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Old 07-22-2011, 10:23 AM   #1
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Pros and Cons of 4.0 vs. 4.2 vs. 2.5

Newbie here.

What are the pros and cons of the three engines offered? I'm only intersted in having a 5spd too.


Thanks!

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Old 07-22-2011, 10:29 AM   #2
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What are you looking to compare? 4.2 has a carb, 4.0 is FI, and the 2.5 is a 4cyl FI

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Old 07-22-2011, 10:30 AM   #3
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4.0= FI, I6 most HP, torq
4.2= carb, I6 decent HP and torq
2.5= FI I4 has the lowest
anything else ya looking for?
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Old 07-22-2011, 10:36 AM   #4
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I'm looking to find out if there are any real downsides to owning one of these. Obvi the 4.0 seems ideal, but is the 2.5 powerful enough to beat around town happily?

I've hard legend of the 4.0 reliability, but I haven't ever heard anything about the 2.5, good or bad.

Just looking for personal experiences from some folks with more experience than myself.
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Old 07-22-2011, 10:38 AM   #5
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2.5's IMO are not worth the money...I have had 6 in the last 2 years with a rod hanging out of the block. Reliability? 1 out of 10 compared to a 4.0.
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Old 07-22-2011, 10:39 AM   #6
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well i have the 4.2 in my 87 and wish i had the 4.0 because i prefer the FI. and what ^ ^ said.
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Old 07-22-2011, 12:42 PM   #7
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Yeah. Good to hear some folks with experience confirm my suspicions. I'll hold out for a good 4.0.

What's the typical mileage where a 4.0 starts to need fixin? Or, is that a trick question?

Also, read on here that in 95' the YJs were frame and body galvanized. Is that true? Makes me think that living in the north east, I have to find a 95'.

Thoughts?
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Old 07-22-2011, 12:57 PM   #8
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350,000 is around when the 4.0 needs a rebuild,
93 they galvanized the body, 95 the body and frame. though it really depends on how they were taken care of i have a 93 with a clean frame and body, and i've seen 95's with rust holes in them.
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Old 07-22-2011, 01:13 PM   #9
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The 4.0 was one of the best engines America has ever made. It's not even worth the time comparing it to the 4.2 or 2.5. It's basically a tractor engine, but it has decent low end torque and reliability/dependability that would make a Honda cry.
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Old 07-22-2011, 01:17 PM   #10
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but is the 2.5 powerful enough to beat around town happily?
Yes, it's plenty to boot and scoot around town. Does well on the freeway too, as long as you don't have too big of a tire, or you have re-geared it. Mine with 29's scoots easily up to about 70 and cruises there. I've maxed it out somewhere in the mid 80's, but Jeeps don't need to go that fast anyways.

Going up steeper hills though it kinda drops off, and you gotta run it around 3500rpm to keep your speed up.

As far as reliability goes... Mine starts first time every time, has always started first time, and has never let me down. Keep them tuned up with oil changes, cap/rotor changes, and new air filter here and there and it will go forever.

If you're wondering about offroad capability.. The 2.5 has lots of torque offroad in low gear. I don't need to use the gas to crawl up hills, it'll just idle along in 1st low.

The only wish I have is that the power was more linear like most other engines are. It has a flat spot right in the middle from about 2000-3000rpm which i'm sure could be tuned away with a decent aftermarket exhaust. Lots of people tell me it's a small engine and likes to be ran up high in the RPM range, but my expeirence with small engines differs from that.
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Old 07-22-2011, 01:53 PM   #11
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Thanks for the input guys. I think I'll keep my eyes open for a rust free 4.0 in the Boston area. I'm not looking to off road this jeep much. It'll be a beach car. Though I'm sure it won't be long till I'm doing oversand driving and such.

Any other big things to avoid when looking?
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Old 07-22-2011, 02:01 PM   #12
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if you plan on using it with a hardtop in the winter, make sure you get one tat has the hardtop from the factory, it saves you the hassle of finding a wiring harness and hooking it up.
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Old 07-22-2011, 02:24 PM   #13
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Never would have thought of that....
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Old 07-22-2011, 04:34 PM   #14
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Quote:
Originally Posted by lsmaclea View Post
Yeah. Good to hear some folks with experience confirm my suspicions. I'll hold out for a good 4.0.

What's the typical mileage where a 4.0 starts to need fixin? Or, is that a trick question?

Also, read on here that in 95' the YJs were frame and body galvanized. Is that true? Makes me think that living in the north east, I have to find a 95'.

Thoughts?
93 jeep started galavanizeing the tubs 95 they.started the frames. I got lucky my tub 83 frame 95. As for the topic 2.5 is good around town and wheeling. But you will be limited to tire size even with rehearing. As for fuel my 4.0 with 36's get just as good mpg as a stock 2.5.
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Old 07-22-2011, 04:50 PM   #15
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Newbie here.

