Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.
When you have a car hard to start cold, hot, or both, trying to find the fault can be frustrating.
Unfortunately, there is no easy solution that can work for all cases. You need to find the fault among many potential components distributed in various systems.
But this doesn't mean you should feel lost.
Most of the time, this type of failure stems from lack of proper engine maintenance, so you stand a good chance of repairing your engine yourself, even if you don't have much experience in car repair.
This guide shows you how to approach your diagnostic procedure to narrow down your troubleshooting to the potential system behind the fault.
Before you start, a suggestion: If your hard-start condition happens only when the engine is cold or hot, concentrate on that particular section first. Also, if you follow the next tests and you find a fault in one specific system, check that system first in the appropriate "cold", "hot", or "cold and hot" section. For example, if the ignition spark is weak, go to the "Check Ignition Components" subsection.
To locate that faulty system, let's try a few simple tests.
I. Locate the Faulty System
II. If Your Engine is Hard to Start When Cold
III. If Your Engine is Hard to Start When Hot
IV. If Your Engine is Hard to Start When Cold or Hot
I. Locate the Faulty System
These series of simple tests may help you in locating where the fault is coming from.
a) Quick Battery Test:
If the engine is hard to start when hot, check for corroded, loose or damaged battery terminals first. Heat acts as an electrical resistance, especially on bad connections.
And one of the first things you need to check when you have trouble starting your engine when cold is to make sure you have a charged battery.
Even a slow electrical drain from a faulty battery or system can leave you without enough power to fire up the engine during a cold start.
For this test, you'll need a digital multimeter (DMM). The test will give you a quick sense of your battery charge condition:
1. Set your meter to 20 Volts DC or a similar low range.
2. Get a voltage reading across the battery posts.
* If you get a reading below 12.4V, you'll need to charge the battery. This may or may not be the cause for the hard-start. Still, you need a charged battery for the different electrical systems to work correctly.
* If you get a reading at 12.4V or higher, turn on the high beams for ten seconds, then turn them off.
* Wait for 2 minutes.
* Take another voltage reading.
* If your second reading is below 12.4 Volts, you need to charge the battery.
Most auto parts stores will charge your battery and check it for free. Take advantage of this service.
If your battery needs charging, make sure to make a detail inspection of your battery. This may be the problem behind your hard to start engine.
b) Quick Fuel System Check:
A hard-start condition may also come from insufficient fuel. This simple test will quickly let you know if you have enough fuel going through the system.
* Engines with Throttle Body Injection (TBI) System:
1. Remove the air filter housing cover and filter.
2. Ask an assistant to crank the engine while you watch the injector.
3. You should see a fine spray of fuel in the shape of an inverted "V". If you see thick droplets or a thin, straight spray, you need to clean the injector, replace it, or check the fuel system for insufficient pressure. For example, check the fuel pump and fuel pressure regulator. Consult your vehicle repair manual, if necessary. Also, make sure you've replaced the fuel filter at the manufacturer's recommended interval. A clogging filter will prevent the fuel pump from building adequate system pressure.
* Engine with Multiport Fuel Injection System:
1. Locate the test or Schrader valve on the fuel rail. This is similar to the air valve on your car tires.
2. Make sure the engine is cold.
3. Stay clear from the valve and depress the valve using a small screwdriver.
4. You should see a healthy spurt of fuel coming out through the valve.
If the squirt is too small, about 2 inches tall, there's not enough fuel system pressure. You need to check the system. For example, look for a clogging fuel filter, faulty fuel pressure regulator, a leaking fuel injector or bad fuel pump.
You can confirm our findings connecting a fuel pressure gauge to the Schrader valve.
Some models may not have this valve. Consult your vehicle repair manual for the best way to check fuel pressure on your system, if necessary.
c) Quick Ignition System Check:
A weak spark will make it difficult for the engine to start, and create all kinds of performance problems. This quick test will give you a general idea about the state of the ignition system.
1. Remove each spark plug and make a visual inspection of it. Look for carbon build up, foreign matter contamination, wear or damage. You may use your vehicle repair manual to help you evaluate each spark plug. Verify each spark plug's gap and adjust it if necessary. Also, check the service interval for your plugs and wires and replace them if necessary.
