Many have asked what upgrades can be done on the 4.0 to increase performance. There are a number of companies that now provide performance products for the 4.0. My plan is to cover parts that not only enhance performance but provide strength and durability as well. I will also try to cover upgrades to systems or parts that may be considered weak or will provide for better function. In the end I hope to increase the performance, strength and durability of the engine without sacrificing gas mileage or daily drivability. Most of these upgrades, if not all, should be viable for any type of motor.

Basic Building Blocks

I had originally gone through and done some simple upgrades. In a previous article I had converted the stock air box over to a custom K&N cone filter. Additionally, I had installed Split Fire spark plugs, a set of Accel 8.8 plug wires and replaced the muffler with a FlowMaster and the catalytic convert with a high flow Dynomax replacement unit. This is the basic foundation that I will start with. Additionally, let me state that I feel very strongly that a free flow exhaust and intake system are a must. Without it any improvements from the changes you make will be diminished. You need to remember that the standard internal combustion engine is nothing more than an air pump. It must be made to breath in and out properly.

Clean Up

I like to start from a good clean base line. That clean base line would be a clean engine. If your 4.0 has a hundred or so thousand miles of work under its belt, its gonna be nasty. So before we get too deep into the grime, its best to clean things up. This will also help you determine where you need to replace gaskets and such as you go. I normally take mine to a local car wash and use either their engine cleaner or buy a can of Gunk. If your unfamiliar with Gunk, just get a can, it really works well. Cover any exposed intake ports. In my case I wrapped the cone filter with some aluminum foil. Spray the engine good with the degreaser and let it sit for a while, to eat at the oil and grime. Then spray it down good. Make sure that you get all the over spray off of painted parts.

After the engine is spick and span, you should take a good look at all of the vacuum and other rubber hoses. Replace any that look worn, cracked or bleached. Check over your electrical cables to see if you need to address any of these as well. This is a good time to get all hoses, belts and wires in shape so that any added horsepower does not cause you any sidebar problems.

Electrical Basics

As I stated earlier, the only electrical changes I have made to date have been the addition of Split Fire spark plugs and Accel 8.8 plug wires. During this project I will be cleaning up a lot of the accessory wiring that I have previously run. I will also be checking all of the existing wiring for problem areas. Keep in mind that you may need to be somewhat adept with a voltmeter to verify that all of your wiring is up to standard. Get an idea of what your plans for wiring are and pickup some flex duct to run your wiring in. This stuff now comes in colors and is available from Summit Racing (1-330-630-3030), and its made by Taylor.

Engine Venting

Ok, this may by my pet peeve about the Cherokee, but I don't believe that the engine compartment is properly vented. Therefore, it builds quite a lot of heat. This along with, again my opinion, a weak cooling system adds to the Cherokee's overall overheating problems with the earlier models.

Step one to correct this problem was to cut a hole in the hood. Yep, you heard it right, I cut a hole in my hood. The hood grill from an '88 Wrangler (part #55025400) makes a great hood vent for the Cherokee. Of course many of you might be a bit leery of cutting a hole in your hood. Luckily, Custom Jeep of Carrollton has a plasma cutter. These things are slick. They will cut the sheet metal easily and hardly even mar the paint.

This is a fairly large vent and has accounted for an almost 10 degree difference in operating temperatures. With my Cherokee operating at an average highway temp of 220 degrees, this is a significant help.

Valve Cover Replacement

We have discussed the problems with blow by on the 4.0 quite frequently on the pages of Cherokee America. In a never ending attempt to correct this problem I have taken the cure one step further on early model Cherokees. Late model valve covers ('94-'96) have twist in crank vent tubes. These tend to give a more positive contact surface and my hope is that the later model integrated PCV valve in the rear vent tube works better than earlier model units.

I first had the valve cover bead blasted and powder coated. I then painted the ribs on top to contrast the new paint. This is not necessary and you can buy these valve covers for around $90 brand new from a dealership. I just wanted to be different. After all of the painting was done, I cut new internal gaskets. The inner vent tubes need to be removed from used valve covers and cleaned. This will ruin the original gaskets. You can use the metal insert as a gasket template and cut new ones. I then purchased a new Felpro valve cover gasket and the new vent tubes from my local Jeep dealer. You will have to fashion some vacuum hoses to work, but this is not difficult. At this point I also took it one step further and replaced all of the valve cover bolts with stainless steel allen head bolts. The end result is very clean looking.

In a future issue I will let you know if this has cured my blow by problems. I will say that the new valve cover looks really great and has spruced up the engine compartment quite a bit, the vent has reduced operating temp's and the overall working environment is much cleaner. We're on our way to better performance!