I have a 2011 Jaguar xjl non supercharged 5.0 L engine. My car runs really good around town and on freeway but as soon as I go down a hill for a minute or two without pressing the gas my car starts to misfire. It will happen every time I can duplicate the problem on the hill by my house. As soon as I stop turn it off they said for about 10 minutes and started up I have to press the gas to connect her out the excessive fuel because of misfires and everything is running normal again. Any ideas on where I should start. Mechanic I have has not being able to pinpoint problem. One thing I did notice is that my catalytic converter is running between 1400 and 1700 degrees Fahrenheit. After it's fully warm and it stays there

 

Most propably you have leaking injector(s). Fuel supply should be cut when engine braking (downhill). When injector is leaking the fuel will accumulate over piston(s) because there are not enough air to burn it away. When you then press pedal again even more fuel is added and because mixture is too rich it dosent burn. = Misfire occurs.
There are two reasons why this is dangerous: 1) Fuel cleans lubrication oil away from cylinder walls. Will destroy your block/piston. 2) Overfueling will destroy your catalysators.
Note: When purchasing new injectors: There are lot of counterfeight in a market. There are several threads for conterfeight injectors on this forum.

 

Can you post what codes you have? Any work done previously? How many miles?
If around 100K I would be putting a set of plugs in as well.

I agree with Vasara it's probably one or more injectors.
It is an odd symptom and I have not heard it before.
.
.
.

 

Like the others said, injectors most likely but a misfire will usually throw codes.

Heed my warning. Do NOT buy the $600 8 injector kit from FCP Euro. Replacing all 8 injectors will with proper ones cost far more than that. I tried to be cheap, like you can get away with on American cars and it bit me hard. In my experience with my car, at least with things under the hood, OEM is required. Chasing miss fires on these cars, also from my limited experience, can take you to and past the breaking point not only mentally but in the wallet. LOL.

It’s also a big job, but can be done at home with some patience and the injector pulling tool (there’s a $30 one on Amazon that kicks butt). If you’re going to do the injectors, you might as well do plugs since you’re in there and right there.

 

Thank you guys for the information. I am thinking that the fuel injectors were leaking also because it only does it at idle and not when I'm pressing on the gas. The hills are extreme because I'm not pressing the pedal and giving it gas but yet the engine is under a load. I will be replacing all eight injectors and seeing if that is going to fix it. Thank you so much for helping me confirm. We will see. Also I am curious my catalytic converter temperatures on my scanner are showing that they are staying at 1400 degrees Fahrenheit. Even goes up to 1700 degrees Fahrenheit sometimes. I'm hoping there was no catalytic converter damage. I think once I fix this rich condition I should be good. My short-term field trims are usually plus or minus 5%, but randomly it is at plus or minus 11 or 12% and it is running rough during those times at idle. Thank you all for the information I will keep you updated on the fix

 

Also one more thing to add when it does start misfiring yes it would give codes every single time for four and six cylinders but if I were to try to keep it running in feather the gas pedal it ends up showing all cylinders misfiring. Every single time also it shows misfire on Startup first 100 Revolution. I'm thinking when I shut the car off the injectors are leaking fuel causing the rough start misfire at the beginning. And one thing I did notice is that there is a constant rough start every single time I start the vehicle

 

Propably the fuel rail pressure will push fuel thru when engine is stopped, so every time you start you have fuel flood on some or all cylinders. You might not see any errors in fuel trim values, since the % of idling or engine braking time is low and when you are actually driving with engine power, the leak is equal or less than needed for moving.

 

One thing I learned…

I had 2 injectors fail on a road trip. Fixed the two problem injectors in a parking lot and when I got home I changed all 8 injectors and plugs. I guess during the time my two injectors were dumping fuel it destroyed my cats (I didn’t know this). Couple weeks after solving my injector saga I kept getting a misfire on cylinder 3. Baffled, I pulled that plug and found electrode pretty much gone. Strange. Guess it was just a bad plug. Changed cylinder 3 for a new one. Drove for a couple weeks and started getting misfires at anything over half throttle. Cylinder 3 and 6. Pulled the plugs and found electrodes were gone - again.

Turns out my clogged cats were causing cray high cylinder temps and eating the plugs. Luckily no other damage was done but I learned a lesson.

So if you suspect your cats are clogged, your misfire problem isn’t going anywhere - in my experience.

 

Replaced fuel injectors
I spent the money and bought eight brand new Bosch original equipment fuel injectors. Started the vehicle up and ran for a while. Fuel trims looked good they were around plus or minus 2 to 3 percent. one thing I did notice is that the Catalyst temperature for Sensor 2 both banks were at 1735° f. It slowly moved to that but I put an infrared thermometer on the catalytic converters and they were reading 300 to 400 degrees Fahrenheit. I test drove the car and coming downhill it started to misfire again there was no change after replacing all a fuel injectors. I have a new question how do I know which fuel injectors to replace or maybe I should just replace them all. Those are also fairly expensive. I'm thinking this is the problem because of the high temperature reading on scanner but not the actual temperature. Any help on this would be appreciated of your thoughts. I haven't put up background on this but I had the head gaskets replaced and I don't think the mechanic replaced the O2 sensors. That would be next for me. Car runs really good outside of coasting downhill and misfiring once I am coasting a little while

 

Your statement is confusing. You said you replaced all injectors. Then you said who do you know which to replace or should you do them all.

Which one is it?

You need to do a drive with a scan tool logging. You can watch the live misfire counter on all cylinders. If nothing is showing I believe the only thing that would cause no CEL is plugs - but that’s my old school thinking.

As I mentioned before, if your cats are clogged - you’ll never be rid of the misfire. Diagnosing these cars without some form of data stream is kind of like pissing in the wind - except the **** is money and it’ll blow out your bank FAST.

 

I meant to say o2 sensors. Which ones are giving wrong temp reading? I replaced all eight fuel injectors and I'm thinking to replace O2 sensors next.

 

Do you not have access to a scanner to run live data when you let off the gas and see what’s going on?

 

I do have a scanner and I was looking at fuel trims and they were going from 1to 4% plus or minus at first and then zeroes out after a few seconds of not pressing gas pedal.

 

I just finished replacing all4 of the o2 sensors Bank 1 Bank 2 before and after. Did a test drive and nothing changed. Car stalled and ran rough at bottom of hill. All cylinders misfires.

 

Have you looked at the fuel pressures in your scan tool?

Trims and cat temps aren’t going to lead you down a road of diagnosing the problem and you’re blindly throwing money at it right now.

 

The car has 6 O2 sensors? You replaced the 4 closest to the engine?
Can you post actual codes? Then clear them and run the car some more?
Not sure why your replacing things unless there is a reason?
.
.
.

 

I was thinking the same thing. My car has a pre and post CAT 02 and then one more 02 further down-stream with a total of 6 02s.

Since he's changed all 8 fuel injectors, I wonder if he ran into the same problem I did with fake Bosch injectors. Since he all of the sudden had all 8 cylinders fail to misfires, it makes me think that's happened. Its the same thing that happened to me. One misfire - changed all 8 injectors with China clones (didn't know) and boom - all 8 failed. If I recall correctly a true genuine Bosch injector for 8 cylinders cost me almost $1,500. FCP Euro is selling their 8 injector kits for about $800 - and they're the Chinese clones that simply do NOT work.

If the OP wants to investigate the clone situations - here's my thread where I found out.
https://www.jaguarforums.com/forum/x...15-xjr-283983/