Good afternoon,

I have installed 2 fuel pressure gauges. One aem digital sensor gauge located on the fuel rail and a liquid filled gauge located on my FPR which is mounted on the firewall. The gauge on the rail reads higher than the gauge on the fpr by about 6-7psi. Is that to be expected? And if so what pressure readding should I use?


Another note... my aem digital gauge is a bit jumpy and does flutuate which I'm not sure is normal.

 

Up to ~5% discrepancy, I'd say is average/normal. But you're seeing a 15% discrepancy (presuming base pressure is ~45 psi) and that's excessive.

First thing I'd do is test each one independently, ie. test the AEM on your FPR and your analog on the rail. Do you still have the same readings / discrepancy?

If so, move on to power/ground for the digital and burp the glycerin. Re-test.

 

Thanks for the input, I'll do some more tests and give an update.

I'm running about 60 psi base pressure.

 

fpr = fuel pressure regulator = regulated by vacuum ... the rail will always be higher unless you pull the vacuum hose

 

No it won't.

You are correct in that the regulator references vacuum and varies the line pressure, but that pressure should be relatively the same all the way back to the tank outlet. OP is seeing a delta pressure between two different measuring devices, the question is which one is right.

 

Buy a different third one to find out...

 

Ok so i swapped the locations for the gauges and they are still getting the same pressures they had had at their original locations. So the aem digital sensor reads 6-7psi higher than my liquid filled gauge regardless of location.

So now the question is, which is more accurate.

I will say this. The aem sensor reads much more stable on the fpr but the reading is pretty much the same.

 

If you are confident that you have it in the same ground plane as the ecu or gauge you are reading it from, I would tend to trust the electronic sender more. I have seen quite a bit of variation in several different brands of mechanical gauges. At the end of the day though, as long as whichever you use is repeatable, predictable, and relatively linear when you adjust it, you should be just fine. Neither is grossly out and any discrepancy will get "baked in" to your tune anyway. It's really just a point of reference.

 

yeah I think I'm going to stick with the AEM. I'm going to borrow a 3rd gauge from a friend and see what reading I get out of curiosity.

On a side note. I've read on another post in the forum that the fuel injectors are only exposed to the base fuel pressure, and the fuel pump sees the total fuel pressure (base pressure + boost pressure via a 1:1 fuel pressure regulator). Does anyone have any thoughts or input on that? My assumption was that whatever the fuel pressure is, the entire fuel system between the pump, fpr, and injectors experiences the same pressure with maybe a marginal variance.

 

If everything is working correctly, the pressure at the tip of the injector is always the same wrt the manifold vacuum/pressure (i.e. the base pressure)... this is what the regulator works to achieve. The pressure gauge you have hooked up is referencing the atmosphere, not the manifold. That is why you have to disconnect the vacuum reference to see your base pressure.