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

[twj347]

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.

[ExDelayed]

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.

Originally Posted by twj347

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.

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.

Originally Posted by MetroMPG

Thermostat stuck open?

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

[me and my metro]

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.

[ExDelayed]

Originally Posted by me and my metro

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.

[ExDelayed]

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

[redneck]

.

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.


>

[ExDelayed]

Originally Posted by redneck

.

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.


>

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.

[ExDelayed]

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.