'86 Comanche DD
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CF Veteran
Joined: Apr 2011
Posts: 1,146
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From: Clayton, NC
Year: 1988
Model: Cherokee
Engine: 4.6L I6 Stroker with port matched +99 intake and 62mm TB
So, being the kind of person I am and never being satisfied with the way things are, I feel the need for a major overhaul on the MJ. I just got the thing in January and have only driven it three times since then. I still haven't had time to figure out my exact issue with the running problem. Anyhow, I just have this reoccurring idea of swapping out the TBI 2.5L/AX4 in favor of a MPFI 4.0L HO/AX15 (with external slave). Thoughts?
Thread Starter
CF Veteran
Joined: Apr 2011
Posts: 1,146
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From: Clayton, NC
Year: 1988
Model: Cherokee
Engine: 4.6L I6 Stroker with port matched +99 intake and 62mm TB
On the '90 XJ/MJ clusters, there's a little potentiometer (or pot) on the backside of the tach that can be adjusted but it's near impossible to adjust with the tach installed.
What I did was mark about where the pot was, removed the tach, drilled a 3/4 inch hole using a hole saw, reinstalled the tach and put everything back together. Then, using a Sunpro tach, I wired it inline with the cluster tach, set the Sunpro tach to 4-cyl mode and adjusted that pot through that 3/4 inch hole with my finger until they both matched through different RPM ranges.
Sounds like a lot of work, but it wasn't too difficult.
What I did was mark about where the pot was, removed the tach, drilled a 3/4 inch hole using a hole saw, reinstalled the tach and put everything back together. Then, using a Sunpro tach, I wired it inline with the cluster tach, set the Sunpro tach to 4-cyl mode and adjusted that pot through that 3/4 inch hole with my finger until they both matched through different RPM ranges.
Sounds like a lot of work, but it wasn't too difficult.
Thread Starter
CF Veteran
Joined: Apr 2011
Posts: 1,146
Likes: 0
From: Clayton, NC
Year: 1988
Model: Cherokee
Engine: 4.6L I6 Stroker with port matched +99 intake and 62mm TB
Painted the tail light borders...
Unmasked...
Painted the whole truck "Heritage Red" and stuck the newer style emblems on it.
A shot of all three... It's an obsession.
Unmasked...
Painted the whole truck "Heritage Red" and stuck the newer style emblems on it.
A shot of all three... It's an obsession.
Thread Starter
CF Veteran
Joined: Apr 2011
Posts: 1,146
Likes: 0
From: Clayton, NC
Year: 1988
Model: Cherokee
Engine: 4.6L I6 Stroker with port matched +99 intake and 62mm TB
Ok, so tonight, after doing some thorough research, I have found out that other guys across the interwebz have had a similar problem with there 2.5L TBI MJ's (this one for instance: http://www.justanswe...-2-5-4-cyl.html), and I ordered a new Bosch turbine fuel pump and new Bosch strainer/sock. Got it all from RockAuto.com and got a 5% discount code from RetailMeNot.com. In total, I paid just shy of $78. Woot! I really hope this solves my issue. Wish me luck guys and gals!
Last edited by onlyinajeep726; Jun 11, 2013 at 10:29 PM.
Thread Starter
CF Veteran
Joined: Apr 2011
Posts: 1,146
Likes: 0
From: Clayton, NC
Year: 1988
Model: Cherokee
Engine: 4.6L I6 Stroker with port matched +99 intake and 62mm TB
Good news guys (I think). So, I tested the old fuel pump with a 12 VDC power supply in a bucket of water and some hose. I turned on the pump, kinked the hose about four times (to build pressure) and the fuel pump went crazy with the shakes for a while, then finally cut off when it got up to "primed" pressure. Well, after I unkinked the hose to lower the pressure (which it should come back on to keep supplying fuel), it didn't turn back on. Only when I cut power and reapplied it did the pump come back on! This explains why when I would restarted the MJ, it would "work" for a while until it started going crazy again. So... hopefully this new Bosch unit is the pot of gold I've been looking for!
Thread Starter
CF Veteran
Joined: Apr 2011
Posts: 1,146
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From: Clayton, NC
Year: 1988
Model: Cherokee
Engine: 4.6L I6 Stroker with port matched +99 intake and 62mm TB
I just got back from picking this bad boy up... It's an AX-15 from a '96 XJ that the seller claims was rebuilt within the last year or so. No bellhousing unfortunately, but I did get the crossmember and a few various other parts. All for $175. This is the start of my '86 2.5L/AX-4 to 4.0L/AX-15 swap. I need to clarify some things and I'm hoping y'all can help me out.
First, I know I need to find a bellhousing, shift fork, external slave and all that stuff. The thing I'm not sure of is if a NP231 from an '88 or '90 XJ will bolt to this transmission since I'm not sure what each of the spline counts are for an '88, '90 and '96 transfer case. So, does anyone here know for sure?
Also, the rear driveshaft is going to be concerning. What do I need to know about that? I should also note that I'm thinking about getting a C8.25 and welding on SUA leaf perches. Also would like to know what gears would work best with an 4.0L/AX-15 combo. I know that stock used 3.07s commonly. I want to make this the most fuel efficient Jeep I can make it. At this moment, I have no intention of lifting or running larger that 215 tires. Also, any writeups on the subject are greatly appreciated. I should also note that I am pulling the engine, computer and all from a '90 XJ with an automatic transmission. Is that going to cause any issues?
First, I know I need to find a bellhousing, shift fork, external slave and all that stuff. The thing I'm not sure of is if a NP231 from an '88 or '90 XJ will bolt to this transmission since I'm not sure what each of the spline counts are for an '88, '90 and '96 transfer case. So, does anyone here know for sure?
Also, the rear driveshaft is going to be concerning. What do I need to know about that? I should also note that I'm thinking about getting a C8.25 and welding on SUA leaf perches. Also would like to know what gears would work best with an 4.0L/AX-15 combo. I know that stock used 3.07s commonly. I want to make this the most fuel efficient Jeep I can make it. At this moment, I have no intention of lifting or running larger that 215 tires. Also, any writeups on the subject are greatly appreciated. I should also note that I am pulling the engine, computer and all from a '90 XJ with an automatic transmission. Is that going to cause any issues?
Seasoned Member
Joined: Dec 2010
Posts: 402
Likes: 2
From: Charlotte/Raleigh, NC
Year: 1999
Model: Cherokee
Engine: 4.0 MPI