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I know the 4.0 has more power and has the reputation to go 3xx,xxx miles or even higher. I don't know much about the 4.2l though, any common problems or issues with this motor? Is it reliable like the 4.0?
 

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I don't think any of them are reputable to go to 300,000 miles.

The 4.0 has more peak horsepower, but the 4.2 has more low end torque.

The 4.2L has a lower RPM operating range than the 4.0.

I have a new rebuilt 4.2L in mine, and after all the upgrades that I have made to it, it produces more horsepower than a stock 4.0 does.
 

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4.2 has a carb. not FI, that can be a problem, other than that, they have the same rod and main bearings, just different bore and stroke. same timing chain set up, same type valve train. If taken care of it should last about the same, I would think.
 

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The 4.2 is a solid engine as well and as you said at a less horse power than the 4.0. The number of miles the engine can get on it is based on proper service on both models. The major difference between the two is the 4.2 is carbureted and the 4.0 is fuel injected. Both has it's own problems. I find the that the carburetor system to be easier to work on. But you will receive varying opinions from other perks here.
 

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The only issue in my opinion, if you want to call it an issue, with the 4.2 is the carb. I have a rebuilt 4.2 and have had no problems in the 2 years I have owned it. The carb can be a problem in states that have emissions laws because they need to be tuned properly in order to pass. I found a reputable mechanic who is a wiz with carbs and had no problems passing smog after he tuned it. Gas mileage is slightly better with fuel injection. I have a 90 4.2 and my daughter has a 91 4.0 and I can't tell much difference in driving the two. I like mine better.
 

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Great motors. I totally agree with Ivie, how long they last all depends on how they have been treated. The 258 in my '76 CJ7 had well over 200k on it when we rebuilt it. It was still running fine, I just wanted to hop it up a little.

I'll be straight up with you, if I could find a good running 258 with a matching tranny & t case locally, I'd swap it in my '95 in a heartbeat.
 

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Very minor problems in over 10 yrs w/ the 4.2 (alternator went, replaced radiator due to my screw up). Driven it over 60K, including a couple 1000 mile road trips and hundreds of wheeling/beach/mountain trips. Has been extremely reliable for me.
 

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Mine is pretty reliable, it had 260,000 before I rebuilt it. As long as they are well tuned, and kept in good maintenance, they'll last a while. The only reason the 4.0l is considered more reliable is that they are fuel injected which prevents rich/lean conditions which wear out carbureted motor's rings and valves.
 

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the 4.2 has great low end torq and fitted with weber carb and a fuel regulater
it's the cats butt.
 

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When my old 4.2 was dead I put a lot of thought into the 4.0.....in the end i stuck with the 258. Love that low end torque!
 

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I just rebuilt my 4.2 and swapped in a 4.0 head. I kept the motorcraft 2100 on it instead of doing the FI. Mine always passes emissions.....on the second trip!
 

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dropping a rebuilt 258 into mine as well-same that was in it with some mods. Did a 30 over on the pistons, new cam, header, intake, dui distributor, weber carb. Asked around much like you are and any "old" mechanics said either motor is reliable if taken care of.
 

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dropping a rebuilt 258 into mine as well-same that was in it with some mods. Did a 30 over on the pistons, new cam, header, intake, dui distributor, weber carb. Asked around much like you are and any "old" mechanics said either motor is reliable if taken care of.
what cam did you use?
 

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4.2 is hindred by a carb and a shitty one at that. Fuel Injection made it possible for the 4.0 to run for 300K+ the 4.2 would as well but excess fuel from primitive carburetor washes oil off the cylinder walls causing more wear and tear over time. So no it's not as reliable as the 4.0 unless you put FI on it.
 

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I have a 1988 YJ with an odometer reading of 125,000, owned for 2 yrs now, daily driver. I'm wary of that ODO reading, although it was mainly used as a toad for past 5 yrs (all wheels on ground, xfer case in neutral -- I hope). It has the original 4.2L, 258 engine. One great solution for the carb problem is to get a Howell Throttle Body Injection (TBI) system. It's a little pricey at ~$1,500, but it is oh so nice. Mine came with it, but due to poor maintenance by previous owner, it needed some TLC. I studied the TBI system and replaced 3 sensors (cheap at Autozone). Very sweet engine now. Next I want to replace the timing chain, tensioner, and possibly the cam shaft to squeak out a few lost HP. Distributor timing curves aren't ideal either.
 

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I have a fuel injected 4.2 and it runs great, very similar to the 4.0 in terms of the build. Easy to work on too.
 

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I have a 1988 YJ with an odometer reading of 125,000, owned for 2 yrs now, daily driver. I'm wary of that ODO reading, although it was mainly used as a toad for past 5 yrs (all wheels on ground, xfer case in neutral -- I hope). It has the original 4.2L, 258 engine. One great solution for the carb problem is to get a Howell Throttle Body Injection (TBI) system. It's a little pricey at ~$1,500, but it is oh so nice. Mine came with it, but due to poor maintenance by previous owner, it needed some TLC. I studied the TBI system and replaced 3 sensors (cheap at Autozone). Very sweet engine now. Next I want to replace the timing chain, tensioner, and possibly the cam shaft to squeak out a few lost HP. Distributor timing curves aren't ideal either.
Welcome to the Forum, from Cave Creek AZ.
 

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I have a 1988 YJ with an odometer reading of 125,000, owned for 2 yrs now, daily driver. I'm wary of that ODO reading, although it was mainly used as a toad for past 5 yrs (all wheels on ground, xfer case in neutral -- I hope). It has the original 4.2L, 258 engine. One great solution for the carb problem is to get a Howell Throttle Body Injection (TBI) system. It's a little pricey at ~$1,500, but it is oh so nice. Mine came with it, but due to poor maintenance by previous owner, it needed some TLC. I studied the TBI system and replaced 3 sensors (cheap at Autozone). Very sweet engine now. Next I want to replace the timing chain, tensioner, and possibly the cam shaft to squeak out a few lost HP. Distributor timing curves aren't ideal either.
Got a question
The PO put the Howell TBI on my 90 YJ and I was wondering what maintenance should I be doing like you mentioned. I am pretty clueless regarding This tbi. I am not sure about any sensors that is associated with the Howell either.
Thanks
 

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I said the PO did poor maintenance because there were so many simultaneous bad sensors when I got it. It ran very poorly, couldn't go much over 45 mph on the freeway (and there was a headwind). Idle varied between 700 to 1500 erratically. The Idle Air Control (IAC) was very dirty, I pulled it and cleaned it thoroughly and also inside the throttle bottle where it mounted. The idle improved but still not stable. I removed and did a resistance check on the TPS, it was very erratic as I moved it off the idle position, replaced it with new. Then I replaced the EGR, new plugs, distributor cap, rotor and wires. Reset the timing to 8-10 BTDC. Then finally replaced the O2 sensor. The engine check light went out with that sensor, and it began running good, passed Calif smog with flying colors. Didn't have to change the Cat. Oh, there were a couple of vacuum hoses connected to wrong ports. Also replaced the coolant temperature override (CTO) just for grins. The takeaway -- as soon as you notice a change in how it runs, zero in on what is failing and fix it, don't wait until it barely runs...
 
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