Hey all. New here. I purchased a 2001 XJ8 about a month ago. It has 115000 miles on it. It was purchased from an individual and the check engine light was on. I drove it and checked it out and it drove like a dream. It still does but I cannot get this check engine light off! The code originally read system too lean bank 1 and 2. I replaced the MAFS and the air filter. Also replaced spark plugs. The light went off for a week. It has come back on but now only for bank 2. I haven't been able to find a vacuum leak anywhere(that doesn't mean it isn't there). Also when I first purchased it the "restricted performance" light would come on at 50 mph or more and would go off when I slowed down. Yesterday it came on while I was stuck in traffic sitting still for 20 minutes. It hasn't come on at all in weeks while moving. Any help at all would be appreciated, as I am at a loss. Thinking of replacing the fuel filter. Might this be the problem? And if it may be a vacuum leak, what's the best way to find it? Thanks.

 

The car will also give like a "bump" when driving. It is that bump you get if you're sitting still and the air compressor clicks on or off, but I feel it also when I'm driving down the road. Don't know if that means anything, but I thought I'd throw it in there.

 

Tom:
Welcome to the forum.
what code?????
Get the code read (free at the chain auto parts!)- Tahts what the light is for - to tell you that there is a stored OBDII code.

 

I haven't had the code checked this last time it came on, but it has been consistently P0174 Fuel Trim too Lean (Bank 2)

 

Oh yea, you said that- sorry!
The intake plenum is known to crack around the damper- the tube looking thing that juts out back near the throttle body intake. Also, the full load breather tube- the one running from the plenum to the valve cover on the right side near the MAF is notorius to split and be very hard to see the crack. Posters in the past have acknowledged finding the leaks on the third or fourth try of inspecting everything.

There are several ways of finding a vacuum leak. My favorite is to connect a scanner to read STFT and then use a unlit propane torch to slide along the plenum and look for changes in the LTFT as the ratio changes to the input-

 

Out of curiosity, how tight should the gas cap be? I've noticed many people stating this could be the problem, and when I mess with mine it seems to tilt up and down. I think it's spring loaded so I figure it supposed to move a little, but how much is too much? Thanks. And thanks for your answers!!

 

Tom, your gas cap sounds like it might be loose, and a loose cap can through a code, but it absolutely, possitively will not set a lean bank code. I promise.

 

I have looked everywhere and can't find any leaks. Is it possible I have fixed it with the MAFS and air filter, but since the codes haven't been reset it keeps popping up? I don't know, just throwing out ideas. I'm lost. Everything looks awesome. What about fuel pump or filter? Possibility?

 

Read document and reset the codes. A vacuum leak encompasses the entire vacuum system. Before you begin checking anything see what the codes are telling you.

 

Tom:
Pump or filter are obvioulsy possibilities, but their symptoms are usually more pronounced. You could test the fuel pressur eat the rail if you want to be sure, but again, get the codes read. It is much better than throwing money at parts.