940 taking longer to start when warm

[Eyebrows123]

I've noticed that when idling, if I remove the vacuum to the fpr the engine rises in rpm, I assume this is normal? There's also defintetely no fuel coming out the vacuum side of the fpr as well.

I think it might be worth doing a fuel pressure drop test and see if it holds the pressure or not.

Rev needle definitely flickering whilst cranking (ok, maybe not for the first 2-3 compressions as the engine gets up to speed but then it starts flickering before starting)

[Laird Scooby]

Originally Posted by Eyebrows123

I've noticed that when idling, if I remove the vacuum to the fpr the engine rises in rpm, I assume this is normal? There's also defintetely no fuel coming out the vacuum side of the fpr as well.

I think it might be worth doing a fuel pressure drop test and see if it holds the pressure or not.

Rev needle definitely flickering whilst cranking (ok, maybe not for the first 2-3 compressions as the engine gets up to speed but then it starts flickering before starting)

Yes, that's normal - it shows not only that your FPR is working but so is your Lambda sensor. If you put your thumb over the end of the vac hose the revs will drop again.

You're letting more air into the engine but also more fuel because without the vacuum, the fuel pressure rises from ~35psi to 45psi, based on the usual figures of 3bar (~45psi) @ atmospheric pressure for the FPR to keep the fuel at.
Normally at idle, the manifold develops ~20"Hg vacuum which is ~-10psi so the idle pressure will be ~35psi normally.
As you open the throttle under normal conditions, the manifold depression will increase to say 10"Hg which is ~-5psi so the fuel pressure will increase to ~40psi - under WOT conditions with no vacuum the fuel rail will be at ~45psi unless it's a turbo and is running 10psi of boost when it will raise to 55psi at 10psi boost.

You could try the pressure test, the NRVs for the fuel pump (called "Check Valves" by Volvo are available, i might have the part number somewhere and when i bought one was about £25 give or take, can't remember now. It didn't improve my hot (or cold) starting though.

As i think has already been mentioned, the first couple of turns on the starter is where the LH2.4 works out where the crankshaft is before actually sending the signal to the ignition ECU to tell it to start firing.

Another thought - do you have a cold start injector on yours? Usually under the throttle body and controlled by a thermo-time switch. If the engine is hot, the switch should be open and if it's cold, it passes a current through a heater in the switch until it has warmed to a preset level. If this is faulty and is constantly letting the Cold Start Injector operate then it will be adding extra fuel to a hot start that won't help. Some cars had this, others not. If yours have you could unplug the connector on it and see if that improves hot starting but it may also make cold starting a pain come the winter if you don't connect it again. By then it should be cool enough to leave it connected even when hot anyway.

[Eyebrows123]

I realise about the LH 2.4 requiring a good crank over, but i'm very familiar with exactly how long this is on my car hence why it's very obvious it is now a lot longer.

I don't think I have a cold start injector but I could be wrong, I'll have a check

I think i'll try get round to a fuel pressure test, at least that'll give me an idea of what's happening and something to hopefully rule out.

Edit: just checked I can't see a cold start injector anywhere

[Rversteeg]

Is it just cranking longer or can you also hear the engine trying to pick up? I can imagine that if it is just cranking the LH2.4 might have missed the reference point the first time due to a weak sensor signal.

[Eyebrows123]

Just cranking, it could be that the CPS is on the way out, what voltage is the wave normally? I could put a scope on it and try sniff out what it is doing.

It seems to have stopped happening so badly the past 2 days, so i'm going to keep an eye on it.

[~Goose~]

Had very similar symptoms with my 940 sport recently.

Changed the crank sensor and it’s all good now, as mentioned further in the thread the outer casing had started to perish.

[Laird Scooby]

Originally Posted by Eyebrows123

Just cranking, it could be that the CPS is on the way out, what voltage is the wave normally? I could put a scope on it and try sniff out what it is doing.

It seems to have stopped happening so badly the past 2 days, so i'm going to keep an eye on it.

It's cooled down a lot in the past few days though.

[Eyebrows123]

So, I've replaced the crank sensor (the previous one's rubber cables were deteriorating). But still I'm having this hot start issue. The other day it really struggled to get going and keep itself going.

When cranking I can see the rev counter bounce a little

[TonyS9]

To test the cps just check it's resistance while wobbling the cable around. Normally rev counter needle movement during cranking indicates it working fine.

The fpr has a check valve that can fail, or it's just part of its normal operation. It's releases fuel back to the tank when above the required pressure. If you can measure the pressure after switching off, it should hold for about 30mins.

Normal slow start is because there is only 1 reference point per revolution of the crank, the ignition computer doesn't know where the engine is until it finds that point, and doesn't start the sparks or fuel. After that it counts the 60 holes for relative position. After its been off more than a few seconds (5 or 10 maybe) it forgets.

[Clan]

Originally Posted by Eyebrows123

So, I've replaced the crank sensor (the previous one's rubber cables were deteriorating). But still I'm having this hot start issue. The other day it really struggled to get going and keep itself going.

When cranking I can see the rev counter bounce a little

whats the fuel pressure after you have left the car for 20 minutes?