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FUEL SYSTEM TROUBLESHOOTING

As more and more setups include aftermarket fuel systems, there is an increasing need for troubleshooting these fuel systems. The following is a simple procedure I have used for decades, and I have added to it over the years as I run into bizarre new problems. I will attempt to explain what each step is trying to accomplish, but knowing how these fuel systems work is really the best bet in effective troubleshooting. In many cases, that knowledge is expanded for people after they run through this procedure a few times and see how things work. This is taken from something I've written with ECMlink DSMs in mind as part of a pre-tune checklist, but it does transfer well to any setup. I will add more info to this to help explain things further.

 

Base Fuel Pressure:

This is something that is still often misunderstood. Simply put, it is Fuel Pressure minus Manifold Pressure, or the actual differential pressure across the fuel injector. It should be the same value at all times, regardless of running conditions, with the exception of the expected brief delays in response during transient conditions. Logging a fuel pressure sensor in ECMlink makes this a no brainer, because the software will do this math for you and provide a Base Fuel Pressure loggable parameter. It should be a flat line, from idle, through cruise, and all the way through WOT! Rather than using a haphazard looking fuel pressure trace and trying to compare it to boost, just look at this Base Fuel Pressure parameter. Anywhere it's not a flat line quickly highlights an issue. A drop at WOT of a few psi is typical however and not cause for alarm. 

Base fuel pressure needs to be set correctly, and FPR/pump/return line functionality verified before tuning any car. Most people will simply start at 42-43 psi and be good to go. Others will run more or less base pressure, but you have to be sure you understand the implications of doing this. Always run it by your tuner.

The following is the procedure I use to verify operation of the fuel system:

  1. Turn the FPR set screw in all the way, or until you achieve ~80 psi (different FPRs have different maximums, consult the documentation for your particular regulator. You should be able to achieve a pressure very near the top end of the base fuel pressure range given by the manufacturer for the spring installed.). This verifies FPR spring pressure, and that the pump assembly has no leaks. Blown out pump hat o-rings or split supply hoses inside the tank often won't let you get over ~50 psi. These are leaks that you'll never see otherwise, and they do happen. It is also possible to have a weak pump, or weak pump wiring show up as a problem here.

  2. Back the FPR screw all the way out. This verifies return line functionality and orifice size. Ideally you will be able to get down to 20-25 psi. This doesn't guarantee you aren't over running the return line, but it's a start. More on this below.

  3. Set base pressure back to 42-43 psi.

  4. Apply 40 psi shop air to the FPR reference port. You should see 82 psi. If not, check for boost leaks in the FPR. A simple jig can be assembled with a good mini-regulator with a 0-60 psi gauge for good resolution, hooked up to shop air (see picture below). In the absence of this, the gauge on your compressor will probably get you close, but don't expect it to be more accurate than about 5 psi.

  5. If possible, apply 20”hg vacuum to the reference port with a mightyvac. You should see pressure drop 10 psi, to 32. If not, check for leaks, or for over running the return line (a larger line or regulator orifice may be required, more on this below).

  6. Reattach the reference line and start the car. Note idle vacuum and subtract it from your set base pressure (2”hg = 1 psi). This should be the pressure you see at idle. If not, check for leaks or return line over run as in step 5. Example: 43 psi base, and 20" hg idle vacuum. 20" is -10psi, so you should see 33 psi fuel pressure in this case. 

  7. If fuel flow from the pump, or through the filter/lines, etc, is a concern, you can also flow test the pump. The easiest way is to run the return line into a 5 gallon fuel jug, kick the pump on in ECMlink, and time how long it takes to move the fuel. You can time it in one gallon increments and do it a few times to be sure you're getting repeatable results, or time it over a larger volume like 3-4 gallons which also increases the accuracy of the test. It's not perfect, but it will highlight a shit pump pretty handily. Note that most plastic fuel jugs will expand with time, making 1 gallon on the markings actually more than one gallon. Also make a note of the battery voltage during the test. If your gobshyte small battery or other POS you have in there isn't a solid 12v+ it will affect the results. It doesn't hurt to put a battery charger on it for the test (5-8 amps at least, trickle chargers will not help). In my experience, I always get VERY close to the advertised flow ratings of the pumps I test, when they are working. By taking the measurement at the return line, you're including the filters and lines and fittings in the test! You can also test at various pressure by adjusting the screw manually to what you want for the test. Some pumps may pass at 43 psi, but shit the bed at higher pressures. 

  8. If all of this passes, you can be reasonably sure that the fuel pressure control system will function as expected. There are a small percentage of potential issues that can still sneak by this test of course. It's still always a good idea to log fuel pressure, since it will show any of a large number of possible problems easily, with no guessing. It will also make step 6 much easier, just stream data and look at the Base Fuel Pressure parameter.

