Scratching my head at ~20% MPG loss. 92 Dakota

[ExDelayed]

It's been going on for awhile now, and I'm just scratching my head. The truck only has 9,500 miles on all of its ignition components and fuel filter. Tires were more on the hyper-miler side of pressure, snow tires were just recently installed. My timing is correct, plugs look good, and the air filter is at about 2,500 miles. It has an upper grille block, WAI and is lowered. Not carrying cargo and driving patterns haven't changed. The only difference around the time of the drop was pulling out year old M1 5w-30, and replacing it with (German) Castrol 0w-30 that I had on the shop shelf, just to get rid of it. Trans bands were adjusted 9,500 miles ago, TV cable adjusted, and the transfer case was rebuilt, both filled with a fresh fill of Castrol ATF+ 4. Power steering was flushed with synthetic and the rear axle is filled with Amsoil.

EDIT: Alignment isn't showing any issues, brakes looked good at snow tire install. Cat and 0² were replace 9,500 miles ago. Exhaust isn't showing any more carbon than usual.

Only issue I could think of, and it's just this tank, is the coolant gauge is acting wonky. Sensor bench tests good, as does the wire feeding it, but the gauge shows well below cold.

Any ideas? I'm a little embarrassed only getting it's EPA estimate.

[M_a_t_t]

How has the weather been lately? In my van I drop 2-4 mpg (which brings me to the EPA rating) when it gets cold out.

[ExDelayed]

This month so far has had the mean temperature in the 30°s, and it's not uncommon for me to be out when it's below freezing. This has been going on since September, when the mean was still in the 60°s. It had a little of this last winter, but it was only two tanks, and it jumped right back up to about 15; it was still in the 30°s last February.

[M_a_t_t]

Have you checked for a gas leak, key on engine off?

[me and my metro]

IF I remember correctly you only have one temp sensor, the ecm reads the temp then drives the gage. You are missing something and your pcm is running rich. I would look there first.

[Stubby79]

I'd check that temp sensor first...spark plug color(carbon build up? leaky injector?) next...and eventually would get around to a compression test if I couldn't find any evidence otherwise.

[ExDelayed]

Plugs looked good about 500 miles ago when I pulled them looking for a problem. I've been wanting to do a new injector upgrade, so maybe that should be in my list soon to rule out injector seals. Can't smell fuel in the engine compartment though.

I'll get to the bottom of that coolant sensor. It's only recently freaked out, but if it had a poor, intermittent connection, the gauge wouldn't have reacted quick enough for me to notice.

[ExDelayed]

Originally Posted by Stubby79

...and eventually would get around to a compression test if I couldn't find any evidence otherwise.

So kinda backwards, but I had some time to burn and the truck was warm. Spark plugs have normal coloration, and are uniform across all eight. Hot compression test had good results, with less than a 5% variation across all cylinders. I'll be able to let her cool down tomorrow to further investigate/possibly change the coolant sensor.

I did find that the hose from the fuel tank to the charcoal canister was disconnected. I reconnected it, zip tied it so it couldn't escape again, and sprayed all the vacuum lines. No other leaks or disconnected hoses were found.

[ExDelayed]

Originally Posted by M_a_t_t

Have you checked for a gas leak, key on engine off?

Yes, no leaks were found.

[MetroMPG]

Thermostat stuck open?