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Your Jeep Tech Questions Answered

Posted in How To: Tech Qa on April 15, 2019
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Lazy Transmission

I have a 2007 Grand Cherokee with the NAG automatic. Sometimes it goes into reverse nice, but most of the time it takes 10 seconds to engage. Won’t engage, reverse lights won’t come on, 10 seconds goes by, finally, reverse lights come on, there’s a “clunk” and then reverse engages. Five transmission shops, filters, fluid changes, and now just changed the tranny and problem’s exactly the same. Put it in reverse and wait 10 seconds.
Joanne R.
Via email

The W5A580 transmission, also known as the NAG1 and 722.6, is actually a transmission with a pretty good reputation. Developed by Mercedes, the tranny is behind quite a few Chrysler performance cars and trucks. It is popular enough that there is some aftermarket support for it, including standalone transmission controllers and harnesses. We asked around a few transmission shops and really didn’t have much luck finding someone who had experienced the exact symptoms you’re describing. Just about everyone stated that these transmissions are extremely sensitive to fluid type and level, and we did find several reports of the transmission feeling like it was slipping when it’s even just a little bit low on fluid. But surely someone among the five transmission shops the Jeep has been to would have checked the fluid level.

To us a big red flag is that you’ve replaced the transmission and the problem persists. That makes us strongly suspect something other than mechanical transmission issues. Take a hard look at the wiring between the transmission and its computer, and maybe even try swapping the TCM to see if that helps. The fact that the reverse lights aren’t coming on could indicate that the tranny isn’t being “told” to go in reverse when you shift it. Unfortunately, that could be many different things, but at this point we would be looking beyond the transmission. We would also go back to the transmission shop that sold you a transmission since the problem isn’t fixed.

Last but not least, you may want to have a look at the rearend. Though it should be making horrible noises if it’s to blame, the four-wheel-drive WKs are equipped with a fairly complex torque-sensing electronic limited slip that has been known to be problematic. Popping the diff cover and having a look inside isn’t a terrible idea.

Gas Mileage Woes

I have a 2013 Rubicon. I recently put 33s on it, from stock 32s. My gas mileage dropped from about 19 mpg to about 15 mpg. I have 120,000 miles on it and I have read the O2 sensors last about 100k. I have no CEL or anything, but after it sits there is a smell of gas when I start, loss of mileage, and when I get on it the smell of something burning. I was thinking O2 sensors. I also programmed it for 33s (measured 32.75 inches) and the plugs have been changed also. Any suggestions to recoup gas mileage? I was just thinking 32s to 33s shouldn’t drop it that much. The Jeep has 3.73s in it but I was considering going to 4.10s to regain 5th on the highway when it’s windy…it seems to like 4th now.
Ron H.
Via email

We all know that we can expect to see a mileage decrease when adding taller tires, but we agree with you that 4 mpg seems a bit excessive for the small change that you’ve made. Since you programmed the Jeep, we’re assuming that you’re calculating off of a corrected odometer. Our first thought is that it might be a good idea to closely examine all of the fuel lines to make sure you don’t have a leak somewhere. Changing the O2 sensors isn’t a bad idea, but most of the time a bad O2 sensor will trigger a check engine light. With the other symptoms you describe, we would take a hard look at the catalytic converters. Though not well known for going bad on a JK, it does happen, and it doesn’t always throw a code. Inside a catalytic converter is a bunch of honeycomb-shaped catalyst, and when it starts to break down it can actually plug up the exhaust. Not only does the converter no longer work properly, a plugged converter can cause serious performance and mileage reductions. This could also account for your burning smell under heavy load as well as your fuel smell on startup. A plugged converter can be difficult to diagnose in your driveway, but one thing we’d do is pull the downstream O2 sensors (the ones after the converters) and examine the tips. A damaged or dented tip is a telltale sign of a converter coming apart. If that’s what’s happening, then you can expect to find plenty of converter catalyst in the muffler as well.

While you are absolutely correct that changing axle gears will help restore the mileage lost with larger tires, in your case we’re not sure it’s worth the significant cost involved to only go from 3.73s to 4.10s. If you were going to do it, we’d recommend going with 4.56s, especially if you think 35s might be somewhere in the future. If everything checks out on the Jeep mechanically, doing things like adding a free-flowing exhaust system and a cold-air intake can help the engine run more efficiently and improve mileage slightly.

