My 2004 XK8 starts easily all the time and idles perfectly. With light, medium, and even heavy throttle, the engine is powerful and smooth. However, with WOT the engine actually slows down and runs poorly, wont increase in speed at all if I keep the throttle wide open, but when I let up to about 3/4 throttle, it speeds up again and all is normal. It seems perfectly fine unless I floor it, which I almost never do, and generally I wouldn't really care about it except it concerns me that something is wrong and could get worse. There are no codes stored, no MIL light, no smoke, etc. Fuel mileage is very good. Use premium fuel. Has anyone experienced this?

 

When I was having problems with my fuel pump the car would cut out or stumble when I floored it. I'm not saying that's what's going on with your's, just a possibility.

I absolutely LOVE my Android phone with an app called Torque installed. It shows me all sorts of data in real time. The bluetooth OBDII scanner plugs into the car, then the phone interfaces with the car to read all of the data. With Torque I was actually able to see my fuel pressure in real time. I think I'll make a video of it this afternoon to show everyone how cool it is.

 

Thanks Sam. I'm afraid it may be fuel pressure, but was hoping someone would tell me it was definitively some other part that is much easier to replace. While I have the technical/mechanical skills to pull the tank, I really don't think I want to tackle the job; even after watching your video. I have not yet purchased the ODBII unit to connect to the carpc/centrafuse, I guess I will do that to monitor fuel pressure. At least then I will know and can tell the dealership.

 

You can get a loaner fuel pressure gauge from AutoZone to put on your car to see what the pressure is now but I want to ask you a question first. Are you accelerating with the shifter in “P” or “N”? If so your car has a limiter that will only allow your RPM’s to go to I think 2500/3000RPM.
This link will show you how to check the fuel PSI Jaguar fuel pressure testing

 

Hi Gus, this is all when I am driving with the shifter I'd D. I ordered the ODBII reader, should have this coming Monday, will check the fuel pressure then. I had brought the car to autozone for them to read any codes, but I didn't think to ask if they could check fuel pressure.

 

They have a loaner program. You can take it home and test your pressure and return it for free.

 

When the car begins to stumble are you getting a restricted performance (RP) indication? When it does stumble what is the RPM of the engine and is it the same every time? I had 2 situations that my car went into RP and both times it was a knock sensor. The last time I found it when I had my OBDII reader connected and it captured the code as I was driving and the car did not set the code or a check engine light.

 

Interesting Gus; your question reminded me of one inststance about 3 months ago (less than 2000 miles ago) when I floored it and I had this same symptom AND that time, but ONLY that time I did get a restricted performance, however, no error codes or lights. As far as the question about rpm's; when driving, if I don't mash the gas pedal to the floor, the engine is happy to rev, but as soon as I go wot, it stumbles and wont rev higher. Hmm, it sounds like I floor it a lot, but I really don't, I actually baby it more than anything.

 

Do your checks and see what you get. Keep in mind that you have a fuel filter and it could be part of the problem. It could allow fuel but on demand performance it may restrict the flow.

 

Fuel pressure readings are normal, 55lbs. What my ODBII readings did show however is that the long term fuel trims are off; bank 1 is 9+% rich and bank 2 is about 5% rich. The rich condition seems higher than it should be and I suspect this indicates a gasket failure, and since the engine runs smooth, I suspect it is intake manifold rather than head gasket. Also leading me to intake manifold gasket leak rather than a leak prior is that the banks are not equal. If it were the intake tube between the throttle body and air cleaner that was leaking, I would suspect the 2 banks to be equal. I suppose this also could be bad (leaking?) injectors.

Does anyone know if the long term fuel trims I am seeing are abnormal? Am I thinking correctly here?

 

Injectors leaking out the top would smell of gas and be obvious. Injectors leaking, out the bottom into the combustion chamber, would probably creating a white smoking exhaust, and likewise be pretty obvious.

 

What is the RPM when it stumbles?

 

The first suspect is a restricted fuel filter or pump getting weak. Thsi suystem also has a fuel pressure sensor that may be malfunctioning. this sensor regulates the pump speed to keep the pressure correct under all engine loads. The next is a TPS going bad and giving a bad signal at WOT. If the stumble happens only at high RPM, it could be bad plugs.

