ronjenx

It helps with cooling on the highway, but you won't notice it is gone until you are approaching the limits of the radiator's ability to do its job (high speed, towing, etc.).
Some people who have removed it have said their mileage went down.

justinstoffregen

i noticed slightly more hood flutter and no real difference in mpg. i have never overheated either.

TINMAN080

At least one hydolocked JK guy swears his engine damage happened because he had removed the shield. When blowing thru any water at all with the shield off, it has a tendency to fly up and hit the underneath of the hood, then back down into the fan assy and the air cleaner intake. I don't know of this personally, call it an Urban Legend if you will, but gosh I'd hate to wipe out an engine by unintended water getting into the air intake because I removed that little panel....

GreytSilly

Ask and you shall receive. Stock X with Rubicon wheels and tires. I haven't noticed any difference with it off. My mileage has stayed the same with and without the plastic crap.

TEEJ

Mine came off, and no difference in MPG was noted, well, it went up a bit, but that might have been due to break in of the engine, etc.

I DO see the extra splash up to the hood underside though, but primarily on the side opposite of the air intake luckily.

I plan on a full set of skids, so, the 3/16" steel should work at least as well as the flimsy plastic crap to block splashes.

racer83l

mine is long gone. no notice with anything really, my first few wheeling trips I went on, It was still on the JK and my engine got real dirty with it to compare having it to not having it, Id say take it off if you wheel the JK and leave it on if you dont. Id rather it be home somewhere then in the woods.

Mark Doiron

The JK wheels fine at 105+ degree temperatures (been there, done that, it does fine). It seems to me that rock crawling at a couple miles per hour in those temperatures would be more demanding on keeping the JK cool than cruising down the highway at 70 MPH, even when towing (also been there and done that).

Mark Doiron

Cheap fix: Two 14" zip-ties.

Mark Doiron

Hood flutter happens even with the air dam on. I had plenty of it before removing mine during a 5K mile trip in 2007. Hood flutter is easily fixed -- see my other post with an image of how to zip-tie the latches.

Widewing

My take on the under-tray air deflector is that it was fitted for a purpose. All Jeep designs are wind tunnel tested for various attributes. These include drag and drag reduction, air flow through the radiator, under-flow, lift, etc. and so on. Virtually every vehicle on the road has been wind tunnel tested or at least subjected to complex aerodynamic computer modeling.

If you have ever examined a photo of a current Formula 1 race car, you will see a large number of small, seemingly insignificant aerodynamic devices tacked on the bodywork of the car. Nonetheless, each one of these tabs, lips, and fins is a part of the total, just like every stud in a load bearing wall is important to the overall strength of the structure. Removing one or more studs may not obviously change the function of the wall, but rest assured that the remaining studs are now compromised due to excess load stress. Likewise, removing one of the aero devices from the Formula 1 racer will change the aero balance of the car. The layman may not see the importance, but it exists nonetheless.

Examining the under-tray of my JK, I can see some obvious reasons for it being there.
1. It directs and smoothes air flow down below the front axle. Without it, air flow will become disorganized, tumbling and allowing for increased aero drag.
2. It also aids in faster, cleaner under-flow, which reduces aero lift.
3. It helps to create a low pressure area behind the radiator, increasing flow and decreasing under-hood air pressure.

If you drive your Jeep at relatively low speeds and don’t tow anything on the highway, you may not see any significant change with the under-tray removed. However, if you spend even a moderate amount of time on highways, especially towing on highways, you may see a decrease in fuel mileage and a probable increase in coolant temperature.

My advice is simple. For most driving and up to moderate off-road work, leave the under-tray in place. If you plan on doing some serious wheeling, crawl underneath and remove it. Afterwards, reinstall it again.

If you’ve lost it or damaged it, order a replacement.

I’ve been in Engineering for a long time. One thing I came to recognize early on is that many people make changes to vehicles without a shred of understanding how those changes will effect the dynamics of their vehicle. Just because they don’t see an obvious difference doesn’t mean there isn’t any. Not having any scientific basis for comparison, they assume too much.

You can rest assured that extensive engineering study went into designing and testing the JK’s under-tray. It is installed for a reason, perhaps beyond the understanding of the layman, but without doubt the reason is valid for general use of the vehicle.

In short, if you don’t know what it does or why it is there, don’t dick around with it.

My regards,

Widewing