 

Test rig, with a 0-60 psi mini-regulator, quick connect for shop air, and a few options for connecting to the FPR, wastegates, pressure sensors, etc. 

 

 

 

 

 

 

 

 

 

A note on return line capacity and regulator orifice size.

Normally, over running the fuel return line or regulator orifice results in too much base pressure. Either your desired base pressure can't be achieved, or your base pressure minus manifold vacuum does not go low enough. This is obvious of course. However, it is possible in rare instances to achieve a proper base pressure, and idle fuel pressure, yet still be outflowing the return line or orifice. This can cause a lean condition at WOT that can be difficult to troubleshoot. With a fuel pressure sensor, it will show up as a drop in base pressure at WOT. The usual suspected cause for dropping base pressure at WOT is a fuel supply deficiency, but in this case, it's cause by return line or orifice over-run requiring you to back the set screw on the regulator out further than it really should be to achieve your base/idle fuel pressure. Once flow increases at WOT, return fuel flow drops, and you get "on the spring" in the regulator, your base pressure is now too low. Upgrading the return line or orifice size (usually requires changing regulators, though some newer regulators offer interchangeable orifices) will clear this up. It's rare, but it happens. 

 

Logging fuel pressure sensors in ECMlink:

There is no arguing that logging a fuel pressure sensor, and using the Base Fuel Pressure parameter in ECMlink, is a huge advantage to anyone willing to make the effort to install one. It will easily point out issues at a glance, eliminating hours of troubleshooting, which usually means throwing parts at the car, often costing more than the pressure sensor would have cost in the first place. And, once installed, you'll always have it. You can also move that sensor around if you're creative and log oil pressure, IC pressure drop, any number of things, very easily. 

In general, a 0-100psig sensor is best for most people. In my case, I run a 150 psig sensor, which covers me to 60 psi boost and 90 psi base pressure. Consider your likely base pressure and boost pressure ranges and choose accordingly. One advtantage to a 0-100 psig sensor is that if it uses a 0.5-4.5v output range, you can simply select the Autometer 2246 sensor in the drop down. 

The Autometer 2246 stainless 0-100psig sensor is a popular choice. It uses the typical 3 pin round metripack connector, but does not come with one, so you'll need to source one. It's about 100 dollars. It's a high quality sensor, from Klavico if I recall correctly. 

AEM's 0-100psig stainless sensor, part number 30-2130-100, is about 150 dollars. It's a high quality SSI sensor and will, in my experience, last pretty much forever. I have these sensors still running at 15 years old. It does come with the connector, and you can use the same ECMlink drop down option for the 2246 sensor since they share the same scaling. 

AEM's 0-100 psig brass sensor, part number 30-2131-100, is similar to the stainless version, but brass, and much cheaper at ~65 bucks. But, it will not last as a fuel pressure sensor. It comes with the connector and uses the same scalings as above, but is not recommended at all. 

There are a number of cheap sensors now available as well. They can be made to work, with vary levels of success. The risk is yours to take. I would aim for something stainless, and be sure it comes with scaling information.

 

Once you have the sensor, you'll need to wire it to a spare input (there are plenty of write ups online showing how to do this, I won't duplicate them here), and set it up in ECMlink as a fuel pressure sensor (linear fuel pressure sensor for custom ranges) on the inputs tab, which makes it loggable and will automatically make it work with the Base Fuel Pressure display item. It will also need to be setup in the logger display if it's a custom linear fuel pressure sensor to show the correct pressure. In all case, using the above mentioned shop air jig will allow you to verify the readings up and down the sensor's range, if you want further peace of mind. This little bit of work will pay off in the long run. It'll change your life, just do it. 

 

Other unusual issues and thoughts:

  • It's entirely possible to have weak connections at the pump causing issues that are difficult to diagnose. Poor contacts, bad or missing grounds, wires with all but one strand broken, things of this nature. Often they will pass continuity and voltage tests! It pays to have a good handle on electrical theory here, it's beyond the scope of this article unfortunately. But, just being aware of this possibility can help a number of people out. Good luck! 

  • Pumps installed or wired backwards usually don't prime. I've dealt with both, it can be a pain in the ass. 

  • Dual/staged pumps can require the use of check valves on at least the secondary pump's output, if the pumps don't have them built in. There are a number of ways for this to go wrong. I may expand on this later. 

  • ECMlink nitrous outputs can be used to activate staged secondary fuel pumps, or dual speed pumps (like fuel lab), I've done both and it works well when done right. Also lots of ways to go wrong here though, I may expand on this later. 

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