftermarket.

No check engine light? PCV might be something to check.

-John

 

Fix the leak and go from there. Use a Stant super stat(of the correct temp) and test it before installing . The colder it runs the more gas it will use. Sounds like you may have a stuck open stat in there now.