Another is it fuel relay or something else post. Fuel pump hose?

[RedBrickCollector]

Update: Car sat for a week and now won't start. Car cranks, "catches" but immediately dies.

[volvolugnut]

If in doubt about fuel supply, do a fuel pressure test at the fuel rail. The pressure can tell you health of the pump and relay. Low pressure likely the pump and no pressure likely the relay.
volvolugnut

[scot850]

+1 with volvolugnut on this. If you open the fuel filler cover and get someone to turn the key to II, listen near the fuel filler cap for buzzing from the pump priming. No buzzing both could be bad. Then use a jumper wire on the fuel relay and see if when you turn the key to II if the pump runs. Note! This is a test, not something to do longer term as it will mean the pump is running all the time and you will fry it.

If the pump now runs the relay is likely bad.

If the car still does not start you will need to check the fuel pressure at the rail for around 43psi. If you get 43psi, then switch the ignition off and see how fast the pressure drops off. If it drops quickly the fuel pump pressure regulator is likely bad. If the pressure holds for 20 mins or so then all should now be good on the fuel side. The only other think would be the injector relay. Check the connectors that like to degrade on it. It sits on top of the radiator.

Neil.

[erikv11]

...
Is there a one way/check valve somewhere in the system? I'm leaning towards a failure for something like that, the hose inside the pump assembly, or the pump itself. Is it common enough an issue to warrant pulling out the assembly to check? Could it be just the fuel filter?
...

Yes, there is a check valve. Turn the key to piston 2, wait a few seconds, don't crank the engine, turn it off. Repeat this process 4 more times. If the car now starts, the check valve was bad.

Twice I've had the little hose come off the fuel pump, you'll know if this is the issue by pulling the pump as your describe.

Be really careful with the gasket at the top of the fuel pump, they get folded easily. I only use Volvo branded fuel pump gaskets.

[RedBrickCollector]

+1 with volvolugnut on this. If you open the fuel filler cover and get someone to turn the key to II, listen near the fuel filler cap for buzzing from the pump priming. No buzzing both could be bad. Then use a jumper wire on the fuel relay and see if when you turn the key to II if the pump runs. Note! This is a test, not something to do longer term as it will mean the pump is running all the time and you will fry it.

If the pump now runs the relay is likely bad.

If the car still does not start you will need to check the fuel pressure at the rail for around 43psi. If you get 43psi, then switch the ignition off and see how fast the pressure drops off. If it drops quickly the fuel pump pressure regulator is likely bad. If the pressure holds for 20 mins or so then all should now be good on the fuel side. The only other think would be the injector relay. Check the connectors that like to degrade on it. It sits on top of the radiator.

Neil.

Thanks for this. I can hear the pump run when in position II which makes me think it's a clogged fuel filter/line or weak pump. It primes and then dies immediately after starting.

What I am not sure about is if the relay can fail in a way that it primes the engine but does not run after the car starts.

Fuel pump has been replaced by a previous owner. Work was done by a mechanic I know, back in around 2019 or 2020. I'm suspecting they used a cheap part. Even before this issue you could hear the pump if you stood behind the car while it was idling. I'll see what it looks like when I get a fuel pressure gauge

[jonesg]

a cheap $15 "elm 327" from amazon hooked up to your smartphone can show fuel pressure at prime and run pressure.
if pressure is good then you might have a leaky injector causing the intake to vaporlock.

[RedBrickCollector]

a cheap $15 "elm 327" from amazon hooked up to your smartphone can show fuel pressure at prime and run pressure.
if pressure is good then you might have a leaky injector causing the intake to vaporlock.

Haven't thought of the leaky injector. One more thing to check.

I have however tried three different elm 327's and none of them work, using torue or the 850 OBD app. Same for my Foxwell OBD Scanner

[RedBrickCollector]

I recapped the fuel pump relay and the car seems fine. I'll still get a fuel pressure gauge for a full check.

What seems to happen is the 22uF capacitor can fail in a way where you can hear the pump prime but won't run when you start the engine.

In more complicated terms my guess is that the pump priming circuit activates the relay separately from the mechanism used when the car is running. When the 22uF capacitor fails, the main circuit that uses ECU signal to activate the relay fails as well.

[RedBrickCollector]

Update: Happened again, both warm/hot start and cold start. Works alright if I give it a little pedal.

I've also cleaned the connector of the throttle position sensor, thought that solved it as it was without issue for something like 2 weeks.

[scot850]

AS this has gone on for a while, I would recheck the fuel pressure at the fuel rail, and check the leakdown in fuel pressure.

Have you checked the ETC (Coolant temp sensor) for it's resistance and the plug at the back of the power steering pump for corroded or broken wires> ETC, even if working may not be giving correct info to the ECU which can cause cold start issues and hot starts.

Neil.