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Discussion Starter #1 (Edited)
Hi, first post on the forum , thanks in advance for any help ! I purchased the RX4 as a non runner , supposedly stood for several weeks with flat battery then wouldn't start after battery replaced , tested pressure at pump ( 300 bar guage direct ) initially 50 bars on crank , interupted the negative wire to the pressure regulator and temporarily earthed it and 200 bars on crank pointing , I think , to an ecu fault ( possibly last owner connected reversed jump leads when attempting to jump start ?)
Can I replace the ecu with a used unit ( replacing the chip in the key also ) ? Do injectors need to be coded ? Will the locking side( BCU ? ) still work ? I am trying to spend as little as possible until I know more about the car !
 

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Super Moderator Technical Supremo Platinum Member
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Just saying, many people blame the ECU when it is something else.
But the black box is the easy thing to blame.

Low pressure can be many things, including leaky injectors, leaky fuel feed so fuel is not getting to HP pump, low crank speed, sticky regulator, duff wiring.....
At the moment, you seem to be guessing and basing conclusions on what the previous may have done.
If you really believe it is the ECU, before replacing it (needs a virgin unit or complete set of UCH, ECU, Keys, Locks), send it off to one of the companies that will check and repair (if needed).
That way it would return as a known good item.
 

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Discussion Starter #3
Thanks for the reply , I had thought that the 200bar on crank with the pressure regulator earthed would be enough for it to inject and fire , could you confirm ( or otherwise ! ) that for me ?, and I do indeed take your point about checking/repair of the ecu and have been looking up local firms who do this !
 

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F9Q? ................... rail pressure around 250Bar+ needed to run I would think.

I would call halt and get a Renault speaking scanner on the thing with a proven good battery in order to be able to inspect all the live injection computer parameters
 

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Super Moderator Technical Supremo Platinum Member
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200 bar is about minimum to fire, would not argue against it being higher but some motors seem to fire on lots lower then expected.
And trying it with the regulator shorted out will not let it run even if it does fire.
Certainly would not argue against a decent diagnostic

I'm not certain but has this got a fuel cut off switch?
Or has it had a shunt where air bags deployed?
Either could be affecting fuel supply.

Filters clear?
Injector leak off test ?
What is the pressure sensor showing... should be able to see that with a basic OBD2 reader.
What is the immobiliser light showing, working as it should ?
 

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I think the ECU on these is one of the spiteful animals - its showing commanded fuel pressure and difference to commanded but not actual (even though it may measure it) so EOBD may not get you there.

Shorts and forcing of pump to run to MAKE the engine start is bad practice ................... way too much fuel and could end up damaging the poor lump as a result of near hydrolocking
 

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Discussion Starter #7
Thanks again everyone , I did realise that it wouldn't run correctly with the regulator earthed but it is the only way to get any reasonable pressure , the pressure sensor only lifted the voltage from 0.5v to 0.65 but that was before I earthed the regulator so would only have been getting 50 bar . It does have a fuel cut off which is ok , it hasn't had a shunt , filters ok , haven't done leak off test , but lastly and possibly most importantly the immobiliser light isn't coming on at all now , I'm sure it did work correctly initially ! All seems to point to an ecu test I think !
 

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Discussion Starter #8
Thanks again everyone , I did realise that it wouldn't run correctly with the regulator earthed but it is the only way to get any reasonable pressure , the pressure sensor only lifted the voltage from 0.5v to 0.65 but that was before I earthed the regulator so would only have been getting 50 bar . It does have a fuel cut off which is ok , it hasn't had a shunt , filters ok , haven't done leak off test , but lastly and possibly most importantly the immobiliser light isn't coming on at all now , I'm sure it did work correctly initially ! All seems to point to an ecu test I think !
Some time later if anyone was interested I bought an ecu/uch/key kit for less than anyone wanted to test the ecu or run diagnostics and once that was fitted the immobiliser light was back behaving correctly and it started first turn ! 😊
 
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