I've now owned my car for two months and starting to think there is something not right with the fuel ecomony.

I average 13mpg (UK!!) Mixed driving, and on a 450 mile trip from Scotland to London, averaged 17.6 @80 mph. This I feel is well below what I should be achieving.

I have on occasion felt small missfires, and think there could be a small manifold crack (although I cannot hear it from the engine bay - only when driving so this may just be a normal XJR noise!)

How can I diagnose coil packs? I've already changed the plugs which made no difference.

The previous owner recently changed the 02 sensors for pattern parts, just a thought that there could be poor quality?

Anyway any comments appreciated. I guess I'm probably looking to take the car to a specialist with diagnostic capability in the London area. Any recomendations?

John

 

Your fuel economy is too low, yes. My XJR get 12-13 mpg in city driving and 20-23 on the highway. In UK-gallons that would be about 14-15 city and 24-27 highway.

A cracked exhaust manifold can trick the fuel injection system into thinking the mixture is too lean. In response, more fuel is added....so this needs to be checked. And, oddly, when they crack they often make no noise!

A faulty coolant temp sensor...one that sends a "cold" signal to the ECU when the engine is actually warm..... will also trick the system into adding more fuel

Coils can be checked with an ohm meter (.75 ohm primary resistance). Of course they often don't outright fail so testing might not be conclusive. Also, look for charring marks in the spark plug wells and spark plug boots. Remove the coil cover and run the engine in a dark garage.....sometimes a faulty coil give a little light show in the opaque epoxy material.

Aftermarket oxy sensors should be OK if they are they correct type. I don't know anyone with deep enough pockets to buy Jaguar-labeled replacements :-)

Lots of other "could be" possibilities. Scanning for trouble codes would naturally be the first step.


Cheers
DD

 

you should take a hard look at the rochester valve, otherwise known as the charcoal canister purge valve. Its right under the passenger seat drivers side underneath the car. If you fuel trims are crazy rich this would be my first fix and its really cheap and fast to do. take a look at my post about the rochester valve. Let us know. David

 

and Andy's bracket won't do you any harm either :P

 

OK great some very good replies.

I've been working away all week but have a few days off now to play about. I shall report back.

As far as andy's bracket is concerned I have emailed him about one already and once he has enough orders will have one. Although I'd like to gte the car working 100% before modifying anything.

Very interesting about this rochester valve. My car does smell very rich, although I've never had idle problems.

Thanks everyone.

John

 

T-stats prone to stick open, which will have the same net effect. Although mine (NA not SC) was stuck and fuel econ wasn't that bad.
Is heater performance up to snuff?