Here's How You Diagnose an Engine That Won't Start
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Every truly crappy day begins with a car that won’t start. We’ve seen it in the movies, we’ve read it in books, and that’s just how it is. So here are some simple steps you can take to nip that crappy day in the bud. Let’s figure out why the heck your car won’t run!
The four main elements needed for a running engine are: spark, air, fuel, and compression. Let’s walk through the steps we took to get his car running. Remember, these are just the basics, and while these steps apply to most cars, some vehicles may require a different diagnosing technique.
Step 1: Check For Spark
Checking for spark on an internal combustion engine is pretty simple. Plug one of the spark plugs back into its spark plug boot, and ground the body of the plug by placing the metal threaded portion against a metal part of the engine. Have a friend crank the engine with the key. Make sure there is no gasoline near the plug, and don’t hold the plug or wire with your hands. Just sit it down and watch. You should see a clear spark between the electrodes. If you don’t, then you have an electrical problem.
Our vehicle showed the nice crisp spark in the image above, so we knew our electrons were flowing properly. Note: in the image above, spark is arcing between the body of the plug and ground; this works too. Also, if you can’t get to your spark plug boots easily, you can use an inductive tool that lights up when it detects a magnetic field from current running through the plug wire.
But What Could it Be?
There are lots of reasons why you might not be getting spark. If it’s an old car, it might be your points. If it’s an old motorcycle, it could be your ignitor. You could also have a bad distributor, a bad coil pack, or even bad crankshaft and camshaft position sensors. The sensors are pretty easy to check, assuming you can get access to them.
First, though, check your fault codes with a scanner and see if anything registers. If not, break out the good ol’ multimeter and check the sensors’ resistances against the published specs. Also check your relays and fuses.
The best way to get to the bottom of your problem, though, is by starting at the spark plugs and working your way back. The plugs aren’t firing, so they’re not getting current. Next thing to check might be the distributor (if you have one). Then check the coil pack. Check to see if they’re getting power. Disconnect the connecter to the coil, and turn the engine over. If you see voltage at the connector, chances are, your coil is bad. If your coil isn’t getting current, your problem lies elsewhere.
Step 2: Check for Air
This one is simple. To make sure you’ve got plenty of clean air entering your cylinders, check your air filter and make sure it’s not clogged. You should also check any ducting or air inlets that might be filled with debris. Our air filter had just been changed, so we didn’t bother checking ours. Lack of airflow is rarely the cause of a non-firing engine.
Also check for vacuum leaks or cracks in the air induction system. This could let in too much air and prevent the engine from starting. You can check for vacuum leaks by listening. If your car fires at all, you can check for vacuum leaks by spraying a bit of starting fluid near suspected leak spots. See if the engine changes its tune. Be careful spraying that stuff near a hot engine, though.
Step 3: Check For Fuel
Usually, you figure out if you have a fuel delivery problem by process of elimination. If you have good spark and clean air, you’re probably not getting fuel. Some folks like using starter fluid as a check. They just spray a quick squirt of starter fluid in the throttle body while the engine is cranking and see if it fires (be careful not to spray on the MAF sensor.)
If it runs on starter fluid, then clearly your issue is fuel related. Because starter fluid can be dangerous, and because I’ve seized an engine using it, I’d recommend using process of elimination and then going through the following steps to pinpoint the issue if you think it’s a fuel problem.
But What Could It Be?
Sometimes, if your engine isn’t getting enough fuel, the culprit could be your fuel pressure regulator, a fuel leak, a clogged fuel filter, or clogged injectors. But most of the time, it’s the fuel pump. The fuel pump can usually be checked fairly easily. First, identify your fuel pump location. The fuel pump and sending unit enter the tank from above, and can be accessed either via an access panel inside the car, from the front side of the tank, or they may require the removal of the gas tank. Don’t worry about removing the tank: the goal is just to get your ear as close to that fuel pump as possible.
On the BMW 325Ci in question here, the fuel pump access panel was under the rear seat. Put your ear next to it, and have a friend turn the key to the “on” position. The fuel pump should prime for about three seconds. There should be an audible whine like the one in the video above. If not, your fuel pump may not be getting power or it may just be dead. Our pump exhibited no noise.
So, how do we check if our fuel pump is getting power? Just connect your handy dandy multimeter to the fuel pump connector, turn the key to the on position, and it should read a voltage for about three seconds (the pump receives current for a few seconds to prime the pump). Our multimeter showed that our pump was getting current from the battery. So that made us think that the pump might be the culprit.
If you’re seeing a voltage reading coming from the connector, the next thing to test would be your pump, especially if you didn’t hear the priming noise. Touch your multimeter leads on the fuel pump terminals and measure the resistance across the pump. Compare this to published values (which you can often find online). If it’s markedly different, your pump may be toast. Our pump’s resistance measurements were off from the published values, so we were suspicious that this might be our issue.
