My 2008 XJ Super V8 (83k miles) is beautiful, but she is a cruel mistress. Several weeks ago it threw P0171 and 174 - lean in left and right bank. Hopped on the old forum here and got instant answers to potential issues. Great advice and I was about to do a smoke test to find the leak and the the codes went away on their own! I for sure did not delete them with my scan tool. So I figured, great! Drive a week with no check engine light and then they came back! Was all set to order a smoke tester (and I do love an excuse to buy a new tool), BUT the codes are gone again! I'm pretty confident that its not the MAF itself based in some diagnostic testing. With the codes gone, I'm not sure a smoke test will reveal anything. Any ideas on what may be causing the codes to go and come at their leisure? Am I crazy to think that maybe when things warm up really good under the hood the leak is somehow sealing itself (temporarily)? The reason I ask, is that I do a lot of short distance driving (live in a small town where my office, gym, and shopping are all less than 10 minutes away). When I have to travel to the next town over or just go for a Sunday drive and let the engine really get up to temp is when the codes seem to go away on their own. Thanks for any assistance anyone may be able to throw my way.

 

Brake booster vacuum hose under throttle body or the rubber seal between the intake tube and throttle body.

Good Luck

 

The rules for setting those codes require you run the engine within a very narrow range before it runs the test and it will only fail if it runs the test and fails on 2 Consecutive Drive cycles

My car does the same thing only it doesn’t go away is easy as yours does but that’s probably because I’m not running the test very often, but I have the same symptoms where it comes randomly after weeks of driving in goes away and its own sometimes. I still haven’t figured out where it is but it’s probably at a very slight leak somewhere that I haven’t it found yet

 

Hoping you have an easy fix.
When all else failed to cure my problem.
Those eight pesky gaskets inside the intake manifold.
Finally extinguished the frigging light, as well as the codes.

 

Well since I had to the audacity to ask for advice on the issue, the soft amber glow of the check engine light greeted me this morning (and you guessed it, P0171 and 174). Smoke tester is on order so I'll report back on my findings. I will pay close attention to the areas mentioned in previous replies first then go from there. Thanks for the assist.

 

Hello

Well if you have a scanner check out the fuel trims... If you have a vacuum leak you will see high positive LT fuel trims(at idle) and as you accelerate the fuel trims will go down to normal levels. At least you can pin it down to a vacuum leak...BUT... If the fuel trims go UP when accelerating it usually is the result of a bad MAF sensor(a main cause). There is a wide open throttle test to check of the MAF is ok.

Mine was both! small vacuum leak and bad MAF. Each happened at separate times.

Good Luck!

 

Does the supercharged variant have those gaskets?
I finally nailed down my vacuum leaks to those upper intake gaskets, as well
-j.