Periodically I keep getting code p0171 - lean bank 1 and sometimes I get "restricted performance" but it comes on and goes away and there does not seem to be any restriciton in performance and the light goes off after about a minute. Long term fuel trims are 14.8 for bank 1 and 14.1 bank 2. Does anyone know what values trigger the DTC for the code -0171 and p0174 (bank 2). The short term fuel trima are 6.8 bank 1 and 4.7 bank 2. Do the trouble codes come on based upon the long term fuel trims or the short term fuel trims?
Anyone have any thoughts on this?

 

The codes are triggered when the engine management system hits the limit of extra fuel it can add to try and avoid a lean condition. The total extra fuel the car is adding is the short and long term trim value added together.

From your figures it does look like the car is hitting that upper limit, probably due to an air leak or bad/dirty MAF as both banks are affected.

When was the last time you cleaned the MAF sensor?

 

Thanks for the input. The MAF was changed about a year ago and I cleaned it since then in an attempt to rectify the lean trims.. My next item to change will be the fuel filter.

 

Check the accordion section of your black plastic air duct tube for splits and cracks, especially on its bottom. This is a very common area for air leaks....

 

14.x - big trims!
Do they drop to near zero if you park with a well-warmed engine and rev to 2000+?
If yes, you've one or more air leaks.

 

Thanks! Yes that is what is happening. Any suggestions on how to go about finding this leak. I am planning to spray brake cleaner around the intake system and see if I can detect anything. Any suggestions you have would be welcomed.

 

Smoke test. Look up my thread on how to make a homemade one.

 

P0171 is going to happen when you have cracks in either of the cheap-*** plastic tubes that disintegrate that run to your either full load breather hose or part-load breather hose (the most common places for vac leaks as they're directly on the intake manifold, and are made out of plastic best suitable for prizes found in gumball machines). On the XKR, as I found out just yesterday, the part-load breather hose on the left hand side runs under the supercharger. If it breaks under the supercharger, it is likely going to require pulling the supercharger to replace. Mine broke right as it exits the front under the supercharger, and I'm going to attempt to hamfistedly bodge expertly craft a repair using either a rubberized vac elbow or a custom vac line insert and extension. I'll record my project as I go when the parts arrive, likely tomorrow.

EDIT: Forgot to mention- the part-load breather restrictor can get clogged. There is a TSB on this which addresses how to clean, using a 2mm or so drill bit that, if dropped into the cam cover while cleaning, will require the cam cover to come off to retrieve so tread exceedingly carefully. If that port is clogged, but your vac lines are still magically intact, you may also get a P0171. The smoke test is a DEFINITIVE way to isolate vac leaks though- and if you don't see obvious problems with your breather tubes, that would be your literal smoking gun.

 

Unfortunately, I suspect you'll find that, if one part of that infernal hose broke, the remainder will be very brittle, too. Mine broke near the valve cover connector. While trying to rig up a patch with electrical tape a big chunk pretty much disintegrated. Of course it's a Jag-only part, and of course it cost 3x what an equivalent cheap-o plastic hose should cost.

 

Yes. The straight-run portion is what remains of mine and is thankfully still airtight. If my re-engineering doesn't work I'll likely cap off that line, and tee it off at the accessible portion near the rear of the engine, and run it around the outside with silicone vac line, reusing the connector at the cam cover. If it didn't connect to something else UNDER the supercharger I'd have already rerouted.

 

That is interesting! I would have thought the lean codes would be caused by intake leak somewhere downstream of the MAF until the combustion chamber. I never thought of the breather tubes! I need to look more closely at the shop manual as to where those breathers connect back into the intake stream,.

 

So my saga continues. I posted a previous thread and got some good replies including one that I thought might nail it; bascially warm up engine to normal operating temp and look at long term and short term fuel trims at about 2000 rpm. My long term fuel trims were about +14-15 and I was supposed to see short trim fuel trims go to about 0 IF there was a intake leak. This what I was expecting. Today I cleaned the engine and examined everything I could see for a crack or vacuum leak not expecting to find anything but prepared to use a smoke machine that I have on order. I ran the car up to operating temperature and used my code reader to see live data. Now when I raise rpms to 2000 rpm the short term fuel trims are NEGATIVE 4-5! Last week they were always positive which of course drove the Long term fuel trims to + 14 - 15. What gives? Does anyone have any thoughts as to why the short term trims would go negative while the lont term trims are big positive. Maybe something fixed itself!

