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non starting 940

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Old Mar 6th, 2021, 19:24   #11
855 tdi
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Ok so bit of an update, using my dads camera on a wire I checked the fuel pump i'd fitted, firstly it definitely pumps, it made a priming noise after refitting and the solder on the fuel relay is all good. Despite my fuel pressure regulator looking a rusty mess, it seemed in working order but I still replaced it anyway, made no difference to starting.

I had a look at my rsr and it had no obvious bad connection or burnt pins etc, I have a listening device (no idea of name, sort of like a stethoscope) but I listened to the injectors and they click on cranking so i'm certain it's not the rsr. The tacho barely jumps when cranking so although I first thought it wasn't the crank position sensor i'm now leaning more towards this, as it's easy and will probably need replacement at some point it seems to make sense to get it done. My gf needs her car back soon so I have a week or so to fix this and book an mot or i'll have to buy a car so i'm against the clock.
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Old Mar 6th, 2021, 22:18   #12
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I’ve been watching this thread with interest. You must be extremely frustrated by now. If I’m completely off the mark just tell me to get lost.
It appears that fuel is not reaching the engine so it will never start.
TonyS9 referred to it in post#3.
Given that the replacement used fuel pump seems to be working.
Have you established that the fuel line from the pump is clear and the fuel that it is trying to pump has somewhere to go?
Is your fuel filter underneath the nearside floor and is the pump managing to pump fuel as far as the filter?
If fuel is reaching the filter is there a blockage after the filter?
I’ve never needed to do it but, is it possible to disconnect the fuel pipe supplying the fuel rail and check that fuel is being pumped as far as that.
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Old Mar 7th, 2021, 09:17   #13
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Originally Posted by Ian21401 View Post
I’ve been watching this thread with interest. You must be extremely frustrated by now. If I’m completely off the mark just tell me to get lost.
It appears that fuel is not reaching the engine so it will never start.
TonyS9 referred to it in post#3.
Given that the replacement used fuel pump seems to be working.
Have you established that the fuel line from the pump is clear and the fuel that it is trying to pump has somewhere to go?
Is your fuel filter underneath the nearside floor and is the pump managing to pump fuel as far as the filter?
If fuel is reaching the filter is there a blockage after the filter?
I’ve never needed to do it but, is it possible to disconnect the fuel pipe supplying the fuel rail and check that fuel is being pumped as far as that.
Definitely getting frustrated, any ideas/help are always appreciated, I honestly haven't checked again to see if fuel came through, mostly due to the faff of mucking with the fuel pump yesterday and my job meaning my hands are currently cut to bits from my job delivering roofing sheets after a customer refusing to help unload and moving them at all hurts. I did however have fuel in the pressure regulator so I do think it's getting through. Also no idea why there would be a blockage of anything but air, tanks plastic so no rust as i've had with motorbikes and the fuel wasn't low enough to suck anything nasty through, obviously I could be wrong but not sure how i'd clear a blockage if there was one. I did research air in the lines but most people seemed to think it shouldn't be a problem on a petrol car..
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Old Mar 7th, 2021, 10:24   #14
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The rev counter won't move much as the rpms are very low, any movement is good. If you want to check sparks properly, the inline lights are pretty good value.

If the CPS isn't working you won't get sparks, injectors or fuel during cranking. It's just a coil of wire, if it has any resistance it is working.

There is little benefit in checking solder condition etc unless you have identified a faulty part. You need to diagnose what isn't working first, ie fuel pressure and sparks.
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Old Mar 7th, 2021, 12:05   #15
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In the olden days before cars had computers I was taught, if the engine won’t start it has to be fuel or ignition. I”m just a nosey interested bystander with some daft suggestions.
First check fuel is being supplied. (I don’t understand why a faulty CPS would prevent fuel being pumped.) If that is the case then try hot wiring the pump to check for fuel being pumped to and arriving at the fuel rail. If it is then that is one problem eliminated. You will not need to worry about clearing any blockage if it has been shown that there is no blockage.
Then move on to the fuel pressure regulator, about which I know very little.
Just a thought, my 1992 940 is reported to be fitted with a Fuel ECU behind the driver’s footwell side panel. That hasn’t been mentioned yet. That’s another item about which I know little or nothing.
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Old Mar 7th, 2021, 12:29   #16
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Quote:
Originally Posted by Ian21401 View Post
In the olden days before cars had computers I was taught, if the engine won’t start it has to be fuel or ignition. I”m just a nosey interested bystander with some daft suggestions.
First check fuel is being supplied. (I don’t understand why a faulty CPS would prevent fuel being pumped.) If that is the case then try hot wiring the pump to check for fuel being pumped to and arriving at the fuel rail. If it is then that is one problem eliminated. You will not need to worry about clearing any blockage if it has been shown that there is no blockage.
Then move on to the fuel pressure regulator, about which I know very little.
Just a thought, my 1992 940 is reported to be fitted with a Fuel ECU behind the driver’s footwell side panel. That hasn’t been mentioned yet. That’s another item about which I know little or nothing.
Good call getting back to basics Ian, the reason why a faulty CPS prevents fuel being delivered is actually very simple although at first glance seems complicated. In short, it provides pulses to the EZK (ignition ECU) unit which in turn sends the firing signal to the ignition amplifier module. It also tells the fuel ECU that the engine is turning and the fuel ECU then triggers the fuel pump relay to switch on.

The good thing from this is the engine shouldn't (in theory at least) flood if the CPS is faulty.
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Old Mar 7th, 2021, 12:44   #17
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Thanks for that explanation Dave.
So to bypass all those items, hot wire fuel pump to establish whether fuel is being supplied. If so move on to each of those items in turn.
Ignition Amplifier and RSR would be the easiest ( and possibly the cheapest) first, then the CPS and then. hopefully not, the ECUs.
Has there been a fault code check done?
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Old Mar 7th, 2021, 12:55   #18
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Quote:
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Thanks for that explanation Dave.
So to bypass all those items, hot wire fuel pump to establish whether fuel is being supplied. If so move on to each of those items in turn.
Ignition Amplifier and RSR would be the easiest ( and possibly the cheapest) first, then the CPS and then. hopefully not, the ECUs.
Has there been a fault code check done?
You're welcome Ian, that's about the size of it. Also on the late cars (96 on if memory serves) the RSR also supplies the coil so could be the culprit although i think this was checked earlier in the thread.

Not sure on the fault code check although sometimes one fault not listed can trigger another one to come up, for example a non-functioning fuel pump (for whatever reason) can often trigger a MAF code because the ECU thinks everything is correct but the MAF isn't reporting increased airflow from the engine starting.
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Old Mar 8th, 2021, 09:54   #19
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The attention has now all been on the fuel system as the moving tachometer needle suggests the ignition is working. However, the tachometer is connected to the ignition module and primary coil (LT part). The fact it is moving does not necessarily mean the HT part is actually working.
Did you already check if the spark plugs are actually sparking? I am thinking more in line of a common failure for all four cylinders like the distributor of HT lead between distributor and igition coil. Or maybe even the coil iitself.
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