Wandering fuel pressure

[walkman]

In my ongoing struggles to get my fuel flow calibrated (may have a bad red cube) I have noticed fluctuating fuel pressure. This is with a carbureted engine with the boost pump off. Sometimes the fuel pressure appears to just drop out, close to zero, for a few seconds. It doesnt just go to zero, it decreases over a second or two and increases the same way. this indicates to me that it is not just a bad connection (new efis/eis 4000).

there is no noticeable effect on the way the engine runs. But is it possible the carb bowl is not getting to empty before the pressure comes back up again?

I had something like this years ago on my old Grumman. The engine driven pump was going out. But, it had a noticeable effect on the engine running.

I'm a bit stumped.

[Sam Buchanan]

Originally Posted by walkman

In my ongoing struggles to get my fuel flow calibrated (may have a bad red cube) I have noticed fluctuating fuel pressure. This is with a carbureted engine with the boost pump off. Sometimes the fuel pressure appears to just drop out, close to zero, for a few seconds. It doesnt just go to zero, it decreases over a second or two and increases the same way. this indicates to me that it is not just a bad connection (new efis/eis 4000).

there is no noticeable effect on the way the engine runs. But is it possible the carb bowl is not getting to empty before the pressure comes back up again?

I had something like this years ago on my old Grumman. The engine driven pump was going out. But, it had a noticeable effect on the engine running.

I'm a bit stumped.

The topic of fluctuating fuel pressure (and no indicated fuel pressure) with carbed engines has been discussed many times over the years on VAF. Run some searches and you will find several explanations for why you are seeing low pressure.

Bottom line....in most cases, it is due to the fuel pressure sensor not reading accurately at low pressure. But eliminate all other possibilities before decided it is just a gauge issue. The fact your engine continues to run properly should lead you to believe that most likely you are dealing with a low indication, not actual low pressure.

By the way, Lycoming sez 0.5 psi is sufficient to keep the carb bowl full. When the mechanical pump started failing intermittently on my O-320, engine rpm would start winding down and then come back up when the boost pump was activated.

[TS Flightlines]

Mr Walkman-
i dont know your specific setup, but I would first verify the fuel pressure with a direct reading pressure guage. You really need to know if its instrumentation, or real fuel pressure loss. I'm sure that more than one member here spent months chasing a bad pressure sender, when actually the pump was failing.
Tom

[s24789]

We had a similar problem and replaced the fuel pressure sender. The new manufacturer is kavlico. I forgot the old manufacturers name.

[GLPalinkas]

How about a fuel flow test like we all did prior to first flight. Calculate how many gallons per hour flow you have with the boost pump on. That ought to at least tell you the whether the cube, sender or gauge is the problem.