Another check would be to look at the fuel pressure via the Schrader valve in the fuel rail (not all cars will have this feature). Rent a pressure tester from the store and see if your fuel pressure is up to spec. If it’s zero, your pump is probably fried. If you don’t have a pressure tester, put a napkin over the valve and push it in with a needle. Check to see if there’s a noticeable pressure release. We pushed the valve and nothing happened.
If your vehicle has a carburetor, you can check your mechanical fuel pump by just unplugging the fuel hose from the carb and feeding it into a glass jar. Turn the car over and watch the jar fill. With carbureted cars, though, chances are your carburetor is the cause of your fuel delivery problem. A sunken float, rust in the jets, or just a “gummed up” carb can cause all sorts of no-start problems.
Step 4: Check For Compression
You can have spark, air, and fuel, but if your pistons aren’t able to contain and compress your air charge, you won’t see combustion. So, an easy way to check compression is to just buy or rent a compression tester. It’s basically just a gauge attached to a hose with a fitting at the end. Pull your spark plug wire, thread the compression tester’s fitting into your spark plug hole, and turn your engine over with the key.
Unplugging your fuel pump relay and ignition coil is good practice when you do a compression test. As a rule of thumb, anything over 100 psi should fire. And uniformity is key: having two adjacent cylinders that differ in compression by 40 psi should be a red flag.
Our 325Ci had an even 180 psi across all cylinders. So we crossed this one off the list. Note: this isn’t straightforward on some engine configurations (boxer engines), as the spark plugs on some vehicles are hard to reach.
Don’t have a compression tester? No fear, there’s an old school trick for that. Just put your finger over the spark plug hole and have a friend turn the engine over. If you can’t keep your finger over the hole when the engine is cranking, you probably have enough compression.
But What Could It Be?
Low compression is usually a result of some sort of mechanical issue. Maybe it’s something as simple as a stuck valve, a valve that doesn’t seat properly, or a timing chain that skipped a tooth. Heck, maybe your spark plug wasn’t in all the way. Or the lack of compression could be the result of wear. Camshaft lobes, cylinder walls, and piston rings tend to wear out over time and decrease compression. If your engine sees high stresses or if you have an exhaust leak, it’s possible you might have burned a valve or cracked a piston.
You can check if it’s piston rings by running the compression test with a little oil in the cylinders. This “wet” compression test basically plugs up any wear passage between the rings and the cylinder wall, theoretically increasing compression. If compression goes up significantly in the wet test vs. the dry test, you probably have bad piston rings. You can check timing with a timing light or you can just open up your timing chain/belt cover and check the slack against the published spec.
You can look for a cracked valve by looking in from the intake or exhaust ports, and a cracked piston can sometimes be seen through the sparkplug hole with a borescope. There are tricks to freeing up a stuck valve as well. But sometimes you’ll need to remove the valve cover or cylinder head to really get to the bottom of a low compression problem.
But What If the Car Won’t Crank?
I’ll get this out of the way: 99 percent of the time, if your car won’t crank, it’s your battery. “But my battery is only a month old,” you say. Check your battery anyway. You can check the battery voltage with a multimeter, or you can just have a friend jump start it. Remember: red good, red bad, black good, ground bad — that’s the sequence for attaching the jumper cables.
If it’s still not turning over, check your grounds for corrosion. Also check your starter relay by swapping it with another relay in the power distribution box (make sure it’s the same relay). Finally, check the starter itself. See that grimey one in the picture above? Yeah, that one doesn’t work.
Check the starter by banging the it with something heavy, then try to start the car again. If you hear a little noise from your starter now, but the engine still won’t crank, you probably have a bad starter motor. Unbolt it and take it to a car parts store, where they’ll confirm whether the starter is toast.
There are many other potential culprits, like a neutral safety switch that’s malfunctioning or a bad ignition switch, but frankly, the problem is usually the battery.
What Happened to Our Car?
In the case of my friend’s BMW 325Ci, we heard no fuel pump whine. We confirmed that the pump was receiving current and that the pump’s internal resistance was not matching the spec. We also noticed no fuel pressure in the fuel rail. We had good spark, a clean air filter, and solid compression. This drew us to our conclusion: it was the fuel pump. We found a fuel pump, dropped it into our tank, and sure enough, the car fired up.
So there you have it, folks. If your car isn’t firing, just remember the four ingredients: spark, air, fuel, and compression. Check them one by one, and eventually you’ll figure out what’s going on.
Geeking Out is former Fiat Chrysler engineer and current Jalop scribe David Tracy’s weekly missive on all things intensely technical about cars. He can be reached at firstname.lastname@example.org.
This story originally ran on July 16, 2015.
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