I have a '00 XJ8 that is running perfectly and getting about 19 mpg combined city/highway. I posted about a CEL read as P0174 only a couple of months back. My Jag mechanic diagnosed a vacuum leak and replaced a hose, I believe the breather hose but maybe not. He warned P0171 and P0174 are very common for X308's and it may come back but if it's running fine and good mpg, it's probably not a concern. About one month after the repair, the P0174 came back - again only the single code. I cleared it with my code reader a few times since then and it comes back a few days later with the same restricted performance message.

I'm inclined to ignore it. Am I being foolish?

 

I use to see it every three or four months in my 08 RRS/SC. Always, a can of Gumout's Best or Sea Foam would clean up the injectors and the problem would be gone for months.

Suggests to me that an injector (or injectors) is spitting instead of spraying. I would try and treat it chemically then hunt down air leaks (generally from air filter to throttle); also clean the MAF sensor.

 

Ignoring it may cause damage e.g. to cat conv but yes you can ignore it if you don't mind.

 

yep go ahead and ignore it. (sarc) because a lean mixture creates a blow torch inside the cylinder and how piston get melted and knock from high cylinder temps which then can also damage pistons/ bearings. a partially lean code say up to 10-20%% is not a big deal(Do you know what percent the engine sees? you have a scanner, most can look at fuel trims) as the engine will increase the injector duty cycle. but as it gets too lean it cannot comphensate enough and mixture becomes truely TOO LEAN. This is why it will then kick on a restricted performance. restricted performance is the cars way of saying YOURE POTENTIALLY DRIVING ME WITH A DAMAGING CONDITION, QUIT IGNORING ME. Kinda like getting a SMALL pain in your chest and saying I feel fine and ignore it. Then your gonna get(IF lucky) a heart attack(restricted perf)> WHich is our bodies way of saying quit ignoring me! You might not survive the next warning if you keep ignoring.

 

Brutal is correct. You can ignore it, but at a price.


This link will help fix it.
https://repairpal.com/OBD-II-Code-P0171-and-P0174

 

You could try cleaning the MAF (with MAF cleaner), and change the fuel filter. Do a smoke test, if you can, if the issue is still there. I had this problem for a while until I found an air leak and replaced the intake manifold seals. That's not a terrible job on a naturally aspirated X308, but probably quite expensive at dealer or indie.

 

Brutal, please give me a call when you get a chance. Its a personal matter I think you should know about. 832-606-6138

Thank you,
Brett