Fuel pressure fluctuating

[carusoam]

11 hours ago, Ragsf15e said:

Yes, I’ve been reading through his old posts.  I think there’s also some differences in the mechanical gage discussed in this thread vs the electronic transducer in my jpi.

The big difference.... is where the sensor is....

When the sensor is in the instrument panel... we can plumb it to make sure a big air bubble resides where we want it...

With an electric sensor... no fuel or oil lines need to come into the cabin... 

But, often... the sensors are on a rack mounted vertical to the firewall... include an air bubble if you want to...

I would think our friends at JPI know this, but don’t have the CS skills to explain it to the customer...

For that service... you need to discuss this with our EI guy... he knows customer service....

Oddly... the noise in the lines has been around for a century... it’s nothing new, or important...  the numbers can be massaged by the software driving the JPI... a great example of useless precision... in place of useful accuracy...

If you are interested in experimenting...  buy the various snubbers and test them out...

They will all tell you what size hole is in each one...

The hole will be based on the viscosity of the fluid...

Air has the least viscosity...

Oil will have the most...

Fuel will be something in between... less viscosity than water...

Either way... it is really easy to catch a big air bubble at the end of the line... it will stay there for a very long time...

PP thoughts only, not a mechanic...

Best regards,

-a-

[Ragsf15e]

42 minutes ago, carusoam said:

The big difference.... is where the sensor is....

When the sensor is in the instrument panel... we can plumb it to make sure a big air bubble resides where we want it...

With an electric sensor... no fuel or oil lines need to come into the cabin... 

But, often... the sensors are on a rack mounted vertical to the firewall... include an air bubble if you want to...

I would think our friends at JPI know this, but don’t have the CS skills to explain it to the customer...

For that service... you need to discuss this with our EI guy... he knows customer service....

Oddly... the noise in the lines has been around for a century... it’s nothing new, or important...  the numbers can be massaged by the software driving the JPI... a great example of useless precision... in place of useful accuracy...

If you are interested in experimenting...  buy the various snubbers and test them out...

They will all tell you what size hole is in each one...

The hole will be based on the viscosity of the fluid...

Air has the least viscosity...

Oil will have the most...

Fuel will be something in between... less viscosity than water...

Either way... it is really easy to catch a big air bubble at the end of the line... it will stay there for a very long time...

PP thoughts only, not a mechanic...

Best regards,

-a-

I definitely agree they have known about these issues for a long time and could have fixed most with software.  We don’t need instantaneous readout of our fuel or oil pressure.

However, they also use transducers that rely on millivolt differences to report pressure.  Those transducers live in our engine compartment along with a whole host of emi generating stuff - ignition wires, spark plugs, magnetos, alternators, etc.  I bet I’ll install snubbers and still have some issues.  Already I’m planning to move my jpi ground wire.  Currently it comes through the same firewall hole as my ~50 year old P leads.

[M20F-1968]

12 hours ago, Ragsf15e said:

Can you point me to the exact snubber you used?  I am looking for a fuel pressure one as well since my jpi 930 has some erratic indications.  I see one for air and one for light oil and water.  Not one for fuel?  Jpi had steered me toward Omega snubbers but said Chemiquip also had them.

I am using the Chemiquip snubber for light oil (which is what fuel is - a light organic petroleum product).

John Breda