Low fuel pressure on start

Hi,
On the last few initial starts on my IO 360A1B, I have noticed an abnormal low fuel pressure… starting at around 3-7 PSI. I watched and timed it today and it slowly rose to its normal value of 29-30 psi after about 90 seconds. Boost pump was off on purpose.
I initially prime with boost on and FP was nominal at about 35psi before I turned it over. Engine ‘sounded’ normal on start, no apparent falter while the pressure slowly rose. I also got a low FP alert after start which never happens.

Is this a sign of an impending mechanical pump failure?

Taking off the cowl now to inspect

I went through this. My bet is “yes”, bad engine-driven pump.

That is a tough one. A lot is going on at start up. If the pump never losses pressure at WOT, it is tough to put the blame on the pump, as it should have many more problems keeping pressure at 16 GPH than at 3 GPH. The unique challenge at startup is purging air out of the system. Mechanical pumps don't like air. In theory, the priming with the boost pump should purge the air out, but no guarantee.

I would start by trying to find a leak source in the upstream fuel lines first. A tiny leak can cause the fuel lines to drain and introduce a lot of air. It will take some time for the mech pump to get rid of all the air and when it does, the pressure will rise back to normal.

Not saying that this isn't a warning that the pump is going bad, but it is far from a classic sign of mech pump problems. My first guess is that this is a priming issue and that fuel is draining back to the tanks and introducing air into the system that must get worked out after start before pressure rises.

Your IO-360 is fuel injected: 3 to 7PSI will not open up the needle in the fuel divider.
So the indication is incorrect, if it were correct, the engine would have no fuel flowing to the injectors

BoydBirchler said:

Your IO-360 is fuel injected: 3 to 7PSI will not open up the needle in the fuel divider.
So the indication is incorrect, if it were correct, the engine would have no fuel flowing to the injectors

Click to expand...

Needle is just about fully down at idle and should not rise at idle. This is the primary job of the spider - to close down the V shaped port at idle, as the restrictors/injectors cannot do this on their own at low flows seen at idle. Pressure at the spider is very low (~ 2 PSI) when below 1000 RPM. Input pressure at the servo is not linearly passed to the output and is VERY low at low power settings. The servo controls fuel flow by varying pressures on the output. Idle is almost nothing and WOT is around 12-14 PSI at high MAPs. The standard spring in a spider is designed to hold a 2 PSI pressure at the lowest flow rates. Mechanical injection is very different than electronic, which typiclly has a constant pressure and instead varies the opening time of the injector. Though sometimes pressure changes are used for enrichment when injector range is limited. This is why some pressure regulators have a vacuum port.

FI engines routinely start on just the mechanical pump and that is producing very little pressure at the 250 RPM cranking speeds seen at start up.

Larry