Big find! I have a vacuum leak. There is a tire valve stem apparatus attached to the front of the left fuel rail. When I spray near that, the car revs up noticeably. I’m sure it is not the valve thing leaking, because that is not vacuum, it’s got fuel pressure behind it. I suspect it is for checking fuel pressure or something. Pretty sure it’s not the problem here. But there is either a vacuum line near it that is leaking, or the intake itself is leaking.

For now I’m going to assume it’s not the intake but we'll see. Do you have any ideas what vacuum lines are in that area? There is a plastic vapor line coming out of the left cam cover, and that plastic line goes under the supercharger to places unknown. But I don’t really think it’s a vacuum line, is it?

Other ideas? Thank you for all your suggestions. We’re getting there....I did not originally spray starting fluid in this area because it’s a fair distance from the intake system, and I didn’t think to spray there.

Here is a picture of the valve stem thing. It is in this area that the starting fluid has the biggest effect.

 

May also have a bad MAF (if it didn't clean) as those trims at 2500 are terrible. Or even a fuel pump problem (or filter).

Still, one fix at a time and then re-check trims. (It'll need to re-learn so after a few warm ups or disconnect the battery to zero them, then again a few warm ups or be cautious about believeing yet-to-be-learned values.)

 

I am going to use a mineral oil smoker to locate the vacuum leak. I want to blow the smoke into the system thru the vacuum brake booster line. Does it just pull straight out of the booster drum? I don’t want to pull too hard without knowing how it comes out. Lots of threads on removing the hose from the OTHER end at the throttle body elbow, but I haven’t seen anything about the booster end...

Thanks

 

It was the part load breather tube. It came out in two brittle and cracked pieces, and it was broken at the T fitting going into the intake elbow. The car has never ran this good. I’m posting the new fuel trim readings. The short terms both seem goood, moving between -1.6 and +3.8 as I operate the throttle, then seem to settle around zero at idle. The long term still seem a little high, but dramatically lower than before. I smoked the vacuum system, and there are no more leaks.

I also cleaned the part load breather port as per the service bulletin. It was plugged, but cleaned pretty easily.

Your thoughts? Thank you!

 

Long term are named because they settle over a long (well, not short) time so they may well still be learning the fix.

A few more warmups should let them settle.

(Or you can force a total relearn by disconnecting the battery as it will zero the trims - and then wait at least as long as the above, so not worth it I'd say.)

Once settled, anything within (say) 5 of zero is good enough generally.

 

You were right, and thanks for all your help. I learned exponentially as I went thru this. I drove the car at various speeds including a stretch of interstate today. I had the app open the whole time so I could observe the fuel trims. I’m calling it fixed! The readings generally varied less than 4% whenever I was watching the screen.

Thanks to everyone for your suggestions. I learned a lot about fuel trims and what they mean. Thank you to rocklandjag for suggesting a vacuum leak. That suggestion stuck in my head. Based on the trim readings, it was likely a vacuum leak, so I kept looking until I found it.

 

Hello Eric from Ogallala, Nebraska.

It has been more than 6 months since your last post on this subject. Did the replacement of the part load breather tube solve the problem completely?

Very respectfully, Bob G from Florida

 

Yes, solved it completely. I routed the new tube above the inter cooler instead of below it, but it worked fine and looks good.

 

Thanks for replying so quickly. That's great news Eric. Very respectfully, Bob G in Florida

 

Any good resources for learning how to read fuel trims on XJR? I have watched a few videos and get the general concept, but not sure if there is more specific guidance for these cars. i.e. it sounds like ceratain RPMs should be used to measure and long term trims may need to be 0'd out by resetting with battery disconnect and ground. Any pointers would be great.

 

It's a normal car.

You would not normally disconnect the battery as you lose all the learned values AND clear all the monitors (used for so-called "smog" tests some places).

The idea is a hot engine and look at idle. A happy engine should have trims near zero. If they're not then rev to 2500 or so and if they head near zero you can be near enough sure there's an air (into vacuum) leak.

Other odd readings can provide useful clues but it gets harder and needs more data to figure what's occurring.