Making a Fuel Pressure Regulator Tester

After all the modifications I’ve done to my engine it’s been left running a bit marginal on air fuel ratio on the rollers, and since I’m not running a fully programmable ECU I wanted to tweak the fuel pressure a little to enrich it. I calculated I needed about 5-6% increase in fuelling which meant an increase in fuel pressure from the standard 44psi to about 49psi (√(49/44)=1.055).

I planned to do this by squashing the fuel pressure regulator as suggested by Dave Andrews and others, but not having done it before I wasn’t sure how far to go and taking the regulator on and off the car for trial and error would be awkward due to poor access to the clip when installed. I also wanted to be able to test the calibration of the spare pressure regulators I had to make sure they were good.

It occurred to me that a pressure regulator is just a controlled leak that leaks at a constant pressure, so I thought an engine leak down tester should read a pressure regulator correctly. It shouldn’t make much difference whether it is regulating air or fuel, pressure is pressure. So I set about making an adapter as shown below.

I simply sawed the end off an old injector rail, screwed a pneumatic fitting into the cut end to match the fitting on my leak down tester and screwed a sump plug into the one injector port left to blank it off, using a little PTFE tape to seal all the threads. I bent the ends of the retailing clip up so that it no longer locks in place, allowing me to fit and remove regulators easily.

The adapter is shown below with a regulator fitted and plugged into my home-made leak down tester.

Things looked promising; with 50psi or more on the input gauge, the output gauge read a steady 44psi which is exactly what the regulator was showing on the inline fuel pressure gauge I have on my car.

The next job was to compress the internal spring inside the regulator a little to raise the operating pressure. This can be done by sandwiching the regulator between two suitable sized sockets (23mm works well to support the base, in my case an 11mm socket was just about the right outside diameter to press the top in). You can them squeeze the regulator between the sockets in a vice. There’s a little plastic ring around the regulator which supports the sealing O-ring, this need to be removed whilst compressing it and refitted later to avoid damaging it.

The aim is to roll in the top of the regulator as shown below.

After a few cycles of trial and error I had it reading 49psi on the tester.

At which point I fitted it back in the car and checked the inline fuel pressure gauge. For this the little vacuum line needs to be left off the regulator as the vacuum signal lowers the fuel pressure under the high manifold vacuum level at idle. As you can see below, it showed a nice 49psi in agreement with the reading on the tester.

The engine fired up and ran smoothly. Idle fuel trim as reported over OBDII had reduced, suggesting that the injectors were now flowing a bit more fuel but well within what the ECU could trim out when operating closed loop. This should leave me with safer lambdas under high load conditions. We will see when I get it back on the rollers later this summer.

Job done!