This is on our 2001 XK8, 50 K on the clock. I made the mistake of putting in a mid grade gas last fill up, and don't know if that is the problem or not.

Have put in high test at the half tank point and erased the code. Code was P0174. Check engine light went out but I have no clue as to weather it will return to warn of a more serious problem.

Did have a issue prior where at full throttle from the get go it would stumble, then accelerate. at normal throttle it ran fine.

Any ideas??

 

This is a popular subject - the P0171 and P0174 lean codes. There's a lot in the archives on this, but not a lot of fool-proof answers. The first thing to check for is vacuum leaks, at the VVT O-rings, the accordion hose, the part-load breather hose, the dipstick tube. Another possibility is the MAF. You can get MAF cleaner at most auto parts stores. Don't use carb cleaner. Another thing to try is fuel injector cleaner. If you've had anything apart or disturbed, or if you can hear a vacuum leak that can point you in the right direction.

 

I'm dealing with the same codes showing a lean condition on both banks. The fact that it's showing up on both banks would lead me to believe that there's a vacuum leak somewhere between the air filter and the throttle body. I've gone over that part of the intake system several times, tightened everything up, cleaned the MAF sensor, and installed a new fuel filter (fuel pressure is about 35 psi). Car idles perfectly, good initial throttle response, but an obvious flat spot around 3,500-4,000 rpm. While driving at normal speeds, the Restricted Performance light will come on, then go away. STFT's are -.8 to +3.9, LTFT's are at 7.8.

My VVT seals are seeping oil; is that enough of a potential air leak to cause this problem? Going to throw a can of BK44 fuel injection cleaner into the fuel tank to see if that helps. After that, ????

 

Yes, it is.

 

A plugged fuel filter or failing pump can also cause a lean condition. You need to look at the fuel trims to see if you have the lean condition at idle, indicating a air leak, or under load, indicating a fuel delivery problem.

 

The fuel trims at idle seem to be fine; -.8 to +3.9, consistent and equal, I haven't checked them under load as yet. Fuel pump pressure is 35 psi, fuel filter has been replaced. I don't think that the VVT seals could leak enough air to be a problem under load at elevated rpm's. Wondering if a there could be an issue with the throttle body. As previously mentioned, there are numerous threads on this problem, with multiple causes.

 

The mafs and throttle body are possibilities. Pressure should be over 40 lbs. It might be worth checking the pressure while driving, but that can be complicated.

 

Back in February, the P0171 and P0174 codes thrown by my wife's 2006 XK8 were caused by leaking VVT O-ring seals that were only two years old but were already baked and brittle. A smoke test showed the leaks. A couple of new seals instantly cured the issue....

 

Leaking seals should have caused high fuel trims at idle. This car has low fuel trims at idle.

 

Yes, usually the fuel trims will go high at idle if there is a vacuum leak. It's when they hit +25 the lean condition is set, I believe. That's what makes fuel delivery more likely the problem. It seems strange it would kick in (lean) at higher RPMs. Does it get worse when the car is warmed up ? Hate to say it, but it could be fuel pump time.

 

If the right side trims are worse than the left, closely inspect the EGR pipe from the manifold to the EGR valve. One of the little bellows on mine had cracked almost half way around. After I replaced the pipe, the codes were gone and the trims were MUCH better (and balanced).

 

Great suggestions, thanks! Drove it around yesterday, got the P0171 & P0174 codes. Checked the whole intake system all over again and found nothing. Took it out for a "spirited" drive. No codes. I'm going to check the fuel pump pressure again, both at idle and with the engine revving, along with my Torque app. I agree with the suggestion that the fuel pump may be marginal. As far as the throttle body goes, based on some paint markings, it appears that the previous owner had the original replaced with a used unit from recycle yard.

Just hate to throw parts at the problem without actually knowing what it is.

 

The 2001 XK8 does not have a EGR valve.

 

Thanks RJ - didn't realize that only the later cars had the EGR system. Just trying to help, of course. The 171/174 problem is so common place - maybe someone with a 4.2 will benefit from the info.

 

Good luck Max. We're all rooting for you. (and curious too)

 

Max,

You acknowledged that your VVT O-ring seals are seeping oil, so bite this relatively inexpensive bullet and replace them with new ones. That small action may indeed cure your P0171/P0174 issues....

 

My (superficial) understanding is that you always need the EGR itself (the re-circulation of exhaust gases).

This is because the NOx pollutant generated at high heat under lean conditions cannot be eliminated through the catalyst. The only option to lower those emissions is to lower the combustion temperature, in this case by putting inert exhaust gas back into the combustion chamber.

One way to do it is to add the whole EGR valve setup of course, but the other way is through VVT. This is explained in the Jaguar engine training book. This explains why all XKRs have EGR valves as XKRs have no VVT. For XK8s, this is more complicated apparently. I believe early and late cars have EGR valves, but some in the middle do not. Taking a stab at it, somebody correct me if I am wrong, AJ26s and AJ34s do have the EGR valve, whereas AJ27s do not.

 

AJ26 does not. My car may have had one when built, but there is a patch on the exhaust manifold where the pipe would have been mounted. Whether this was modified before or after shipment to the US I don't know.

 

I had those issues/ codes, found them to be caused by an aftermarket air filter system, the MAS censor had a bad O-ring around it.

 

Thanks for all of the help everyone. A couple of clues stuck out; i have an aftermarket intake tube and air filter which has given no problems for the six months I've had it installed. I did, however, recently spray-clean the MAS (didn't physically touch it) and re-installed it. I didn't notice an o-ring anywhere, so I'm going to check it again to see if it might have fallen out when I had it apart. I'm also going to change the VVT seals; couldn't hurt. At least none of these conditions require crawling under the car to fix; getting under isn't the problem, getting out and standing up with a 75 year-old body is.