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

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. :o

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.

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.

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

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.

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.

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.

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.

Yes, no leaks were found.

Thermostat stuck open?

Are your winter tires new or did you use them last winter also? They could have significantly higher rolling resistance than your summer tires. That, combined with cooler temperatures could explain the drop in fuel efficiency. It's a good idea to do a coastdown test when switching to new tires so you can quantify the difference.

European 30W oils are a little thicker than 30W oils in North America, but you wouldn't notice more than a 1% difference in fuel economy because of this.

Getting to the bottom of the list one thing at a time. I knew the injector seals were old and hard, so it got new ones installed today. The truck's coolant sensor showed proper resistance and the ECU's reference ground is good. The only thing left would be the gauge, that is almost a full sweep under cold, even when off. I'll hit up the wrecking yard tomorrow and find one. If that doesn't change anything, all that is left is the cold. The only thing I can't figure out, is why I had the drop before it started to get cold.

It's thermos is almost complete. It's going to go from full rich, to having plenty of warm coolant on hand to help it on the cold days.

The tires were used, but they weren't on the truck at all last winter. I knew of the oil being slightly thicker, but along those lines, I drained out 5w30 Mobil 1, for the 0w30 Castrol, which adds another (small and irrelevant) variable.

Already looked there, and the truck does get to operating temperature fairly quickly.

The gage is a tattle tail after the fact. It is an output from the pcm not an input. Whatever it reads does not affect the way it runs. You might check the readings from the temp sensor at the pcm to verify the harness.

I did that last night. Grounds are good, feed from the ECU to sensor is good. Having the gauge function properly was done in part to satisfy me. It was broken, and just staring at me.

I'm still going to cross my fingers at this. I had an old SAAB, years ago, that wouldn't charge if the alternator light was the wrong item, or missing.

It's all put together now. I'll fill her up and see what happens now.

She's full, time to see if any of the recent work helped.

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Just a thought.

How many amps is your alternator pulling ?

How old is the battery ? And have you checked it?

A battery on the way out will pull more amps from the alternator because
of the extra load, leading to a lose of mpg.

Cold weather exacerbates the problem.

Like I said, just a thought.


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Battery is about two months old. There seems to be no strain on the electrical system when it's loaded (heater/radio/brights & driving lights). Easy to have it tested though. :thumbup:

Thanks for everyone's help! She seems to be doing a little better. My fuel log at the start of the thread isn't showing my two most recent fill ups as of 1/2/18, 14:00 MTN. However, on this tank I did make four trips over the mountain near my home (3 mile climb from 4,500' to 6,200', 2nd gear), and my mileage was still better than the previous tank.

I'm going to say any low readings now are from the cold and E10 fuel.