Thread: D2 running rough under load after 4.6 conversion

I've just changed out the 02 sensor that was throwing a code so I'll see if that makes any difference.

I'm not hopeful

What mark Adams says here may be some of your symptoms
Having the Injectors serviced may also help
and a 4.6Maf sensor.

Thank you for your message - it's good to hear from you. Although this is a simple question at first glance, it does deserve a detailed answer. I hope I am not swamping you with information!

Whilst the Motronic system will run a 4.6 engine even with a 4.0 Litre tune, you certainly won’t be getting the best out of it. As your motor has Oxygen sensors in the exhaust, the fuelling runs in closed-loop which gives it the ability to trim the fuelling to suit the engine. Of course it will only be using the preset factory targets for this function. The ignition has knock detection to prevent detonation, which will also trim back any dangerous levels of ignition advance.

A couple of years ago when nobody could do anything with these systems, the stark choice with a capacity upgrade was to drop it in the hole and see what happened. As you can see from the last paragraph, Motronic will have a good shot at making the best of it. Indeed there are hundreds of these vehicles running around with 4.6 engines operating on 4.0 Litre tunes.

The fact that they work has lead to the general opinion that the conversion works, which is evidently true. However I will explain why you can get a very big improvement by reprogramming the ECM to suit the new larger engine.

As a bit of background, there are over 150 versions of the software for the Disco and it is important that you have the correct one. Earlier systems such as GEMS had software that would cope with pretty well any configuration of the vehicle, with just a few software switches. Although there are four types of Motronic ECM, the software is unique to each possible configuration of the vehicle.

There are some solutions for this one, although none of them are ideal. Sending the original ECM puts you off the road for several days at least, even if using a courier such as DHL. As you say, for a daily driver this is a no-go.

A couple of my clients have sourced a similar ECM locally, and used that whilst they sent their own over here. As mentioned above, it does need to be the same version. The chassis number and software version are always printed on the casing at the factory, but the label is made of paper and often peels off. It is theoretically possible to obtain the correct ECM, and load it with software over here and send it out. However you would lose the programming history of the vehicle this way. I am working on a way around this, but haven’t got there yet due to the encryption involved.

In all cases when using the non-original ECM, it is necessary to use specialist test equipment to re-learn the security code for a Discovery. This is an added pain and expense unless you already own such equipment.

Anyway - that is all the negative stuff. There are considerable benefits to be had if you can take the hassle.

The ECU mapping will need revision to accommodate the larger capacity, and here are the reasons why you would get a bad result from a 4.6 engine on a 4.0 map. The engine's fuel and ignition requirements are determined by engine speed and load. Engine load is determined by measuring the true mass of air that the engine is consuming at any particular speed, using the "Hot Film" Mass Air Flow (MAF) meter.

Mass airflow is proportional to the torque that the engine is producing (for this type of engine), and hence it is closely related to the engine capacity. This is also heavily influenced by atmospheric conditions such as barometric pressure, temperature, etc.

Therefore it is vital that the software is programmed with the correct maximum mass air consumption of the engine, known as airflow meter scaling. This is vital for good driveability and fuel economy especially on large capacity engines, and allows correct control of part throttle fuelling. Note that airflow meter scaling determines when the top of the fuel map is reached. When using an engine of larger capacity than the software was designed for, the top of the map will be reached too soon. This leads to over-fuelling at part throttle, and under-fuelling at full throttle.

A re-profiled Fuel Map which provides good fuel economy on part throttle, and allows engine to rev more freely (standard profile reduces fuelling heavily after 4000RPM). This produces quicker, more progressive throttle response and better mid range torque.

Although the ECU can work around large errors in the fuelling where Oxygen sensors are present where the map is significantly incorrect, when it reaches the limits of adjustment it will upset the entire fuelling learning process.

When re-calibrating the Motronic ECM for a larger capacity engine, there are actually over 38 maps that need re-scaling for both engine load and speed. This level of detail is one of the reasons why the Tornado upgrades are not cheap, although I do believe that the quality is unmatched.

The following information refers to either upgrading a 4.0 or 4.6 vehicle in the same capacity, but the gains are much larger when going from a 4.0 to 4.6. If you want the technical details of why this is so then I would be very happy to provide them.

The Bosch engine is mapped in such a way that it produces maximum power when it hits the Rev Limiter, which is set at 5500 RPM. At this point the mixture is rather rich, but the ignition timing is fine for good power production. Therefore the peak power will not improve more than 2-4 BHP with any upgrade, unless the Rev Limiter is moved up to say 6000 RPM. Although I can do this and the power is still increasing when it hits the Rev Limiter, it would be something of a “smoke and mirrors” or Snake Oil improvement. Power increases here are largely academic, since the vehicle will change gear long before this limit is reached.

