Lag Hard Start Stall Diagnose Car Truck Low Fuel Pressure

Hey, friends, it's Len here from 1A Auto. This video is going to be all about checking out low fuel pressure on your vehicle.

So, now, the first thing you need to figure out is if your pump's working at all, because if it's not working at all, of course, you're going to have low fuel pressure/no fuel pressure. We're going to go with the assumption that you do have fuel pressure. So, there's going to be different ways you can tell if your fuel pump's even running. Some vehicles, like older ones, like my '96 Dodge Ram, if I turn the key to the on position without actually starting it, I can hear that fuel pump running right down inside that fuel tank. It's nice and loud.

Some newer vehicles, you're going to have a little bit harder of a time, and you might not even hear anything in all honesty. Some vehicles are going to have a little Schrader valve generally up along one of those fuel rails and, of course, you could just hook up one of those fuel pressure gauges to it and that's going to be your best inclining on whether or not you've got any fuel pressure at all.

Okay. So, a couple of things that I've come to learn in my experience. One of the things would be, if you frequently run your vehicle with low fuel, you could potentially cause premature damage to your pump. Essentially, it's just not going to last as long. The reason for that is because the fuel inside of your fuel tank is actually supposed to be cooling that fuel pump. Of course, as the pump's running, it's going to be generating heat. The heat needs to be able to dissipate. If it has nowhere to go because it's not completely coated in fuel, of course, it's going to heat up and it could potentially cause damage to the inside of that pump. And if you're the type of person that likes to run your fuel tanks, so the fuel light turns on, and then, of course, you wait another 30 or 40 miles to get to the fuel station, you're, of course, going to have an issue.

The reason for that is because your fuel pump might actually draw a little bit of air or even sediment from inside the bottom of the tank and, of course, that's gonna cause an issue. If, of course, you're drawing air into the pump, it's going to be fuel mixed with air, trying to get through there and you're not going to have a consistent flow of fuel. And, of course, the fuel is going to lubricate the inside of the pump. So, when you have the paddles going around on the inside of your fuel pump module, it's supposed to be spinning very close to the inner walls of the fuel pump. As those paddles get worn down, there's a possibility that fuel could get rushed by those paddles and it's not getting forced up into where it's supposed to go up towards where your injectors are.

Now, in my experience as a mechanic, I've come to notice that fuel pumps tend to like to go bad, especially after a fill-up. It seems as though the consumer just went and filled up their tank, next thing you know they're calling a tow truck, bringing it down to me, and I'm telling them that their fuel pump's no good.

All right. So, we got all that out of the way. One more thing that I want to reiterate to you is this video is going by the assumption that you know that your fuel pump runs, we're just trying to figure out some of the reasons why you might have low fuel pressure. But first, you need to know the optimal mixture between air and fuel. Essentially, you need to have 14.7 parts of air compared to 1 part of fuel. If any of that's off, even one little bit, you're going to have a running condition. Now, to start off, I want to give you a brief overview of the difference between a lean condition or a rich condition.

If you have a lean condition, that would essentially mean that you have either too much air coming in, or you don't have enough fuel, which essentially would make your vehicle run a little hotter. Or, if you have a rich condition, that means you have too much fuel coming in in comparison to the amount of air, in which case, you might see some black smoke coming out of your tailpipe. More than likely your catalytic converter is going to be clogged up, which will, of course, make it turn probably a little red because it's going to be getting super hot. And even you might notice that you have fuel inside of your engine oil, which, of course, would be very bad. Either way, you should figure out what's going on ASAP.

So, let's talk about some symptoms of having low fuel pressure. Some of the things that you might happen to notice is when you try to turn that key, it just [vocalization] and almost sounds like it wants to stall out sometimes, or maybe even it's just a long crank, but then it does start and it runs just fine.

