Cheaper 4.0 ECU upgrade for a 4.6??

I know people commonly use the 4.2 ECU from a LWB Range Rover Classic when running a 4.6. Might be a more cost effective option

It won't run lean. They use mass air flow control with oxygen sensor closed loop. They compensate for the higher air flow automatically. A re-map helps it work a bit better, but it is not a big difference.

Tyler Wirken said:

I am working on my rebuild from a 4.0 gems to a 4.6 and have done all the research I can find about the ECU and how it really needs to be remapped for the 4.6 but all I can find for solutions is to either purchase a remapped ECU or have mine sent off to the UK which is proving to be costly.

So, anyone have a better solution to avoid the cost of a new ECU and the issue of the engine running lean?

I read that using the PROMS out of a Range Rover 4.6 is problematic as it throws transmission codes. Other than the CEL coming on is there any other issues with considering that as an option?

Lots of great info here especially on the thread by ECR just hoping someone might have a better option to save some cash. This rebuild came at the wrong time.

Thanks for the help. Oh and if anyone has any questions about these motors and the conversion feel free to contact me. I have learned quite a bit.

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Sent you a PM.

Isn't the 4.0 vs 4.6 just a toggle in the TestBook 4 ?
I have only used discovery 1 ecu's, never used a p38 one.

I ran a 4.6 with a 4.2 chip for years. I went with a custom chip from Pendy eventually, hoping for a performance upgrade, but it ran about the same. FYI.

Thanks for the responses. I have read that you can toggle the ECU to 4.6 but it actually does nothing but I have also heard that is all you need to do.

Plus this statement from a company in the UK that I inquired to- "I would not advise you drive round on the 4.0 litre map long term. A few miles to ensure the engine is not knocking etc is OK but long term and any real load will start to see the engine running lean which could of course result in you needing to do another rebuild."

Thus my confusion on this subject. But I appreciate all of the help so far.

------ Follow up post added April 11th, 2018 08:06 PM ------

Those 4.2 chips are only for the 3.9 14CUX ECU's correct? Or can they be used in a GEMS ECU?

Logic tells me that if I want to remap the ECU I could use a 4.6 Range Rover Gems PROM and all would be good. My CEL is always on anyway due to a bad rough road sensor computer. Has been for at least a decade. So having a CEL on won't be an issue with me.

Davis said:

I ran a 4.6 with a 4.2 chip for years. I went with a custom chip from Pendy eventually, hoping for a performance upgrade, but it ran about the same. FYI.

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The 4.2 chip that I am referring to is only for the 14CUX.

Davis said:

The 4.2 chip that I am referring to is only for the 14CUX.

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Thanks. That is what I thought.

I've been using my 4.0 ECU with the reman. 4.6 for almost a year now, and seems to be running fine.
Truck cruises at 75-80mph on flat roads, whereas the 4.0 would struggle to keep 75 before.

The 4.0/4.6 ECU are the same. There are actually 2 mapping chips inside the GEMS ECU that need to be changed if you are feeling like spending the $$$. Alternatively you could do the "slide" with Testbook/Auto Logic and get the similar results as rechipping it. I have done more than a handful of these over the last 15yrs and had zero adverse effects to the engine by just doing the "Slide" instead of the rechip. These ECU's are adaptive and will make up for the .6 of a liter of extra engine. Rechipping only comes into play when you really start messing with the displacement and modifying the OE fuel demand...5.2 and bigger engines. You'll be fine with a 4.0 to 4.6.

What is the “slide”?

Go in with Testbook/Auto Logic and slide your finger across the button that tells the ECU that you have a 4.6 instead of a 4.0

I think thats only for p38s and is really picking a different set of maps similar to a 14cux tune resistor. A GEMS 5 speed shows up as using the 4.6 map so Im thinking the original intent was one set of chips for all 4.0 discos and the map flag selects 5 speed or auto, but there must have been a problem with the base code as there are seperate chips used for auto and 5 speed, but the flags and targeted maps were left in use. So all a disco/90 flag change does is pick the second 4 liter map tuned around a 5 speed.

The auto/manual option interestingly is locked at auto on all ecu’s Ive encountered

This is based on looking at it with a faultmate. They may just have just misidentified the data and not realized it

Thanks for all the help and insight. Makes me feel a bit better.

So what about using the chip out of a Gems P38? Heard that is a great way to go as well.

Might get the more out of the motor without having to spend the $$$. If so anyone have a source for those chips?

Tyler Wirken said:

... If so anyone have a source for those chips?

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I've got my left over stock chip set someplace. I replaced them a few years back with a Tornado Gems P38 chip from Dan Harris (TigerDan) here on the forum. He's a distributor out west for Mark Adams. Very happy with performance since.