What are the pros and cons of the three engines offered? I'm only intersted in having a 5spd too.


Thanks!
I find that unless you have some sort of background to contrast opinions with, that one is better off getting first-hand experience before deciding.
If you can, go out and test drive each. The internet is a wonderful resource, but only goes so far. It would be a shame to miss a good deal simply because someone convinced you of what "they" don't like.
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Old 07-22-2011, 05:57 PM   #16
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Valid point.
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Old 07-22-2011, 06:22 PM   #17
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4.0 is awesome. Others are ok. If $ permits get 4.0 that was taken care of.
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Old 07-22-2011, 06:31 PM   #18
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4.0 is awesome. Others are ok. If $ permits get 4.0 that was taken care of.
I want to drop an HO 4.0 in my jeep, but idk If I want to do all that work yet
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Old 07-22-2011, 07:18 PM   #19
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My friends 2.5 had piston slap and all the litters were bad. That thing still ran and drove every day lol. But I love my 4.0. If you need to jump out into traffic, it's got a lot more pick up than the 2.5.
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Old 07-22-2011, 10:47 PM   #20
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Only real downside to the 4.2 is it's carbed. And the 80's emissions feedback carbs sucked horribly! If you change out the carb you can have a pretty good running/driving package but still not as hassle-free as a good working 4.0 efi system.
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Old 07-22-2011, 10:49 PM   #21
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i need to find a way to get rid of my carb so i dont have to deal with it anymore
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Old 07-23-2011, 07:44 PM   #22
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i love my 4.2. it leaks oil like a battleship and dont get the best fuel milage but you cant kill the damn thing. it wont stop. it has plenty of power and thats with an auto tranny, 33s and 3.07 gears.
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Old 07-23-2011, 07:54 PM   #23
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I think the best would be to get a 4.0 (did they come in high output for YJ's??) head with all of the intake and exhaust pieces then slap it onto a 4.2 block. Best way to improve the 4.2. Makes the best of the torque, and gives you the power of the 4.0.

Definitely look for a 6 cylinder if you want to boot and scoot around town with ease. It's not that the 2.5 isn't capable of it, it's just that the 6 cylinder does it a lot easier so you save gas
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Old 07-23-2011, 08:52 PM   #24
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I think the best would be to get a 4.0 (did they come in high output for YJ's??) head with all of the intake and exhaust pieces then slap it onto a 4.2 block. Best way to improve the 4.2. Makes the best of the torque, and gives you the power of the 4.0.

Definitely look for a 6 cylinder if you want to boot and scoot around town with ease. It's not that the 2.5 isn't capable of it, it's just that the 6 cylinder does it a lot easier so you save gas
It it easy to put a 4.0 head on the 4.2 block? And how would it work emission wise?
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Old 07-23-2011, 09:07 PM   #25
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Originally Posted by ClayL

It it easy to put a 4.0 head on the 4.2 block? And how would it work emission wise?
Someone's getting a little happy with this new knowledge! Haha. Idk about where you are but here in MA you don't have to pass emissions after 15 years
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Old 07-23-2011, 09:14 PM   #26
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It it easy to put a 4.0 head on the 4.2 block? And how would it work emission wise?
Fuel Injection is generally better for emissions, and easier to pass. Normally for smog states (I.E. California), any engine upgrades have to be done with engine parts of the same year or newer.
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Old 07-23-2011, 10:20 PM   #27
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Fuel Injection is generally better for emissions, and easier to pass. Normally for smog states (I.E. California), any engine upgrades have to be done with engine parts of the same year or newer.
Ok, idk what it is in Colorado, so I'd have to look it up. And I am getting excited about it, so tired of a carb
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Old 07-25-2011, 12:49 PM   #28
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Here we go again.

For what it's worth, I just bought a '95 with the 2.5L and I couldn't be happier. The specs don't look impressive on paper but the engine is tuned for low-end torque so it has enough power to get out of its own way in stop and go traffic, and with the 5-speed the first two gears are low enough that it actually feels quick off the line (sorta). On the highway, she'll cruise at 65 all day, though I had her up to 75 once (on a downhill with a stiff tailwind, though I'm never doing that again). I do need a downshift on even mild inclines, though that's more of a product of the YJ's brick like aerodynamics.

I test drove a few 4.0L before I bought mine and the extra power is nice, but the 2.5L is more than sufficient. (The lower insurance rates, and the fact that it makes a leadfoot like less likely to get speeding tickets was a factor ) As for reliability, I've read on this forum (and others) of the 4-cylinders lasting 300k miles.
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Old 07-25-2011, 12:54 PM   #29
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Ive never seen a documented original 2.5 with over 200K on it.....I have personally owned 5 now that have had one or more rods blown out the side of the block at 150K and less.
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Old 07-25-2011, 01:30 PM   #30
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^You must have gotten ones that were treated like crap. Mine still runs strong at 96,000 miles.

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