4.2 or 4.0

4.2 or 4.0
R

RP1

Jeeper
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Location
SO.Cal
Vehicle(s)
82 CJ7
258 CID
Just bought this and its time to start rebuilding! This is a 82 CJ7 that has been sitting for 8-10 years. Was parked with motor problems so I am just going to pull it and rebuild it.
Should I redo the stock motor or upgrade to the 4.0 FI motor, looking for reliability and simplicity.
Thanks
 
I vote for fuel Injection.:)
 
I see some doing the FI conversion with the AMC 258 i6 / 4.2l short block and 4.0 head, good or bad?
It seems simpler to use the complete 4.0 since everything is coming out already, your opinion?
 
RP1 said:
It seems simpler to use the complete 4.0 since everything is coming out already, your opinion?
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I agree. I know a few people with 4.0's, they are happy with them.
 
RP1 said:
I see some doing the FI conversion with the AMC 258 i6 / 4.2l short block and 4.0 head, good or bad?
It seems simpler to use the complete 4.0 since everything is coming out already, your opinion?
Click to expand...

For me I am doing a 4.0 head and Mopar mpi injection system on my lower milage AMC 258 i6 / 4.2l motor, not the cheapest way to go but easy enough to do...
That said my wife used to drive a 95(I think) Cherokee with the 4.0 HO motor and that thing ran very very good for a straight 6 and if I needed a complete motor swap that would be something to consider.
I guess it comes down to what your motor might need? Do you have access to a good 4.0 motor? Do you have to get it smogged? How much do you want to spend?
The best 6 would be the AMC 258 i6 / 4.2l with the 4.0 head and F.I.
 
I will need to smog it here in our wonderful golden state but as long as I get everything from one doner vehicle it shouldn't be to bad.
Is there a preffered year? why?
 
RP1 said:
Why do you feel that is the best way to go (AMC 258 i6 / 4.2l with 4.0 head)?
Did you use the electronics that match the head?
Click to expand...

The AMC 258 i6 / 4.2l has a longer stroke and has more torque than the 4.0 motor.
The 4.0 head is better than the AMC 258 i6 / 4.2l head from the intake port shape and is a must for a multi port injection system.
The electronics need to match the fuel system used and not head.
For Ca. like me you have only a few choices....
1. AMC 258 i6 / 4.2l motor with ether the Howel T.B. injection or the Mopar MPI(both CARB legal)
2. 4.0 complete swap and a visit to the smog referee for a beating.
3. A donor jeep MPI system transplanted onto your AMC 258 i6 / 4.2l and again a visit to the smog referee for a real beating this time.
 
RP1 said:
I will need to smog it here in our wonderful golden state but as long as I get everything from one doner vehicle it shouldn't be to bad.
Is there a preffered year? why?
Click to expand...

The Mopar MPI system is based on the system used in a 95 or 96 year 4.0 jeep motor and just sold as a pakaged system ready to install.
Can't say what years are best for a donor but should be safe with the 95-96
 
I to am from the great state of California. Wouldn't live anywhere else. I talked to my smog mechanic because my AMC 258 i6 / 4.2l is getting real old, and he said the easiest as far as smog goes is to replace the AMC 258 i6 / 4.2l with the 4.0 as long as the donor meets the California emissions. The 4.0 being a newer motor would not be a problem as far as smog goes. He said it causes more heartburn when you try to change out parts. That just makes it harder to check and raises flags. I have my eye on a donor already but mine is still running good enough to try the Rubicon again next month. If it holds up I may just leave it until I can't go anymore. The more stock it is the easier it is to work on and get parts. My opinion anyway.

Tommy
 
me too as far as a Californian.
Does the 4.0 have inherent smog problems? or do they pass easily. I am also considering a 4.3 Vortec conversion. I am fairly impressed by the V-6 Engines.
 
81laredo said:
me too as far as a Californian.
Does the 4.0 have inherent smog problems? or do they pass easily. I am also considering a 4.3 Vortec conversion. I am fairly impressed by the V-6 Engines.
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After spending a summer with the Mopar MPI on my AMC 258 i6 / 4.2l I would do it exactly the same way again. I have been to Death Valley at -250' up to 10,200' in the Big Pine Ca. valley and it starts and runs flawlessly allways as well as the fuel consumption is approx. double what it was.
 
Sasquach said:
After spending a summer with the Mopar MPI on my AMC 258 i6 / 4.2l I would do it exactly the same way again. I have been to Death Valley at -250' up to 10,200' in the Big Pine Ca. valley and it starts and runs flawlessly allways as well as the fuel consumption is approx. double what it was.
Click to expand...
Double the fuel consumption? if that is true and the system almost $3,000.00 for the system. I would rather make an engine swap.
 
Oops, I meant double the mileage. It is mostly a guestimate but my range is much improved, it's pointless to say I get ?MPG because I don't do city or highway driving much and may spend an hour in 4L and only go a mile or 2 sometimes.
 
