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Some quick background:

While idling, my 2007 Ford Focus "revs" at about a 1-second cycle, you can hear the engine subtly go "wroom / wroom / wroom". It has issues with long crank times to start and sudden loss of power while driving.

My question:

I have recorded about a minute of the engine idling. You can see the fuel pressure cycle as the engine revs - as the fuel pressure goes up, the engine revs up, and then back down again.

What is a "normal" fuel pressure reading? Is the fuel pressure cycling like this indicative of a fuel system problem or something else?

The data

Here's the fuel pressure:

enter image description here

And here's the RPMs overlaid on them. Note that they diverge towards the end - I'm not sure this is correct; while recording it I was watching the pressure chart and the sound of the engine correlated exactly with the pressure, so I'm wondering if the drift is a delay in the recording.. not sure.

enter image description here

In case that's not helpful, here's the complete reading of all the metrics: https://docs.google.com/spreadsheets/d/1AahZZ-ru4P5d3NkOs5fiPIgjKXZeIMLhq7RFlKQYxj8/edit?usp=sharing

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  • So a fuel pressure reading of 40 +/- 1.5, don't really see an issue there as 1.5 is less than 4% of 40... If the reading was fluctuating 20% or30% then there may be an issue. – Solar Mike Jan 20 at 21:04
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doesn't sound like a fuel issue to me. Electric fuel pumps and regulators rarely just lose pressure. They fail altogether. I realize there isn't much data to go on here, but do you not have any trouble codes? Fuel system out of range would trip one. I am leaning more towards something causing extra air into the system such as a cracked vacuum hose, or perhaps the MAF is lying.

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  • Agreed. Also, check for a dirty throttle plate. – Carguy Jan 21 at 22:16
  • Since the question was if this fuel pressure looks correct, marking this as solved since you're answering that, thank you! On follow-on exploration: I did another run recording the fuel trim, MAF and O2. Long term fuel trim trends around 0-3%, avg. about +2.5%.. That doesn't seem totally out of the ballpark, but does seem to imply it's running lean? I had a vacuum leak on the PCV hose a while back, dried-out O-rings, so it would make sense if other vacuum parts are worn out.. – Jacob Davis-Hansson Jan 21 at 22:42
  • well it's funny. In theory the oxygen sensor should detect lean also and pump in more gas. I would have a hydrocarbon test done to see if you are actually running lean. – John Lord Jan 24 at 22:04

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