Apologies in advance for the long post but lots of detail necessary.

I am looking for some suggestions as to where to go next with a minor problem with my 3.9EFI Discovery 1 please. It’s completely standard with no modifications. I have had the vehicle for five and a half years and covered around 14,000 miles in that time. Total mileage is now 164,000.

The problem is intermittent with idle on start-up, both hot and cold. The engine does not always settle to a steady idle for the first fifteen seconds or so but will occasionally hunt between almost stalling and running at 1,000 rpm. Sometimes it’s fine and does what it should do, particularly from cold. The engine runs fine, pulls well with no hesitation and manages just under 16mpg in general usage. Out of town, consumption improves significantly.

The basics like timing, plugs cleaning the MAF etc. have been done. The fuel temperature sender was changed a couple of years ago as was the fuel filter, coil and the ignition module bolted to the side of it.

I purchased a Hawkeye Total and the only fault code that comes up is ‘23, fuel pressure’ but once cleared it doesn’t come back for a few days. I suspected that given the age of the vehicle, it could be the injectors dribbling and making the engine run too rich on start-up until it had cleared, so I checked the fuel pressure in the rail. It’s a steady 32psi with the ignition on and also with the engine running. If I disconnect the vacuum hose to the fuel pressure regulator, it increases to 40psi. After the engine is stopped, the pressure in the rail remains at 32psi for at least five minutes which I believe rules out leaking injectors.

I am going to change the fuel pump relay and the fuel filter as the next step.

I have run the Live Data scan on the Hawkeye; this showed up a problem with one of the Lambda sensors which was found to be loose in the exhaust pipe. Once cleaned and tightened, the Lambda readings are equal.

I have cleaned and checked the Idle Control Valve. This is showing ‘180 steps’ on the Hawkeye live data. I swapped it for another (used) one that I had and this is reading around the same level. Am I correct in thinking that this should be around 110 rather than 180?

I am beginning to think that the idling problem and the fuel pressure fault code may be unrelated. I am planning to remove the fuel tank and replace the rusty fuels lines at some stage this autumn and will probably change the fuel pump at the same time.

I would appreciate any suggestions as to where I go next with the idle problem please.

First make sure the base idle is correct, the ECU then controllers the idle, based on information received. Cleaning a hot wire MAF has been known to give issues because of damage to the sensor.
Also have you cleaned the crank case breather. flame trap and it's hose to the plenum, a clogged breather is also know to give stalling issues.
There's no need to replace the fuel filter if it was changed a couple of years ago or the relay for that matter, if as u say the engine is running fine. The air filter is more important.
Also the ignition amp should be mounted remotely, as standard it would be on the near side of the engine bay next to the coil.
As as your vehicle has such a high mileage I suspect that more than one item could be faulty.




Sent from my iPad Air on a train.

Thanks for the response. The base idle has never been touched and 99% of the time, the vehicle idles around 700rpm. it's the cold start and hot start idle immediately after the engine fires that is the problem.

Breathers and hoses have all been checked. I think that the ignition module is bolted by the side of the coil as you suggest rather than to it. I will check tonight.

Regarding the MAF, this was cleaned a year or so ago and hasn't been touched since. This idle problem started after a period of a couple of months in a bodyshop having the sills professionally replaced.

It maybe an early LR V8 thing but I've found that over the last 29 years of owning one, other than the cleaning the crank case breather when the idle gets a bit erratic it's time to use some injection cleaner in the fuel, which soon puts things back to normal.

I have just checked the ignition module and, as you said, it's beside the coil rather than bolted to it. I have checked to operation of the idle control valves and both of them work in the same way in that they both extend when the engine is started and retract when it is shut off.
I took your advice and checked all of the breathers, removed the crankcase breather and cleaned it out in the wash tank. Reassembled everything and it does seem much better including a hot start fifteen minutes after a ten mile drive.
The previous owner used a garage who used a large dose of injector cleaner prior to every MOT presumably to help with the emissions test. it still ran like a bag of nails when I got it but the problems were everywhere else but the injectors.
I actually have a 30,000 mile factory replacement engine to fit in the Autumn but wanted to clear up the idle problem before doing so. I have had the injectors checked and ultrasonically cleaned prior to install of the engine.
Thanks for your suggestions. The next few days should see whether the problem is resolved.

Also resetting the EFI ECU back to default is something that may help, just disconnect the vehicles battery, have a cup of tea/coffee and reconnect and start the engine to let the ECU decide where and what it needs to do, as simple as that and you should see some benefit on doing that.

Thanks for the breather suggestions Discool. I cleaned the breather on the offside rocker box and it seemed to resolve the problem ..... until the hot weather disappeared and then the intermittent cold and hot start up idle problem came back. I checked everything again, fuel pressure, fault codes, live data and nothing showed up.
I did some more research on line and discovered that the V8 uses two coolant temperature sensors. One of these links to the fuel injection ECU along with the fuel temperature sensor. Apparently they are both prone to failure and do so by going open circuit. My fuel sensor was giving readings in the correct range but the coolant sensor (PRC3505) was open circuit hot and cold. This one is positioned in the thermostat housing. The fault symptoms in the article were almost identical to the problem that I have. New Lucas sensor tracked down and on it's way.

The temp sensor PRC3505 in the thermostat housing controls the A/C fans in an over temperature issue, nothing else, the EFI temp sensor is AMR1425 it has a black top and is positioned just behind the distributor, PRC8783 is the dash temp gauge sensor for an auto, that's just from memory so numbers may have been superseded.

In my experience of EFIs of past, the ECU temp sensor indicates it's iffy when the engine reaches working temp with engine misfire...