Hello. I have a 2004 XJ8 that has 151,000 miles on it. It had been running fine for the past couple years, but lately I've gotten the occasional CEL or "Restricted Performance" warning. I had specific codes that led me to replace an ignition coil which went fine. I had also redone the spark plugs, fuel filter and mass air flow sensor in the past couple years. Lately, the issue is that the car has had the issue of starting normally cold, then if I go run an errand, it will start and sputter for a few seconds, then even out. I've timed this to the point that I can reproduce this sputtering if I wait just about 13 minutes between the time I turn off the car and wait to start it again. Sometimes, but not always, it will throw the P0171 (lean on bank 1) code. If I do this repeatedly, it will throw the CEL.

I have an OBD scanner that will let me watch short and long fuel trims. The car will show Long term fuel trims of around 9 on bank 1 and around 5 on bank 2. It will show short term trims of 0 to negative 5% while driving normally. If I am up to high speed, accelerate, and then let off the gas to coast, I can see STFT of. -25% for a second or two. The various posts on here suggested that there was a vacuum leak somewhere. I went and purchased a smoke machine and found that there was visible smoke in three places. Two small leaks by the throttle body and one massive leak by the oil filler tube. The oil filler tube probably leaks 100x more smoke than the other two. You can see the three smoke test videos.

Thinking that I was on to the problem, I thought I would test by sealing up the leak with metallic duct tape. I then reset the battery and ran the car for a test drive. To my surprise, it only increased the long term fuel trim to higher numbers, like 12 and 10%. So that didn't seem to make any difference.

Do you think the smoke test is pointing me in the right direction? I don't want to take more costly steps like replacing fuel injectors until I have some better sense of what is actually the problem. The car is so old that I can't really justify investing a ton of outside labor in it. I can possibly swap injectors but I don't know about cracking open the intake manifold and looking for problems inside like failed internal gaskets.

Please let me know what you suggest.

 

If your car had a major vacuum leak than your long term fuel trim’s are already way out of whack because it’s been constantly adjusting due to the leak so I would keep things sealed up, reset your long term fuel trim’s to zero and start over looking at your fuel trims. What were the short term fuel trim does looking like after you’ve sealed everything up?

 

After I sealed up the major leak at the oil filler pipe, I reset the battery and then went for a drive. After about a half hour of mixed driving, I saw very little difference in the short term fuel trim. It will still go to -25% after acceleration then coasting at highway speeds but the short term fuel trims are almost always around 0 with little variation. The long term fuel trims show positive at 10% at most but not always. During driving, it will go down towards 0% when driving at a steady speed. If left to idle, it will inch back up from 0 to positive 10%, then sit there. From this, I conclude that the LTFT is only high when it is allowed to idle for a few minutes at a time; when driving, it doesn't really seem to be large issue based on the data.

The fuel trims are interesting to watch, but I"m trying to determine if the fuel trims are out of whack due to the vacuum leak or due to something else like a leaking injector. That was mentioned in some other forum posts as the cause of the sputtering after sitting. It could be that these are separate problems and not that related to each other. I don't care too much about the fuel trim data per se, but I do care about the sputtering because it seems like it could be damaging to the engine long term and it makes me have low confidence in the vehicle as a daily driver. I would like to nurse the Jag along for a couple/few more years, if possible.

Thanks for the input!

 

There is also gas tank ventillation system that gets connected to engine vacuume. It has it's own DTC's. Can leak there cause lean condition?

 

@Big Koshka , I was thinking of buying a new gas tank cap to see if that would help.

 

Hi G.M. Webster,

That leak at the oil filler tube is very common. There is a small tab at the bottom of the tube that you can carefully pry on so the tube can be threaded off. You'll find a large O-ring there that is not an easy size to find, so what I do is add a second, thinner O-ring, which usually solves that leak.

The throttle body is easy to remove, just four screws. With the TB off it should be easy to see where that leak is.

The leak below the EGR valve could be the gasket between the valve and the pipe, but you may need to pull the valve off for a better look.

Some of the other common leak points are the VVT solenoid seals, the part- and full-load breather hose fittings (there are O-rings inside the fittings that can be replaced), the two O-rings on the crankcase vent valve fitting that inserts into the right camshaft cover (the crankcase vent is the disc-shaped part often called a PCV valve). Also check the main intake pipe for cracks in the accordion folds and at the seams where the Helmholz resonators are ultrasonically welded on. Also, the camshaft cover gaskets can allow unmetered air ingress. Anywhere oil is leaking out, air can be inhaled under engine vacuum.

Kudos for investing in a smoke machine. A helpful companion tool is an endoscope (a.k.a. bore scope or inspection scope), which can help pinpoint the source of smoke without having to remove any components. I have a couple of high-end ones that work with my expensive Autel scan tool, but I recently saw a very affordable scope in a YouTube video and decided to purchase one as a backup. It's a pretty impressive tool aside from some slightly counter-intuitive controls, and it's very affordable:


For even more affordable units, search eBay and Amazon for scopes that connect to your phone.

Cheers,

Don

 

Cancelled

 

I had leaks everywhere Don B mentioned. And if you only fixed that one leak but you had any other leaks I can assure you that you will continue to have problems with your short term and or your long term fuel trim.

it only takes one leak for them to start going off in the wrong direction in your long term fuel trim‘s will always suffer because you’re short term fuel trim’s a telling the PCM that he needs to adjust so until you get all the leaks fixed your long term fuel trim is gonna always creep up because the short term will always see a leak.

right now my vvt solenoid might be leaky bc I see oil residue. How do those solenoids and o rings come out? They look and feel more than just pushed in. I dont want to break em

 

Aarcuda, those vvt o rings are a doddle; just unplug the vvt and use a pick to take your worn ones out...The new ones go in as easy as pie!

 

The amount your fuel trims are off will not cause a sputter. When you take your foot off the accelerator the fuel is cut off during coasting so it should always show -25 until fuel is reapplied or RPM drop to about idle range.
Sounds like heat induced vapour lock in fuel rail. Possible you have a leak in the one way valve and your pressure is dropping when sitting. Put a gauge on it and watch for a pressure leak down in the fuel line.