944S2 running problem... help please!
12-26-2012, 01:00 PM
#16
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Join Date: May 2008
Location: Gloucester, UK
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on the face of it this sounds like a problem that should be now easy to track down . lets face it you have replaced practically everything now, bar the loom!
you've eliminated a lot of stuff so you should be getting closer to solving this
You have replaced the ECU without improvement so it should be ok to dismiss that as the direct source of the problem however everything to do with the fuel delivery and ignition pulsing is controlled by the ECU which in turn is controlled to a large extent by the various sensors so it could still be the indirect culpret.
The ECU not only earths the injectors and ignition amplifier but also earths one of the DME relays internally so if for example the earths of the ECU were poor or intermittant the ECU would periodically fail to earth the DME relay so the ECU would lose not only its own power supply but also the fuel pump, injectors and ignition amplifier supplies would fail also
The symptoms would be that the fuel delivery was faulty ie faulty pump,FPR, blocked filter, sticky injectors etc etc whereas the true fault was electrical ie bad ECU earth or DME relay earths
To ensure that the DME is not the source of the problem i would advise that you do all testing and running with a 3x wire bypass fitted. This will force the operation of the fuel pump and provide the ECU ,injectors and ignition with a constant 12v+ supply at all times. At least you should not have an intermittant DME relay operation to worry about. If you fit a simple switch in the wire between terminal 30 (constant live) to the other 2x wires (terminal 87 & 87b) you'll be able to switch the bypass on and off without having to take the damn thing out every time.
Next I would check and clean the 2x critical earth points MPll & MPlll which ground the ECU and DME relay . They are located under the dash and next to the fuse/relay box. (big brown wires)
It might be worth substituting a good ignition amplifier to eliminate that. Its bolted to the wing panel in the pass headlight well.As you say it looks like the problem is heat related and the IA runs hot ! which is why it needs a good heatsink fixing to the wing
Have you checked the hall sensor behind the distributor ? ....wiring, electrical connector etc ?
A couple of other obscure intermittant problems that occur are blocked fuel tank vent and intermittant vacuum ie brake master cylinder diaphram .
I think the fact that the engine will both idle and rev at full load but not in between is very diagnostic but I'm not fully awake yet and need a coffee so I'll come back to this later
you've eliminated a lot of stuff so you should be getting closer to solving this
You have replaced the ECU without improvement so it should be ok to dismiss that as the direct source of the problem however everything to do with the fuel delivery and ignition pulsing is controlled by the ECU which in turn is controlled to a large extent by the various sensors so it could still be the indirect culpret.
The ECU not only earths the injectors and ignition amplifier but also earths one of the DME relays internally so if for example the earths of the ECU were poor or intermittant the ECU would periodically fail to earth the DME relay so the ECU would lose not only its own power supply but also the fuel pump, injectors and ignition amplifier supplies would fail also
The symptoms would be that the fuel delivery was faulty ie faulty pump,FPR, blocked filter, sticky injectors etc etc whereas the true fault was electrical ie bad ECU earth or DME relay earths
To ensure that the DME is not the source of the problem i would advise that you do all testing and running with a 3x wire bypass fitted. This will force the operation of the fuel pump and provide the ECU ,injectors and ignition with a constant 12v+ supply at all times. At least you should not have an intermittant DME relay operation to worry about. If you fit a simple switch in the wire between terminal 30 (constant live) to the other 2x wires (terminal 87 & 87b) you'll be able to switch the bypass on and off without having to take the damn thing out every time.
Next I would check and clean the 2x critical earth points MPll & MPlll which ground the ECU and DME relay . They are located under the dash and next to the fuse/relay box. (big brown wires)
It might be worth substituting a good ignition amplifier to eliminate that. Its bolted to the wing panel in the pass headlight well.As you say it looks like the problem is heat related and the IA runs hot ! which is why it needs a good heatsink fixing to the wing
Have you checked the hall sensor behind the distributor ? ....wiring, electrical connector etc ?
A couple of other obscure intermittant problems that occur are blocked fuel tank vent and intermittant vacuum ie brake master cylinder diaphram .
I think the fact that the engine will both idle and rev at full load but not in between is very diagnostic but I'm not fully awake yet and need a coffee so I'll come back to this later
I have a fair amount or experience in many different vehicles but knowing exactly what triggers what inside the ECU isn't always written down or easily available.
MG