Essentially, what that means is you have low fuel pressure inside of the fuel system to help prime that engine to start. So, essentially, the fuel's all the way up at the top, by the fuel rail just waiting to get out and burn up inside that combustion chamber. It's sitting there waiting. It's waiting too long. It kind of just seeps back on down. Next thing you know it's all the way back down into the fuel tank before you even try to start up the vehicle. Essentially, there's a one-way valve on the fuel system that's supposed to kind of make sure that the fuel pressure stays up there by the fuel rails waiting for the fuel injector to just open up and say, "Okay, it's go time. Here we go." Now, if your fuel system loses pressure over time, it's more than likely due to a faulty fuel pressure regulator or even a leaky fuel injector. In which case, you're probably gonna notice that you have a long or extended crank when you try to start your vehicle.

Another symptom you might find is a stalling condition. Essentially, if your engine stalls while it's running, it means you have a fuel delivery issue, whether it's a bad pump, a clogged fuel filter, or even a sticking fuel injector. If your fuel pressure is a little low, you might notice a little bit of a hesitation when you try to accelerate, but under heavy acceleration, you might notice that your vehicle wants to stall out. Your vehicle needs fuel to be delivered to each cylinder under the proper amount of pressure to be mixed with the amount of air for proper combustion. Now, remembering that you need 14.7 parts of air compared to 1 part of fuel, which is basically like 15 times more air compared to fuel, if the mixture is even slightly off, you might think that you have a misfire, but really it's just uncontrolled combustion inside the engine. Either way, it can be damaging to your car's engine or even the car's exhaust system such as your catalytic converter.

Another quick symptom that you might happen to notice, and it kind of goes along with one of the other ones that I had mentioned would be throttle response. Maybe you're sitting at a red light and the vehicle is just idling, and it's idling just fine, the light turns green you go to take off and the vehicles just kind of like, [vocalization] and it starts wanting to take off, but it's very laggy throttle response where you kind of really want it to go when you say go. Essentially, why this might happen would be possibly because of the pump having low fuel pressure or even maybe there's a clog in the system such as maybe your fuel filter, which would be a very common cause for this and a pretty cheap fix overall.

Now, the next thing we're going to talk about is a possible check engine light. And the reason why I say possible is because some vehicles, you're probably not even going to see a check engine light for low fuel pressure. More than likely on a newer vehicle you will, but an older vehicle such as maybe a '96 Dodge Ram, more than likely you won't. You might still feel that stumble, but no check engine light. You might think it's a misfire, but maybe it isn't. Like I said, it's probably due to low fuel pressure.

Okay. So, most modern vehicles are going to have a very sophisticated computer system in it. That computer system is going to be trying to do a lot of different calculations. One of the things it needs to do is talk to that fuel pressure sensor that I was just talking to you about. What that's trying to do is find out if there's going to be enough fuel pressure in the system for the engine to be able to do its job. Kind of like your boss asking, "Is everybody getting what they need to be able to do their job?" When the fuel pressure sensor finds that there either is too much or there isn't enough fuel pressure inside the line, it's gonna, of course, tell that computer something's wrong and the computer is going to turn on that check engine light for you so you know that there's something you need to look into. Now, if you have low fuel pressure, you might get a couple codes. One of them might be P0087. That just basically stands for low fuel pressure. Another code you might get is P0190. And essentially, what that stands for is you have low fuel pressure inside your fuel rail, which is basically where those fuel injectors are.

Now, if you have a newer vehicle, something newer than a 1996, more than likely you're going to have an OBD-2 connector port. That port is generally going to be found somewhere around the driver side of the vehicle, generally underneath the dash. If you have access to a newer scanner or at least maybe a mid- to high-range scanner, you should be able to get onto it and, of course, you'll be able to pull your codes. When you pull the codes, it'll kind of guide you in the direction that you need to go for diagnosing your particular issue. If one of the codes that I mentioned before comes up, then you know you more than likely have a lean condition in your vehicle. And, of course, if you don't have access to a higher-range scanner like this, you can get yourself one of these gauges right here. It's a nice fuel pressure gauge.

