4.0 head and EFI on a 4.2 block

Swaping 258 to 4.0, 4.0 parts to 258 etc!
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Racer117
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Re: 4.0 head and EFI on a 4.2 block

Post by Racer117 »

markncarj1 wrote: February 13th, 2019, 10:22 pm Not sure if anyone still checks this but I am having a hell of a time with this swap. Everything seems to work except when I run it for a min I reverse it to a steady throttle it starts misfiring. And exhause turns red as hell. It starts and idles perfect even at dead cold. Runs for 3 mins or so and dies. Turn it over again and fires right up and after a few mins at idle it dies again. I can hear the icv open to try to keep it going. After the 3rd start it will idle forever with no problems. Biggest problem is the first one. The misfire when heald at steady rpm. I did not cut the ear on the dizzy so it may be causing the problem. I can put a link up to a YouTube video if need be. This is a setup my son started and screwed it up so now I am back tracking his problems. Pls help
[/quote



I know this was a year ago, but what was your fix??? I was working on a buddies xj with a stick engine. His did the same stuff. I didn’t think an OBD1 engine relied on the O2 so strongly. Anyways. I unplugged the O2 and it would stay in open loop. Runs perfect. He is short on cash and it’s not his daily, so he left it unplugged.

Just a thought.
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Re: 4.0 head and EFI on a 4.2 block

Post by Jeep-Power »

Yep, still checking it. :-)

Glowing manifold indicates retarded timing. I had that problem.

You have to cut the ear on the distributor. Ultimately it sounds like your distributor indexing is off. Indexing it is a bit of a trick and usually it takes one or two tries to really figure out what's going on. I don't know where to find it these days but there is a specific set of instructions. Serch it up & see what you find. Worst-case let me know and I will post a photo of what's in the factory manual
Racer117
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Re: 4.0 head and EFI on a 4.2 block

Post by Racer117 »

There’s actually a link on I think the second page of this post on setting your distributor. The k ok not thing I’d add is that when you’re rotating the crank to TDC, you go in direction of rotation. I’m not disputing a possible timing issue. My thoughts are that timing should effect it right away, and not after it runs for a while. IE goes into closed loop. Now not standing there in front of the vehicle. It’s a hard call. I was thinking it’s an easy and free test. But then again. So is yours.
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Re: 4.0 head and EFI on a 4.2 block

Post by Jeep-Power »

The quoted message threw me off Racer117- I didn't see your addition to markncar's post. :doh:

Yeah - the OEM systems rely pretty heavily on the O2 sensors. I am surprised how well they do on narrow band O2 data.

I think there is a cranking advance setting and then an afterstart setting, before the O2 sensor warms up enough to give accurate data - I have my MS3 set up this way
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Re: 4.0 head and EFI on a 4.2 block

Post by ccpanel »

Urquiola wrote: February 25th, 2019, 2:05 pm Hi!: for inspection purposes, it would be good to me having the Jeep Cherokee 4.0 HO head and camshaft in the 4.2 Pacer engine, with the AMC double vase carburetor, but I ignore if this can be done while keeping the existing intake and exhaust Pacer manifold, necessary not to make the changes obvious to inspectors, as installing the injectors, intake and exhaust Jeep manifold would show. Any suggestion? Thanks. Salut +
4.2 intake fits just fine on 4.0 head.

using 4.0 factory header/manifold and 84 intake with holley 2bbl carb.
see my sig for build thread.

its power is weak but its not a issue with intake to head, its an issue with a slippy tranny and carb instead of EFI and weak renix 89 cam.... bunch of issues.
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Re: 4.0 head and EFI on a 4.2 block

Post by ChisholmYJ »

Jeep-Power wrote: March 3rd, 2020, 8:13 pm The quoted message threw me off Racer117- I didn't see your addition to markncar's post. :doh:

Yeah - the OEM systems rely pretty heavily on the O2 sensors. I am surprised how well they do on narrow band O2 data.

I think there is a cranking advance setting and then an afterstart setting, before the O2 sensor warms up enough to give accurate data - I have my MS3 set up this way
Do you still check this thread? I'm in the process of doing the efi swap on my 4.2, I've followed this exact thread from the start, in part with some other one. I current have the grounds in the harness tied into the block, the PDC tied directly to the battery, and my ignition line from my 89 spliced into the ignition start and run circuit on my donor 91. I keep getting voltage on the grounding side of my ASD relay and no injector pulse, if I bypass the ASD I get injector pulse but still no fire. Any ideas?
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Re: 4.0 head and EFI on a 4.2 block

Post by Jeep-Power »

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Re: 4.0 head and EFI on a 4.2 block

Post by Jeep-Power »

ChisholmYJ wrote: July 4th, 2022, 3:47 am
Jeep-Power wrote: March 3rd, 2020, 8:13 pm The quoted message threw me off Racer117- I didn't see your addition to markncar's post. :doh:

Yeah - the OEM systems rely pretty heavily on the O2 sensors. I am surprised how well they do on narrow band O2 data.

I think there is a cranking advance setting and then an afterstart setting, before the O2 sensor warms up enough to give accurate data - I have my MS3 set up this way
Do you still check this thread? I'm in the process of doing the efi swap on my 4.2, I've followed this exact thread from the start, in part with some other one. I current have the grounds in the harness tied into the block, the PDC tied directly to the battery, and my ignition line from my 89 spliced into the ignition start and run circuit on my donor 91. I keep getting voltage on the grounding side of my ASD relay and no injector pulse, if I bypass the ASD I get injector pulse but still no fire. Any ideas?
Yep, still here, if not a bit late in getting notifications.

Honestly, it has been more than 10 years since I fiddled with the OEM electronics, but I will do.my best.

Just to confirm- you still have the Jeep PDC in place?
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