**12 Volt System**

We know it's gonna be less than 20 volts. So I've got selected the 20volt range on my volt meter. So we're good to measure anything below 12 volts. Let's go ahead and check battery voltage. So the battery's got about 12 and three quarters volts. Nice and charged, that's sweet.

**Fuel Pump Voltage**

So now we're gonna power up the fuel pump and see how much voltage it's getting. Now we can see the fuel pump's getting about our battery voltage, it's getting 12 volts, so that seems pretty normal. So that is just another thing checked off. The pump is getting voltage, so why is it not making pressure? Let's get down to the pump, get it out of the tank, and take a closer look.

**Voltage Drop Test**

All right, so it seems that our fuel pump has voltage. So that sounds good, it sounds like the problem must lie with our fuel pump. But before we go pulling it out and replacing it, we need to do one more test and it's a really important test. It's called a voltage drop test and basically what this is gonna do is allow me to measure the circuit the fuel pump is in in terms of wiring and make sure that the problem doesn't lie within the wiring.

**Testing**

And basically what we're doing is looking to make sure that there's no large use of voltage anywhere in that circuit. We're gonna want to see about a half a volt at most in terms of voltage drop on this circuit. So let's go ahead and do it. Positive on the positive terminal and negative on the positive terminal of the fuel pump. And again we're looking for half a volt or less.

**Results**

So we've got .49 so we should be okay. It is a little bit high though. Now let's do the same thing with the ground circuit. Remember the fuel pump is still running. Negative onto the negative terminal of the battery, positive to the negative terminal on the fuel pump and we're getting about half a volt here too. Just a touch over actually, .53, which is on the upper end of how much voltage drop I'm okay with seeing.

**Identifying the Problem**

But it should be okay for a fuel pump. So now that we know that we don't have a huge resistance in our wiring and everything else seems to be okay, now it's time to pull that fuel pump out and see what's going on. Now basically all we need to do is jumper our relay to turn the fuel pump on, go to the front, and see if we've got fuel pressure.

**Finding the Problem**

Hey, 44 PSI. That's what you want to see. We've got fuel pressure, which means this thing should start pretty easily. What was that, was it a spider? Yeah. Bro, I felt that crawling on me. That was a genuine scream.

**Starting the Engine**

I know, it was great. Was it a black widow? Just tell me yes. Uh, yes. Okay, cool. All right, let's see what we got. (engine starting) Just been a minute since she's been alive. All right, she's running, let's let it warm up.

**Conclusion**

Aye, it's finally not lean at idle. So this is an untuned car. I've got it on a base tune, I've got it tuned to the point that I'm able to drive it around the block. But is not quite tuned. So we're gonna let it warm up and then we'll see if she goes around the block.

**Final Test**

All right, so we're just taking it for a quick spin around the block to make sure that things are running right. (engine revving) I'm feeling pretty good and starting to think that it's time to go get this thing tuned.

