Breaking in a crate motor?

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Arod1243

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Contributor I

Hey all,
I recently installed a new crate motor on my 97 wrangler (2.5l 4cyl). Everything I've read says these motors should get around 20 mpg's hwy, however I am only getting around 12.5. The new motor only has about 400 miles on it so it is pretty fresh. My questions are...Is this normal to have such low gas mileage on a new motor? Will I see any increase in power with breaking it in? and how many miles usually until you can get the max out of it?
Thank you
 

smritte

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The motor is still tight. Give it some time. I don't really notice a difference until over 1k miles on a new motor. I would on the other hand be surprised with 20mpg. I'm sure its possible but with you getting 12.5 now, I would guess broken in it would only go up 2-3 mpg. Possibly 5 tops.

My new Cruiser motor has about 2k on it now. I can tell the difference when I start it by the speed the engine turns over. I picked up about 2mpg from the beginning. Depending on how yours was built will determine how long until its completely broken in.

The question is, were you getting 20mpg before the new motor? There's quite a few things that effect mileage. Age of your injectors, Cat..ect.
If the issues are subtle, they wont kick up a check engine light. They commonly can add up to less fuel mileage.
 
Last edited:

Arod1243

Rank 0

Contributor I

The motor is still tight. Give it some time. I don't really notice a difference until over 1k miles on a new motor. I would on the other hand be surprised with 20mpg. I'm sure its possible but with you getting 12.5 now, I would guess broken in it would only go up 2-3 mpg. Possibly 5 tops.

My new Cruiser motor has about 2k on it now. I can tell the difference when I start it by the speed the engine turns over. I picked up about 2mpg from the beginning. Depending on how yours was built will determine how long until its completely broken in.

The question is, were you getting 20mpg before the new motor? There's quite a few things that effect mileage. Age of your injectors, Cat..ect.
If the issues are subtle, they wont kick up a check engine light. They commonly can add up to less fuel mileage.
I am not sure what it got before since I bought it with a blown piston. The other items I have not looked over yet but definitely plan too. I would be ok with 17mpgs. Thanks!
 

Dilldog

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So question, how are you breaking it in? You should be giving it lots of hard acceleration and deceleration to seat the rings. Basically beat on it a little. When we rebuild engines at work they are run up to temp to check for leaks, then they get strapped to the dyno and made to work, literal 100% load factor. Loading an engine HARD is the best way to break it in.
Now to the actual question, yeah until things wear into each other a bit you will see poor millage. Give it a few thousand miles and it should come up. Also its never a bad idea to run a shorter oil change interval the first 2-3 changes.
 

Arod1243

Rank 0

Contributor I

So question, how are you breaking it in? You should be giving it lots of hard acceleration and deceleration to seat the rings. Basically beat on it a little. When we rebuild engines at work they are run up to temp to check for leaks, then they get strapped to the dyno and made to work, literal 100% load factor. Loading an engine HARD is the best way to break it in.
Now to the actual question, yeah until things wear into each other a bit you will see poor millage. Give it a few thousand miles and it should come up. Also its never a bad idea to run a shorter oil change interval the first 2-3 changes.
Ive been doing kind of what your saying. Running it hard and doing quite a bit of highway driving where im steady at 3200 rpms. Also i am doing frequent oil changes. First change at 600 miles next will be at 1500. Im running conventional oil for break in but plan to go full synthetic after a few thousand miles so hopefully that helps a bit too with mpgs.
 

old_man

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I have decated of experience with the 2.5L. as well as the 4.0L/Strokers. I call BS on that low of gas mileage being acceptable. I got 28 with my 1986 2.5L throttlebody. If a scan does not show up anything, take it and have an emissions test done. You have to be running insanely rich. I recommend running a can of BG44K through the fuel to clean out the injectors. Also double check the timing. Pull the plugs, post up pix. Do a compression test.

I get 20 mpg on my 4.7L stroker here at 5000 ft elevation.

What size tires and gearing are you running?
 

Arod1243

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I have decated of experience with the 2.5L. as well as the 4.0L/Strokers. I call BS on that low of gas mileage being acceptable. I got 28 with my 1986 2.5L throttlebody. If a scan does not show up anything, take it and have an emissions test done. You have to be running insanely rich. I recommend running a can of BG44K through the fuel to clean out the injectors. Also double check the timing. Pull the plugs, post up pix. Do a compression test.

I get 20 mpg on my 4.7L stroker here at 5000 ft elevation.

What size tires and gearing are you running?
It past emissions already. I am running the stock gearing. I think 4.11?? Also have 32×11.5 tires and 2.5 inch lift.
 

smritte

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I have decated of experience with the 2.5L. as well as the 4.0L/Strokers. I call BS on that low of gas mileage being acceptable. I got 28 with my 1986 2.5L throttlebody.
I don't think anyone said it was acceptable. In my decades of building motors the best increase I've seen is 2-3 mpg. If he's at 12 now I doubt the new motor is keeping the mileage that low. If he was dumping that much fuel, he would most likely see smoke. Before I finished my degree, I spent 25 years in a shop diagnosing things like this. If he passed emissions, odds are the fuel system is good. There's too many factors to really guess communicating over a forum and we cannot use how we correctly build things to assume other people build the same. You should see a bad cam grind that was indexed properly, it was almost 20 degrees retarded. That threw me for a loop.
I hope for the OP that more than anything he screwed up the math. For what its worth, I have had people in my jeep club with 13-14 mpg 4cyl manual transmission wranglers.
 

Dilldog

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I would be willing to bet its the tires. From what little experience I have with the Jeep 4 cylinders I remember them being good engines but not the most powerful. You would be amazed how much added load even a slight increase in tire size will put on things. If I remember correctly those Jeeps shipped with like a 29" tire that was around 9" wide. If you are still running stock gearing, youre asking a lot of that engine to move a tire that much bigger.
 

old_man

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I ran 35's with my 2.5, but it was geared 4.56s. My only complaint was a 80 mph top speed here at altitude. I was running a 5 speed and had a 11 lb flywheel weight to help low end torque and rock crawling, Pulling the Eisenhower Tunnel at 35 mph sucked.