walfreyydo

I see a lot of folks with engine running issues but never test the actual fuel pressure. Below is a list of parts that I used to install a fuel pressure gauge on my fuel rail. The position of the gauge will vary between the 8V cars and the 16V cars due to the position of the bung on the rail.

Measuring fuel pressure is essential to determining poor running issues - mainly the condition of the FPR, whether the pump is producing the correct pressure, and whether there are any obstructions. Just "having pressure" often doesn't tell the full story. High fuel pressure can cause rough running or the injectors not to fire, adding confusion to diagnosis and leading you down rabbit holes. Low pressure can also lead to rough running and a rail that doesnt hold pressure after shutoff (faulty check valve) can lead to hard starts.

As opposed to getting an external gauge kit, an even better option is installing a gauge directly on the rail. The bung on the rail is a M12x1.5 and you will need an adapter to whatever size (typically 1/8" NPT) gauge you are using.

Gauge (1/8 NPT fitting):
Adapter (male 1/8 NPT to female M12x1.5):

Adapter (female 1/8 NPT to female 1/8 NPT) - S2 uses an elbow version of this


Note on adapter: I had to grind down my M12x1.5 adapter because the threads were too deep to make a correct seal on the rail. The head would bottom out on the rail bung before fully seating. After modification, things worked well. I also used a 1/8 NPT female to female elbow for the S2. Your mileage may vary, or a shorter adapter may be sourced.

Cost: approximately $30

Result (yes this is an S2 but also workable for 8V cars):


The benefit is always knowing if you have fuel pressure and what the pressure is. Hopefully this guide helps others with poor running issues properly diagnose their fuel pressure.

Additionally, if you just want to use an external fuel pressure gauge with 1/8" or other fitting, you can just get the appropriate M12x1.5 adapter to get your fuel pressure gauge connected.

Safety Disclaimer - you MUST make sure that you have no fuel leaks after you install the adapter and gauges. It may require a bit of plumbers tape to fully and safely seal the connections (being careful that the tape is only on the threads and cant get into the fuel system). Pressuring the system before starting is recommended. If you dont have a pump-prime solid state dme relay (F9Tech), you can make a DME relay jumper to activate the fuel pump and pressurize the system (and also a good way to test whether your DME relay is bad or not).

Jfrahm

I'm not generally a fan of underhood fuel pressure gauges. The threads are not designed to seal, using them requires a lot of faith in the PTFE tape. Lots of leak potential and fuel pressure is not something most people need to check other than the occasional diagnostic.
In the case of big boost or N2O you need to know your fuel pressure under load and an underhood gauge doesn't help with this.

Connecting a gauge for diagnosis is fine but IMO not wise to leave under the hood all the time.

orig944

I would think a nitrile or viton flat washer as a seal would work for a permanent install, though I would want to loctite the threads to insure it didn't vibrate off. I rather agree that there is no need to do this permanently, and it adds more points of potential failure. The gauge setup can easily be carried in the car for use as needed. I've ordered the gauge and adapter from Aliexpress for even less out of pocket. Thanks for the idea.

walfreyydo

Yes - all good points.

I think the main point of this post is to bring clarity for folks who dont know how to measure fuel pressure due to the rail bung and unsure of what adapter(s) are needed. My hope is that this post removes those barriers for folks so that they can accurately measure fuel pressure and correctly diagnose any fuel issues related to a bad FPR, high pressure causing injectors not to fire, obstruction, check valve or pump issue instead looking past these potential causes to their issue, which I see people commonly doing.

I will say that Lindsey has a similar kit for permanent install and I have been running this for a number of years with no issues. I think the points are valid however, as any potential point of failure or fuel leak should be considered, taken seriously, and checked regularly (which I do). Folks can use their own judgement if they want a permanent gauge attached to the rail or not and this post offers the knowledge and flexibility to go either route, permanent or temporary.