WEBVTTKind: captionsLanguage: en- This is my RB25 swapped S14and it's gonna make abunch of power, I swear.Once I figure out what's wrong with it.So what do you do when all of a suddenyour project car runs low on poweror even worse, won't start at all?Well, maybe you considerpushing it into the ocean.Or even worse, pushingit to a mechanic shopand paying them to tellyou what's wrong with it.Well that just won't do.So where do you start when your car won't?So today we're gonna give youa proper introduction to my S14,and then we're gonnause it to show you guyshow to diagnose a carthat's running poorlyor just won't run at all.I'm Zach and this is anotherone of my money pits.(thunder)(hip hop beat)(house music)If you've been watching this showyou've probably seen this little Nissanin the background staring at me,green with envy whileI spend countless hoursworking on the Miata.Well today that all changes.So this is a car I've always wanted.It's a 1997 Nissan 240SX,which means it's a koukior a face lift front end,which means it's thebest looking front endof all the 240s we got here in the states.But where I'm fromyou really don't comeacross these that oftenso I never had one.But as soon as I moved out hereI knew it was only a matter of timebefore I got sucked into one.And I sure did get sucked into one.First off, the body is a huge positive.It's super clean and youreally don't find thesein this good a shape that often.And if you do people are askingway too much money for them.But the other biggestpositive is under the hood.It's got an RB25.(crowd applauding)(engine revving)It's a straight sixturbo and it is awesome.It makes all the noises youcould ever want it to make.But when I bought the car thepositives really ended there.It was a clean body and it had an RB25.But it needed a lot ofwork to get it roadworthy.We've done a lot ofstuff since I bought it.I've replaced the turbo,I've done the whole ignition system,I've done the whole fuel system,I did a new throttle body,I did a new catch can,I did a new wastegate.So much stuff has been doneand we are so close togetting this thing on the dynoand actually making power.But then the other day it fell on its faceand has been having issues running at all.(engine cranking)Now I've got a pretty goodhunch of what is going onbut I thought this wouldbe the perfect opportunityto show you guys how to do some diagnoseson a car that's running poorly.So to just jump right into,in terms of diagnoses,the first thing we need to understandis what an engine needs to run.It's pretty simple.It needs air,it needs fuel,it needs spark,and it needs compression.So we'll take a look atthose four overall systemsand then we'll figure out whichsystem the issue lies withinand then we'll take adeeper look at that systemand pinpoint the issue.It's gonna be a day of doctoring.Call me Dr. House.First let's talk about air.I'd say usually in myexperience it's not air,but let's talk about a few issuesyou could have with the air system,especially on a turbo car.Your boost couplers can come looseand you can have a boostleak, which is not good.Then you're not getting allthe air into your enginethat you're supposed to be.If you've got any sort of a leakafter the thing that meters your air,like if you've got a massairflow sensor on your carlike the Miata does,if there's a leak afterthe mass airflow sensorthat can confuse the ECU.If the mas airflow sensor's dirtyor maybe the wires are worn through,that can also confuse the ECUso that it doesn't knowhow much air it's getting.So it could be air in those senses.This car doesn't havea mass airflow sensor.It runs on manifold absolute pressureand I just installed thatas part of a new ECU.So I know that's not the problem.I know all my boost couplers are tight.I know that air is not theissue in this situation.Okay, next, spark.Spark is maybe the simplestbecause all you need is a sparkand it's pretty easy to check for.I'll show you a couple really simple ways.The first is with this tool,this is a spark tester,and you can get theseat any auto parts store,Harbor Freight, theyonly cost a couple bucks.So real quick I'm gonna popoff one of my coil packsand this is gonna go inlinein between the coilpack and the spark plugand we should see if it's sparkingby this lighting up in here.(engine cranking)- We got it.- Okay, so that's one way to testwhether or not you have sparkand it's pretty easyif you have that tool.But I'll show you an even easier waywith just a spark plug.Okay, so the idea hereis just to insert aplug into your coil packor into your plug wireand then the only weneed to do is make surethat somehow we ground theground side of the plug,which is the threadsup to the ground strap.So easy enough actually,I guess here we'll justsit it on the turbo.And now I'm gonna go crank the ignitionand we should see spark hereat the plug if we have spark.(engine cranking)All right, so that is how you test sparkand we've got spark.So that was pretty easy, right?Now let's move on to compression.So doing a compressiontest can test your valves,your piston rings, your head gasket,all sorts of thingsinternally in your engine.So we're gonna do aquick compression test.I've got my tester here,I think I bought this at Harbor Freight.It was probably $40 or $50.It's pretty simple.We just got to remove a sparkplug to test a cylinder.We'll take this hoseand it goes right downinto the plug threads.And now we've got a hoseattached to our cylinder.So we'll put our pressure gauge on it.Nice and easy.Thumbs of steel, don't fail me now!Okay, so now we don't have any sparkso we're good to do our compression test,the engine's not gonna start.Generally, in any gasoline engine,you're gonna want over 100PSI in all of your cylindersand you're gonna want them all to bewithin about 10% of each other.So without further adoI'm gonna go crank it,you're gonna keep an eye on it, right.(engine cranking)Okay, how'd we do?- About 120.- 120, that's a littlelower than I was expectingbut I know that compressionisn't the issue on this,so we're gonna move to the fourth thingthat an engine needs torun, and that's fuel.(funky music)The first thing we're gonna want to doin terms of checking our fuel systemis to check and make surethat there's fuel pressure.I bought this kit from Harbor Freight,I think it was about $40,or you can rent this fromany auto parts store.It's a fuel pressure gaugeand you can put this rightinline of your fuel linewith some supplied hose,or it comes with a bunch of fittingsthat will screw into a receptacleor a Schrader valve on your fuel rail.But if you're like meand you're fiddling with this stuff a lotand you want to be ableto keep an eye on your fuelpressure pretty regularlyyou've got a fuel pressure gaugeon your fuel pressureregulator in your engine bay.An engine usually wantsabout 43 and a half poundsof fuel pressure at idle.Really at all times.So when I turn the key onI should see this come upto about 43 and a half PSI,that's what I have it set to.(man gasping)Okay, so what happened?