I have had this issue for a while. It passed a smoke test, and I can not find a vacuum leak with carb cleaner. P0171 and P0174 keep putting it in restricted performance mode. I know this is a very common issue, but I can not figure it out after reading the threads. I think I can hear a vacuum leak from the throttle body/intake manifold area on the drivers side, but no smoke when tested and no response to carb cleaner. LTFT is at +25 on both banks in closed loop. I have it hooked up to my laptop, and I think I should be able to confirm a vaccum leak based on the the manifold pressure and other values, but I do not know the correct values or exactly how they relate to each other.

Before I started on this my brother had brought it to the dealer a bunch of times and they would replace sensors and clear the codes. It would go into restricted performance on his drive home from the dealer everytime. I'm not sure of everything that they did, but I do know they replaced the MAF 3 times and 02s multiple times.

I am leaning towards a bad throttle body or intake manifold gasket (or cracked manifold), but I am confused by the results of the smoke test and the carb cleaner test. It was overheated when the radiator cracked before all of this, so a cracked intake manifold would make sense.

Any suggestions would be greatly appreciated. Also, it would be great if someone could let me know were to find what the OBDII values should be, because I have not been able to find them.

 

See https://www.jaguarforums.com/forum/g...17/#post304460

Also check the vacuum routing diagram on the left side shock tower and verify all vacuum hoses are connected and intact.

 

Xkrmaster, intake manifold gasket leaks often won't rear their heads during a smoke test. I'm also leaning to this as your leak.

 

Ok. This suggestion might seem a bit of a reach, but since you have checked most everything else....! Be certain that you don't have an upstream exhaust leak, which could cause the O2 sensors to pick up more than the normal amount of oxygen. This could skew the readings enough to give you those codes. I havent seen this on Jaguars, but did have it happen on a few German cars. They had split converters right along the weld lines. Showed up as an almost invisible crack.

 

Your fuel trims should be around zero both short term and long term as the % measure the balance.

Now I have just gone through similar issues with a faulty MAF sensor. Once I got a good MAF sensor I found another problem caused by Jag dealer back in 2004 (TSB to replace EGR pipe) EGR bolt had worked loose and was sitting on my gear box. Now to fix this I did take the throttle body out and cleaned simple job about an hour. Also found I had loose spark plugs another common problem.

My short term fuel trims now read 0% each side but my long terms are 4% one side and 7% the other so I still have a slight out of balance condition. But nothing I'm worried about as car runs perfect.

I'm also in CA bay area you near me ?

Simon

 

Thanks for the suggestions. The part load breather hose (AJ84880) has a small hole in it (it is not really even visible). It is the part of the hose behind the throttle body and pretty deep in there. I had someone block the hole with their finger and the hiss went away. STFT started to come down some. I don't know how this got by the smoke test. It looks like the trim might still be off with the hole blocked, but I am just going to replace it before I do anything else. I will try the other suggestions if the trim is still off after I replace the hose.

 

The other trick to find leaks is to watch the STFTs while spraying carb cleaner rather than looking for rpm change - I don't know if you were doing this.

 

It is not uncommon for the accordion pleats in the intake hose to crack open on the bottom. This is between the MAF and the throttle body and will definitely generate a lean condition.

 

Agreed - check those accordion pleats. I discovered that my wife's 2006 XK8 intake hose pleats had cracked and split back in October. I wouldn't be surprised if every one of these things is split on the older cars. Cheap plastic combined with constant engine heat equals cracked pleats every time....

 

I replaced the part load breather and checked all the potential vacuum leaks again. Thanks for all the suggestions, I had already checked all of that stuff, but I decided to go back and check it all again anyway. At idle the LTFT stays at 19 something on both banks. When being driven it goes to 0 and everything looks good. There is obviously still a vacuum leak, but I smogged it without any problems (even though the secondary air monitor never turned "OK").

I can take out the oil dipstick and trim does not really change whether or not the tube is blocked. I know it usually would impact trims. Is it just because there is already a vacuum leak, or is the connection above the oil pan bad? I popped it out and reseated it and everything seemed fine, but I can't really see the connection from underneath to check it out.

Thanks for all the help, if anyone has any other ideas I would like to try them. For now I am just happy that it is smogged and drivable.

 

I used to get these codes all the time, and oddly enough one time I tried a different pump brand and higher octane and the issues stopped. With 94 Chevron I'm all good, but codes come back if I switch down to 91. Try a different pump and higher octane to see if that fixes it!

 

Dipstick has an o-ring too, there should be a noticeable change in STFT and engine sound when pulling it free. I found mine missing a long time ago. There are o-rings on those Norma connectors for breathers, one under the throttle body at the lower elbow, and even an o-ring on the brake booster vacuum line on the passenger side of the TB. Many of them get hardened over the miles and heat of the engine.

This video doesn't show the oil dipstick, but does show the trims going crazy negative when shooting a leak with some TB cleaner...but barely audible engine change.

 

I don't think it is fuel because the trims look like a vacuum leak, and this has been happening on multiple types of fuel (all 91, I'm in California.)

The carb cleaner/STFT is the method I have been using the entire time. The most I saw was a +7 when I was spraying the small hole in the since replaced part load breather.(this did not noticeably effect engine noise or the rpms as viewed on the tach) Needles to say, replacing the part-load breather did not fix the problem since it was such a small vacuum leak.

I had already checked everything that has been suggested, but I went through it all and checked again. I introduced a couple of vacuum leaks and the results are confusing. At idle LTFT sits at +19.5 and STFT is around 0. With the oil dipstick out the STFT does actually go up a little, but only about +3. When I take off the nut and remove the dipstick tube altogether STFT goes to about +6. This seems significantly off because I would expect over +10 from only removing the dipstick. When I remove the oil cap STFT goes to over +20, which seems about right.

Does this mean I have narrowed down the leak to where the dipstick tube attaches above the oil pan, or is it possible to get these readings just because there is already a large vacuum leak in the system so vacuum is already reduced? I tried to pull the tube out to check the o-rings, but I am having trouble getting it all the way out of the engine bay to look at it.

I really appreciate everyone's help. I have never had this much trouble tracking down what looks to be such a large vacuum leak. I may be wrong, but I think it will set the lean codes and put it in restricted if LTFT is over +20, so atleast I am avoiding restricted performance with LTFT at +19.53 on both banks at idle, and pretty much normal when driving.

 

My STFT/LTFT is going wacko. LTFT is avg 8/7.6 and STFT can do some odd stuff. I remember when I bought my car my dipstick O-RING was missing and I used an A/C o-ring. Dipstick seems nice and fitted, but eh who knows it may have a tiny gap.

edit: dipstick o-ring. lol i would hope the tube wasnt missing.

 

Nice pic p-Rock. Do you just pull the dipstick tube out? I tried working it out, but it binds up after a few inches and it feels like I will have to force it pretty hard.

 

I had the same codes earlier this year and it turned out to be a vacuum hose that connects to the fuel pressure regulator. This hose is easy to miss - it has an elbow that goes into the fuel pressure regulator, which is located on the passenger side near the front of the fuel rail. I had big changes in the trim readings on my scanner when I sprayed carb cleaner near the dipstick as well, even though this is on the other side of the engine! I would check that vacuum hose! Hope it helps!!

 

She was the one who told me to post it after the shoot haha. Thanks.

Yeah it comes out pretty easy man, and you can feel the gasket giving it a good seal.