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So I monitored my voltage at the pdc, alternator and battery (optima yellow top approx 2 years old). With no load at all I have 12.95v from the battery which is right on. When idling i have 14.28 until I turn on AC, fans and headlights. It drops to 14.17 and hold steady for about 1 minute. Over the next 2-3 minutes the voltage steadily drops to about 13. This 1 volt of difference is enough to drop my fan output below what it needs to keep up with this engine (fairly heavily built). I sent off my Singer 270a to get rebuilt and plan on putting it back on either way but am seriously debating where to go for cooling, the factory fans didn’t cut it when I had that setup. Kind of back to the drawing board on this one. Again, idle over temp is my only issue.

I know the vast majority of MJ’s are renix era and @cruiser54’s tips have proven to be invaluable. However, for the HO crowd, it would be nice to have some tips as well since there are still plenty of little problem areas that could be addressed in the same fashion as the cruiser tips. Any chance we could compile this info and then consolidate it for reference? @Pete M please feel free to move this topic if it’s better in another location. Calling all HO MJ owners

It’s also an option to do stand alone efi either using the 4.0 HO harness and ecu from 91-95 or a stand alone unit from Holley, mega squirt, hall tech etc. That way you can keep your engine running happy and your original wiring in-tact. The factory harnesses just fused together chasis and efi harnesses. They can be separated though as it doesn’t actually take very much to run an HO engine

On a serious note, for reference, note that most injectors are rated at 3 bar (43.5psi) and not 39psi which is what our stock FPR flows with no vacuum attached or at WOT. 24lb injectors rated at 43.5psi obviously will flow less at 39psi. On my latest build I got some 6 hole 27lb injectors which give me around 24lbs at 39psi. Some Ford injectors are also rated at 39psi but always worth confirming

Yikes those lights are rough! I used an M6 socket head screw. It has a perfect size and flat shoulder so it doesn’t split the plastic. Only thing you’ll need is a m6 nut clip.

Is your PCV working correctly? A plugged system will blow oil out every seal

It’s more of a brainstorming session regarding the HO charging system. I’m also running a Warn 10k winch along with all the other electrical add ons. The electric cooling fans have always been my number 1 issue. They have a real performance difference between 12.5 and 14.25v. My Singer alternator needs a rebuild so I sent it off and installed a new Napa 90a. While reviewing the spec sheet that came with it it got me thinking about what is actually happening with the voltage in my MJ. I just got done completely revamping and relooming the entire under hood harness. Replaced connectors, repairs and broken insulation and tested every single FI and charging system circuit. Alternator output is 14.25 at idle but once I turn on ac, fans and lights I drop down to about 13v. Not a big if it wasn’t for the fact that the e fan output drops quite a bit with that 1.25v difference. Enough that it can’t keep up with the cooling demands in traffic at idle.

Please elaborate. The the only energy dissipated by the radiator is thermal unless you are referring to a disruption of flow???

I have a burnt out Singer 270a alternator I’m getting rebuilt. The ultimate issue is these fans. They are marginal at 14v and the cfm is too low at 12.5v. All the added electronics (HID lighting, arb compressor, triple fans, audio system) I’ve integrated are just too much to maintain a good voltage.

Sounds like it has power on crank but not run. Does it sound like it just sputters out?

Here is an output printout of a new 90a HO alternator. It would stand to reason that, at idle with all accessories running, a 90a alternator is not up to the task.

Yeah check them out. Also keep in mind that drilled and slotted rotors offer less pad contact area and typically reduced stopping power. Conversely, they are superior in high performance applications where over heating is common. They can also combat brake fade. But if this is a normal daily driver, a good set of centric pads rotors, calipers and some black magic pads is the way to go.

Only big brake setup for 15’s is the Vanco setup as far as I know. Blaine at black magic brakes sells them. He also sells the best stock brake components available for the 90+ knuckles.

Just a note in the HO FI system. Contrary to popular belief, it is completely tuneable. It can be socketed with a flashable eprom and setup for pretty much anything. There are several people who offer the service or you can diy. For Jeep specific tubing of the sbec2 controller (91-95 HO Jeeps) Reach out to Chris at christuned.

Not personally but if it’s prepped well it should hold up just fine. We used to use JB weld to seal up cooling ports on the 4.0 head when installing them on a 4.2 block

Yes it does

Agree with all the above. A cowl intake is noisy but does offer some noticeable gains. Also, look into stand alone efi. I’ve run Holley efi with great success.

That makes sense. I would think the fuel pump would be at least 5a also. Ac clutch has got to be a bit of a draw also. AC on idling in traffic and there is definitely a significant voltage drop on a 90a alternator. Those FF dynamics fans need a solid 14+ volts to keep temps in check. I have a singer 270a alternator I’m sending in for a rebuild but I seriously considering switching back to a mech fan setup. The ff system was a big investment to just dump though.

If that’s the case then no accessories would run at full alternator voltage (~14.25a). The alternator needs to be able to handle the full load of all accessories. That’s why it goes directly to the PDC I believe but perhaps I am mistaken

Replies posted in the quotes above. I’ll try to edit them to make them stand out better

99+ intake is the best flowing electric vacuum pump pulls a lot of amps. Think hydroboost or stay conservative with the cam specs for good vacuum

Any thoughts here guys?

Agreed. The info above is assuming that you have a good running truck that has been properly maintained all its life.

For parts, most xj cherokee front lifts will fit. There are few complete kits for the MJ available so you’ll have to likely source the lift for the rear. The rear uses a longer leaf springs than the Cherokee and the springs run under the axle. If you convert the springs to ride over the axle (known as soa) then you will gain approx 5.5” of lift in the rear. If your looking for 2-4.5” you could go with either an add a leaf setup (adding a new leaf your your rear spring pack to provide more lift) or you could go with an aftermarket leaf spring pack which replaces the whole spring and provides the needed lift. SD truck springs makes an HD 5 leaf sparing pack that will give about 2.5” of lift to a stock truck. Hells Creek off-road makes springs in the 4-4.5” variety iirc.

Welcome aboard. There are so many different ways to go with the build. If your looking to build an off-road or trail rig then a long bed is always a bit of a pain to work with. I always like the idea of a long bed 2wd towing rig. That was the real “work” truck model. But with time and money you could build anything you want. A mild 2” lift will let you run 31” tires. 4.5 would be decent for 33” tires.