- We got about 20.- 20, that's low.Our fuel pressure's only at 20 PSIwhen it should be at about 43 and a half.So that means we've got anissue with our fuel systemand there's only a handful of thingsin our fuel system that it can be.So let's work our waythrough the fuel systemand check them off.So the first we're gonna dois actually visuallyinspect the fuel systemfrom front to back to make surethat we don't have anyjust straight up leaks.And also use your nose'cause, you know, gas stinks.So,okay, no fuel leaks.I've actually been throughthe fuel system front to back,I've had the gas tank outbecause that's a whole thing on these.I know there are no fuelleaks in the external sense.But now it does kind of sounds likewe've got maybe an internal leak.The next thing we've got isthis fuel pressure regulator.All right, so I guess there are two waysthat our fuel pressureregulator could be the problem.For one, the diaphragm insideof the regulator could be tornwhich would let it leak up into this top,which would mean that thishose would fuel of full,full of fuel and it isn't.Doesn't even really smell like fuel.So that's fine.Not the diaphragm there.So the other thing that could happenis that the fuel pressure regulatorcould be stuck open insideand just bleeding pressureback to the tank constantly.So what I'll do is pop the return lineoff the bottom of the regulator hereand then I'll go cycle the key.And if it basically justdumps fuel out the returnthen we know that ourfuel pressure regulatoris just letting pressure off.You good?- Yep.Just key on.(beeping)- So you wantgas coming out, right?- No. I know.- I'm just kidding.- Okay, so that rules outour fuel pressure regulatorwhich is to be expected.I mean is a new regulatorand didn't really go bad that frequently.So this moves us furtheralong in our fuel system.We've ruled out leaks, we'veruled out our injectors,and we've ruled outour pressure regulator.So now it's time to go back to the tankand look at the thingthat makes the pressurein the first place.The fuel pump and its circuit.All right, so now we'reback here at the trunk.We've worked our way down the fuel systemand we've gotten back towhere the gas tank is,and in the gas tank lives the fuel pump.So we're gonna test thefuel pump and its circuit.Which should be pretty easy,this setup's pretty good.My battery is right here, myfuel pump relay is right here,and my fuel pump itself is right in there.So the first thing I'm gonnado is pop out this relayand I'm gonna use this jumper I made,just a little piece ofwire with two connectors.So I'm gonna jumper the poweron the relay to the fuel pumpso I can power up thefuel pump constantly.And then I'm gonnacheck and just make surethat the fuel pump isgetting battery voltage.I'm gonna do that with my multimeter.So to check voltage,in this case we're gonnabe checking voltageon an automotive system whichworks on a 12 volt battery.So this is not an auto ranging multimeterlike some of them are.You have to select the rangewhich you're gonna be measuring in.So since this is a 12 volt systemwe know it's gonna be less than 20 volts.So I've got selected the 20volt range on my volt meter.So we're good to measureanything below 12 volts.Let's go ahead and check battery voltage.So the battery's got about12 and three quarters volts.Nice and charged, that's sweet.So now we're gonna power up the fuel pumpand see how much voltage it's getting.(hip hop beat)Now we can see the fuel pump's gettingabout our battery voltage,it's getting 12 volts,so that seems pretty normal.So that is just another thing checked off.The pump is getting voltage,so why is it not making pressure?Let's get down to the pump,get it out of the tank,and take a closer look.(house music)All right, so it seems thatour fuel pump has voltage.So that sounds good,it sounds like the problemmust lie with our fuel pump.But before we go pullingit out and replacing it,we need to do one more testand it's a really important test.It's called a voltage drop testand basically what this is gonna dois allow me to measure thecircuit the fuel pump is inin terms of wiringand make sure that the problemdoesn't lie within the wiring.And basically what we're doingis looking to make sure thatthere's no large use of voltageanywhere in that circuit.We're gonna want to seeabout a half a volt at mostin terms of voltage drop on this circuit.So let's go ahead and do it.Positive on the positive terminaland negative on the positiveterminal of the fuel pump.And again we're lookingfor half a volt or less.Ooh baby!So we've got .49 so we should be okay.It is a little bit high though.Now let's do the same thingwith the ground circuit.Remember the fuel pump is still running.Negative onto the negativeterminal of the battery,positive to the negativeterminal on the fuel pumpand we're getting abouthalf a volt here too.Just a touch over actually, .53,which is on the upper endof how much voltage dropI'm okay with seeing.But it should be okay for a fuel pump.So now that we know that we don't havea huge resistance in our wiringand everything else seems to be okay,now it's time to pull that fuel pump outand see what's going on.(hip hop beat)- (groans) Whatdo we got, what we got?- See that hose?That should be all the way downon this little outlet here.Instead it's blown off.AKA not gonna make much fuel pressure.AKA our problem.This is a kind of inconvenient spot.All right, so that is everything.Now basically all we need to dois jumper our relay toturn the fuel pump on,go to the front, and seeif we've got fuel pressure.I think we're gonna.Hey, 44 PSI.That's what you want to see.We've got fuel pressure,which means this thingshould start pretty easily.(Eddie yells)What was that,was it a spider?- Yeah.- Bro, I felt that crawling on me.That was a genuine scream.- I know, it was great.- Was it a black widow?Just tell me yes.- Uh, yes.Okay, cool.All right, let's see what we got.(engine starting)Just been a minute since she's been alive.All right, she's running,let's let it warm up.Aye, it's finally not lean at idle.So this is an untuned car.I've got it on a base tune,I've got it tuned to the pointthat I'm able to driveit around the block.But is not quite tuned.So we're gonna let it warm upand then we'll see if shegoes around the block.(engine revving)All right, so we're just taking itfor a quick spin around the blockto make sure that thingsare running right.(engine revving)I'm feeling pretty goodand starting to thinkthat it's time to go get this thing tuned.(engine revving)I need a diff.Well, sweet.That's a productive day in my book.We got this thing ready to go to the dynowhich is all I really wanted.And I think along the waywe've hopefully taught you guyshow to diagnose some stuff.And the fact is diagnosesis a very deep well.There is a lot of stuffso we've really onlyscratched the surface.When issues pop we'llcover some more diagnoses.I hoped you guys like this one,I appreciate you watching.Follow me on Instagram @zachjobe,follow Donut @donutmedia andI'll see you guys next week.