Hi I have tried to fix this but with no luck. I checked all vacuum lines replaced one and still get check engine light. Codes always say lean. I went around and sprayed all vacuum lines with carb cleaner no change in engine speed. I really don't know what to try next. Please let me know your thoughts my wife drives this car and she said it seams to miss some times also. Out of ideas here thanks for help. Sorry it 0171,074 1996 x type with 3.0 with 101,000 miles

 

Please post the codes you are reading. Also putting up year, miles and engine (2.5 or 3.0) of your car would be helpful, along with what work you've done on it.

 

I recently had a mystery vacuum leak with 0171 and 0174 codes. I searched and search all the usual leak spots and changed several hoses and parts which probably were ok but old enough to be replaced anyway. My vacuum leak ended up being a small one but caused the lean codes and CEL. I only found my leak by pumping low pressure smoke into the intake manifold to find the smoke seeping out of the connection of the purge valve vacuum tube into the intake manifold. I tried the carb cleaner trick but it didn't show itself with that test.

 

Since you have codes for lean both banks about the only thing the problem can be is a vacuum leak. The main culprits are as follows:
1. Brake booster line. Check that the line is seated and tight in the manifold. There is an O-ring that can dry out and leak.Press on the outer plastic ring to push it against the manifold and gently pull it out. Apply some silicone lube on it and reinsert it.
2. The PCV hose. If you have the smooth. Replace it with the update ribbed one. If it isn't leaking now it eventually will leak.
3. Check and make sure the dipstick is seated fully. If it isn't seated it can draw air thru it to the PCV valve.
4. IMT o-rings check that they are not the source of a leak. If you don't know for sure that you have the update green O-rings change them.
5. Fuel pressure regulator vacuum line. This is a tough one to find. It is on the rear of the fuel rack and runs to the back of the manifold. It is a small diameter plastic line and easily broken during any tune up work.
6. The least likely is the convoluted rubber connector from the throttle body to the air cleaner. Check it for tears and check that the 2 pipes going into are not leaking.
7. Check the air cleaner resonator that goes in the convoluted rubber connector to make sure it doesn't have any cracks in it.
It is a tedious process but take your time you will find the leak.
The SMOKE TEST is the best method.

 

When an X Type is lean everyone seems to shout Vac leak, but MAF sensor fault clogged fuel filter, fuel pump low pressure, Oxy sensors can also cause a lean burn.
It is less likely to be oxy sensors unless 2 or more have gone down otherwise only 1 bank would be affected.
You need to check codes, and check fuel trim values.
Vac leaks affect this too so I'm not saying its not a vac leak, as they do suffer with this.
STFT is affected by VAC leak, if the leak is big you should find it easily, if it is small then the STFT will be affected more at lower revs than at higher revs, this will help isolate a vac leak from other issues.

Post any codes, and if you can the STFT values at idle and 2500 RPM along with O2 sensors voltages for both upstream and post cat sensors, and LTFT too.

Reset the codes, and make a note of STFT at the same revs above after resetting the codes, this should give enough info to help further.

 

It's because an air (vacuum) leak is easy to test using a $5 tool. And it commonly shows that an air leak is present.

 

there was a large hose right behind air cleaner that was not connected. I am not sure what is for but plugs into a small box. We replaced radiator in this car and moved that out of the way and must of unplugged. So thanks agin I hope this fixed the problem. Kirk

 

I have to agree, just that to say it has to be a vac leak could be misleading, although the Jag does suffer with vac leaks throwing things off, as you know there are many other things.

You obviously know exactly what your talking about, it would be good if you could do a whats what on "cruise not available" which is often vac leaks but also not always, a really good procedural check list would be great for this which would cover misfires etc. and flashing CEL etc. A pinned post would be great as there are a lot of threads covering the same thing with different conclusions, 1 really good detailed post would be an advantage to a lot on this forum.

 

Absolutely. That's why I recommend the $5 OBD tool. I keep one in the car, though have not so far needed it in 7 years.