 

Don’t overthink the short term fuel trims. They are only there to make you crazy.

Only the long term trims matter. AFAIK, only the long term trims will set off a code……

……right or wrong o dear wise ones ???

pay attention to any long term trims that add up to
more than 10 from both sides. I know I know that won’t set off a code, but if you are headed in that direction it won’t be long before they get large enough to do so. So find the root cause before you get stranded…..


as many have pointed out already, a large positive long term trim is almost always a leak in the intake pipe.


Z

 

Hi Alan,

I have moved your latest post from the new thread you started to include it here, so others can see the resolution to your issue. One of the most frustrating things in any forum is for progress reports to never be posted in the same thread in which all the symptoms and suggestions have been discussed. Please do not start any more threads on this topic but keep the discussion here.

Thanks!

Don

 

When you get your smoke machine use it to find the air leak/vacuum leak you still have. Maybe you have already fixed one, but there is still a leak somewhere. Drive your car for a while and see where the long term trims end up to see if you have really fixed anything.

 

Keep in mind the best way to correlate trims with air leaks is to check at idle. Idle is high vacuum, inducing more air leaking in, and low rpm, where the contribution of leaking air is the highest in comparison to the overall flow.

As already pointed out, focus on long term trims for now. Try and bring them to 5% or less.

Trims depend on load and rpm. Staying at idle sets the rpm. Make sure you pick a load and stick with it during the repair. IOW, decide upfront if you want to establish baseline in D or N. D drives the torque converter and increases the load.

The long term trims are updated every 2 min by adding the average of the short term trims at that load and rpm. For example, if your LT trim is, say, +10 (10% more fuel is added because of leaks), and your ST trim averages -5 after a repair (Reading the o2 sensor now makes it look like 5% fuel has to be removed because you plugged an air leak), the LT trim will be updated to +5 after a full 2 min (120 s at the rpm and load). Now, in this example, the LT trim becomes +5 and the ST trim should average near zero from that point forward.

Best of luck, keep us posted.

 

Occam's Razor dictates that I ask if you changed the XKR with the appropriate MAF sensor, and did NOT use an XK8 MAF sensor. They will physically interchange I believe, but they are not electronically the same, and will throw off mixture... unfortunately the XKR MAF is more expensive and harder to source.

EDIT: added illustration. Same same, but different:

 

Thanks for the input! The short term trims really can drive you crazy. I will focus on the LT. I also plan to change fuel filter.

 

Just a bit more food for thought. I smoke tested the snot out of my engine with a high-quality Snap-On smoke machine, but it still missed some vacuum leaks. O-rings are odd animals and sometimes they'll seal under vacuum and not under pressure, and vice versa.

I smoke tested my engine and found leaks coming from the pipe that connected the RH valve cover to the intake tube, where it connects to the tube. I replaced the pipe with a new one, and still got leaks at the same spot, only a bit less. I also got leaks from the dipstick tube, so I replaced that O-ring. Yet I still have vacuum leaks. So, I used brake cleaner spray and found leaks at the left front corner of the intake, the pipe that plugs into the LH side of the throttle body, and the pipe that plugs into the RH side of the throttle body. None of which the smoke detector found. The T connector on the LH side wasn't connected correctly, so I plugged that in tightly, That, and the new O-ring on the dipstick, fixed the ones causing the lean code. I replaced the intake seals on Sunday and the leak improved but I don't think it's entirely gone. I wonder if the intake is warped.

Anyway, my point is that you should also use some brake cleaner, or similar, on likely leak spots to see if you have leaks the smoke machine doesn't find.

 

I had no idea there are so many vacuum hoses that could leak! While I am waiting for my smoke machine to arrive I will try the brake cleaner test. Do I just run the engine and while running spray the cleaner at each connection and watch for a change in rpm? From your experience I don't see how the smoke machine would get smoke to all those little hoses. I expected I would connect the machine to the intake pipe at the point the pipe comes from the air cleaner box and watch for smoke to appear somewhere between that and the intake manifold. I never thought about leaks in the system after the combustion chamber. Also, should I clear the DTC codes or just let the long term trims come down as I make "repairs"?