Techline: Your Top 4x4 And Off-Road Tech Questions Answered Here


Whistling Wrangler

I have a ’04 Jeep Wrangler. Every once in a great while the engine starts to whistle. I can’t find the problem. I tightened the two front and rear exhaust manifold lugs with no success. It’s for sure not a squeak. It’s a whistle.

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There really aren’t many things that can cause a whistle under the hood of your Jeep. It’s either an intermittent vacuum leak or some anomaly on the intake tube. You’ll want to start by checking for vacuum leaks. Look for and repair any popped-off, loose, cracked, or missing vacuum lines. If the whistle persists, you’ll have to take the search to the next level. You may be able to find the offending whistle with an automotive stethoscope. Companies such as Harbor Freight ( offer inexpensive automotive stethoscopes. Check carefully around the intake manifold and don’t forget to poke around the vacuum-operated brake booster too.

If you’re still unable to find the leak, pull the plug on the throttle position sensor or some other sensor that will keep the engine in open-loop mode. Start the engine and let it idle. Spray aerosol carb cleaner on and around areas with the potential for vacuum leaks, such as around the intake manifold sealing surface. Keep in mind that this is dangerous—the carb cleaner could flare up if ignited. Use only short and direct puffs of carb cleaner. Don’t hose the whole engine down. The combustible carb cleaner will be drawn into the engine through the leak, and the engine will pick up its idle speed momentarily.

You can also use an unlit propane torch to search for vacuum leaks. A short piece of hose attached to the torch valve outlet will let you poke around more precisely in the engine bay. When the combustible propane is sucked into the lean-running engine, it will smooth out and speed up slightly. Water, lubricants, or motor oil can also be sprayed onto a suspected leak and achieve the same end. You’re looking for a change in engine idle when you spray the potentially leaking area.

Slick Taco

I have a ’08 Toyota Tacoma 4x4. There is a clunk when shifting from 4WD to 2WD. Also, the truck struggles when in Reverse in four-wheel drive. I have to give it more gas to move backwards.

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The two problems you have with your Tacoma may or may not be related. Keep in mind that there is inherently more gear noise and drivetrain slop when the transfer case is in low range. Also, it wouldn’t be unusual for there to be at least some noise during a transfer case shift. However, your Tacoma should not struggle in Reverse. There are a few things you can check yourself. Are the brakes sticking? Is the emergency brake sticking? Is something worn in the axles? You should inspect the brakes and axles by first safely raising the truck off of the ground. Spin each wheel by hand with the transmission in Neutral and the emergency brake off. If they all spin backward freely and you don’t see anything unusual, the problem is likely in the transmission.

If it’s a manual transmission, you may have a slipping clutch, although the clutch would also slip in the forward gears too. If it’s an automatic transmission, it’s possible that the transmission is low on fluid, has the incorrect fluid, or is simply worn out. Check the fluid level and inspect the transmission fluid. If it has metal bits in it or smells burnt, you may need to dive further into the transmission. Reverse is typically the first gear to go out on an automatic transmission, which might explain the slipping that it sounds like you are experiencing. On most transmissions, Reverse gear utilizes less fluid line pressure than the forward gears. The reduced line pressure allows the worn clutches in the transmission to slip.

Merit Badged

I am a member of Boy Scout Troop 148 in Temecula, California. I am currently working on our Communication Merit Badge and need to contact the editor of a magazine. What would be the best suspension option for the ’03 Jeep TJ I have? I plan on using the Jeep off-road in areas like Big Bear and the Sierras. The Jeep currently has a 2-inch spring lift on it and some 31-inch tires. I think I want to run 33-inch tires. Is that a good size?

Craig Andersen
Via email

Thanks for choosing Four Wheeler to reach out to!

The Jeep TJ is a great platform to build off of when assembling a 4x4 for use on trails and street. The later-model TJ Wranglers like yours are the best of the bunch. If the 31-inch tires aren't enough for the type of off-roading you like to do, you might consider stepping into 32x11.50 tires instead of the 33-inch tires. The typical 33x12.50 tires are wide and heavy. They really only offer an extra 1/2-inch of ground clearance over the 32-inch tires, which will be much lighter than the 33s. The lighter, smaller tires won't punish your throttle and braking performance as much as the 33s. Also, the 32-inch tires will fit the Jeep better with your current 2-inch lift. You may even be able to reuse your existing wheels. The 33-inch tires will rub off-road with only 2 inches of lift and may require different wheels than what you have, depending on the backspacing.

Good luck with your Jeep project and your Communication Merit Badge!

XJ Driveline Vibes

I have a driveshaft question that I have received different answers on. I have a ’97 Jeep XJ Cherokee with about 4 inches of lift. I can feel a vibration starting at like 59 mph and it gets worse the faster I drive. I can take my rear driveshaft out, driving in four-wheel drive (front axle only), and the vibration goes away. I’ve had the driveshaft rebuilt. It turns out that they used a smaller U-joint, so I went back and got the correct U-joint, but the vibration remains. Should the pinion of the factory Chrysler 8.25-inch rear differential be pointed at the NV231 transfer case, or should it be on the same degree plain as the transfer case output? I’m not sure how to find the degree at the axle and transfer case. The videos I have seen never really tell you where to put the degree finder or what the angle should be.

I know my transfer case is at 0 degrees. I have a 1-inch transfer case drop. I put 4-degree shims in the rear leaf packs to bring the pinion back down, but then I was told that I should point the axle pinion at the transfer case. I’m trying to keep it cheap for now, what are my options?

