My gut tells me Vector is right; an air leak of some sort. Something that happened today seems to confirm that hunch.

I changed the oil and filter today because, well... it was time. When I started the engine my fuel trims went about 10 higher positive than normal. The only thing I touched that could have effected vacuum was the oil filler cap (and the dipstick, which has new O-rings top and bottom). I inspected the filler cap and the rubber seal is as hard as a rock. It feels like a piece of hard plastic rather than a seal of some sort. I cleaned it thoroughly and coated it with Vaseline. The fuel trims went back down to what they were before, which is a little high but not bad. The bank that runs consistently higher is the bank with the filler cap, coincidence?

I'm going to order a new cap. Mine is red, but all the ones online appear to be black. I'm guessing that really doesn't matter?

What's still confusing to me is that when I bought the car it had +25 fuel trims, everything was leaking...yet, the engine idled smooth. Now with a less than +10 fuel trim there is the occasional bumpiness. That makes me think it's not a vacuum issue, but it SO feels like one.

 

Don't overlook the transmission mount(s). If the X308 has them, that is.

 

Well, I'm really perplexed. My fuel trims were not perfect, but they were pretty good before today. All I did was change the oil and filter, that's all! The fuel trims are crazy high now. I had a spare oil filler cap in the garage that I forgot about, so I tried that one and it made no difference. Granted, that one is about 14 years old also. My new oil filler cap should arrive in a couple of days.

I have never seen any codes, except today I got P0305 (misfire cylinder 5) and P1316 (various misfire possibilities, it had a long description). These were only pending codes however, the check engine light never came on. They occurred right after the oil change and went away by the next startup. I had a spare coil so I switched the one on cylinder 5 out. It seemed to idle better at first, but when I went for a drive nothing had really changed. When I changed the coil, I also cleaned the MAF and did a hard reset. Same occasional stumble at idle and high (positive) fuel trims.

I have an old XJ8 MAF in the garage. I figured "it measures air volume, it should need to be the same accuracy regardless of the engine, right?". I swapped it out just to see what it would do. The car started and ran exactly the same, no hick-ups and no codes. The fuel trims showed slightly better with the XJ8 MAF, but not a big difference. I switched them back.

The idle stumble seemed to start after I ran a can of BG44K through the gas tank. The high fuel trims started after today's oil change.

Tomorrow I plan to do ANOTHER smoke test and possibly check the compression in cylinder 5. If that cylinder is bad then I may have to get out of the Jaguar game. Hopefully it's something simple that I've just missed.

 

Dipstick not sealing?

 

The dipstick O-rings are all new, top and bottom.

I still suspect the LHS valve cover, only because it was more recently loosened and re-torqued. It is not leaking any oil and no matter how much carb cleaner I spray around it there is no change in fuel trims. That's why I hesitate to remove it and re-seal it; if it ain't broken don't fix it, you know? But at this point I'm willing to trouble shoot pretty much anything.

I want to conduct a smoke test today, but my garage is full and it's challenging to do the test outside because direct sunlight makes it hard to see the smoke. We are taking my oldest out for his birthday this afternoon and tonight I promised my wife I'd change the spark plugs and oil in her 2004 XJ8, so this morning is the only time I have available for my XJR.

Yesterday when the fuel trims were going crazy, they would bottom out at -50 STFT on both banks occasionally, then they would shoot up to +35 etc. It was wildly swinging around. After cleaning the MAF again it only seems to go high positive, but this leads me to also suspect the MAF as the problem. If my budget wasn't so tight right now I'd just order a new one, but sadly that isn't an option for a while.

 

Well, I decided to go check JagV8's recommendation anyway. Yes the O-ring is new, but it was ripped! Already! I found two O-rings of similar size in the garage, I put them both on there and the dipstick was actually difficult to push all the way in because of it...

I will drive it to the auto parts store now to shop for O-rings.

I just drove about 10 miles and the fuel trims are much closer to what they were before the oil change. They are still just a bit higher than before, but the LTFT now stay at +10 or below. They only go to +10 at idle and sometimes during coasting. Under throttle they are closer to 0.

I still suspect my MAF AND my LHS cam cover. A helpful forum member is sending me a known good MAF for testing, so I really appreciate that.

