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July 2006 4x4 Tech Questions - Techline

Posted in How To: Tech Qa on July 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 own a Jeep Liberty with the diesel engine. I tried to order it with the five-speed manual, but had to settle for the automatic. While it's not what I consider a real four-wheeler (I have a CJ-5 for that duty), it's a great commuter to work. I'm a ski instructor.

What I would like is a bit more power, as it just never seems to be in the right gear. Any thoughts?
Jimmy Long
Reno, NV

Answer: We will have to agree with you-it does seem to lack power right after a gear change. At one time we even considered a Liberty as a project vehicle.

During our initial investigation into increasing power output, we uncovered several companies who were in the process of developing "black box" computer modifiers to increase power. TS Performance (270/746-9999, has its plug-in Powerplay MP-8 now out, which the company says will make 50 more horsepower and 100 lb-ft of increased torque.

Question: You guys are my last resort. I have read articles on 4x4s in your magazine and others, called Ford directly, asked a local mechanic that works on a lot of 4x4s, and none could answer my questions. I have an '02 Ford F-150 FX4, with rear limited-slip. I put a 3-inch body lift on it to fit 35-inch Pro Comp XTerrains.

Will both front tires spin at the same rate even if one is completely off the ground? Second, in a stuck situation that I was in recently, I had my left front tire on the ground with the truck frame stuck on a 3-foot embankment. One of my back tires was on the ground but way up in the wheelwell, and the other rear was up in the air. The one in the well was not turning-I opened the window and looked-and the other on the passenger side sounded like it was just spinning in the air. I thought the limited-slip should have prevented this? You are my final hope at understanding.

Answer: Let's see if I can clear this up for you. The differential, as its name states, is made up of a case, sometimes referred to as a carrier. To this carrier is mounted the ring gear, which is driven by the pinion gear, which is directly connected to the driveshaft. Within the carrier are two sets of gears. One set rides on each axleshaft, and the other set is mounted to the case. When the case turns, the differential gear turns the axle gear, which in turn drives the axle and, in turn, the wheel. The design is such so that when you go around a corner, one wheel can turn faster than the other to make up for the radius of the turn, or differentiate the two different wheel speeds.

The problem is the differential also will send power to the wheel with the least amount of traction, as it can't tell the difference between this and going around a corner. This means if one wheel is off the ground, as you experienced, that wheel will spin freely. Now, in the rear axle where you have a "limited-slip," the same thing happened. Surprise? No, not really. Limited-slips really don't work all that great. They still have the differential gears, but there also is a series of clutch packs that put pressure on the gears through various methods when a spinning tire is detected, and try in somewhat of a ho-hum manner to send power to both wheels. They are designed to limit tire slippage to some degree while not causing any undesirable driving characteristics. As you found with one wheel in the air and the other on solid footing, the tire with the traction got no power, and all of it went to the free wheel. The same thing will happen with one tire on ice and the other on dry pavement. One way you can cheat and fool the limited-slip to help out a bit is to slightly put some pressure on the brakes. You can do this with the foot pedal or with the emergency brake. Still, factory installed limited-slips just aren't worth the money you had to spend as an option.

Auburn makes a clutch-pack-type differential that is claimed to have the best of both worlds-that is, of a clutch-pack-type limited-slip and a true locking diff. Also on the market is the totally mechanical Detroit Truetrac differential. This uses some special-cut gears that allow normal differential action under turning, but will lock up so that both wheels get power sent to them when needed. It has been my experience that sometimes it's necessary to add a bit of brake pressure to get this to happen under certain situations. The Truetrac has none of the harsh driving characteristics that some people associate with its famous big brother, the Detroit Locker. The Detroit, and others of its type, lock both axles together until you make a turn, such as going around a corner, and then release but will apply equal power to both wheels when in such a situation as you encountered.

Question: I recently bought an '01 Ford pickup with the 5.4L engine. It seems to run really well, but coolant seems to just disappear, though I can find no leaks. Apparently, the first owner had the same problem and that's why I bought it so cheaply.