However the dominant engine characteristic is actually a huge hole in the torque curve, centred around 2500 RPM. This is filled in very well with the upgrade, delivering an extra 10-15 lb/ft of torque and much faster throttle response. Economy improvement is 10-15%.

When driving a standard vehicle it actually pulls away from a standing start quite well initially, but loses interest once it gets to 3000 RPM. Personally I find this trait especially annoying, and it doesn’t even sound like a V8 either! All the action happens on the first or last quarter of the throttle pedal movement, and the bit in the middle doesn’t make a lot of difference. It is also a real nuisance to my clients who tow, since it often means the gearbox will kick down two gears when they only wanted a bit of extra effort (on a gradient for example).

Once I have extracted the software from your ECU, I can clone it in future if you should ever lose or damage your original. However, if it gets lost on the way up then it would be necessary to have a new one installed and programmed using TestBook or RovaCom – expensive hassle you don’t need!

Although I can happily send you the graphs for a 4.6 Range Rover or 4.0 Discovery (since we don’t have the 4.6 Discovery over here), they aren’t very helpful due to the fact that the vehicle is automatic and we have to test it in third gear. This brings the attendant problems of kick-down in the middle of the run (right where the best gains are), and the graphs are a bit of a mess. Whilst vehicles with manual transmission do exist which would give the most meaningful results, I have yet to see one.

While I should be the last one to poo poo Mr Adams,, the 4L ECM will run the 4.6 under load,, trust me on this.
so its something else.
running old and new cam gear is never a good idea,,
new O2 sensors usually help the error code readout-- but I couldnt tell any difference by the seat of my pants
running the 4L pistons with the 4.6 crank will give higher than standard compression.
If you cant hear it ping/knock dont worry about the compression.

I agree Pedro I was thinking the same. and the cam thing too. I just wanted to try a standard cam again after the Crow cam as I was SO completely underwhelmed by it.

I changed out the 02 Sensor so no codes but no change.

I'm thinking stuck lifter at the moment. Maybe holding a valve open. Although it didn't show up in the compression check I did the check cold and sometimes it will take up to a minute for the lifter noise to start. I'll do another check at temperature.

Then tonight it's off with the intake manifold and I'll see if I can see anything. I'll probably change the cam out as well if I can't see anything obvious and put the Crow cam in (it's only got about 3K on it but at least it was quite). I'll also pull the sump off and have a look at a big end and main just to see it there is anything going on there. I dropped the oil out last night and put it through a paint strainer and didn't find any bits in it except for the normal running in sheen in the oil. Also cut open the oil filter to check, again no bits. (now thinking it might be just as easy to pull the engine out completely)

Cause I'm new here, just for a bit of my history. I'd just like to mention that although I'm not a auto mechanic I am a marine engineer & diesel fitter and I'm not a Rover V8 virgin. I've had Rover and Leyland V8s continually since I was 14 , now 47 (my first car was a P76 And I've still got one as part of my fleet). I've rebuilt dozen or so Rover/Leyland V8s and converted my first 81 RR 2D to a 4.4 with Rover heads. My fleet of cars is of course the Disco 01 ES V8 (wife), my daily is a 01 XJ8 4.0 Jag, my toy is a 81 Factory Turbo Sigma (1 of only 500 made), the hobby car in the garage ATM is a 74 XJ6 Jag and out the side of the house waiting for some love'n is the 74 P76 4.4 auto Executive.

I'm thinking stuck lifter at the moment. Maybe holding a valve open. Although it didn't show up in the compression check I did the check cold and sometimes it will take up to a minute for the lifter noise to start. I'll do another check at temperature.

hi FisherX and welcome
re: lifter , usually if lifters are noisy they will not hold a valve open , quite the opposite as the noise is indicator of excessive clearance (bent pushrod , collapsed lifter ,worn cam lobe ,bent valve , low oil pressure etc...)pulling engine out/apart sounds a bit extreme especially if no other knocks/ rattles and good compression

Thanks for the welcome Discorevy,

I'm thinking it just might be easier pulling the engine as it only takes 2 hours and saves sore knees and back working from above and so much easier working on it on a stand.

The engine only has 300km on it since rebuild and the noise and roughness has me troubled. I'm thinking I'll change the cam regardless as it was the old std cam and is probably worn out even though it doesn't show much wear. With the engine out I can just double check everything for $80 worth of gaskets.

But I'll pull the intake manifold first and then decided if I'll go further depending if I find anything.

I'm not sure if you can pull the cam with the engine in without disturbing the A/C condenser.

I had a 4L cam in the 4.6 with worn everything and since replacing it sucks ass almost as much as fuel (@33L/100)

???????