 

Another thing to check are the pins of the MAF plug. These can get loose, and the fuel trims will become intermittent...they will flair to very rich and back. with tiny pliers you can bend them down or up a bit so they make better contact. I had weird trims and intermittent searching idle, and low mpg, and this fixed it.

 

My new oil cap should arrive any minute. When it does I'm going to thoroughly inspect the connection pins on my MAF and put on the new oil filler cap. Maybe that will make a difference.

I have to say, I just spent the morning servicing my wife's 2004 XJ8 x350 (I know, wrong forum) and have a few observations. I put a new set of plugs in her car, rotated the tires and changed the oil and filter. That car has 27,000 more miles on it than my XJR but it idles like a sewing machine. It is ridiculously smooth! It seems to be almost as darn fast as my car too, but the ride is infinitely smoother. I can only imagine what the XJR version of her car must drive like. But, as pretty as her car is, it just doesn't hold a candle to my X308 when it comes to appearance.

 

The new oil cap didn't make any discernible difference. I'm not sure what is causing the +5 to +10 LTFT. I will test with the known-good loaner MAF tomorrow night when I get home from work; it should arrive during the day. If that drops the trims to normal then I'll by placing an order for a new one I guess.

The STFT are very inconsistent. Sometimes when I coast they'll go high positive, sometimes when I coast they'll go high negative and sometimes when I'm coasting they stay on zero. No rhyme or reason.

The stumble only seems to be really prevalent at start-up. If I drive it for a while and come to a stop, it idles pretty smooth. When I first start it however, hot or cold, it will get lumpy.

I have one other random theory. When I first start the engine the driver side of the supercharger makes noise. It clangs a little from within. Could that be putting undue load on the engine? It clears up and gets quiet after the car has been revved a little, and the car is still getting from 0 to 60 mph in the 5 to 6 second neighborhood. In other words, the supercharger certainly seems to be working fine.

Oh well, I'm done rambling. This is what happens when I have 4 days in a row off work! Back to work tomorrow though....

 

I put on the loaner MAF and nothing changed much. The loaner MAF actually showed slightly worse high positive fuel trims. The flow readings on the loaner MAF looked lower than what I would expect from specs online. I think possibly my current MAF might actually be the more accurate of the two, but the difference is minimal.

I hooked up the smoke machine, again. I created a slightly higher pressure than I usually do to try and find even small leaks. I saw smoke coming from nowhere, nowhere at all. So, while watching fuel trims closely, I covered the engine with carb cleaner. There is a spot right on top of the EGR valve that if spayed, causes the STFT to go about 10 to the negative. I smoked it again, NO SMOKE from that area. I started the engine again and spayed carb cleaner; that spot causes a negative fuel trim again.

I replaced the throttle body gasket and all EGR gaskets about two months ago. They are brand new! Clearly though, one of them is leaking. Could the EGR valve be cracked or something? Could it be malfunctioning in some way?

While I had the system pressurized with smoke I traced the entire length of the part-load breather hose with an inspection camera... no leaks. I also closely inspected the throttle body to brake booster connection with the same inspection camera. No leaks. All the usual suspects look innocent. I sprayed directly around the connection between the throttle body and the throttle body adapter, no change in fuel trims. Etc. Etc. Exhaustive search for leaks found only the one area, top of EGR valve.

 

Sounds like it leaks only under vacuum not positive pressure.

 

Would a faulty EGR valve (I AGGRESSIVELY cleaned it when I replaced the seals, maybe I damaged it?) cause wildly varying fuel trims? The only time all four trims are good, all within 2 of zero, is under at least mild acceleration. When coasting they usually go high positive but sometimes go WAY negative.

At idle they usually settle out around +10

 

Harvest, I know you are concentrating on vacuum leaks once again, and that the MAF sensor replacement did not improve things for your lean issues.

However, I think I just solved my rich-issue, and you might want to have a look at this as well.

I was travelling last week, and did not manage to confirm the order for a new MAF sensor yet.
Then, while waiting on the airport, I stumbled on a forum thread (forgot which forum, and if it was an old or new post).

Anyway, the poster advised to check on the connection of the MAF connectors, especially the pins.
My connector showed perfectly clean, and I was already a bit disappointed.

