Oxygen sensor & fuel mixtures
 

On a '99 XK8 which of the two right side O2 sensors would most likely detect or signal a lean condition on that side? Would it be the heated upstream or the downstream or is it a toss-up?

With a persistent but somewhat inconsistent P0171 indicating a lean condition on the right bank only, some of the probable causes that don't make sense if the problem is restricted to that single side of the system. With the pinpoint tests sometimes being difficult to carry out by oneself and parts as expensive as many are I trying to go after the most likely targets first.

The upper heated sensor controls fuel mixture and the downstream determines the efficiency of the catalytic convertor. With an OBDII scanner you should be able to reaf the voltage to the sensor and determine if it is functioning. Before condemning the sensor I would make sure that you don't have a vacuum leak at the manifold gaskets on that bank. The best test is a smoke test for a lean condition. You can also try spraying starting fluid to check for leaks around manifold. Another possible source is the full load breather since it pulls off of the right bank.

Bank 2 Upstream has been the most common one I've seen fail - the one under the expansion tank.


On rare occasions I have seen MAF sensors cause lean codes to be logged.


+1 with avern - check for vacuum leaks first, even the slightest leak can cause havoc.

Get a cheap ELM327 and read the fuel trims. This code means the ECU is tired of adding fuel for you to compensate for un-metered air. The code means it is already adding 25% more than it should. Check for air leaks all around the head, including dip stick. Should be mostly o-rings. Harbor Freight has great kits with all kinds of sizes. VVT seals should also be high on the list. Keep in mind this is typically not a code that comes up because of this one thing. The problem is that the sum total of ALL leaks are too much to compensate for, i.e. not necessarily limited to just one bank. For all we know, the fuel trim on the other side is high too, but not quite 25%.

Best of luck, keep us posted.

It's been on my list for over a year
 

+1 with avern - check for vacuum leaks first, even the slightest leak can cause havoc.[/QUOTE]


fmertz----- tried all of those things yet so far no luck. The problem is so erratic that the only consistent code I get is P0171 and even that isn't every day

I've been searching for such elusive vacuum leak for near a year now without any luck. I've read and re-read the svc. manual to the point I almost know the page #'s by heart. I've gone over every hose and fitting I can reach, replaced any that look suspicious and still no joy.

Some days it's there and other days it's not. Recently during Florida's dip into sub-30 degree days it would stay all day and yet the next day disappear entirely. At some point when it warms up a bit I'm going to remove the right front wheel, fender inner liner, and check the emissions plumbing looking for some telltale.

I've got it in my head that given that it is ALWAYS just the right bank that goes lean and to me that indicates that those things that should affect BOTH sides aren't worth chasing after. That would eliminate fuel pressure regulator, fuel pump, throttle position sensor, coolant temperature, etc.


I've replaced ALL the injectors, removed and replaced both valve covers and their associated gaskets because of slight oil seepage at the rear corners, replaced all the plugs.


If there's a vacuum leak it's a sneaky one so I'm left thinking that either there's an ECM problem (very isolated I'd guess) or one of the sensors in the exhaust system on that side.

If you were able to monitor short term fuel trim ( STFT ) this would tell you if its compensating for vacuum leak. Anything over 5% would indicate a problem.


Remember Bank 1 is RH side of vehicle ( air box side ). Because we're all human, I've seen people make the mistake of looking at wrong side - including myself.


See attachment - might shed some light, hope this helps. When was the last time the fuel filter was changed?


Also - the air intake pipe itself, in the accordion section, have been known to crack and leak on the underside, maybe take it off and have a good look at it, after all it is plastic.


You " could " be looking at a MAF problem as well. I've had several MAF sensors cause revving problems, simulate a slipping trans, and even cause P0171 & P0174 codes to be logged. If you've checked meticulously on Bank 1 and have found nothing, this may be where your problem lies. Might be able to clean it..... sometimes works, sometimes doesn't, still worth a try.

I have not done this myself, but you can look into doing a smoke test. The idea is to push smoke through the intake and see where it seeps out. It can be done professionally, but youtube has lots of videos for home-made devices.

You should really get a tool to read fuel trims. These numbers (short/long term, for both sides) have powerful diagnostic abilities.

Another trick I have learned lately: Check the OBDII monitors and see which one is not completing. Could be another hint as to an area of focus.

Last, remember that some of these codes have a 2 trip logic. The underlying condition has to be present for 2 consecutive trips for a code to show up.