New engine, LESS mpg? Huh?

I’ve recently replaced the engine in my 2007 Pacifica. The newer one has half the mileage, yet the mpg has reduced by about 25%! What gives? I figure it must have something to do with the computer but if I knew for sure, I wouldn’t be here asking your advice! Help is greatly appreciated.

We need more info. Is the check engine light on? Is this new engine warming up as quickly as the old one did? A simple bad thermostat can drop mpgs the amount you post.

Have you questioned the shop that installed this engine about this problem?

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Nope. No check engine light but, I do have a check tire system! But nothing is wrong with the tires. They all keep pressure accordingly. Once I clear it, it’s gone.

2004-2008? Did old motor slowly die? Take a few years? Or just explode? It burned oil for 3 yrs? Killed cats? And so on.
Or, it’s a 2019 and has a fresh motor under warranty?

What about my other 2 questions?

I didn’t change the belts like I should have at 200K, so, 90 miles from home on my way back from a trip, well… you get the picture. I suppose the good news is the tow truck company was happy!

100k used motor might be less then great shape. I was thinking 3.8 but it’s 4.0? Ok

It’s a 4.0

Oops! The engine cranks up just as quick at the previous one and also runs smoother. The shop’s response is it can be any number of reasons so it would be hard to pinpoint. As for the shop, the owner has always given me confidence in his knowledge and ability. Unfortunately, we all don’t know everything and this is one of those instances. Lastly, the old engine had around 255k miles and the replacement is a rebuilt with 130k. In general there does not seem to be any other issues with the car.

I didn’t ask if it cranked up quick, I asked if it warmed up quickly. An engine should be fully warm in summer weather in about 2-3 minutes… the temp gauge needle should be in the middle of the gauge. A bad thermostat will not warm the engine as quickly or to the proper temperature. That is what you should be checking.

Also take a 15 minute drive, try to use the brakes as little as possible so look for a deserted rod. Pull over and carefully touch each wheel. Is one warmer than another? especially when compared to the wheel on the opposite side? If so, you have a dragging brake and that will hurt your fuel economy, too.

Gotcha. I’ll try that. Thank you. And I’ll check how quickly it warms up.Seems to me no differently than before.

Giving further thought to your “dragging brake” assessment, that may have some validity. As I mentioned, the tire system light does come on. I’ll make sure to reply here once I try this.

If your driving habits are identical to Previous pattern then most folks would notice a 25% mileage drop. And not blame it on short trips or other reasons.

Yep. That’s exactly right and why I’m here.

Even if the “new to you” motor has been rebuilt, it’s not brand new. Aside from the rebuilt factor, you don’t know anything about the engine’s previous history.

I tend to agree with the shop, there are just too many unknowns here. I’d just monitor the situation over time, myself, and drive on.

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Current motor was rebuilt 130k ago?

When the engine was replaced, the battery was disconnected.

This causes he computer to lose its KAM.

That means the computer has to relearn all the operating parameters to make adjustments so that it returns to where it provides the best engine performance.

The vehicle may have to be driven for a while under various conditions for that to happen.



No, the current motor had 130k when it was installed recently.

Hmm! I read the article, thanks. I’ll run it by the shop next week. Hopefully, they can reset the KAM using the “capable scan tool”.

KAM, keep alive memory. You don’t reset them. It’s the set of parameters your computer “sees” while car is running. Sometimes cars need to relearn them after a battery is changed. Or you replace a motor. Which 98% of folks never do. Your car will relearn them while you drive for a day or week or month.
How old is timing belt on your 130k mile used motor?

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You need to clarify something as your entire complaint is a bit murky.
You say new engine and then said it had half the mileage of the old one. That does not make it new.
Then you said it was a used engine that was rebuilt.
Then you said it had 130k miles on it.

If the assumption is made that this engine was rebuilt then a lot depends upon who rebuilt it. A rebuild could still be garbage.
Have you got a receipt for this “rebuild”?