2. Check the spark strength using an adjustable spark tester. You should get around 30K to 40K Volts from the ignition coil for a good spark. Otherwise, check the spark plug wires, coil(s), distributor and rotor, if your engine has them.
d) Quick Emissions System Check:
Your vehicle engine control unit (ECU - car computer) monitors important emission control systems through several sensors and actuators to make necessary adjustments.
When any one of these sensors or actuators deviate from its normal parameters of operation, a Diagnostic Trouble Code (DTC) is set by the computer in its memory. This means your check engine light (CEL) will light on your dashboard.
However, a fault may come and go. An intermittent fault like this could set a pending DTC without making your CEL illuminate. Still, you can scan your computer for pending codes that may point you to the source of your hard to start problem.
If you don't have a scanner, take your vehicle to an auto parts store. Many of these stores will scan the computer for you.
II. If Your Engine is Hard to Start When Cold
A hard to start engine when cold is usually associated with certain components or systems that can give you trouble when they begin to fail or wear out. Although not exclusive to this condition, the following components are worth checking because they can help you zero in on the fault.
Check Ignition Components
Bad spark plugs are blamed for many performance issues: Poor fuel economy, low power, misfiring and, yes, hard starting on a cold engine.
Depending on the particular type of spark plugs your vehicle model is using, you may need to replace spark plugs every 2 or 5 years. A worn out plug will make it difficult for a spark to jump between electrodes. Make sure you've replaced your spark plugs on schedule. Consult your car owner's manual or vehicle repair manual.
You can pull one spark plug at a time and make a visual inspection of it. If the spark plugs are good, check the spark plug wires resistance and physical condition, and then check the coils. Consult your repair manual for this.
Also, keep in mind that problems in other systems like fuel or emission may affect an otherwise perfectly good spark plug. Too much fuel in the system or an oil leak into the combustion chambers, for example, can lead to carbon build up around the electrodes, effectively blocking the spark. Your repair manual can help you inspect your spark plugs.
Check Fuel System Components
Lack of necessary fuel system pressure will make it hard for your engine to start. This may happen on a cold engine if pressure is slowly leaking or not building appropriately, affecting your engine after being parked for a couple of hours or overnight.
There may be several reasons why system pressure is leaking. For example, you may have a:
- weak fuel pressure regulator
- leaking fuel injector
- clogging fuel filter
- fuel pump with worn bearings, motor commutator, brushes
- bad fuel-pump check valve that allows fuel to drain back into the tank over time
- 'lazy' fuel-pump relay
- high resistance in the fuel pump electrical circuit
- leaking fuel line
A quick test can help you confirm that fuel is lacking during a cold start:
Ask an assistant to crank the cold engine while you spray some starting fluid into the throttle body. If the engine catches while spraying fluid, there's insufficient fuel during the cold start.
However, remember that a flooded cylinder, from a leaking cylinder for example, can also cause starting problems, even if you have good fuel pressure. Once the extra fuel is spent during the initial start, normal operation resumes, until the cylinder is flooded again while the car is parked. Check for a possible leaking fuel injector.
Fuel System Checks:
* When was the last time you replaced the fuel filter? Check your car owner's manual for the service interval and replace it if necessary.
* Check the fuel system for pressure, fuel volume and, if necessary, the fuel-pump electrical circuit.
* Cold start injector, if applicable. Basically, the injector sensor will activate this special fuel injector when the engine is cold to richen up the fuel mixture. Once the engine begins to warm up, the injector's switch opens to cancel the cold start injector.
To check the cold fuel injector, or other fuel injectors, for operation:
1. Ask an assistant to crank the cold engine while you listen to the cold injector ticking, using a length of rubber hose. A piece of vacuum or fuel hose will work fine here.
2. Place one end of the hose against your ear and the other one against the sensor.
3. You should hear the sensor ticking while the engine cranks. Otherwise, the injector isn't working.
You can check the other injectors in the same manner. Consult your vehicle repair manual to locate the injector and other possible tests you can do to confirm proper operation of the fuel system in your particular model.
See the next video for some advice on diagnosing fuel system problems that may cause hard to start issues.
