4.0 head and EFI on a 4.2 block

[Jeep-Power]

Welcome to the board & thanks for the props.

Wow-- that is a serious build you have.

post some photos of what you have left of the wiring & we might be able to say what's NOT there.

[n_stockstill]

I'll try and get some pictures posted of the entire build. Also in the mean time im looking for a donor vehicle with the head and EFI I will need. Was planning on buying a whole jeep that was in less than satisfactory condition, get everything I want off, maybe part out a little bit and then junk the rest and maybe recoup some cost. From what I understand I need a Cherokee from 91-95, 4.0. Are all these rigs the "HO" version or not? If not, is there an easy way to spot the donor vehicle I need? Without looking at the casting #, that I believe is 7120? Thank you again!

[SilverXJ]

Yes, 91-95 are HOs

[TXslayer]

Hey guys, New to the forum and also a new Jeep owner

I purchased a 1990 Cherokee with motor that had rod knock on all 6 cylinders (not sure how the tough gal still ran)

Anways, it had the 4.0 Non-H.O. motor in it and my buddy who owns a few jeeps gave me a carb'd 4.2 motor to slap my 4.0 head on. i was wondering if it would be the same steps as this build or if I would have to do anything different since it's a Non-H.O. head.
thanks.

[Jeep-Power]

it should be mostly the same-- the casting/coolant passages might be a little different. I'd check that with a 4.2 head gasket laid on the 4.0 head.

[TXslayer]

The coolant passages are the same as how they were described, I'd have to plug a few and such.

I'm just wondering if there was any difference in the combustion chambers that might cause the swap to be any more difficult than what you have done?

[rshaffer49]

Hello, this has been a great help and I was able to get my YJ fired up and driving pretty quickly. I pulled my harness out to clean up the installation a bit and now when I hook it back up I'm having problems. I have it all connect as I did previously but I cannot get the fuel pump to come on. I get voltage on all legs of the ASD and three of the four on the fuel pump relay, of course the one leg of the fuel pump relay that does not get voltage is the green/white wire to the pump. Any thoughts? I have several extra relays that I have tried so it doesn't seem like a faulty relay issue. I've jumpered the voltage to the pump and the jeep fires right up that way, I'm at a loss.

[Jeep-Power]

sorry for the delay-- did you get it figured out?

[rshaffer49]

I have not figured it out yet. I have tested the relays and they seem to be working ok. My initial description of the problem is a little off though on the ASD I get 12V to three of the four legs, the dark green / orange wire is not getting any voltage, and on the fuel pump relay I am getting voltage on all legs to the relay except the dark green / white to the fuel pump. I have a feeling that the DG/OR wire on the ASD is the culprit, but I'm not 100% sure, any thoughts? thank you.

[76jeepcj8]

do you have a breakdown on the wireharness what to keep what to ditch ??

[YJfor2]

So here is my question, I am in the middle of this build myself, as far as lifters, pushrods and rockers go, what has to be changed to make this work? did you change any of it? Stock 4.0 lifters, rods and rockers? or did you use 4.2? i have a 7120 head wit the valves and rockers installed.
Really appreciate the help. Oh and I have a brand new cam from a 1993 4.0.

[jsawduste]

I know who you are..........

[YJfor2]

Haha. I figured Id find you eventually. Read into another post and found your Jeep on some rocks in a debate with another guy on crankshafts. He didnt seem to like the 12cwt.

[ranger3157]

Great post! Good job on the pictures. I am building up a 4.0l into a stroker to replace the 4.2L with Mopar Mpi in my AMC Eagle. Will the exhaust manafolds interchange between the two engines or will I have to use the 4.0 exhaust manafold with the 7120 head? Reason I ask, on the Eagle, the front diff support bracket is bolted onto the the two bosses on the side of the block just about where the 4.0 manafold comes out. Would be handy for me to use the 4.2 manafold on the 4.0 head. Any ideas?

[Jeep-Power]

thanks.


i am pretty sure the 4.2 manifold will not work on the 4.0 head. The 7120 4.0 manifold does line up pretty well with all the 4.2 bracketry.

have you checked in the rest of the Swapology forums?