I pulled the cylinder heads looking for a leaking head gasket, as per information from my dealer's service department. There was no bad head gasket or visible cracks in the heads or the block. I put it all back together hoping for the best, and it still has this mysterious leak.
Marshal Spiek
Little Rock, AK

Answer: It took me a bit of research to find an answer for this one, but I like challenges. This is what I came up with.

First, it could be a crack in the cylinder head, not visible to the eye, which only opens up enough when the engine is warm. But I also found that there is the possibility of some very small pinholes in the head casting under the exhaust valve seat. The coolant leaks out here and is vaporized as it leaves the tailpipe. It's such a small amount that you never notice any visible vapor. It's too bad that when you had the heads off, you didn't have them pressure-tested-the leak would have most likely shown up.

There are three solutions, the best being to buy some new cylinder heads. Second best is to again pull the heads and remove the exhaust seats, weld up the hole and do the machining for a new insert. However, before I went to that much work, I would try a can of KN Block Sealer. I've had remarkably good luck using this for various mystery coolant leaks in the past. Be sure to follow the instructions as printed on the can. It just may work and if it doesn't, you're only out a few bucks.

Question: I've got an '83 Ford Bronco with a 351 Cleveland engine and C6 box. I'm rebuilding and modifying it until it's the truck I want. I've already fitted 35-inch Mudders, given it 5 inches of suspension lift and fitted a rear Detroit Locker. I'm currently rebuilding the C6 and making a reasonable amount of mods to the engine, such as an aftermarket carb, a Weiand Accelerator manifold, extractors, and a cam that makes power from 2,500 to 5,500 rpm.

My problem is whether or not I should fit a 2,500-rpm stall-speed torque converter. I'm not sure if the benefit of getting my revs into the correct power range will be outweighed by the problems caused by more traction loss. I'm currently running 4.11:1 gear ratios but plan on dropping them to around 4.55:1 in the future. I live in Carnarvon in Western Australia, which is about 500 km (310 miles) away from another town. Most people run stock four- or six-cylinder diesel Toyota or Nissans with standard manual gearboxes and minor suspension mods, so it is hard to find someone out here who knows much about real 4x4s with decent power/drivetrain combos that actually work.
Ben Jansen

Answer: That sounds like a nice mid-range cam with most likely a max rpm limit of about 6,000 rpm. Hopefully, it's not something designed for a circle-track racer that never sees rpm below that. If that is the case, and you're trying to build a trail machine, I would say reconsider the cam choice, or better yet, pick up the phone and talk with the manufacturer of the camshaft and see if this is really the cam you want to run. Most likely, they will also give you some clues as to what is the proper stall-speed converter to use. Just remember that the converter is going to be slipping at any rpm below that 2,500 figure, and most of the time on the trail you are going to be below that unless you make a point of keeping the rpm up, either out of necessity for power or to prevent excessive slippage. Slippage makes for heat, and heat is a destroyer of transmissions. For instance, I have seen a drop of over 50 degrees of fluid temperature at highway speeds between full lockup and non-lockup on a lockup-style torque converter, and that is only about a 200- to 250-rpm difference. With the 35-inch tires and 4.11:1 gears, you're just going to make the lockup point at 65 mph, so the 4.56:1s would definitely be a good choice to swap to.

Also keep in mind that just because the torque converter is rated at 2,500 rpm, torque characteristics of the engine can alter that rpm. Even the axle ratio will affect the load that the converter sees and change the stall speed. Under full-throttle acceleration, the stall speed will be higher under a heavy load, such as is generated by a heavy vehicle and a high gear ratio, compared to a light vehicle and a low gear ratio. For instance, this 2,500-rpm stall converter in your Bronco with the 4.11:1 gears and 35-inch tires may turn into a 3,000-rpm converter. However, in a lightweight roadster like a Cobra with 28-inch tires and 4.11:1 gears, it becomes a 2,000-rpm stall converter.

Picking the right torque converter is really a hard decision so my best advice is to spend some time on the phone with both the cam manufacturer and torque-converter manufacturer's technical department and go by their recommendations. You want to be sure to tell them the application, engine specifications, axle ratio, tire size, and vehicle weight.

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