URD fuel rail tap - Pressure question

I installed fuel rail tap and an AEM fuel pressure gauge. I was getting readings in the 40s and then suddenly it's pegged at 100. I pulled the fuel pump relay and cranked and it came down to 35, but as soon as I put it back in and cranked it's up to 100 again. Any thoughts?

 

I'm thinking fuel rail pressures will be high, the injectors use what they will and any excess to ran back into the tank

 

FPR issues, I would rec to confirm with a second gauge and/or sensor. it is boost reference?

 

It’s very likely the fpr like Lucario said.

What type of pressure gauge setup is it? A mechanical gauge mounted directly on the fuel rail? A mechanical gauge in the cab? A sending unit attached with wires to a gauge face in the cab?

AEM made quality products in the 90s. Since then I don’t know if they were bought out or made changes.

You can probably pick up a liquid filled 100 psi gauge for less that $30. To eliminate the possibility the existing gauge is bad. Or if you have an air compressor hook it up next to the existing one on the compressor to see if they’re reading the same.

 

it’s a digital and I have spare sensor and another gauge to check with, I guess that’s the next step. It is indeed boost referenced, but these pressures were at idle, fwiw.

 

If this is a regular mechanical pressure gauge, hopefully liquid filled, you could hook it up where the sending unit goes to see what it says. You might need a pressure snubber so the needle doesn’t bounce like crazy.

What I’ve seen, the sending unit gauges are less accurate and way slower to react. Once I put a mechanical boost gauge next to the existing stock one. The mechanical would instantly react, the stock sending unit gauge would take near 2 seconds to catch up.

When I felt the need for a fuel pressure gauge, I hooked up directly with a hose long enough to reach the cab. Ran the hose out and in the open window. I didn’t use a snubber. Made my test pass on high boost, pressure didn’t drop. Then I left it connected, rolled up the hose & left it under the hood. My point being I don’t think monitoring fuel pressure always is really necessary. Even though it has a cool factor.

 

you could be blocking some of the pressure to the rail depending on how far down the fitting goes into the rail

 

It's a little extra, that's for sure. I mainly added it to watch fuel pressure under boost conditions and try and prevent engine damage if anything weird happens. The spare gauge is a digital oil pressure gauge that sits next to it.

 

Shouldn't be an issue? I would think URD would have tested their design for things like that.

 

Had a similar issue with my digital fuel pressure gauge. Ended up having a bad sensor