Hi,

As I mention in a thread about brakes, my friend has the same car as me, he drives his XJR in similar conditions mostly shorter local trips with a day a week being driven longer distances.

His on-board computer shows average fuel consumption of around 21, my car goes around 12 MPG (Imperial).

I also have oscillating revs at idle (when in D at lights, etc.)

Can anyone suggest what I can do to possibly improve this?

Thanks,
Nick

 

Hey Nick,

The way I improved my fuel consumption is replacing sparkplugs, coils, and air intake. My mpg went from 15.8 to 19.6 (Here in US, where I live is almost flat so could vary).

Sparkplugs
E3 sparkplugs

Spectre cone air filter

6 coils from amazon

it'll run you about $130 USD

 

Hi jacklynthejag,

Thanks for that, I forgot to mention, I had the original Champion plugs fitted recently and all coils are new (they are the made in Japan ones sold by SNG Barrett, which are rumoured to be made by the people who made the original Jaguar Lucas branded items).

I also have a K&N air filter.

The car idled worse before the coils were changed but fuel consumption is still awful.

Cheers,
Nick

 

Has the fuel consumption always been bad, or has it dropped after adding the K&N? I've heard a number of reports of the oil from the K&N fouling the MAF, so that's a possible culprit.

The other suspects would be the coolant temp sender (best to check with a scan tool what temp it is reporting to the ECU), a stuck open thermostat, bad MAF or bad ECU.

 

Engine Coolant Temp sensor ( solid state thermistor ) biases the fuel ratio rich if it sees cold coolant .

Ice water 32 deg F = 5900 ohms

Boiling water 212 deg F = 0190 ohms

Pull the sensor and calibrate against the numbers . A bad connector goes high resistance mimicking cold coolant thereby enriching fuel mixture .

Clean the MAF sensor as my be oil contaminated by the K & N oil soaked air filter . Should read 1.2 volts ( center Green / Pink wire ) at idle at assumed correct idle speed

 

You may need new Throttle Control valve and recalibration , exclude any air leaks. Also make sure temp sensor working properly. Sometimes new lambda sensors are needed. I get my mechanic to do all this stuff as he knows what he's doing. I can fix basic mechanical things but hate electrical stuff

 

Hi Jagboi and Lady P,

(AL NZ thanks). Regarding your comments on the K&N filter you may have a point. However I don't know if that is the cause. I have had the car for just over a year. When I go it I had to drive it from Devon to Kent (across most of Southern England) got 25 MPG. When it arrived home one of the first things my 11 year old son and I did was fit K&N filter and Andy Bracket...

So don't know...

Will investigate further...

If that is the issue do I simply revert to the original paper panel filter or is there another performance option?

Thanks,
Nick

 

You can revert to the paper filter, but the MAF will need to be cleaned. The paper filter isn't as restrictive as you might think, especially when clean.

I myself don't like K&N's because I knew someone who was a mechanic on a race team. As an experiment they ran 2 identical cars for a season, one with a K&N the other with a paper filter. Filters were changed and cleaned according to the instructions. At teardown, the engine with the K&N had measurably more bore wear than the engine with the paper filter. In his words the K&N only keeps out "boulders", but clearly let in quite a bit more abrasive dirt.

 

+1 on a stuck open thermostat and/or failed engine coolant temp sensor (ECTS). Change the thermostat as a matter of course.

You can perform a basic test of the ECTS without removing it by using an ohmmeter to measure the resistance across the terminals when the engine is cold, then again when the engine is hot. The resistance should be significantly lower when hot, per the range posted by Lady P.

Cheers,

Don

 

Hi,

My XJR6 is now with the mechanic I use. He says he is getting some odd readings from the MAF sensor

At tick over 0.0 g/s
At high revs the highest reading = 0.2 g/s

Is this normal or is the MAF dead?

