You seem to have everything else checked, so there isn't much left! Fuel side seems ok, so all that is left is breathing. Perhaps EGR? My car doesn't have that, so not sure of that can cause a problem.

 

EGR is also new, maybe it is just clogged up...

 

"my car drives like it has a sock in the intake"
Also, the MAX I can get my throttle open is roughly 72%.

Well if your throttle is not opening wide that would be like having a sock in the intake!
Maybe your traction control servo is preventing full throttle either because it is broken or under control of ecm/tcm and maybe that is a
result of something happening in the gearbox?

 

I have re-read the entire thread and do not notice anywhere that you say whether or not you have any OBD codes. So, I will ask the obvious???"

 

No codes, I think that the coil packs I bought from these guys in November may have reached their point of diminishing returns perhaps?
http://www.buyautoparts.com/buynow/1...ml?xref=228228

I seriously can't figure anything else out....

 

I don't think I have traction control on my car. The throttle cable issue wouldn't cause the mileage to plummet...

 

Could indeed be a factor, but they usually throw a code when they fail (at least on my BMW). Do you have the old coils to swap back in?

You might also try pulling one coil to see what code comes up, if any. Also, maybe pull them all out and clean them (oil?).

Padre

 

They are clean. The Jaguar doesn't do the best at identifying the culprit via the computer too well. I have the originals and will swap back tonight. The company I bought them from (who claim to have great customer service and a 1 year warranty) want me to pay to have them tested at the dealer!?

 

Vehicles with OBD2 protocol can identify the cylinder in question based on the DTC. Based on my experience, a failing coil unit does not always set a DTC nor turn on the MIL.

What is the test the vendor wants performed?

 

He wants it "bench tested" at a Jaguar dealer. Really!? I have to spend money to prove their progressively bad product performance claim? I called them back in January about the same issue, they have my notes on file. Has anyone bought these coils?

 

You may want to consider another source for these coil units.

 

i see in the first post you had the MAF tested
why not just try a new/secondhand one? (part LHE1620AA)

Assuming you have a N/A car, they are easy to come by (XJR AFMs are not easy (LNA1620AA) - I am struggling to get another genuine used one at the moment)

It just might be the MAF - no promises, but easy to swap in and see

 

It's not the MAF, my Jaguar mechanic tested it. The car idles beautifully, in fact, you can barely tell the car is on it's so smooth.

 

Given the amount of money it sounds like you have spent, getting a used MAF and swapping it out seems worthwhile to me. I did that when I had a problem (that was resolved with a new crank sensor) and it eliminated the MAF from the equation. Testing is one thing, in use is another.

I think it's OK to run the engine with the O2 sensors disconnected (although check first) - maybe that would tell you something? My local mechanic did that on his own car when it was playing up and its performance perked up a lot, but it was some crappy euro box.

However, surely you should address the fact that your throttle only opens to 72% That is a clear fault and without eliminating things you know are wrong you could be in danger of wasting time an money.

I am following this thread with great interest but I'm glad it's not me.

 

All six plugs looked like this after 100 miles or so.
New Champion RC12YC plugs @ .035. Think I need to increase the gap .01-.02?


Champion RC12YC

As far as throttle cable adjustment, I'm just out of room. Is there an adjustment at the pedal as well I'm missing?

 

Another left field suggestion:
Idle good, driving bad - could it be the gearbox ?

 

Is the operative word here not the "suddenly" which appears in your title to the thread?
Going back to first principles, it is worth retracing your steps just before the "suddenly" moment to look for clues. You havent provided much information beyond the fact that you used some form of additive made by Chevron. How have you been able to eliminate that as the possible cause? As I said earlier, I am not familiar with the additive in question, and hence don't know how it is administered or what it is intended to do, but it would seem to be something of a smoking gun in this mystery.
When you are trying to accelerate, and the car feels like it has a sock in the intake, I am taking that literally, ie that it is not misfiring, running rough etc ? Is there any visible exhaust smoke?

 

We don't have that Chevron additive in the UK but I did find a review on Amazon US that said they too suffered an mpg loss after using it as well as some posts saying it 'wrecked their engine'. Apparently you can over use it but it's also an additive they put in their fuel anyway. Most reviews are very positive and the Aston Martin forum recommends it.

 

It's Chevron Techron. My Jaguar mechanic told me to use it as my EVAP monitor wouldn't reset and he thought it could make a difference. It didn't ruin my engine.

 

Just a shot in the dark. Could it be possible that your injectors are partially blocked? I am thinking your additive may have clogged things up allowing enough fuel for smooth idle but not capable of delivering enough fuel when demands are increased. Could this also keep the injectors from delivering a proper atomized spray of fuel and just dumping fuel into the cylinder, leading to unburned fuel, thus the poor fuel mileage.