P38 Engine Management ECU Compatibility?

Hi all,
first off, hello. I've been reading the forum and the pages for a few weeks since I acquired a P38 (and new way of life, in garages, trawling breakers yards, etc), and I'm hoping that someone here can point me in the direction that I need to go!

I already love my P38, but it does have a few 'issues'. Many I've learned and fixed ... mirrors, keyfobs, battery, slight EAS droop, etc, alone or with some help from the garage. I have a major future challenge with the HEVAC (the well known blend motor issue), but the problem I'm looking for guidance on is more complex and serious .. the engine management ECU. Here goes....

Short history:
- car drove fine, so far as a novice could tell, on test, except a slightly low battery
- drove 100+ miles at Xmas in Scotland, on New battery, no problem
- then coolant temp guage flickered intermitenntly, faulty engine coolant temp sensor was diagnosed, has been fixed.
- however, garage advise other problems
- starter motor.. apparently always drawing max current (hence the flat battery?)
- stepper motor (Engine Idle Air). Was apparently 'in bits'.
- Engine management ECU ... apparently is not setting the correct engine idle target speed

The garage seem to know their stuff, specialise in 4x4's, esp. Landies. However, Engine ECU is serious stuff, and a new one would be ludicrously expensive hence I need to understand if I really need to replace, and if so, with what 2nd hand part.

So, the questions:

1)is it reasonable of the garage to say that I definitely need a new ECU, before the new stepper motor has been fixed? I guess so, since the ECU sets the speed and the air?

2) What ECU do I need exactly? To suit a P38 1995/M 4.0 SE. Garage thinks Part# ERR5760? I've also seen reference online to ERR6645, ERR 5499, etc. How interchangeable are the ECU's?

3) Can the ECU's from 4.6 HSE's be used instead? (many breakers/ebay sellers suggest so, garage says no)

4) Can the ECU's from 4.0 v8 Disco's be used?

5) What do the various codes on the ECU label mean?
i.e. "ERR6645 GEMS 8 XXXXX", where XXXXX is a 5-digit code. Are the X's important, or is just the
part number important for assessing suitability?
Similarly, "Type code : 2796". Is the 'type code' relevant?
What is GEMS anyway?

6) Is there a fountain of knowledge on ECU's online somewhere? The service manual is a bit light on the topic.

I'd really appreciate any guidance anyone can give. I'm quite willing to spend some more time reading and learning, if someone can point me in the right direction. I couldn't find the answers I'm after in the forum pages or other sites, though I have tried hard.

Crossing my fingers now....
R

Bump (initially posted this late last night, when the forum was quiet)

Firstly-starter motor, o.k it's drawing excessive current to start the motor, the alternator should replenish this after a short while. A new starter will cost you around £60 from Island 4x4. Secondly, replace the idle stepper-cheaper than an ECU and the ECU's rarely go wrong.
If you want first class, second opinion, ring Jameson 4x4 in Summer Lane, Birmingham (John,Paul or Andy) and they wont bulls**t you or rip you off.

Replace the IACV and starter and see what happens.If it still plays up then look for a replacement Gems ECU for a 4.0ltr.There will probably be 3 or 4 part # changes from 1995 -1998. From then on It was Bosch Motronic as on the DII 4ltr -totally different to your RR,even different on 99-02 RR's.(Different immobilisation etc)
Any of the Gems 4.0 ECU's will code to your car with 2 mins on Testbook,the 4.6 variants will work but have different fuel/ignition maps.
As said tho,they are reliable -fix the easy bits first.

Thanks Guys,

suggestions are very sensible. Have already acquired new IACV and a secondhand starter (as well as spending all morning picking over the remains of crumpled P38's at Douglas Valley Breakers in Wigan) , which I'll switch for new as suggested. I'll put these on and then get it tested again. I'll also run it a bit, after all I can't see how the ECI behaviour can't change in response to new inputs (coolant temp sensor + indirect impact of more air via IACV.

What surprises me is that when I start her up, she runs at ~900 rpm, which seems Ok to me, nothing like the 2,000rpm+ I see on other posts relating to similar problems. But then I guess I haven't seen the Testbook output, will make sure I'm there in person next time. I'll also check up on Jamison's, and I see there's a bloke from Northampton posting elsewhere on the forum also, which is close to me, so I'll drop him a line too.

It's just occurred to me that the trip computer and cruise control don't work either, and I wonder if this might be connected, not that I need them.

On the potential replacement GEMS unit, can you tell from the labels if the units/part# if the unit was on a 4.0 or a 4.6. It would appear not from the ERR number, but do any of the other labels give a clue? (of course I guess there is no guarantee that an ECU off a 4.6 didn't start life on a 4.0.

Thanks for your guidance and reassuring words,

R

Wills is the guy from Northampton. On the ball with the electrics, may be wort a PM,