P0128 is either a temp sensor or a thermostat in all likely hood. That is the reason the car is using so much fuel. When the PCM thinks the engine is cold it will run it rich until it warms up. A bad thermostat or temp sensor will make the PCM think the engine is always cold, hence the running rich/burning way too much fuel. Is the P0128 a pending or permanent code? If it resets immediately after clearing, then you may have sensor unplugged or broken wire or something like that.

 

The P0128 code is a permanent code. I had to drive about 75-100 miles after clearing the codes for it to show again. The ECT sensor is only about $20 so I'll replace it and see if that solves the problem. If not, I'll check the thermostat. (I'm thinking the sensor will be easier to replace?)

 

I have replaced the ECT sensor and cleared the codes again. The car is driving very well and I am waiting to see if the code reappear in the next 100 miles like it did the last time.

 

Argh, so frustrating...the engine light came back on with the following codes:

P0175
P0456
P0172
P0442

The Jaguar Codes list does not include any of these codes. Any help is appreciated.

 

Well not so bad. You did get rid of the PO128 so you are making progress. Good repair on that one.

The two 400 series codes indicate leaks in the vapor recovery system. This can as simple as tightening or replacing your gas cap.

The other 172 and 175 are rich codes. Have you looked at or cleaned the MAF?

I am not sure but is the problem at 5500 RPM still there or is that fixed?

Sorry just losing track of where you are at!
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Yes, the P0128 code is gone I will check the gas cap. Let's hope it is that easy. I have not had a chance to check the problem at 5500 RPM as traffic has been busy and cops have been out lately, doh! Restricted performance keeps going off and on if that makes any sense? Also, I have replaced the MAF sensor and that made a tremendous step in the right direction...almost night and day difference!

 

Sometimes the MAF plug can be a problem. On the STR it points straight up so if it's not sealed properly water will get right in.

People have replaced the plug or used dielectric grease to keep the water out.
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I'll definitely try the grease. Thanks for the tip!

 

Your model and year is the same as mine, so I may as well chip in here. I had restricted performance go on and off, on after 1-2 seconds at WOT, off after another hour or so.

Turned out to be the S/C coolant auxiliary pump, not to be confused with the regular coolant pump. Only on STR's.

Does not sound like your problem, but you never know.

 

Thanks for the info Robinb. I'm still trying to narrow things down at this point so I will consider that too. Thanks!