Just bought a '91. 4 Wheel drive with the 4.0. About 200k miles. It starts and runs fine, has a small lift and larger tires, nothing particularly special done to it.

I am really just looking for advice on where to begin modding for performance. Want to start with the engine. Has no problems that I can tell other than a squeal from the belt that I can fix. Looking for ideas on engine mods that are gonna save me from headaches in the future and improve performance overall. I'm new to the jeep world, be gentle.

 

Welcome to the XJ world!

Reliability Mods: Rather than make a long list, click here and read Cruiser54s XJ tips which target the most common issues that lead to reliability problems. If you follow those and make the recommendations you will have a better running rig and save yourself a lot of headaches. Most translate to your year rig. One word: vacuum issues. Vaccum issues are a common source of headaches and trip up a lot of early XJ owners running old brittle vacuum lines. The front/rear intake nuts can (and do) come loose too (more vacuum issues). Be sure the little hole in the rear engine grommet isn't plugged (clean with a nail or small drill bit). Pre-clean IAC too. A guy could distill a lot of engine-realted posts here to: Bad vacuum, Dirty IAC and EGR, CCV, and O2 sensor.

Scanner: If you are committed to your XJ, you might consider planning ahead and get a scanner which will read your pre-OBDII system. A popular one is an older Snap-on scanner MT2500, which can be bought cheap these days on ebay (I've seen sell as low as $60). There are XJ related youtube videos which show how. Cartridges in the 1999-yr range would be ideal, but newer cartridges are backward compatible and will work. A multi-meter works too, but not as convenient. Some newer, pricier, scanners like the Snap-on Solus Ultra for example are backward compatible to like 1980 vehicles and will read XJ systems fine in case you had access to, or were inclined to invest more $ to also be able to read other newer vehicles you may have. There are several scanner options. When ya run into a headache it's nice to be able to throw on the scanner and get a read on things.

General Assessment: Since the XJ is new to you, you might consider doing an initial compression test of the cylinders, get a read on the vacuum levels, charging voltage, etc. Check driveshaft and axle 'play'. Just to get an understanding where things are at with your rig right now. For example, if you discovered you had one cylinder low on compression, it might help you plan for repairs or mods going forward rather than being tripped up for a while. Everything will probably come up looking good, but can't hurt for the little time it'll take.

Rear End: The rear end pinion bearings (and bearings in general) are a weak point to keep your eye on especially running larger tires. Keep on the lube schedule.

Performance Mods: I'm sure others will chime in on higher performance-oriented mods better than I.

Heater Core: If yours is original, they seem to be failing about now. Just be aware of it. It's a fairly big job but not as bad as it seems once ya dig into it. Hopefully yours has been replaced. You CAN get brass/copper heater cores despite what some people may say. Check on ebay if/when the time comes. Don't forget to flush your coolant right away and check pH of the new stuff you put in.

Cracked Exh Manifold: These are really common issues on 4.0 engines, even on Grand Cherokees. The engine is very long and the expansion/contraction amount is great and eventually leads to a crack or multiple cracks. When these occur, exhaust leakage can also screw up the O2 sensor reading, which may result in a hidden poor engine operation source. I've replaced several over the years and re-welded a few only to have them crack again near the welds in a year or three. Last time, analyzing the cracking situation, I took my original 87 manifold and rewelded it, but also welded *stitches* cross-wise to where the normal cracks occur. I was going to weld in a stainless steel expansion bellow, but it didn't arrive in time. This will be the 3rd year and so far no cracks yet.

Those are some things which come to mind. ENJOY!

 

Do you have a budget for engine modification?

For example, if you want to do bolt on modifications, the following is available.
- Freeer flowing exhaust. I used the Borla cat back on my 2001 XJ.
- Exhaust header. I used Doug Thorley on my 2001 XJ.
- 4 hole / 12 hole fuel injectors. You might have to figure out the fuel injector plug / harness. I used the 4 hole.
- Larger throttle body. I used the BBK.

Without a dyno & reading internet lore, I MIGHT have picked up 20 hp, and I feel this number could be on the high side. I did the Borla exhaust, because my original exhaust was giving up the ghost. I did the header on my XJ, because I was replacing my cataltic converters. I figured why not at this point since it was getting torn apart. I did the fuel injector upgrade, because it was only about $100 for me and wanted an easy project at the time. I did the BBK throttle body to complement the previously mentioned upgrades.

For your older XJ, I believe the newer style of Intake manifold can be used.

You could always do a Stroker kit. Somewhere I read, it really cuts down on life expectancy of the 4.0 engine, but I don't know. Internet lore...
You could always do the V8 swap of some sort.

Honestly, spend your money on neglected maintenance first. Figure out your skill level and how long you can afford to have the XJ in non-drivable conditions, as you do your modifications.