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April 2011 Techline

Posted in How To: Tech Qa on April 1, 2011
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Wrangler 4.0L Gets Poorer Mileage, Less Power
Q I am the original owner of '98 TJ with the 4.0L Six and auto trans, 105,000 miles and all stock except for the header that replaced a cracked factory piece a couple years back, and 31x10.50/15 tires. It started getting poorer gas mileage and then developed a hesitation that built into a popping (on the intake side, I think) with a loss of power on take-off. I took it to Chrysler/Jeep dealer; their first diagnosis was it had the wrong plugs in it (too short?), so they changed them. After a few miles it did the same thing, so I went back to the dealer. The second diagnosis was the catalytic converter "breaking up" inside and plugging the exhaust (there was a rattling sound in the converter, and, they said, not enough exhaust coming out of tail-pipe). Now the Jeep has a new cat and after-cat exhaust, and on cold take-off, it intermittently shows the same symptoms. After spending almost a thousand dollars at the dealer, I fear letting them throw more parts at it in hopes of getting lucky. Gas mileage does not seem to have improved, and I'm not sure I have full power. The air cleaner and box and tube are clean and not plugged in any way; and vacuum lines and connections look good and are not cracked. The spark plug wires are two years old (best from NAPA) and look good also. The wife is on me bad to trade in my baby for something new, but says I can't have the new Mustang GT, so what do I try next? (P.S. Must stay with the wife, she makes the money around here.)
Wilford Mangold

A There are so many things that could be wrong. The first thing that I would do is to check to see if there were any factory service bulletins dealing with your problem. I checked and found none. Doesn't the dealer have a scan tool? You made no mention of them checking for trouble codes. With poor gas mileage and lack of power and hesitation, there is a pretty good chance that some type of a trouble code would show up on their scanner. Hopefully, when they changed out the spark plugs, they also checked out the rest of the electrical system. Like you said, I also think that they are just "throwing parts at it" in the hopes that something will work.

I can only suggest that you take it back to the dealer, or perhaps an independent garage, and have them hook up a scan tool to see if there are any trouble codes. If they can't determine the problem from this, then perhaps it's time to do a compression test and/or a "leakdown" test to determine the internal condition of the engine.

I would be guessing from here, but my first inclination is that you have an intake valve that is not seating properly, hence the popping from the intake side, but there could be a lot of other issues that only the proper testing will uncover.

Info on Positive-Ground Electrical Systems
Q I recently bought a battery tender for my wife's car and my four wheeler. While reading the instructions, it described how to set the hookups for both a positive- and negative-ground electrical system. Now I know that the industry standard for all cars and trucks is to have negative-ground electrical systems, so why do they include details for a positive ground? Also, what would a positive ground be used in, and why?
Barry Grable
Hattiesburg, MS

A Back in the old days when I was growing up, cars came both ways-that is, with positive and negative grounds. In fact they were built that way until about 1955, when most major automakers switched over to negative-ground/12-volt systems. However, a lot of British and European cars still held on to positive grounds until about 1965 or so. I think the main reason for the switch was that the carmakers, at least those in the U.S., got together in one big meeting and said, let's set a standard.

Some electrical engineers believed, but never could totally prove, that cars with positive grounds suffered from increased body corrosion in relation to the opposite polarity. This well may be true because anodes have an oxidation reaction, and cathodes have a reduction reaction. In a battery, the cathode is the positive terminal. If you notice, most of the corrosion on your battery terminals occurs at the positive terminal; this alone gives some positive speculation that this may be somewhat true.

Now there are also a bunch of other theories as to why the positive-ground system was used in the first place, one being that the British had some theory that somehow electricity flowed better in that direction. But as for the reasoning why anyone thought this was so, I can't find anything plausible that sustains that theory. However, for years a lot of vehicle manufacturers figured that positive grounds were the way to go. Perhaps someday you will run across an older vehicle that needs its battery charged, and you can be the hero with your new charger.

Speedo Recalibration Options
Q I am looking into purchasing a used Jeep Wrangler-either a TJ, LJ, or JK. I hope to install a small lift to accommodate larger tires, probably 33s. I know there are a variety of axle gear ratios available in these models. The Rubicon package includes 4.10:1 gears. If I were to add a lift and larger tires to the Rubicon with an automatic transmission, would the transmission need other modifications to ensure the vehicle has plenty of power for the highway and shifts correctly? I do not want the automatic transmission to have issues searching for the right gear.

With a manual transmission, I don't see any issues, but I don't know about what problems I might cause with the automatic transmission. I want to save money and not have to re-gear the axles. Besides changing the speedometer gear to match the tire size, will I need to purchase a programmer also? This vehicle will be a daily driver and see much more street use than off-road use.
Dustin Searcy
Tulsa, OK

A Yep, you're right-you will have to correct your speedometer reading, and you cannot do it with a speedometer gear change. You have two choices. Compare your speedometer reading with that of a GPS unit, and keep track of the percentage difference and do the quick math in your head when you want an accurate reading. The second is to buy a controller that will fix the problem. Superlift ( has what is called a TruSpeed box, and Pro Comp ( has a Power Pro Performance tuner that will not only correct the speedometer but also allow you to adjust shift points on the auto trans and even get about 13 more horsepower out of the engine. Your transmission will do just fine as it is.

Where To Write
Address your correspondence to: Four Wheeler, 831 S. Douglas St., El Segundo, CA 90245. All letters become the property of Four Wheeler, and we reserve the right to edit them for length, accuracy, and clarity. The editorial department can also be reached through the website at Due to the volume of mail, electronic and otherwise, we cannot respond to every reader, but we do read everything.

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