'91 Pioneer 2wd Long Bed Renovation

[Randy90MJ]

1991 Jeep Commanche Pioneer - 4.0L Rennix - 2wd - Long Bed - Automatic

 

Alrighty Folks, 

--I have been so lucky to acquire this Jeep from a very good friend of mine right before they passed away.  He and I had a few plans for the Truck.  But him being the Jeep finatic of us both leaves me in a pickle for what parts to buy, what parts to upgrade, where to buy said parts from etc.

--The Engine deifinately needs a makeover.  A little over 200k on the clock.  First time i crank it when its cold, it will fire up, sometimes a small skip, but not common.  After running for 60 seconds or so the engine will start surging.   If i unhook the IAC it will stop surging but won't run right.  If i leave it alone and try to keep it running with the accelerator itll run about 5 mins and shut off.  Not completely sure...

--------I know there was a fire under the hood a few years back that burnt up the Air Intake Piping to the Throttle Body.  This is one of the Engines that has a flange fit over a rubber foot extended from the Air Intake Hose.  Nothing else looks like it got crispy but hard to say.  

--My questions being:

1. WHAT replacement parts should i buy for this and WHERE should i buy them from?

2. WHAT Test would you recommend I do to narrow down the problems?

3. I also have another 4.0 High Output that need rebuilding. WOULD it be better to rebuild it and put it in or keep the Rennix?

[89 MJ]

Looks like a good starting point. 
 

I would recommend you go through all of Cruiser’s tips first. In the grand scheme of things, the HO engine really isn’t any better than the Renix. The cylinder head is better depending on the year, but that’s it. 

[Randy90MJ]

On 8/8/2025 at 12:13 PM, 89 MJ said:

Looks like a good starting point. 
 

I would recommend you go through all of Cruiser’s tips first. In the grand scheme of things, the HO engine really isn’t any better than the Renix. The cylinder head is better depending on the year, but that’s it. 

Thanks I appreciate that.  I have read over the tips and made myself a list of things to conquer.  What about which parts to buy?   Like what brand of sensors. 

[Randy90MJ]

On 8/8/2025 at 12:13 PM, 89 MJ said:

Looks like a good starting point. 
 

I would recommend you go through all of Cruiser’s tips first. In the grand scheme of things, the HO engine really isn’t any better than the Renix. The cylinder head is better depending on the year, but that’s it. 

Also about the HO head.  If its better should i use it on the rennix block? 

 

[89 MJ]

15 minutes ago, Randy90MJ said:

Thanks I appreciate that.  I have read over the tips and made myself a list of things to conquer.  What about which parts to buy?   Like what brand of sensors. 

 

14 minutes ago, Randy90MJ said:

Also about the HO head.  If its better should i use it on the rennix block? 

 

I bet a lot of the sensors that you have will probably be ok. If you need to replace any, I like Standard parts and you can typically get them at any parts store. 

To be honest, I don't think it would be worth swapping an HO head onto the Renix block. The HO Jeeps made 13 more horsepower than Renix. Some of that came from the head, some from the cam, and some from the different fuel injection system. In the words of Cruiser54, "HO Stands for Highly Overrated".

 

Are you sure this truck is a 91 though? All signs are pointing to it being an 89 or 90. 91 was the first year of the HO Jeeps, plus the Pioneer stripes changed in 91 as well. @fiatslug87, any thoughts on that?

 

@jdog or @JMO413, can we get a build sheet and selling dealer when one of you has the chance please? 

[jdog]

3 minutes ago, 89 MJ said:

 

I bet a lot of the sensors that you have will probably be ok. If you need to replace any, I like Standard parts and you can typically get them at any parts store. 

To be honest, I don't think it would be worth swapping an HO head onto the Renix block. The HO Jeeps made 13 more horsepower than Renix. Some of that came from the head, some from the cam, and some from the different fuel injection system. In the words of Cruiser54, "HO Stands for Highly Overrated".

 

Are you sure this truck is a 91 though? All signs are pointing to it being an 89 or 90. 91 was the first year of the HO Jeeps, plus the Pioneer stripes changed in 91 as well. @fiatslug87, any thoughts on that?

 

@jdog or @JMO413, can we get a build sheet and selling dealer when one of you has the chance please? 

As long as the Vin is posted in the registry. 

[89 MJ]

3 minutes ago, jdog said:

As long as the Vin is posted in the registry. 

My bad, I'm an idiot and thought I was in the registry when I asked this.

 

@Randy90MJ, could you post the VIN in the registry?

[fiatslug87]

It’s a ‘90, bench with headrests started in the ‘90 model year.

[89 MJ]

57 minutes ago, fiatslug87 said:

It’s a ‘90, bench with headrests started in the ‘90 model year.

Good catch

[Eagle_SX4]

9 hours ago, 89 MJ said:

The HO Jeeps made 13 more horsepower than Renix. Some of that came from the head, some from the cam

 

The cam profile is identical from 87-96. The H.O. engine only made more power in the upper RPMs because of the improvements in the head to flow better when the RPMs are closer maxed out. It could be an improvement if you regularly reach 3500+ rpm but other wise you probably won't notice a difference.

[89 MJ]

11 minutes ago, Eagle_SX4 said:

 

The cam profile is identical from 87-96. The H.O. engine only made more power in the upper RPMs because of the improvements in the head to flow better when the RPMs are closer maxed out. It could be an improvement if you regularly reach 3500+ rpm but other wise you probably won't notice a difference.

That’s interesting. I’d heard before that they were different, but never saw any concrete difference. I just assumed it was true because my truck doesn’t seem to make power with its 96 engine until 2500 rpm. Good to know that they’re actually the same. 

[Eagle_SX4]

17 minutes ago, 89 MJ said:

That’s interesting. I’d heard before that they were different, but never saw any concrete difference. I just assumed it was true because my truck doesn’t seem to make power with its 96 engine until 2500 rpm. Good to know that they’re actually the same. 

I was surprised to learn that the cam shafts were the same as well.