Does your 4.0L idle very rough once its up to temp? Or maybe after you shut it off and start it back up before it’s had a chance to cool on a hot day? I have been fighting with an idle problem on my 2005 Jeep Wrangler TJ Unlimited (LJ) for quite some time. In the past 6 months or so it’s finally started throwing the codes P0171 and P0174. Most of the time it happens when I’m out on the trails on a hot day, and after the engine has been hot for a little while, if i shut it down (like to eat lunch for example) and start it back up before its really cooled down. It’ll idle very rough, and in some extreme cases actually stall out. It does also happen on the road though. It left me sitting on the side of the road one hot day this past spring. I had to have the LJ towed home by AAA.

jeep wrangler idle air controler and map sensor locations
jeep wrangler 4.0l engine with IAC idle air controller and MAP sensor hilighted

These codes translate to a lean fuel condition in the injector banks. Usually when the codes are triggered, I get both of them at once. This tells me that either I have issues with injectors in both banks, or the problem is in something that controls the injectors. Or i suppose it could be the fuel pump… My engine needed a little maintenance anyway, so the first things I did were a quick tune-up (plugs and boots) and i ran a bottle of fuel injector cleaner through it. This seemed to help, but the problem wasn’t gone.

So, the next recommendation I found online was to replace either the Idle Air Controller (IAC), which is a valve on the throttle body that does what the name suggests, controls the amount of air at idle. OR Replace the manifold absolute pressure (MAP) sensor. Both of these modules live on the 4.0L’s throttle body, and are actually relatively easy to get to. All it took was a flat headed screw driver, and torx T20 and a little patience. These sort of things are best demonstrated in video, so I made the following video to show how to replacement goes.

I don’t drive the jeep a whole lot, so the jury is still out on whether the issue is fixed. I will say that on my test drive, it behaved pretty well, which is a good sign but not enough to convince me that it’s fixed yet. I will report back once I have some confidence in the repair.

Thanks for reading!

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