1994 Jag XJS v12

In my opinion my Jag is using too much fuel in the city.

Trips are usual European style - 35min in a traffic jam and 15 min normal driving (speeds up to 50km/h).
Average - 5-6MPG (35L/100km);

If I'm driving on the motorway faster speeds - 100km/h and more - consumption seems to be normal - 10/14l/100km
But traffic jam - 5-6MPG (35L/100km);

Any ideas if this is normal or not? Otherwise car drives really well, no petrol smell.

 

My '85 XJ-SC 5.3 is doing about 23-28L/100km around the neighbourhood, according to the in car computer. I need to get it on a freeway and see what I can get it down to.
Hope this helps.

 

Sounds a tad thirsty, BUT, traffic snarl with gulp the fuel, especially with the A/C running.

My HE's ran around 15l.100kms in traffic, and Adelaide does NOT have traffic of our eastern states.

11 - 12 L/100kms was the average on highway, and on C/control.

The 85 returned 11.2 for our 25000km round OZ road trip a good many years ago.

Many things can influence this above what I mentioned above.
Rtre pressures
engine oil needs refreshing. Surprising the drag oldish oil has.
Air filters too old
Spark plugs too old.
All small things, but they add up.

 

Jammed solid or nearly so fan coupling will affect it too.

 

Hi Yoopas

Unless I'm much mistaken, a 1994 is a SIX Litre XJS so maybe not so Surprising about that Fuel Consumption

Alex

 

Hi all.
My car according to the fuel computer averages around 19 mpg mixed driving. Sorry I don’t work in Ltrs 100 km😂pretty good I think.
Rob.

 

Yes, car is 6L V12.
60% of the car is newly changed Including spark plugs, injectors, full cooling system, 02 sensors and etc.
Maybe something hidden is causing ECU to think that engine needs to work harder (More rich) during traffic jam situations?
On highway - consumption is really good. But traffic jam situations - 35-40L - computer shows 999 )

 

You could rig up one of those exhaust gas measuring thingies and see what it reads in traffic?

 

Another thought, and related to 6ltr, DUNNO

The Vac hoses to the 2 ECU units, are CRITICAL. As in integrity.

The fuel ECU has the brain wired as such that LESS Vac = more fuel. Maybe these are "leaking". Not enough to screw the engine, but your wallet for sure.

Vac test the line from the I let to the ECU, if it holds Vac, all good, if not, go looking.

Not sure on the 36CU, but the 16 has a short Vac hose INSIDE the unit to the MAP sensor, and that also could be not being friendly.

 

Dear Grant, you are as always very responsive and helpful - appreciate it.

My Car is Convertible - so ECU should be in the trunk (Boot) in the right cornect i assume, yes? - here Vacuum line should go directly to intake manifold - yes?

 

I would assume so.

Never had a Ragtop, dont like them.

Our Lucas cars have that Vac supply under the car, in the tunnel, as a steel pipe, with hoses at each end. These hoses deteriate with age. Also, ours have a habit of oil building up in that line, fumes from the balance pipe slowly building up and restricting the opening bore, and fuel issues appear.

Alex, AKA Orangeblossom, will know, he has Ragtops.

 

Hi Yoopas

My 1995 4.0L has the Ecu on the Passenger Side (UK) Car on the side of the Door Jamb near the Floor and behind the Trim Panel where it is so well hidden that it could take you for ever to even find it

Have a look at this post as it will show you where it is: Location of ECU on my 1995 4.0L XJS

 

Yes RH side just to the side above the battery on my car hose runs from the centre of the crossover pipe under the car then up to the ecu.

Rob.

 

Dear Grant,

Thank you for the information. Both Vacuum lines to fuel ecu and to transmission ECU had some leaks, thus consumption was waay out range. Interestingly engine run perfect. Additionally i have changed both Intake Air Temperature sensors.
Now average city driving consumption is around 27L/100 KM (Around 11-12MPG) - i assume its absolutely normal for 6L V12.

Once again thank you and case closed.