Solving Automotive Performance Issues
The fuel pressure regulator (FPR) is a device mounted on the fuel rail and is usually connected to intake vacuum. The internal diaphragm inside the regulator dictates the amount of pressure in the system.
Remember, pumps don’t create pressure; they create volume. The resistance put on the output volume of the pump creates the actual pressure. In the case of automotive fuel systems, this is often the job of the fuel pressure regulator. Some fuel pressure regulators are adjustable and you can dial in the proper pressure. If this is the case, try to dial in your FPR and see if your performance issue goes away. If it does, great; you can move on with your life. If not, we need to keep working.
As I said, many FPRs are hooked up to intake vacuum. If there is a leak in this vacuum line or it’s not getting vacuum to the FPR, then what you would normally see is high fuel pressure all the time. In fact, when you pinched off the fuel return line, you would likely see little to no change. High fuel pressure can cause issues just like low fuel pressure. So if you find a vacuum feed problem, address it and recheck for your performance issue.
While we’re in the topic of the FPR a good visual inspection is also in order. Remove the vacuum line and check for fuel. If you see fuel here, your FPR is bad. There should be no fuel leaking out into the vacuum line. Also, when doing your fuel pressure test, remove the vacuum line going to the FPR. When you do, there should be a rise in fuel pressure. If not, this would indicate a problem with the FPR and it should be replaced. FPRs are not usually expensive, so if you suspect a problem with one, it’s usually not much of an expense to replace it.
Another fuel pressure test is the leak down test. To perform this test, hook your gauge up and take your readings while the engine is running. Then shut the engine off and observe the pressure reading. It should hold steady for at least 30 minutes. It will drop a slight amount when you first shut the engine off, but it should hold steady after that. If it doesn’t and you see a steady drop in pressure, you have a leak somewhere.
One of the most common leaks is the check valve inside the fuel pump. Each electric fuel pump contains a check valve designed to hold pressure in the system after shutoff for a faster start-up later. One of the symptoms of a bad check valve is a long cranking time when you first start the engine after it’s been sitting for some time. The reason for this is that the fuel pump needs to fill the entire system with enough pressure for the engine to run before the engine will start. Here is a video showing what’s inside a fuel pump.
The check valve isn’t the only thing that can bleed off fuel pressure. You could also have a leaking fuel injector or bad FPR, as we talked about earlier. To separate the two problems, simply perform the same test above, but this time pinch off the return line and feed line going back to the tank when you shut the engine off. This will isolate the fuel rail from the rest of the system. If the leak goes away and you don’t see a pressure drop, your problem is in the tank and is likely the check valve in the fuel pump. If the problem is still there, then you know the issue is somewhere in the fuel rail, which could be a leaking injector of FPR problem.
So how do you find the leaking injector? I often find them with the power balance test listed above. A power balance test can isolate a problem cylinder. With a leaking fuel injector, I often see a wet spark plug on the cylinder where there’s a fuel leak. It might also have lots of soot caked on it from running rich. Sometimes you can remove the fuel rail and observe the injectors after you pressurize the fuel rail. You need to be careful with this, as fuel injectors are often just held in by the fuel rail. Once you remove the fuel rail, the injectors aren’t secure and can pop out if pressure is applied behind them. Only do this test if the injectors are held onto the fuel rail in some way. You might do this with a piece of wire or something. You don’t need to start the engine with the fuel rail removed; in fact, you can’t. All you need to do is turn the key to the ON position for three seconds or so to pressurize the fuel system. For the most part, I think a wet or sooty spark plug that sticks out from the rest is all the info you need.
From time to time you run into a fuel injector that fails. It can fail and start leaking fuel externally, or the electronics can fail and it can stop working. It’s usually pretty easy to find a bad fuel injector if it’s leaking externally, just put your eyes on it. If you see fuel leaking out of it, replace it. The electronics can be a bit harder to track down. I normally find a bad fuel injector when doing a power balance test. If I find that I have spark but no fuel on a particular cylinder, the next place I look is to the fuel injector. If I suspect a bad fuel injector the first step I often take is to see if the injector is getting a signal to fire. If it’s not getting a signal, it’s not the fuel injectors fault. It could have a wiring or a driver problem that’s causing it not to fire and deliver fuel as it should. There is a tool for this test, it’s called a noid light.
Noid lights plug into the injector harness in place of the injector. When the injector is sent a trigger signal, the noid light will blink to indicate each time a signal is sent. If you have a fuel injector that’s not working and the noid light lights up when you run or crank the engine, you likely have a bad fuel injector. If however the noid light does not light up when you crank or run the engine, you likely have a signal problem and you should focus your search on the wiring and any components associated with delivering the signal to the injector. Some people use a test light to perform the same function as a noid light. I suppose it is possible to see an injector signal this way but I don’t think it’s as good or as accurate. Sometimes the test light will not light up enough for you to see the injector signal. If that’s the case it might lead to a misdiagnosis of the injector.
I haven’t had much luck checking injector resistance. Sure there are times when you find an open injector but there are other times where the same test can show a good injector but in fact the injector has failed. I won’t get into the specifics of why this is, just know that it’s a possibility. So if you’re checking an injectors resistance take the information with a grain of salt and see what you can do to confirm the injectors operation.
One last note on fuel injectors. Be careful of the seals. Fuel injectors often have 2 main seals. The upper seal is where the fuel injector attaches to the fuel rail. This seal keeps the fuel in the fuel rail and directed toward the injector. If this seal fails it can cause a fuel leak. The lower seal helps prevent vacuum leaks into the intake. If during your testing you damage one of these seals you can create a vacuum leak or a fuel leak. Either one is a problem that needs to be taken care of if you’re looking to solve a performance issue. I like to put a small amount of silicone paste on injector seals before I install them. This way I’m not installing them dry and in doing so I reduce the risk of damaging the seals.
Video Title: Solving Automotive Idle Problems – EricTheCarGuy Video Description: In this Article, Solving Automotive Performance Issues, Eric looks at what can be the cause of Solving Automotive Performance Issues with your vehicle .Thumbnail: http://www.ericthecarguy.com/images/faq_buttons/Large_FAQ_Images/Performance-and-Driveability-icon-large.png