IIRC correctly, and I might not (and my GEMS experience is P38, not Disco), there is no fuel filter other than integral to the fuel pump.

 

No the D1 and Defender's have an external fuel filter in the line - D1 is located passenger rear wheel well area - probably classics too

 

For fuel trim data it is important to note whether negative or positive. Negative values are pulling fuel because the engine is running rich. Positive values are adding fuel because the engine is running lean. Ideal value for LTFT is zero for factory conditions. Normal good values should be + or - 5% or so, extreme limits are + or - 25%. STFT values depend on operating conditions. Extreme limits for STFT are + or - 25%. If STFT rides at the limit for long enough time, then the LTFT gets adjusted. Since LTFT are negative, that shows rich condition. That could be impacted by faulty ECT sensor, if it reads cold even when the engine is hot, like Walt suggested above. ECT replacements are inexpensive and easy to do. A faulty ECT might not throw a code. The ECT sensor is not the one terminal temperature sender that feeds the dash gauge. It is the two terminal sensor, near the thermostat. Is your engine reaching the normal operating temperature? Around 170 to 200 deg F on the scanner (minus 40 is bad). Rich condition is indicated by low oxygen in the exhaust gases, since over fueling burns up all the oxygen. Rich condition shows up as O2 voltages near 1 volt, lean condition goes down near 0.1 V (approximate values). A good running engine modulates between those values. Middle of range (stoichiometric) for the O2 sensors is 0.45 V or so.

If MAF readings flash to zero intermittently, that could indicate a problem with signal or ground wires, or the connectors, or the MAF. MAF signal voltage range is zero to 5.0 volts (not sure how many g/s). In the garage, I get about 1.3 V at 700 rpm, ramping up to 2.3 V at 3000 rpm. Sensor ground is the red/black wire, sensor signal is blue/green. Use pins to back-probe the connector, then clip on the voltmeter with alligator clips.

 

A lot of life happened in 3 weeks, so sorry I did not reply sooner, asI had not had chance to touch the discovery. I really appreciate all the help and guidance.
The MAF was the problem, I found a genuine used MAF which arrived yesterday, put it on last night and now 70 miles of driving later we are going good.
Fingers crossed the problem is fully solved.
The MAF which failed had been in the vehicle since October and lasted 11,000km, it was a Walker brand.
Thanks again
Dave

 

Great news. Thanks for letting us know what it was.

 

Walker seems to be crap

I would go with an original LR part if you can - or Bosch
LAND ROVER DISCOVERY 2 TD5 GENUINE LAND ROVER AIR MAF SENSOR MHK100620LR | eBay

 

Congrats on the DIY repair! And thanks for the feedback on the MAF. MAF hot wire elements need to be cleaned from time to time if grime builds up. MAFs produce a voltage signal, around 0.3 V with key on, 1.6 - 1.8 V at idle, then ramps up to nearly 4.5 V at full engine load around 4500 rpm. Low voltage signal due to dirty hot wire elements might have contributed to your problems, as this causes low fueling problems. I need a magnifying glass to clean the hot wires, but some just spray cleaner when doing oil changes. Cleaning is especially needed when an oiled aftermarket air filter is used.

With things running well, now it the time to familiarize with correct O2 voltage trends (real time data), and Fuel trims. Do a couple scans and write a few notes. That base-line information makes it easy to see when things are all good, as well as when trouble pops up. The whole system needs to work in concert in order for O2 trends to look right. Glad you can enjoy driving it now.