Check Emission Components
Within the emission control system of your car, you'll find several components that, when going bad, can start to make the engine hard to start. Sometimes, a component only needs some cleaning or a simple hose or grommet needs to be replaced.
Keep in mind that sensors in the emission systems will cause the computer to set a trouble code when operating out of normal parameters. So scan your car computer even if you don't see the check engine light (CEL) coming on. You may have a pending code.
Here's a list of the most common faults you can look for during your diagnostic. Again, if you haven't paid attention to one or more of these components or the specific system they belong to, that's a red flag:
1. Mass Air Flow (MAF) sensor: Over time, dirt or foreign matter can cover the sensing element, preventing the sensor from operating correctly. Check the sensor, clean it if necessary, and make sure to reinstall the air cleaning assembly properly to prevent unmetered air from entering the engine.
2. Throttle Position Sensor (TPS): Usually, a bad or failing TP sensor will trigger the Check Engine Light (CEL). So scan the computer memory for diagnostic trouble codes (DTCs). Your vehicle repair manual may tell you how to test the TPS, if necessary.
3. Check the Air Temperature (AT) sensor, if your engine is equipped with it.
4. Positive Crankcase Ventilation (PCV) valve and connecting hoses: If the valve is stuck open or one of the connecting vacuum hoses is torn, it can have an adverse effect on engine starting.
5. Air filter: If the paper element seems covered in dirt or clogged, install a new one. If you haven't replace it in more than two years, replace it.
6. Vacuum leaks: A vacuum leak will make it hard for the computer to figure out the correct air-fuel mixture for a cold engine. Trace every vacuum hose with your hand, trying to feel for rough, soft, hard, or uneven spots that may indicate a damaged hose.
7. Throttle valve (throttle plate) and bore: Whether your engine is fitted with a carburetor, TBI system, or throttle body, inspect the throttle valve and bore for carbon buildup. Carbon deposits in the valve or body can interfere with the valve. Use carburetor cleaner and a shop rag to remove deposits. NOTE: Some vehicle manufacturers apply a protective coating to the throttle bore that makes it hard for sludge to accumulate. Cleaning the bore with an aftermarket chemical may remove this coating. If necessary, consult your vehicle repair manual.
8. Idle Air Control (IAC) solenoid: The solenoid is used in most fuel injected models. It allows extra air to enter the engine at idle, depending on operating conditions.
Over time, the valve passages may fill with carbon deposits that can interfere with its operation.
If you can start the engine while accelerating when starting your cold engine, this may be an indication of carbon buildup in the valve passages or a bad IAC solenoid. Remove the solenoid and inspect the passages. You can find it near the throttle body. If necessary, check the solenoid for proper operation. Consult you vehicle repair manual.
9. Coolant Temperature Sensor (CTS): The sensor is in charge of reporting to the car's computer, just like its name says, what's the coolant temperature at any given moment.
The computer uses this information, along with information from other sensors, to add more fuel to the mixture when the engine is cold, and to gradually lean the mixture as the engine reaches operating temperature. Even more, the computer uses information form CTS and other sensors to adjust ignition timing as needed.
As you can see, a bad CTS can make it hard, not only to start the engine but to operate the engine efficiently.
This test is for a common thermistor type sensor. Consult your vehicle repair manual, if necessary.
* First, make sure coolant is at the correct level. Insufficient coolant will interfere with CTS proper operation.
* With the engine cold, unplug the CTS electrical connector. Check the resistance between the two sensor terminals using a digital multimeter. At around 70F, you may read around 40K Ohms of resistance on a good sensor. If necessary, check resistance specifications in your repair manual.
* Then, run the engine until it reaches operating temperature. You can idle the engine for about 15 or 20 minutes or run some errands and come back to finish the test.
* Again, unplug the CTS and measure resistance between the two sensor terminals. At about 200F, you may get between 2K and 4K Ohms of resistance.
Compare your results to the specifications listed in your vehicle repair manual.
III. If Your Engine is Hard to Start When Hot
Some components or systems that cause you problems to start your engine when cold, may cause you trouble when the engine is hot as well. If a component you suspect in this section also appears on the "My Engine is Hard to Start When Cold" section, refer to that part as well.
Here is a list of the most common system or components that you may want to check, if your engine is hard to start when hot.