Of course, it's going to be helpful to know what your fuel pressure should be at though. Different vehicles may vary. Most vehicles, especially modern vehicles are going to be between 55 to 65 PSI, or pounds of pressure, in your fuel system to start the vehicle. If you have less than that, you might notice an issue starting. You might notice a stumbling starting. And you might have also notice stalling conditions. So, like I said, fuel pressures may vary. If you have a direct injection vehicle, more than likely your fuel pressure is going to be much higher. Or as opposed to that you might have a carbureted engine, which those fuel pressures would be very much lower, like 4 to 5 pounds, really.

Now, you're probably wondering, can a lean condition cause spark plug issues? The answer would be yes. A lean condition can cause spark plug issue. Maybe you keep up with your maintenance just like you should, but you still get that pesky check engine light, or even a murmur or stutter or even stalling conditions from your vehicle under load. Let's say you risk it and you drive a bunch more miles, but you can't take that shudder anymore so you bring your vehicle to your local trusted mechanic. And, of course, they're going to do all the usual tests. Not only do they find that you maybe have low fuel pressure, but they're probably also gonna find that you have odd signs of wear on your spark plugs.

When you have low fuel pressure, that means you have a lean condition. Once again, that means that you have too much air getting mixed with the amount of fuel that's getting burnt up inside your engine's combustion chamber. This causes your engine's combustion chambers to run extra hot. Spark plugs are designed to be the weakest link in any cylinder. The reason for that is to help prevent further damage, and it's also one of the cheapest and easiest parts to fix in this case. Basically, the ground strap where your spark jumps to from your electrode melts off rather than causing potentially detrimental damage to your engine's internal parts, such as your pistons, your valves, or even your internal engine cylinder walls. So yes, a lean condition can potentially cause your spark plugs to fail. If you find that you need to replace your spark plugs more frequently than the recommended interval, you should definitely check out your fuel pressure.

Okay, friends, a couple of things to think about when you're diagnosing. First of all, let's talk about safety. You want hand and eye protection, especially if you're going to be doing anything with fuel. With that said, it would be a great idea to have a couple of these tools, or at least one of them. It would be a good idea to have a multimeter. The reason for that is because you want to be able to check that battery. You want to make sure you have approximately 12.4 volts with the vehicle not running. You're also gonna want to have an option to be able to check your fuel pressure, whether it's a manual gauge like this one, or even if you have a scanner, something like this where you can hook into your newer vehicle's OBD connector.

So, one of the first things we're going to do is go ahead and check that battery voltage. We want to make sure we have perfect battery voltage because if the voltage is low, there's a possibility your computer might not know exactly what's going on. Let's go ahead and connect our two leads to the battery terminals, and we're going to pay attention to that multimeter. Like I said, you want it to be approximately 12.4 or maybe even a little bit above. This battery looks great. So, that tells me I'm clear to continue diagnosing. Once you know that you have a good known battery, you're going to want to move along to trying to pull those engine codes. Some vehicles aren't actually going to show a check engine light code for this, but if you have a newer vehicle, you should be able to, at least on a higher-level scanner, check the fuel pressure and maybe even graph it. This, of course, is going to be a great starting point in your diagnosis process.

Now, of course, you're going to want to know what your specific vehicle's fuel pressure is supposed to be at. There's going to be a little range. Generally, you can either find that in a repair manual, or if you needed to, you could, of course, scour the internet and more than likely, you're gonna be able to find it. To check your vehicle's specific fuel pressure, you're going to want to do that after your vehicle has been sitting for a while. You're going to go ahead and hook up this little gauge to the connection point on your fuel rail, assuming you have one. Once you've done that, what you want to do is turn the key to the on position without starting. The fuel pressure is going to come jumping right up and then you want to watch it. You want to see exactly how far it came up to and take note of that, but watch it for maybe 15, 20 minutes and just see if it starts slowly going down. If the pressure goes down to 5 or even 0, which would be, of course, very bad, that means you have a leak at your fuel pressure regulator.