Definitely 4.0l! i just put one in my 84 jeep CJ7 my self. it's easy to do, lot's of parts, plenty of them around, starts every time, makes your jeep more desired if you go to sell but what am i thinking who sells a jeep cj. but over all Best up grade i have done to my jeep cj. i will never put another carb in a jeep again. good luck
 
81laredo said:
Double the fuel consumption? if that is true and the system almost $3,000.00 for the system. I would rather make an engine swap.
Click to expand...

Schmah I built a junkyard FI injection for under 2k. For you cali guys what if you kept the AMC 258 i6 / 4.2l block and used a 4.0 head/injection and accessories( I still havent quite figured out how to run a 4.0 alty bracket on the AMC 258 i6 / 4.2l block but the rest of the brackets are a direct bolt on) and just telling the smog people the engine is a 4.0 from whatever year you use?

To address some of the 4.0 questions here.

91-95 are the preferred FI donors because they are the OBd1 system and use less sensors and therefore are less of a hassle to wire. You will need to use the fuel rail and sensors from whatever a 91-95 4.0.

91-95 is also the preferred head to use, alot of sources recomend a 3 angle valve job. This is supposedly because 91-95 heads flow better or something.

You can use any year 4.0 intake on a 91-95 head. I just installed a 01 Intake onto my head which came from a 95 xj. the Power steering pumps are different on a 91-95 intake compared to 96+ ps pumps. so make sure the ps pump/bracket matches the year intake you use. If you retain the AMC 258 i6 / 4.2l short block the 4.0 PS pump from all years 4.0s will bolt right up the AMC 258 i6 / 4.2l has all the bores needed.

If you are using a AMC 258 i6 / 4.2l block and you want to use the 4.0 serp set up. you will need a spacer to place the harmonic balancer correctly (hesco sells them) All the brackets from 4.0s are direct bolt on I have found except the alty brackets. I originally used a complete xj setup which has a one piece alty bracket. That failed horribly. SO i have not figured out if 4.0 serp can be sucessfully run long term on a AMC 258 i6 / 4.2l block. I am currently using all the brackets from a97 wrangler that uses the ac bracket as the upper alty bracket and then has a lower bracket, i am hoping this will stabalize everything enough but havent had alot of road time with it.

So now recommendations for what you should do? If you can get a junkyard AMC 258 i6 / 4.2l /4.0 head mpfi setup to pass smog go for it. I am totally happy with the results in mine. But if you have to buy the mopar kit to do it and spend 3k? I would probably just pick up a 4.0 and yank the crank and rods out of my AMC 258 i6 / 4.2l and have them swapped into the 4.0 and build a poor man's stroker as all that would probably run the same price as the mopar kit or close too it.
 
i put mine in for under a $1000. and it was easy to do. there is a lot of information on the web on how to do it. i bought my 4.0l harness and computer for $500 and i paid around $200 for a fuel pump, $75 for a electric fan and a few odd's and ends like fuel lines ext. like i said you won't regret it. fuel economy vairies but i got better fuel economy with the 4.0l efi.
 
Tommy said:
I to am from the great state of California. Wouldn't live anywhere else. I talked to my smog mechanic because my AMC 258 i6 / 4.2l is getting real old, and he said the easiest as far as smog goes is to replace the AMC 258 i6 / 4.2l with the 4.0 as long as the donor meets the California emissions. The 4.0 being a newer motor would not be a problem as far as smog goes. He said it causes more heartburn when you try to change out parts. That just makes it harder to check and raises flags. I have my eye on a donor already but mine is still running good enough to try the Rubicon again next month. If it holds up I may just leave it until I can't go anymore. The more stock it is the easier it is to work on and get parts. My opinion anyway.

Tommy
Click to expand...

The other posters need to remember that this post is about Ca. and building a stroker motor for the same cost as a Mopar kit or junkyarding an mpi system does not address the issues faced with the smog police.
 
Just a little input from someone that doesn't have a dog in the California smog fight.
I would go with the AMC 258 i6 / 4.2l if it is rebuild-able. longer stroke, more torque, original equipment and it is paid for. I would also go with the 4.0 head just because it is a better design. I would go with the Howell TBI, least expensive option and it is CARB certified. So far I have found no advantage to MPI over TBI. If I thought the junk yard conversion would save some bucks that would be one thing but so far that is not the case.
By shear coincidence this is pretty much my build plan. I would like to think that, for the most part, I don't change a lot of things with out good reason. So unless it represents a major technological advancement or is the only option to keep the Jeep running I like to go with what the Automotive engineers that designed my CJ first came up with. I believe they knew what they were doing.
Here in Texas there is a seldom quoted section of the emissions rules that reads something like this. If the motor is replaced with something other than the stock it will be subject to the standards of the year the swap was performed. That is to say the year of the vehicle and the year of the motor are not relevant. How they would police this I do not know, but it is in the rules as they are published. I am sure there are all kinds of obscure little clauses in the Cali. rules that would that would make your head hurt and come back to bite you in the butt one day. :cool:
 
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