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February 2006 4x4 Tech Questions - Techline

Posted in How To: Tech Qa on February 1, 2006
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Address your correspondence to:
Four Wheeler
6420 Wilshire Blvd.
Los Angeles, CA 90048-5515.

All letters become the property of Four Wheeler, and we reserve the right to edit them for length, accuracy, and clarity. The editorial department also can be reached through the Web site at Due to the volume of mail, electronic and otherwise, we cannot respond to every reader, but we do read everything.

Question: I have a '77 Jeep CJ-7 and want to know if it is possible to put a diesel engine in it.
Clint Kartchner
Mule Creek, NM

Answer: Anything is possible, but not always practical. Jeep has actually used a diesel in Jeeps before. From 1961 through 1969, they used a Perkins four-cylinder with 192 cubic inches (3.15 liters), which was rated at 75 hp and 114 lb-ft of torque. I don't think that many were produced for the U.S. market, as I have only ever seen two-and one of those was on the Jeep assembly line in 1964. In the early '80s (and maybe longer), Jeep offered in their overseas market another four-cylinder diesel in the CJ line but it was never imported to the U.S. Then in 1986-87, the Cherokee line had a Renault aluminum-block and aluminum-head four-cylinder turbocharged diesel. This was a 126ci (2.1L) motor which made 85 hp and 132 lb-ft of torque. Again, not a very popular option, so finding one would be quite difficult. I have heard of and even seen a few conversions using a Nissan six-cylinder diesel, but can't tell you much about them.

In Montana, there is a flatfender Jeep built by Advance Frame Works (406/538-4996, that has a Cummins four-cylinder diesel in it. While it's a great engine, it does weigh in at over 900 pounds! Some special considerations have to be made in regards to this extra weight as far as frame, suspension, and axle use are concerned.

If any of our readers have any information on diesel swaps for Jeeps, I sure would like to hear from them on the subject.

Question: One of my favorite columns is Willie's Workbench. In the November '05 column explaining carburetor sizing, there is a small error in the cfm calculation: Half the engine size times the maximum rpm (corrected for units) should be multiplied by the VE (%), not divided by the VE. Dividing by a number less than one (1) will increase the product. Here, a VE less than unity should decrease the estimated airflow of the engine. This is properly calculated by multiplying by the VE. Willie's example gives the correct value if multiplied by the VE (i.e., 665 x 85% = 565, not 665 / 85% = 565 as written). On the other hand, 665 / 85% = 782.

Another factor that I've found important, other than shoe size, is the size and type of air cleaner used on the carburetor. Since many designs can reduce the effective airflow through the venturis, a larger carburetor may be necessary to achieve the cfm predicted by the calculation. As Willie said in his limited space, there are many variables, some easier to quantify than others.
Doug Taylor
Rio Rancho, NM

Answer: Thanks for the good catch. I went back and looked at the original copy, and while I wish I could blame the error on the copy editor who may have changed the "multiplication" for a "division" sign, there is no one to blame but me. I screwed up. (OK, maybe my wife who typed up my scribbles did it wrong. Yeah, that's who I can blame!) Actually, I should have caught it when I proofed it before I sent in the story.

Hey, I like it when readers catch my mistakes and keep me honest, but I sure hope my mistake didn't cause someone to buy a carburetor that is way too big for their engine.

Oh, and you are so very right in that the air cleaner can have a major effect on airflow, especially those that drop the filter down slightly below the carburetor's air horn where the air has to make a turn to get into the carburetor.

Question: If I get a 4.5-inch lift and 2-inch coil spacers on my '89 Jeep Cherokee, will I have to get new upper control arms too, or just lower ones?
Cody Hansen

Answer: While it can be done-and I am sure some people have done it-I don't think that using a 4.5-inch lift and 2-inch spacers is a good idea for several reasons. A lot of people will stay with the stock arms with a 3- to 4-inch lift for mild four-wheeling. However, the aftermarket provides stronger tubular arms because they are a lot stronger and resist bending like the stock arms will over time.

Another problem is going to be ride quality. With that much lift and "short," stock-length control arms, the angle of the arms is going to be very steep. Take a minute to make a simple drawing of the control arms in relationship to the ground in stock ride height. Note how they are parallel to the ground. Now raise the body up 4.5 inches and note the angle of the arms. Pretty darn steep. Take your compass, putting one end at the arm attachment point on the unibody frame and the other on where the wheel would be. Draw the arc that the wheel will now travel upward in. You can easily see that the wheel has to move forward as it goes upward. When the vehicle is going forward and you hit an obstacle, the wheel also has to go forward when the suspension tries to compress. Talk about forcing something in the wrong direction! Not only does this greatly affect ride quality, it also puts a lot of stress on all the mounting points.

My personal opinion is that 3 to 3.5 inches is just about maximum lift height with stock-length arms. Any more lift than this, and you had better look into a "long-arm" suspension kit. The long control arms reduce the operational angle so that it's not perfect, but much better than the stock arms with a given amount of lift, and allows so much better articulation without putting stress on the mounting points.

Question: I bought a '98 Jeep TJ used with a four-banger and automatic. It now has a vibration between 40 and 50 mph. We changed the tires because it had some cheap Wally World tires, and also changed the U-joints on the rear. A mechanic checked it, but could not find nothing. Could the driveshaft be out of balance? If so, would a centramatic balancer help? Or do you know of any problems that the Jeep could have to cause a vibration? If you let up, or give it gas, it will quit for a while.
Barry Bivens
Leoma, TN

Answer: Vibrations can be hard to identify, especially with the limited information you have given me. While you changed tires, you didn't say if you changed rims, but hopefully the technician who balanced your tires would have mentioned if the wheels were bent or if he had difficulties balancing them.

You also failed to say if the Jeep had any type of lift on it or not. A suspension lift kit will change the driveshaft angles, and unless corrected, these can very much cause vibrations. Depending on the amount of lift, you could simply lower the transfer case to obtain the proper angle, or better yet, buy one of the slip-joint eliminator kits that are on the market. This will gain you several inches of driveshaft length, which in turn will reduce the driveline angle. The problem with dropping the transfer case is that it then puts the front driveshaft at the wrong angle and you could get some vibrations from it.

The vibration could also be coming from one of the front axleshaft U-joints or a bad wheel bearing in one of the front hub assemblies. Even a bad bearing in the front or rear differential can cause a vibration.

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