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940 taking longer to start when warm

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Old Sep 21st, 2020, 12:17   #1
Eyebrows123
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Default 940 taking longer to start when warm

Recently my 1996 940 (B230FK) has been taking longer to start (a few more turns of the starter motor before firing happily as usual). It's happening only when the engine is hot and I go to restart it. Starting from normal ambient temperature seems to be fine, I don't yet know if cold weather affects it as we haven't had any cold mornings yet. Any idea what could be causing this? As soon as it fires, it starts and runs happily.

Engine (lambda) light is not on

Last time I had symptoms like this it was the ECT sensor (which i replaced 1 year ago), but that triggered the engine light and it was replaced 1 year ago with a FEBI/bilstein item so I'm doubting it would fail that quickly?

What do you reckon it would be?

Last edited by Eyebrows123; Sep 21st, 2020 at 13:02.
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Old Sep 21st, 2020, 14:13   #2
Clan
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Quote:
Originally Posted by Eyebrows123 View Post
Recently my 1996 940 (B230FK) has been taking longer to start (a few more turns of the starter motor before firing happily as usual). It's happening only when the engine is hot and I go to restart it. Starting from normal ambient temperature seems to be fine, I don't yet know if cold weather affects it as we haven't had any cold mornings yet. Any idea what could be causing this? As soon as it fires, it starts and runs happily.

Engine (lambda) light is not on

Last time I had symptoms like this it was the ECT sensor (which i replaced 1 year ago), but that triggered the engine light and it was replaced 1 year ago with a FEBI/bilstein item so I'm doubting it would fail that quickly?

What do you reckon it would be?
a classic symptom of fuel pressure draining away when you turn off , it should hold for at least 20 minutes otherwise the fuel oils in the pipes with engine het and leaves an airlock to clear through before it will start .
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Old Sep 21st, 2020, 14:36   #3
Eyebrows123
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a classic symptom of fuel pressure draining away when you turn off , it should hold for at least 20 minutes otherwise the fuel oils in the pipes with engine het and leaves an airlock to clear through before it will start .
Possibly but it happens if I switch the engine off then instantly try to restart it (I think, I'll check this later when I next drive the car though)

Also, I always wait for the fuel pump to do it's 2 second prime thing when I turn on the ignition before starting (you can hear it start, then turn off again), would have thought this would be sufficient to build up any lost fuel pressure?
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Old Sep 21st, 2020, 15:09   #4
Eyebrows123
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Having said that, i imagine it could be a failed FPR or check valve in the in tank pump, should probably test the fuel pressure and see what it does
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Old Sep 21st, 2020, 16:12   #5
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Also check for the rev needle to flicker while its not starting. If its flickering it means you are likely getting a spark and therefore more likley to be fuel.

The 940 normally takes along time to start due to a single reference point on the crank. The spark doesn't start until then. The hall effect or Renix systems are twice as fast.
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Old Sep 21st, 2020, 16:35   #6
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Default Bad luck?

If the reference point on the flywheel just passed the sensor when you stopped the engine last time, it can take almost a whole revolution of the engine before the sensor spots the point again and the ECU starts injecting / firing. It could just be "bad luck" that the engine has stopped just passed the ref point a few times.
I would be more concerned if it would pick up right away, but run rough for a few moments.
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Old Sep 21st, 2020, 16:47   #7
Eyebrows123
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I've had the car for almost 5 years now, I'm very familiar with how long it takes to start, normally you can tell it has to turn over a few compressions before it starts but it's always done so exactly the same amount (and continues to do the same amount when cold). I know about the issue of only one trigger point for the sensor. When warm it has suddenly started requiring considerably more cranks, when it first started happening, my muscle memory would turn the key out of the start position before it had started (because i'm so used to the exact amount of time between cranking and starting). So I'm sure something has gone somewhere or is on the way out.

I keep meaning to check the rev counter needle, everytime i get in the car i forget until it's too late, i'll have a look but i think it's flickering.
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Old Sep 21st, 2020, 17:59   #8
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I had this problem on my last 940 and it got to the point that it wouldn't start when warm.
The solution that worked for me was to replace the crankshaft position sensor. Apparently the insulation on the sensor lead breaks down over time and the pulse becomes weaker. A short term fix was to wrap the lead in insulating tape.
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Old Sep 21st, 2020, 18:19   #9
rogerthechorister
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Default Keep an eye on this!

I have exactly this symptom on my tatty Sport. Nice one goes first turn every time, tatty one goes first turn when stone cold, but needs quite a while (maybe 4 or 5 seconds) when hot.
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Old Sep 21st, 2020, 21:40   #10
Laird Scooby
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Given the hot weather we've been having recently mine has also been doing this. Also the fuel return line from the FPR had perished badly in the 2-3 years it's been on there so i not only bought a replacement hose but also some heatshield sleeving.

As an experiment i also covered the fuel rails with bacofoil, shiny side out. Vast improvement and back to normal again!



Oven ready!

I'll try and find the ebay link for the heatshield sleeving, wasn't cheap but still cheaper than renewing the fuel hose every 2 years because the heat had perished it! Also cheaper than an engine bay fire!
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