Nope and Nope

I wanted to comment on the many questions and responses for “Death Wobble” that I have read in this and other magazines. One possible cause that I do not recall being mentioned is wheels with incorrect backspacing. My first (and so far, only) experience with death wobble was when I tried to use wheels with much less backspacing than the stock wheel. Scared the crap out of me, as I was just a teenager with very little driving experience at the time.

My question is regarding our 1997 Jeep Grand Cherokee with the 4.0L engine. We have over 220,000 miles on the original timing chain. Should I replace this ASAP? I have been told that this is an interference motor by one person and told that it is not by another. I think that if the chain breaks and the pistons do not hit the valves, then I would not worry about it. If the pistons could hit the valves, then maybe it is time to replace the chain. Can you tell me which is correct?
Jeff B
Via email

Sorry, but wheel backspacing by itself will not cause death wobble. Using wheels with less backspacing can aggravate an already existing death wobble problem, but backspacing will not cause it. Using wheels with less backspacing moves the contact patch of the tires outward relative to the wheel hub, which places more stress on wheel bearings, ball joints, steering components, the track bar, and other components that can cause death wobble, but that’s where the connection between backspacing and death wobble ends.

We wouldn’t worry about the timing chain on your high-mileage 4.0L. While 220,000 miles is a lot for some engines, it’s not uncommon to see 4.0Ls go 300,000 miles or more with nothing but oil changes and regular maintenance. We’ve also never heard of a 4.0L timing chain breaking, and the timing sets will usually last for the life of the engine. The chains themselves are fairly short, so there’s not a lot of opportunity for them to stretch, and they get plenty of oil. In the highly unlikely event that the timing chain does let go, the 4.0L is a non-interference engine, so the worst thing that will happen is having to call a tow truck.

Confuser Location

I’m in the process of swapping a late-model fuel-injected engine in my Jeep, but I’m not sure where I should place the computer. Should I put it somewhere that will keep it protected from the elements, such as inside the cab, or is it OK to put it under the hood? I’ve done a lot of searching, but there’s not really a clear answer.
Richard B.
Via email

Where you need to put the computer depends a lot on the computer itself. While you are somewhat limited by the length of the harness you’re using, the main priority should be whether or not the computer is weatherproof. If the computer doesn’t appear to have a sealed case, such as the computers used in TBI Chevys, then it should be placed in the safest place possible. Usually this means inside the cab, but if you have an open-air Jeep, then we’d recommend putting it up high somewhere under the cowl or in the center console. This will keep it protected from the elements and reduce the chance of it taking a dip if you decide to dive off in a big mudhole one day. Conversely, if the computer case appears to be well sealed, and the connectors have O-rings as well as other weather-proofing features built in like late-model Chevy LS computers, then under the hood is just fine. While mounting the computer to the bottom of the oil pan probably isn’t a good idea, anywhere reasonably high on the firewall is fine.

We like to use the donor vehicle’s computer location as a general rule of thumb. If the original location was inside somewhere, then that’s where you should put it on your rig. If it’s under the hood, then anywhere in the engine compartment is just fine. In underhood situations, it’s a good idea to select a location that’s going to be reasonably clean and dry, and at least several inches away from the exhaust manifolds. Mounting it in a location where it’s going to be subject to heavy spray from the front tires or down low where it could be completely submerged should be avoided if possible. It’s also preferable to mount the computer vertically or at an angle as opposed to flat. The thought process here is that vertical or angled mounting allows any water to run off the computer, rather than sitting where it could eventually compromise a seal.

Older Jeep, Better Ride

I have a 1975 CJ6 that I special-ordered and bought new in April 1975. It has a 360, 3:73 gears and the old narrow springs. It sat for years while we raised four kids, but now it’s my time! I am not a rockcrawler or heavy mountain climber, but dirt roads and tackling some hills is great. I want a decent ride and better handling on the road while maintaining good off-road manners, and I would like to update the Jeep with a new spring setup. I understand that coils and a four-link are the way to go, but I would prefer to keep it on leaf springs. Should I use Wrangler springs or do you have a better idea? It’s my under-standing they ride better and have a more progressive spring rate.
Wayne C.
Via email

We’re sure many of our readers wish they had hung on to a 4x4 from their past, so congrats to you for keeping your somewhat unusual Jeep all these years. CJ-6s make great trail rigs due to their longer wheelbase, and the production numbers were low enough that you don’t see very many of them on the trails these days.