Nevertheless, I bent the pins just a little bit down to have some better contact pressure.

Guess what; Long term Fuel Trims dropped from -15 back to ~-5 at warm idle for both banks, and the MAF reading itself dropped just below the factory advised value for idle.

I still need to make a ride, but based upon previous readings, I am confident that I will be back at 'around zero' readings once I give it a bit of gas.

All that after disconnecting and connecting the MAF sensor several times, which never gave any improvement, so your sensor and the replacement sensor might be perfectly alright, but just not connected well.

Perhaps a long shot, but wanted to share with you, it could explain the erratic behavior you noticed.

 

Sorry for this
8 cylinders = 8 spark plugs - Yep

 

I bent the connectors very slightly, I think the connection is good.

The connection between the EGR valve and the EGR pipe is my suspect leak. I just put that new gasket in there a couple of months ago, but it sure seems to be where the leak is located. I wish I could just tighten the bolts, but there is just no way to get to the bottom one without removing the EGR valve, again.

Fuel trims are consistent now. They are very good under load, but between +7 and +12 at idle. Coasting can go as high as +18, briefly.

Any advice, or should I just order all new gaskets, again, and re-seal everything, again?

 

Those egr bolts have a history of coming loose. Use some locktite on them. I worked on a XKR a couple of years ago that had the same problem, and others have mentioned it.

 

The bolts holding the EGR valve to the throttle body adapter were loose when I bought the car. I know those can be problematic. The ones that appear to be leaking connect the EGR valve to the EGR pipe. I guess those can be problematic as well.

The top bolt is tight, I can't verify the lower bolt.

The gaskets that came off were the old style (01 engines and older). Since the engine was assembled in 2002 I put the new style (metal) gaskets on when I changed them. Not sure if that makes any difference...

 

It appears I improperly identified the air leak. It is in fact the EGR valve, but the location is actually on the body of the valve where the black plastic cap meets the metal from of the unit. I will blame poor visibility due to it's location under the throttle body for my mis-identification. Sounds good, right?

The gaskets I installed do not leak. When I spray those locations the fuel trims didn't change. It's when I spray the plastic part up top, near the electrical connector, that the fuel trims change. I looked closely at the location of the leak with my inspection camera (that thing really comes in handy, great Father's Day gift). There is a cracked section where an actual piece of the plastic is missing. It looks like a small hole in the plastic. I don't know how that would create an air passage to the inside of the valve, but I can't deny that's the spot that causes negative fuel trims when sprayed with carb cleaner.

For now, I filled/covered the spot with black RTV. I will let it dry overnight. When I drive to work tomorrow if the fuel trims are good, or at least better, then I will know I'm on to something.

A new EGR valve is very expensive. I am not particularly happy with this discovery. It has always leaked from that area; I'm guessing it was cracked when I bought the car but I never noticed. If the RTV were to solve the problem, I would probably be tempted to just leave it rather than shell out for a new EGR valve and gaskets. Not exactly "professional" mechanic work, but just being honest.

 

If you used a high temp RTV it might be a permanent fix, but if the fuel trims go up in the future you know where to start looking.

 

Well, that spot appears to be completely sealed now, but it had no discernible effect on my fuel trims. When I sprayed carb cleaner on that spot before the trims went negative. Now I spray there and the trims don't change, BUT my fuel trims still run about the same.

After 15 minutes of targeted spraying, as best I can with the little red straw that came with the can, it seems the fuel trims go high negative when I spray around the back of the throttle body, just above the EGR valve. I checked the throttle body bolts and one was a little loose, but after tightening it the leak is still there.

Either my throttle body gasket has failed, or was the wrong one to begin with. There was ambiguity online as to which should go on my car, the metal gasket or the paper one. I went with metal.

Or the leak is somewhere else in that area. The EGR to throttle body adapter gasket is new and I tried targeting that area but could not find a leak there. That leaves the adapter underneath? I don't know of a possible leak location in that area. The brake booster line into the adapter is definitely not leaking. I showered that spot with carb cleaner and...nothing.

Should I try a paper throttle body gasket?

Edit: Okay, I just called the local Jaguar dealer. The parts guy checked with the technician and said the 03 XJR does not use the metal gasket which many sites list for my car. Therefore, the one I have on there is the wrong part.