Donovan Bone

It sounds like you have successfully isolated the vibration to the rear of the Jeep. Considering your setup, I think it’s safe to say that the vibration problem is in the rear driveshaft. Typically, any lift over about 2 inches or so will cause at least some vibration to emanate from the stock XJ rear driveshaft. With your lift and transfer case drop, you’re at about 3 inches of lift over stock as far as the rear driveshaft is concerned, which is not an ideal setup. The vibration can eventually lead to a leaky transfer case rear output shaft seal, worn output shaft splines, and in some extreme situations, complete transfer case failure. The best way to correct the vibration is to install a slip yoke eliminator kit on your transfer case and add an aftermarket double-cardan driveshaft. On the plus side, you’ll be able to remove the T-case drop and pick up extra ground clearance.

If you are on a budget and want to experiment with what you have, you may be able to decrease or possibly eliminate the driveline vibration with some careful measuring and pinion angle adjustment. To do this, you’ll need equal angles on the upper and lower U-joints of the rear driveshaft. This can be measured with an angle finder on the flat machined portion of the yokes. If the transfer case yoke is at 0 degrees, the rear axle yoke should be at 0 degrees too, or negative 1-2 degrees to compensate for axlewrap under load. Tapered pinion angle shims can be installed between the leaf springs and the leaf spring perches on the axle to alter the pinion angle. However, if you go this route, you’ll need to carefully cycle the suspension at full droop to see if the driveshaft binds. If it does, you may be able to clearance the yokes a little with a grinder and or install suspension limiting straps.

Diff Swap

Is it possible to put the limited-slip differential from the rear axle of my ’06 Chevy 2500 in the front IFS axle? I know there are strength issues, but can I upgrade the axleshafts perhaps? I would then put a locker in the rear axle.

Name withheld
Via email

Unfortunately, it’s not as easy as moving parts from the rear axle to the front axle to get a limited slip. In most cases, the front and rear axles of a 4x4 are vastly different and utilize completely different parts inside. In your case, you should have a 10.5-inch GM 14-bolt full-floating rear axle, or an AAM 11.5-inch full-floating rear axle. Up front is a GM 9.25-inch IFS axle. The rear axle features either a 10.5- or 11.5-inch ring gear and carrier. The front axle has a 9.25-inch ring gear and carrier. Obviously, they are not interchangeable. However, there is a locker available for the ’89-’10 GM 2500 9.25-inch IFS front axle. ARB ( offers an air-operated selectable locker for this axle. You have many locker options for the AAM 11.5-inch rear axle including an ARB Air Locker, Eaton ( Detroit Locker, Yukon ( Grizzly Locker, or a Yukon full spool. The GM 10.5-inch 14-bolt axle has even more aftermarket locking differentials available for it.

The survivability of the front axleshafts will depend on the tire size you choose and your driving habits off-road. If you are concerned about the strength of the stock axleshafts, RCV ( offers Ultimate IFS CV replacement axleshafts for the independent front suspension under GM 2500 and 3500 trucks. You may also want to consider upgrading the tie rods and ends. EMF ( offers a complete tie rod upgrade kit with greaseable, rebuildable chromoly joints and solid 1018 steel links. These are much less prone to bending and breaking than the stock steering parts.

1-Ton Parts Truck

What would be the best truck to buy and get as many good parts to build a rockcrawler with 40-inch tires? My wish list would be a 14-bolt rear axle, NP205 transfer case so I don’t have to buy an Atlas 2, and a Dana 60 front axle. I’m assuming I’d invest in heavy-duty end forgings and kingpin high-steer knuckles for the Dana 60. I’d like to run 5.71:1 ratio ring-and-pinion gears to avoid having to buy a doubler for the transfer case. I’ll upgrade the driveshafts to at least 1350 U-joints. Since my knees are shot, I’d like to have an automatic transmission with a granny-low First gear. Thanks for all you do. I've been an avid reader for many years.

Alfred Ricci
Via email

Given your list of desirable parts, it sounds as though you should start with a 1-ton truck. I am assuming the frame and body will get tossed and the parts will make their way onto whatever rockcrawler chassis you have planned. Dollar for dollar, it will be pretty hard to beat an ex-military M1008 1 1/4-ton GM truck or any of its derivatives. These came with a kingpin Dana 60 front axle, GM 14-bolt rear axle, and the bombproof TH400 transmission. The only hang-ups are the NP208 transfer case, which can be swapped out relatively easily, and the pathetic 6.2L diesel. You can usually pick up a non-running M1008 for less than the cost of the parts. The second option would be the ’77-’91 GM K-series 1-ton truck. This truck would have come with the same Dana 60 front and 14-bolt rear solid axles as the M1008, but could have been had with a gas V-8 engine. Some even have a big-block 454. Most of the 1-ton GM square body trucks came with a TH400 automatic transmission backed up to the NP205, but the NP465 four-speed manual transmission was also available. The TH400 doesn’t really have a granny-low First gear, it’s only 2.48:1, but TCI ( offers 2.75:1 and 2.97:1 aftermarket First gear ratios that can get you a little bit lower.

The Dana 60 front axle has gear ratios available down to 7.17:1; however, the 14-bolt only has gear ratios available down to 5.38:1. With 5.38:1 as your deepest available axle ratio and a stock TH400 First gear, you’ll be looking at a crawl ratio of about 26:1, which really isn’t low enough. Switching to a Dana 60 or Dana 70 rear axle and 7.17:1 ratio axle gears would get you a crawl ratio of about 35:1, which also still isn’t all that low. You may need the transfer case doubler to get the crawl ratio into a suitable range. You’ll want to aim for at least a 50:1-60:1 crawl ratio with an automatic transmission for extra-heavy rock use. A low crawl ratio like this will give you the slow speed control you need in the boulders and help keep the slushbox transmission from overheating.