A four-link suspension with coil springs has its advantages, especially in the ride and handling department, but that’s probably not the best option for your needs and uses. First, it would take a whole lot of custom fabrication work to pull off a four-link on one end, let alone both ends, of your Jeep. Aside from the extensive fabrication involved, you will quickly find that the narrow frame under your Jeep (among a few other things) doesn’t lend itself well to packaging a properly designed link suspension without basically throwing everything away but the body and drivetrain, and then starting over. Second, setting up any type of four-link correctly takes a lot of knowledge; there are only a few ways to do it right, and a whole lot of ways to do it wrong. But third and most important in our eyes—you just don’t need to go through all that work. You would be taking away from the cool vintage aspect of your Jeep, and you can easily work with a leaf-spring suspension to deliver the comfortable ride you want while maintaining off-road capability.

Lots of people with early CJs like yours have had great success using 2 1/2-inch-wide springs from ’87-’95 YJ Wranglers. The wider springs, combined with the newer designs and technology that go into them, generally yield vast improvements in both on-road ride and off-road flex. You didn’t mention your desired lift height or tire size, but there are excellent YJ springs available from several suspension manufacturers in a variety of lift heights, so you have a lot of options.

To do the conversion, you are going to need new, wider spring hangers for the frame; wider spring perches for the axles; and wider U-bolt plates with new U-bolts. Both the hangers and the perches can be purchased from shops that supply suspension builder parts and brackets, such as Rusty’s Off-Road (rustysoffroad.com) or RuffStuff Specialties (ruffstuffspecialties.com). You can also make your own with some time, knowledge, and plate steel. Attaching these components to the frame and axles will require welding and possibly some light fabrication, but nothing like a four-link suspension.

Unless you plan on going with some really tall tires, avoid the urge to do a spring-over conversion, as this opens up a whole different realm of things that need to be addressed. A shackle reversal is another popular way to improve ride quality, but depending on your tire size, a reversal can cause clearance problems with the front fenders. If a shackle reversal is something you want to explore, check out what M.O.R.E (mountainoffroad.com) has to offer. If fabbing up new mounts is more work than you want to do, then you can always consult with a custom spring manufacturer such as Alcan Spring (alcanspring.com) about having a custom set of narrow springs built to your specifications.

Missed the Brakes

Why is it you almost never hear about needing a brake system when flat-towing a vehicle? There should be a provision for applying the brakes in the towed vehicle when the driver hits the brake pedal in the towing vehicle, as well as a breakaway system that will apply the brakes automatically in the event the Jeep breaks loose from the tow vehicle. Not only is this required by law in most states, it’s also a pretty good idea. There are various types of systems available to make this work.
G. Blair
Via email

You are 100 percent correct. Though not required in every single state, most states do have some sort of law that requires towed vehicles of any kind (trailer or otherwise) over a certain weight to have a brake system that will apply the brakes on at least one axle of the towed vehicle when the towing vehicle’s brakes are applied. The requirements vary quite a bit from state to state. California requires brakes on anything over 1,500 pounds, while Texas doesn’t require brakes on any trailer under 4,500 pounds. Still others have braking requirements for trailers (cargo), while specifically excluding towed vehicles like a car or truck. But the majority of the states require brakes on anything being towed over 3,000 pounds, and since most of our 4x4s are over that, just about anyone reading this should have a brake system on their 4x4 when flat-towing it. It has been our experience, and those of many others, that these laws are not enforced, but even so, flat-towing without brakes in a state that requires them opens you up to some serious liability in the event of an accident—even if the accident is not your fault. But your last point is the best one: It’s not just the law; it’s a good idea. Not only do these brake systems enable you to stop more quickly, they’re just safer in general. Most of these systems also have a breakaway function that will apply the brakes on the towed vehicle in the event that it comes loose going down the road. We’ve all heard horror stories about that happening.

There are a variety of electric, hydraulic, and vacuum-assisted systems on the market that can be temporarily or permanently mounted to a flat-towed vehicle. We discussed flat-towing equipment and usage in our “How to Flat-Tow Your Jeep Anywhere” article (Oct. ’17), and RV supply stores and shops can help with more information on a towing system that works for your needs and budget.

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