If your fuel system loses pressure or has too low of pressure, your fuel system is not going to be primed up along those rails where it connects to your fuel injectors. If the fuel injectors don't have enough pressure and they're trying to push fuel into your combustion chamber, the injectors aren't going to receive enough pressure and you're going to have uncontrolled explosions or combustion inside of those cylinders. Now, where our fuel rail actually has a nice Schrader valve connection point on it, we're going to be able to go right here with our fuel gauge. Some vehicles you might find that you don't actually have a connection point up on your fuel rails, such as this Schrader valve right here. If that's the case, you're going to probably need to get a different kit, and, of course, you're going to need to tap into that fuel line somewhere. We're pretty lucky in this instance. So, I'm just going to go ahead and screw this on here, and we're going to watch that fuel gauge.

So, now that we have our fuel gauge hooked up to our fuel system, the first thing we need to do is go ahead and jump inside the vehicle and turn the key to the on position. We don't want to run the vehicle, we just want the second click. That's going to build up a little bit of pressure and it's going to force fluid inside of the line of the gauge itself. But we're not going to stop there, so I'm gonna go ahead and do that real quick. You're gonna see the fuel pressure come up.

Now, I'm going to release the pressure into a collection receptacle. There we are. I'll set this back up here. And at this point, we should have plenty of fuel inside the lines of the gauge itself. So, we'll be able to take a proper measurement.

So, now, this is our second time around and what we can see is the gauge jumped right up to the 40 PSI, maybe even a little bit above that. Well, that looks pretty great for this particular vehicle. It might be a little bit different for your particular application, though, so just keep that in mind. Now, the next thing that I want to do is watch this gauge to see if the needle drops down. You don't particularly need to sit here and wait for it, you can maybe go for a little walk, get a drink, maybe a snack, come on back, and if you happen to see that the needle came down, maybe below the 30 down to the 20, the 10, or even worse, you know, 0, you know that you have an issue with your fuel pressure regulator.

Okay. So, here's where our needle's at right after we've let this thing sit for a while. In all honesty, I went and had lunch, I came back after an hour, and here's where it's sitting, approximately 15 PSI. That's very low. Now, the next time that I jumped inside the vehicle, when I went to start it, it's going to have to try to build that pressure all the way back up to the 45 to 55 area that I want to be in before I can start the vehicle.

Now, with the pressure dropping the way that it is that kind of tells me that we have something that's not exactly doing the job that it should be doing. Whether it's a fuel injector that's for some reason sticking and staying open in some way, just allowing the pressure to kind of drip right down into that cylinder, even though the vehicle is not running, or you could have a pressure regulator either generally on your fuel rail somewhere or even inside your fuel tank, that's supposed to be kind of like a one-way valve, and if it's not working the way that it should, only allowing fluid to go one way, well, of course, you're going to have a seeping issue and it's going to make its way back down into the tank. The only other thing I can think about is maybe you have a fuel leak someplace, in which case, you probably see some dripping and you definitely smell fuel.

All right. So, for my particular vehicle, what I've come to realize is I have an issue with my fuel pressure regulator. It could be one of those other things that I had mentioned to you before. Something such as maybe you have a leak somewhere or even a leaky fuel injector. So, for your application, you're going to have to figure out what's going on if you're losing pressure from your fuel system.

All right. So, now, let's get back to talking about if you happen to notice that your fuel pressure was going up slowly after you turn the key. Essentially, what that would mean to me is there's more than likely a restriction someplace. A very common place to have a restriction is through a maintenance item, such as a fuel filter. Sometimes it's easily overlooked; people don't necessarily think about it. You don't have a fuel filter oil change sticker up there that says, "Hey, don't forget about your fuel filter." So, unless you're keeping up with your maintenance, you might easily forget about it. But the fuel filter is generally going to be one of the easiest things to fix on your fuel system. The reason for that is because it's generally going to be out in the open underneath your vehicle, usually along the frame of some sort. There are going to be some instances where your fuel filter is actually going to be located inside of the fuel tank, which, of course, that's going to be much harder to do and, of course, it's going to be much more labor intensive.

