• Mon-Fri 8-6

Why it’s Important to Monitor Fuel Pressure

Is your fuel system working as it should?

Having a good working fuel pump is critical to a proper tune. A lot of the cars we tune have upgraded injectors but the owner is sometimes unsure if the fuel pump is upgraded as well. Since we monitor fuel pressure on pretty much every car we tune, we can see if fuel pressure is bad/low while tuning.

Turbocharged Subaru’s and Mitsubishi’s operate with a manifold pressure referenced 1:1 ratio fuel pressure regulator. They need this manifold referenced 1:1 ratio because as the manifold pressure goes up the fuel pressure needs to go up to maintain the correct fuel pressure differential, keeping the fuel injectors flowing the correct amount. Fuel pressure differential is the delta between manifold pressure and fuel pressure. An example would be if your base fuel pressure is 43.5 psi and you are running 19psi of boost your fuel pressure should hit 62.5psi (43.5+19=62.5). Another example is a base fuel pressure of 43.5psi and if you have 10psi of vacuum at idle it would be 33.5psi (43.5-10=33.5). Fuel pressure can be higher and lower than the 1:1 ratio. If it’s higher than the 1:1 ratio it will run rich, lower will cause it to run lean. Since fuel pressure is always varying, having a way to adjust the tune based on fuel pressure changes will make the fueling model that much more accurate and keep your AFR’s hitting the desired target.

-Blue line shows good fuel pressure.

-Red line shows a failing fuel pump.

The red lines in this picture show the fuel pressure hitting about 65psi, which is good, but then drops to around 50psi by redline. Even with the fuel pressure dropping so badly we were still able to hit close to the target AFR of 11:1. This shows you how important monitoring fuel pressure is. If we weren’t monitoring fuel pressure, we may have thought this tune was good. Of course, there are other indicators that a fuel pump is failing. If you are very familiar with a set up you can know if something is wrong by the injector duty cycles needed to hit a certain AFR.

Looking at the fuel pressure curve it was obvious that the fuel pump was bad. We installed a new Walbro 255lph and finished this tune in one session.

This picture shows how fuel pressure is supposed to follow the boost curve. You can see the blue line hits a peak of 70psi and holds that all the way to redline following the boost curve hitting 19psi and holding that all the way to redline.

When making more than stock power, maintaining the correct fuel pressure under boost/full throttle becomes very critical. Too little fuel pressure and your fuel injectors are left not injecting enough fuel which causes your engine to run lean. Potentially causing major damage. Cobb Tuning has added the ability to monitor fuel pressure and make real time adjustment based on fuel pressure differential to the Subaru ECU logic. It definitely doesn’t get the attention it deserves but it’s a great feature.

With the Cobb Custom Features we can add fuel pressure to be monitored on the Accessport and also set up the tune to make real time adjustments from fuel pressure. We input the base fuel pressure and if the fuel pressure is more or less than expected from the formula above, the ECU can make the needed changes to keep the AFR at its target. This feature is very helpful when doing flex fuel tunes as we typically raise the boost on the E85 portion of the tune and fuel pressure doesn’t always follow exactly since it’s controlled with a mechanical spring. This also has the benefit that if you have a fuel pump/fuel pressure regulator issue the ECU can compensate and potentially save your expensive engine from blowing up. The system can only compensate so far and shouldn’t be relied on if you are having issues. It’s really to keep your AFR’s at the tuned targets as fuel pressure varies slightly under full throttle. If your car is constantly making large adjustments from large fuel pressure differential errors it’s a good idea to get your car checked out, find the issue, and have it repaired.

Below are some pictures illustrating what happens. The graph shows fuel pressure, boost pressure and fuel pressure compensation. The blue line is losing fuel pressure from a failing fuel pump and the red lines hold fuel pressure. You can see that as the fuel pressure drops the fuel pressure compensation starts rising and ends up at 50%! That would have left your engine running dangerously lean but as you can see in the second picture the AFR stayed almost identical.

This feature is only available on manual transmission 04-current STI, 06-current WRX, 04-08 FXT, 05-12 LGT/OBXT with a V3 Accessport.

We always recommend running a fuel pressure sensor when running flex fuel or any car with larger than stock injectors.

Have Questions? Call Today!
(763) 316-5882