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Controlling Underhood Heat on a Jeep Wrangler 4.0 – Part I

Controlling Underhood Heat on a Jeep Wrangler 4.0 – Part I
By Patrick 18 days ago No comments

The Jeep 4.0L inline-six was the last true Jeep/AMC engine design before Chrysler bought everything and folded it into the Pentastar family. It was the final derivative of the old 4.2 inline-six and debuted in the 1987 model year. It’s last year in an American Jeep would be 2006, when the TJ Wrangler ended production. But you’re not here for a history lesson, so for the complete background on the 4.0L click HERE.

What does this have to do with heat?

The 4.0L is a massive hunk of cast iron American greatness. But all that cast iron means it’s a giant heat source under the hood, and once it gets hot it holds that heat for a while. And everything around it underhood soaks up all that heat, which can negatively affect performance, reliability, and component life. But with some application of Heatshield Products thermal barriers, the amount of heat being radiated under the hood can be kept in check, and more sensitive components protected to make sure performance isn’t hurt nor does reliability suffer.

A friend recently picked up a very low mileage (only 18k on the clock) 2006 Jeep Wrangler Unlimited with the sturdy 4.0L for motivation. One of the first things we noticed when checking it out was how hot the hood was after the Jeep had been running, and the hood prop rod was scorching hot when we opened the hood. Didn’t take more to tell us it was time for some Heatshield Products for this Jeep.

Part one of our blog will cover the different temperature readings we took of the Jeep with our trusty laser temp gun. In part two we’ll show the difference after we’ve installed some different thermal barriers to help things out.


We started the Jeep and drove it some to get it up to operating

temp before taking some temperature readings.


The air intake tube was reading 135 degrees F, and within a couple of degrees across the length of the air intake tube.


The fuel injector rail read 183.5 degrees F. That’s pretty hot, and a lot of heat that is inevitably transferred to the fuel flowing through the rail, even while the engine is running.


Yes, the upper radiator hose is going to be hot we know. Why we took this reading was to verify the accuracy of the Jeep’s temp gauge, and to go with our next picture.


This is the high pressure line for the A/C system. It rests right on top of the upper radiator hose. Besides needing to do something about the direct contact so neither hose or line was damaged, the heat transfer between the two could hamper the A/C system’s efficiency. We’re going to address that with some thermal barrier sleeving and take measurements to see the difference it makes.


The airbox was reading 180.5 degrees F. Definitely need to install something on the box to shield it from underhood heat. As the cooler air flows through the box, that heat will get transferred to the air, raising the intake air temperature which can hurt performance, especially on an already hot day like we were experiencing. We’re going to regroup, figure out which products we want to install and where, then take some after readings and see what we’ve got.









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