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4.2 engine swap

Jeepmedic46

NAXJA Forum User
NAXJA Member
Location
Orange, Ma
I tried searching but couldn’t find anything in regards to a 4.2 swap into my 2000 XJ which currently has the 4.0. Looking for more horsepower and I was thinking the 4.2 with a carburetor gives me more options. Plus I want to take out all
Plus I would like to take out the computer. I only have to pass safety no emissions.
 
That’s a downgrade not an upgrade.

If you’re looking for performance build a stroker with the crank from the 4.2
 
It's sure not a upgrade, after 50 yrs of wheeling I will never go back to a carburetor specially if you wheel at different altitudes. In my old J-2000 I had a half a tank of gas to get up the mountain to the ski resort (26 miles), I never made it!
 
Jeep 4.2 is rated at 200 gross HP.

Jeep OBD-II 4.0 is rated at 190 gross HP.

Carburetors suck for cold weather starting and for anything with altitude.
 
Last edited:
Jeep 4.2 is rated at 200 gross HP.

Jeep OBD-II 4.0 is rated at 190 gross HP.

Carburetors suck for cold weather starting and for anything with altitude.

Shouldn’t the 4.0 number be net crank , with all accessories installed?
 
Thank you, Carburetors do stink in the winter. Was thinking of swapping engines because when I start the Jeep now the engine is running very rough and seems like it’s not running on all cylinders plus looking to add a some more horsepower.
 
Figure out what is wrong before you decide you need to make a substantial change.

I really doubt you can come out ahead building a carbureted 4.2. I could see having an adaper plate for a carburetor and a points style distributor you could drop in if an EMP hits, but for normal use in the world as we now know it the H.O. 4.0 is the better motor.
 
Thank you, Carburetors do stink in the winter. Was thinking of swapping engines because when I start the Jeep now the engine is running very rough and seems like it’s not running on all cylinders plus looking to add a some more horsepower.

All the time or just hot restarts?
 
Figure out what is wrong before you decide you need to make a substantial change.

I agree, putting in a 4.2 or even just a carb on the 4.0 is really going backwards and a substantial downgrade in reliability and power. It would be a challenge for a 2000 which runs the dash based on data from the engine computer, and the automatic trans computer uses input from the throttle sensor.

Can you elaborate more on your symptoms, including when it's stumbling, mileage, etc? Knowing the original head on the 2000 is prone to cracking, I would suggest checking the oil for coolant and checking if you have an 0331 head. Most of the problems we run into are fairly easy to resolve.
 
When I start the Jeep it seems like a misfire and the whole vehicle shakes and it seems like it’s running on 3 cylinders then it smooths out but is still running rough. Hasn’t overheated and I just changed the valve cover gasket.
 
Any codes?

I agree with the rest - this is going to be a lot of work to make the vehicle worse.
 
If it started after you changed the valve cover gasket I'd suspect it's a wiring issue. There's often wiring harness problems right between cylinders 5&6 where the harness passes under the fuel rail.

To see if you have a dead misfire, pull injector connectors one at a time to kill fuel to that cylinder. If it gets worse, that's not the one.

As was mentioned before, the 258 (4.2) is not an upgrade. It's much lower on power than the 4.0 and carburetors are always a hassle.

For about $10 you can get a bluetooth code scanner from Amazon or eBay and it'll read codes and display live data from the computer. Look up elm327 scanners.
 
If it started after you changed the valve cover gasket I'd suspect it's a wiring issue. There's often wiring harness problems right between cylinders 5&6 where the harness passes under the fuel rail.

To see if you have a dead misfire, pull injector connectors one at a time to kill fuel to that cylinder. If it gets worse, that's not the one.

As was mentioned before, the 258 (4.2) is not an upgrade. It's much lower on power than the 4.0 and carburetors are always a hassle.

For about $10 you can get a bluetooth code scanner from Amazon or eBay and it'll read codes and display live data from the computer. Look up elm327 scanners.
Thank you, I’ll check the wiring. I looked at the scanners on Amazon and they are for Android phones. Do you have any recommendations for scanners that work with I Phones?
 
I've got a cheap ELM327 from eBay as well. It works on most of my vehicles, except the 2014 Cherokee, where it freaks the computer out and make all the warning lights on the dash light up.
 
Yeah it's a crap shoot with CANbus vehicles. I haven't found an OBD2 vehicle that flips out on it yet, but CANbus can get hairy sometimes. My brother bought one for his Mazda 3 and it went all wacky, threw a bunch of codes for TCM communication, etc. I came over and plugged mine in and it worked just fine. I traded scan tools with him and still never had a problem with any of my older OBD2 stuff.
 
I found a scanner at Summit that wasn’t to badly priced that will link up to my phone. Did manage to find someone to scan the code it’s showing number 3 cylinder misfire. Now I get to try and find the issue with that cylinder.
 
It's either spark, fuel, or compression. Check compression first, then cap/rotor, plug wires, spark plugs, then play musical chairs with your injectors.
 
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