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Fuel Pressure Erratic during startup

Help, Advice, Owners' Discussion and DIY Tutorials on the third generation Volvo V70 wagon and XC70 crossover, and second generation S80. Are you an owner? Prospective owner? Get in here and join us! Start a new topic or comment on an existing thread.

2008-2016 V70
2008-2016 XC70
2007-2016 S80

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Fuel Pressure Erratic during startup

Post by pbierre » Sat Jan 25, 2020 11:18 am

This is a 2015.5 XC70 T6. A couple of months ago, I noticed that it was taking longer for the engine to "catch" (begin firing) during startup cranking. It was delayed by an extra second or two.

This didn't happen on every start up. Cold starts were normal, as were warm starts. But, if the car had been cooling off 4-6 hours, that was the condition that brought on the delayed starts.

I hooked up my graphing ODB-II, and noticed what was happening on such a long startup....the Fuel Rail Pressure was bouncing up and down (a full range ziz-zag) until it flattened out high when the engine kicked in.

I just replaced the Fuel Rail Pressure Sensor, but no improvement. The pressure zig-zag doesn't make any sense from a physics standpoint, as there should be smoothing in the pressure where it is being sensed.

Aren't there 1-way check valves in fuel pump? In the fuel filter?

Need help -- which way to turn next to get to root cause?

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Re: Fuel Pressure Erratic during startup

Post by pgill » Mon Jan 27, 2020 8:26 am

I don't own the Turbo version of the engine.

But I do own two of the normally aspirated version.

I am not sure that this will help but it might.

The Fuel system is return less and the pump is turned on and off by the pump controller to maintain the pressure.

When the Engine is cold started the pressure is raised above the normal level to achieve better atomization.

The fuel pump runs very briefly to prime the system and the ECU then monitors the Crank sensor to see that the engine is running and then the fuel pump is turned on.

Here is what I wrote on another Forum

"What I noticed was that the fuel pump module doesn't turn on until the engine is cranking and that as soon as the meter registered that the fuel pump module was using current (a few amps) then the engine started.

The delay was inherent to the design.

As soon as I saw that the starting delay corresponded directly to the current flowing to the fuel pump module I stopped worrying about it.

This is as the Volvo engineers intended it to be.

And only Land Rover owners with Volvo Engines even notice the delay."


Now to answer your question as to what to do next

1. Measure the voltage drop for all of the big cables while cranking (Remove the fuel pump fuse to make this easier to measure)

- Fix anything above 0.3 Volts

2. Replace or rebuild the starter motor (faster cranking = faster starting)

- When I installed a new starter on my LR2 3.2 it started noticeably faster, under all conditions

3. Install new Denso Iridium TT spark-plugs

4. Replace the Crank Shaft position sensor

Let us know what you find out

Good Luck


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