Thanks,
Nick

 

Hi Nick,

A general rule of thumb is that the MAFS reading at idle in grams-per-second will be approximately equal to the displacement of the engine in liters, so on your 4.0L AJ16 engine the MAFS should read around 4.0 or 5.0 g/s at idle, and at higher revs the reading should climb to 20 or 30 g/s or so. Either your MAFS has failed or there is a problem with its wiring or electrical circuit.

Cheers,

Don

 

Hi Don,

Thanks for the information, it did indeed look like a dodgy MAF (or circuit), will share this information tomorrow.

Now if the MAF is dead, then the old quandry arises second hand or non-branded one...

Thanks,
Nick

 

The EVAP valve taps into the MAF sensor wiring . Possibly the EVAP us sucking the MAF signal down . Try disconnecting the EVAP valve located under air filter assembly . Run the engine at idle and see if you read the 1.2 volts on the center ( Green / Pink ) wire of the MAF

For non - S/C engines the shared wire also goes through the Canister Close Valve through connector Papa Indy 161 pins 1 and 2 . This valve not always installed .

Also check the ECU connectors for corrosion

 

Good points, Lady P. The fuel injectors are also spliced to the Black/Light Green wire shared by the MAFS and EVAP Control Valve. It's late and my brain is tired, but at first glance it appears this Black/Light Green wire supplies B+ voltage to the injectors, Carbon Canister Close Valve, and Evaporative Emissions Control Valve, the EGR Valve, and possibly other components I missed.

Cheers,

Don

 

Don ,

Something is not right , the sensor and injectors pulse width should be feed by a fixed reference voltage as B + changes . My Brain Hurts

I did find an error on the EGR valve pin location / wire color on my Lady P . as I have wrang all the wires on the engine harness before .

Papa Indy 104 - 24 and Papa Indy 105 - 17 are not listed in the wring out sheet as to their values .

 

Hi Lady P,

What I interpret from the schematic is that the BLG circuit supplies B+ voltage via the fuse you note and the ECM-Controlled Relay to the injectors, MAFS, EVAP Control Valve and EGR valve. The connections to the ECM in both the High- and Low-Power sections allow the ECM to monitor B+ voltage. According to the EMS manual, the ECM operates each injector by connecting it to ground/earth, and it adjusts the pulse width in response to the current state of B+ voltage. I assume but have not confirmed that the ECM may also adjust its interpretation of sensor values relative to B+ variations.

Cheers,

Don

 

Don ,

Never could determain where the ECU gets its first power from as the ECU controlled relay must first get a commanded ground by the ECU to make the relay close which the goes to the 2 monitor pins in the high and low halves of the ECU . I can see where the monitor pins can also be used as a power source but then the ECU must have a first power source .

Saw a pinout of the Lucas / Sagem GEMS -8 versions in my files for the Morgan Plus 8 sports car .

http://www.gomog.com/allmorgan/GEMS/GEMSbyPoole.pdf page 39

and GEMS - 6 for the X300

http://www.jagrepair.com/images/Trai...20-%202000.pdfPage 59

Found it , I think

Editing

 

It looks like the ECM receives its first power via the ignition-switched circuit 32II on White/Pink (WK) wires, which also supplies power to the ignition coils and connects to the ECM at PI 103-33. It appears the ECM operates the ECM-Controlled Relay by grounding the Brown/Purple (NP) wire that connects to its pin PI 104-18. Does that make sense?

I have that Morgan document and EMS Training Manual; I'm attaching a few other GEMS documents, including one for the XJS and one for the Land Rover implementation, in case you're interested:

Cheers,

Don

 

Thanks , looking into it . But we're digressing from the thread starter original issue a bit . But more information is useful .
What you say makes scene as I cross reference missing information that was found in the XJS doc with respect to the X300 PI105 connector pin 33

 

A couple of left field suggestions re high fuel consumption:
-A binding brake.
- A fuel leak from a rusty fuel line. X300s are well known for fuel lines rusting out, and a pinhole leak would tend only to leak under pressure, so no puddles under the car (yet!)
As I say, just a thought.