So, now, the next thing that we want to try doing is checking the fuel pressure while the is running. What we want to do is pay attention to the fuel gauge itself and just watch that needle. I want to see if it's bouncing around, it should essentially kind of come up and you might see a kind of, you know, moving around just a teeny bit and that's normal because it's running off of a pump, of course, and, of course, the fuel is getting drawn out into the cylinders. But if you happen to see the needle going like this, bouncing around like a maniac, well, you know you have an issue going on.

All right. So, we just started up the vehicle and, of course, I turned it back off because I'm inside an enclosed place. But what I wanted to show you was when you looked at the fuel pressure gauge right here, when I was starting it, the fuel pressure was kind of just [vocalization] but then as soon as the engine smoothened out, the needle actually smoothened out as well. If for some reason you happen to realize that the needle didn't smooth it out and it just kinda kept bouncing around, well, we know that that's another issue and it's something that you're going to have to continue with your diagnosing.

Now, something else I want to talk about is fuel pressure while you're under load. Essentially, if you're under load, you're kind of putting a lot of strain on your engine's driveline system. If you have low fuel pressure overall, well, then your engine might not be able to get the sufficient amount of fuel to be able to function as it should. Now, if you can't get the proper amount of fuel pressure in your system, what you're probably going to notice is a hesitation, possible bucking, and you might even notice a lean condition, which is essentially your engine telling you it needs help.

Now, if you find that you have a bad fuel pump or even a weak fuel pump, of course, you're going to want to replace it. At the same time as you replace your pump, you should always replace your filter at the same time. I don't probably need to tell you any more about the filter. Now, if you're replacing your fuel filter, if you were to take a nice clear cup and just kind of dump out that fuel filter, do it from both sides, you might be kind of surprised what you're going to see in the bottom of that cup. It might almost make you think twice about going to one of those cheapo gas stations.

Okay. So, now, tips for replacing the pump. Of course, first of all, you're going to want to go ahead and disconnect your negative battery terminal. Go ahead and set that aside from the battery terminal so there's no way it's going to be able to make connection. Now, the next thing you're going to need to think about is your fuel tank is going to be located underneath your vehicle. So, you're going to want to make sure you have plenty of ground clearance because, well, to get that tank down, firstly, it's going to be very heavy and secondly, it's going to be fairly large overall. So, you need to make sure that once it comes down, you'll be able to have access to the top to disconnect any lines or connectors, and, of course, you'll need to have access to be able to slide it out from underneath the vehicle to continue working. And with said, it's super helpful to have your fuel level as low as possible during this process.

And one last thing, once you've gotten your fuel pump in, the initial start of it, you might happen to notice that you have a long crank of some sort and it might be a little bit worrisome because, "Well, I thought I was fixing the long crank." Something that you need to think about is when you disconnected those lines down low, there was an opportunity for the fuel that's up inside the fuel lines to make its way down, so it's kind of potentially putting a lot of air into the system. So, go ahead and turn that key, put it in the on position, and wait for the fuel pump to activate. After about three to five seconds, go ahead and turn it back off real quick. Once you've done that, go ahead and turn it back on, listen for the pump, and then go ahead and give it a crank. What you're probably gonna notice is it fires right up.

Okay, friends, like I said, on our particular vehicle, we happen to find that we had an issue with the fuel pressure regulator. Essentially, like I said, it's a one-way valve, and for some reason, the valve isn't just working one way. It's slowly letting the fuel seep back down into my fuel tank. With that said, I'm going to go ahead and replace my fuel sending unit, and that, of course, is going to be replaced with my fuel filter at the same time, because that's just how I roll and I recommend you do it as well.

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