 

Could also be a bad coil, at least the symptoms you describe where very much the same as I had.

 

Thanks guys. Tomorrow I will test to see if it really is specific to an rpm range, although I don't believe it is, but will verify as Gus asked. If it were a weak pump or restricted fuel filter, I would not expect the fuel pressure to be at 55, am I thinking wrong? If it were a bad TPS or bad coil, would it always throw a code (none are stored)? I never have changed the spark plugs, no idea if they have been done before, car has 64k miles, again, would expect misfire codes though. Do the long term fuel trims being uneven and rich mean anything (If they really are rich, I may not be interpreting these numbers properly, looking to confirm). Again, thank you all for your input!

 

Long term trims under 10% are acceptable.

I see a little misunderstanding here - even though it's unlikely to be Steve's problem (although, as mentioned earlier, the TPS could be an issue at WOT) electronic throttle bodies do not allow the throttle to 'snap' open as quickly as you can mash the pedal to the floor. In fact, the main reason for electronic throttles is so you can't do precisely that. Electronic fuel injection has no accelerator pump so a certain amount of lag time exists between one's ability to mash the pedal to the floor and the injection's ability to supply the required extra fuel to keep the engine from stalling or hiccuping from the sudden lean condition. Then there is also the killing of air velocity in the intake... the engineers realize that 99.99% of people that go WOT drive like the Duke boys and think their car's best power response comes with immediate WOT. It doesn't. The throttle needs to open gradually to maintain velocity in the intake tract and proper A/F ratio. And by gradually I don't mean
g-----r-----a-----d-----u-----a-----l-----l-----y, but rather
g-r-a-d-u-a-l-l-y, vs.
WOT.
This is the main reason for the existence of 'drive-by-wire'. It never had anything to do with throttle cables or linkage (as evidenced by our cars' cable to the throttle body.) It also simplifies cruise control and traction/stability controls.

 

Actually not, which made the trouble shooting pretty long. I did a coil swap last as that was the most expensive part. Ideally you need to find 2 new good ones, and then change per 2 until the problem disapears. As I had already about 160Kmiles on them, I just changed them all.

 

This is sort of a let down, almost like a false alarm: I tested the car this morning to see if the problem is prevalent at a particular RPM as Gus asked, and it turns out the problem seems to be gone. I guess I will have to just monitor and see if it comes back.

At least I know some things about the car now that I have the ODBII reader; it really is a great tool. I now know that my cooling system seems to be in good working order, as I never saw the coolant temp go above 186 degrees. I am glad to know I don't have a fuel pump issue at this time, that's some peace of mind.

So, I will watch this and see what happens. I will keep in mind what everyone suggested as possible culpripts: Crank Position Sensor, TPS, Coil, (maybe I will replace the plugs anyhow).

I thank everyone who offered advice on the issue, this truly is a great forum.

 

I suggest changing plugs at 60K, you will be surprised at how large the gap will be. At high cylinder pressures, this wide gap can be cause for misfiring. The R models maintenance calls for plugs at 60K.

 

This is a relatively old thread, but something recent happened which I think could help others. My car has been running well since the last post when the "stumble" seemed to disappear. Today my wife and I went for a 3 hour drive and the car ran well except for the last 5 miles or so when I noticed that any more than 1/3rd throttle it would stumble badly, actually would slow down. I always have my Elm327 OBD plugged in so I switched screens on my carpc from my radio to the elm software and all appeared ok, no codes, engine temp 194, fuel pressure around 55 psi which is normal. Then, while watching fuel pressure I put the gas pedal half way down and my fuel pressure dropped to 13 psi, let up on the pedal, pressure came back to 55-56 again. Looks like a bad fuel pump. Oh well, there goes another grand at the dealer, at least i know the cause now though. Thankful to have the OBD plugged in all the time. maybe this will help others diagnose similar symptoms. The important thing here is that the pressure is fine at idle and small throttle inputs, so don't be fooled by good pressure in your driveway.