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Old 06-14-2011, 09:42 AM   #1
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4.2 Q's

So I may be in a position to eventually buy a 4.2, but I am needing to know some things about it, and just want some answers to some of my general questions.

I see they only put out about 112hp, but they have 210ft. lbs. of torque (as compared to my 112hp 110ft. lb. 4 cylinder). Even with the low HP rating, shouldn't it still be able to get my Jeep up and going easily with nearly double the torque?

What are some common bolt on power upgrades, granted they are California SMOG legal?

How do these engines do on the freeway? Please tell me they don't cruise at 3100 rpm 65mph

Lastly, what is the main issue with these engines? I keep hearing that people hate the 4.2, and would much rather have the 4.0 (although I can see why, with it's higher power output).

That's all for now, until I can think of some more.

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Old 06-14-2011, 03:30 PM   #2
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I have an 89 4.2 and I think it's a great engine. If it's taken care of, regular oil changes, it will last a very long time. The weak link over the long haul is the carb. My 4.2 maybe uses 1/4 qt. of oil in 4,000 miles. With a 5 spd. I'm at approx. 1,800 rpm at 55mph, stock tires and gears of course. I had a 4.0 in a TJ, which was also a good engine, but I always seemed to like the 4.2 a little better. Just a personal preference. I'll catch some heat, but the 4cyl. doesn't hold up as well internally and you have more problems with the FI system as you reach the 20 year mark.

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Old 06-14-2011, 03:57 PM   #3
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Acceleration isnt much of an issue at all. The reason most people hate the 4.2 is they only came with a carb. The best improvements you can make to the 4.2 are the 4.0 HO head swap and a better carb or FI set-up for it. I run the MC2100 and love it. It was a HUGE improvement over the carter. My jeep is an auto and runs at about 1700 RPMs at 70 MPH cruise. Running 85 MPH its at about 2600 RPMs. Also of note, with swapping to the MC2100 and performing the "TFI" upgrade (aka teamrush) I went from getting ~9 MPG city and ~12 highway to now hitting 15.2 city and 19.8 highway.
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Old 06-14-2011, 05:29 PM   #4
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So it sounds like, in order to get a better performing 4.2, that I need to swap in a 4.0 head, 4.0 fuel injector, and just for kicks toss in a performance exhaust system?

I honestly don't know if I would ever go over 4" of lift and 32" tires, so it would make a good DD I'd assume without the lift or tires.
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Old 06-14-2011, 05:42 PM   #5
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You don't have to go as far as changing the head and going efi. Even just upgrading the ignition system and swapping in a better carb will work wonders. The original ignition system isn't the greatest and the factory carb is an electronic feedback carb plagued by emissions equipment.
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Old 06-14-2011, 07:26 PM   #6
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I am in Southern California, so emissions is a must. Also, I'd like the power and fuel economy the FI system would give.
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Old 06-14-2011, 07:53 PM   #7
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The head and carb upgrades aren't necessary, they're simply the best bang for the buck like your looking for. As far as smog legal, I'm not entirely sure there. There are no inspections or emissions here so I don't have much info on that.
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Old 06-14-2011, 09:05 PM   #8
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The 4.2 had a pulse air system that puts fresh air into the exhaust to make it pass smog. Most people toss them though and I've heard can still pass but I'm not sure. I got the mc2150 and threw away the pulse air. Well I have them and it's expensive to fix since most people don't use them. Also going fuel injection you probably wouldn't have to worry about it I guess. But just saying. I'm rebuilding mine doing an HEI swap and motorcraft carb so it should be running pretty awesome when I get it back
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Old 06-15-2011, 05:34 PM   #9
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Here in PA I wouldn't pass emissions without the pulse air on my '87. My mechanic says that it has to have them even for a visual emissions test.
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Old 06-16-2011, 01:35 AM   #10
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The 4.2 had a pulse air system that puts fresh air into the exhaust to make it pass smog. Most people toss them though and I've heard can still pass but I'm not sure. I got the mc2150 and threw away the pulse air. Well I have them and it's expensive to fix since most people don't use them. Also going fuel injection you probably wouldn't have to worry about it I guess. But just saying. I'm rebuilding mine doing an HEI swap and motorcraft carb so it should be running pretty awesome when I get it back
is the pulse air the puts of air constantly coming outta my exhaust?? or do i have other mechanical issues? also what weber carb and air intake combo would one recommend for there 4.2? i hate sounding like an idiot but im still pretty knew to jeeps
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Old 06-16-2011, 06:06 AM   #11
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Originally Posted by Crew_Chief

is the pulse air the puts of air constantly coming outta my exhaust?? or do i have other mechanical issues? also what weber carb and air intake combo would one recommend for there 4.2? i hate sounding like an idiot but im still pretty knew to jeeps
I've heard nightmares about the weber carbs. As for the intake with them most people use an open element filter. I know the motorcraft you can adapt to your stock air box which I will probably do when I get it cleaned up. Then you wouldn't have a problem keeping the pulse air. It's hooked to the intake through the breather box and uses the intake of air to go through the exhaust system at the mid-pipe (just after the headers) and at the catalytic converter. It's to big tubes with balls on them that are valves.
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Old 06-16-2011, 06:48 AM   #12
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ive got a 4.2 with a hei dizzy and a weber 32/36. it runs great (i have to run 93 octane so it wont ping though). im not a big fan of the weber, it seems to leak a lot and i cant get it adjusted right but its better than that carter hunk of crap. the hei dizzy was by far the biggest improvement it really woke it up. i dont have to worry about emissions so all that bull crap is stripped off.
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Old 06-16-2011, 08:14 AM   #13
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Originally Posted by jakec
ive got a 4.2 with a hei dizzy and a weber 32/36. it runs great (i have to run 93 octane so it wont ping though). im not a big fan of the weber, it seems to leak a lot and i cant get it adjusted right but its better than that carter hunk of crap. the hei dizzy was by far the biggest improvement it really woke it up. i dont have to worry about emissions so all that bull crap is stripped off.
Hear that a lot with those carbs I'm glad I went motorcraft. Just that helped a ton. When the motor gets back I'll be doing the HEI so I'll be set. But to the OP you can always modify new carbs to do the pulse air If you have to....
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Old 06-17-2011, 07:40 PM   #14
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Originally Posted by Xpress View Post
So I may be in a position to eventually buy a 4.2, but I am needing to know some things about it, and just want some answers to some of my general questions.

I see they only put out about 112hp, but they have 210ft. lbs. of torque (as compared to my 112hp 110ft. lb. 4 cylinder). Even with the low HP rating, shouldn't it still be able to get my Jeep up and going easily with nearly double the torque?

What are some common bolt on power upgrades, granted they are California SMOG legal?

How do these engines do on the freeway? Please tell me they don't cruise at 3100 rpm 65mph

Lastly, what is the main issue with these engines? I keep hearing that people hate the 4.2, and would much rather have the 4.0 (although I can see why, with it's higher power output).

That's all for now, until I can think of some more.
I have a '90 YJ with a 4.2L 5 speed Trans. I can say I have driven a '00 TJ with a 4.0L, and I LOVE my 4.2L, she has power (I have no fear of pulling out in traffic if I see a hole). I have thought about changing my 4.2L, but I think I am sticking with it & switching over to TBI when I get a chance. Hope this helps.
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Old 06-18-2011, 07:25 PM   #15
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How about towing capacity with the 4.2? All I have right now for towing is a small trailer than can fit a couple of ATV's, and it's being pulled by a 95 Toyota Pickup with a 118hp 22RE. Goes about 60 uphill through the mountains to the desert, but it'd be nice to have something that I don't have to push so hard.

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