The brake vacuum booster. Before doing it use a code reader to read fuel trims. Long and short term should add up to near zero. Mine were near 20. Removing the vacuum line from the brake booster and plugging it brought the trims down (indicating that's where the leak was).

 

Where is the booster located? Tried looking it up

 

Pragmatic a question please;

Is there enough room, without disconnecting the master cylinder brake lines?

 

Brake booster is in front of the brake master cylinder (between the master cylinder and the brake pedal). I would make sure plugging the vacuum line going to the booster solves the problem since the booster is a pain to change. It can be changed without removing the brake lines from the master. You will need to remove the wiper arms, plastic covers, strut bracing, plastic surround and unbolt the master cylinder while under the hood. Then you have to work under the drivers side dash to unbolt the booster (bolts are behind the brake pedal and remove the rod from the brake pedal. Once unbolted its back under the hood to wiggle the booster out. There is just enough room to get it out without removing the brake lines from the master cylinder. Again check if plugging the vacuum line (not easy to remove from the booster) fixes the fuel trim numbers.

 

Thanks pragmatic,
Was just asking for future reference, with the A B S system, would rather not break open the supply lines.

 

OK so i replaced the EGR (with a working used one) and dipstick o ring at the top of the dipstick. i also checked my fuel trims as you suggested. Here is what is happened now.

I now have 2 more codes popped. P0171 (similar to P0174) and P0193. im wondering if i damaged the fuel rail pressure sensor while swapping the EGR or if there's some other cause. an 04 XJ8 just showed up at my local junkyard so ill go grab that sensor for like $4 to see if it resolves that.

Fuel trims after start up at idle are shown in the photo attached. Seems the Long Fuel Trims are high if you say they should be at 0 at idle

 

P0174 is the same as P0171, just on the other bank. Means you only got half the problem.

OK, Your LT trim 1 is high at 11% (don't know if your reader "Bank 1" means the 4.2 bank 1 or bank 2 by this).

Note that LTFT can (but shouldn't) vary widely, since it's derived from your ST fuel trim. So not clear (to me) if your 11% is a freeze frame data point, or the moment you took the pic is a widely varying display on your unit. But it will vary. On a new car, maybe 1-3% either way from the ideal of 0%. On an old one, 10-20% is very possible. Most guides will say that +- 10% is acceptable, but it's not to me.

In the X150 4.2 Naturally Aspirated engine, a lean code (P0171 or P0174) is triggered around 20%, even if it peaks for a short moment at that value. It's almost always a vacuum leak. I've been down this road, and the only practical way to find it is to use a smoke machine and pressurize the vacuum envelope of the engine with sufficiently pressurized smoke. That means blocking off the throttle body with something with heft, not just a rubber glove or similar. I used a 3" rubber plumbing cap secured with a hose clamp. After pressurizing up quite a bit, I found several more leaks, but the big one was a ripped injector oring. I was lucky that the brake booster was fine. In the absolutely worst scenario, a leaking valve cover gasket can contribute to the problem, but that's rare compared to the rest.

The point is that it can be anywhere in the envelope. In a car with 80K miles and 14 years, I found 6 different vacuum related gaskets leaking: The EGR metal ones, the full and part load breather orings, the oil fill, the dipstick (top), and the insanely fragile and tiny AIR vacuum line*. Breathe on it and it broke. The injector o-ring was damaged on dealer installation. To be safe, I pulled all the injectors and replaced the orings.

Yes, we all know that Jag rubber parts go bad. To really do a vacuum refit, you must have strongly pressurized smoke to see it all. Then replace every piece of easily replaceable rubber that interfaces vacuum to normal air pressure. THEN, once you know your vacuum envelope is good, you can consider the MAF or O2 sensors. But way too often, MAFs or sensors are replaced that are fine.

My LT fuel trims went from +20 to +- 1%, consistently.

Good luck


* Note: I don't know if the X350 and x358 4.2 NA has an AIR system. If it does, it's very possible that the tiny vacuum line connecting from the manifold side of the throttle body to it is broken. Amazingly fragile. Look carefully.

 

Would you happen to have a pic of the AIR System and highlight the area where it leaks? Now I want to check on my engine. thanks!

 

# 19, in the pic below. Routes vacuum from the throttle body, to the AIR control switch on the front of the engine, and a device connected to the right front of the fuel rail.

 

Just do vacuum test directly on booster, where the vacuum line is is connected.If it holds vacuum it is fine.