If one or more of the components listed here are part of a system you haven't paid attention lately, check those first. Also, don't forget to scan your computer for DTCs. Even if the CEL hasn't come on, you may find some pending codes that can guide you in your diagnostic.
Check Fuel System Components
Look for an obstruction to fuel flow or a malfunctioning component preventing fuel from properly reaching the engine.
1. Check for a clogging fuel filter.
2. A fuel pump going bad may not show any symptoms until operating for a few minutes. It may even cause the engine to stall.
3. If you haven't changed the air filter in more than two years, or if the filter seems clogged with dirt and debris, replace it.
Check Engine Ground Connections
A bad engine ground connection can work as an unwanted resistance in a circuit. Once the engine has reached operating temperature, the heat will add some unwanted resistance to any wire that is making a bad contact. The same effect can have corrosion, dirt or grease around an electrical connection.
There may be two or more ground engine straps connected between the engine and the body to serve as a return path for electrical current. A torn, damaged, or oil-contaminated strap connection can interfere with this current path.
To check engine ground using a digital multimeter:
1. Disable the fuel or ignition system by removing the fuel-pump fuse or disconnecting the ignition coil. Connect the coil wire to ground using a jumper wire. This will prevent your engine from starting.
2. Set your meter to a low range in the DC (direct current) Voltage scale
3. Have an assistant crank the engine. NOTE: Don't crank the engine for more than 30 seconds at a time to prevent damage to the starter motor. If necessary, allow the starter to cool before proceeding.
4. Connect your meter's black lead to the battery negative (black cable) post, and the red lead to the engine. Any unpainted metal surface on the engine will work.
5. Read your meter display. It should read 0.3V or less of voltage drop. Any reading above this should indicate some unwanted resistance in the ground side of the circuit.
Check engine grounds and battery ground connection. Look for loose, contaminated or damaged connections.
Check Emission Components
Refer to the "check emission components" on the "My Engine is Hard to Start When Cold" subsection.
IV. If Your Engine is Hard to Start When Cold or Hot
There are varied reasons why you may have a hard time starting the engine whether early in the morning or every time you turn the key. Here are some of the most common:
1. Starting system and battery: This is perhaps the most common fault to appear under this condition. You may notice the starter motor not working as usual or weak. Make sure to check that the battery is fully charged before condemning the starter though. Then, have the starter motor tested, if necessary. Most auto part stores will check the starter motor and battery for you.
2. Vacuum leaks: The number of vacuum hoses differ from one engine model to another. But most have an emissions diagram label at the front of the engine compartment, indicating existing vacuum hoses and where they connect to.
Check each of these hoses for proper connection, damage, wear and uneven spots. To make it easier, trace each hose with your hand. You may need a mirror to help you inspect some hoses in difficult to reach places.
3. Ignition system: Check for worn out spark plugs, carbon deposits around the plugs' electrodes, check spark plug wires resistance, carbon traces around the distributor cap and rotor, if equipped. If necessary, consult your vehicle repair manual to test components and specifications.
4. Fuel system: Check for a clogging or leaking fuel injector, bad fuel filter, fuel pressure regulator, or faulty fuel pump.
5. Mechanical Problems: Carbon buildup around the intake valves can also give your problems during a start and other performance issues. Service the valve with a decarbonizing chemical, if necessary. Seafoam is a popular product for this and you can find it at Amazon. You can also add it to the fuel tank to clean the fuel injectors. And, if you want to know the mechanical conditions of the valves before or after you service the valves, do a vacuum or compression leak test.
6. Emission Systems: See the "check emission components" subsection in "My Engine is Hard to Start When Cold" and scan the computer for potential DTCs or pending codes.
When a hard to start condition suddenly appears, the last thing in our minds is the need for engine maintenance. But remember that your car engine and computer rely on sensors, actuators, hoses, fluids and electrical systems for proper operation. And once a component in need of attention begins to fail, it can cause other components or systems to fail. So you end up with more than one problem to deal with. Then, pay attention to the service interval suggested by your car manufacturer. You can find the schedule in the vehicle repair manual for your particular make and model. Most of the time you only need to adjust or replace one or more components or fluids. If you don't have the manual, get an inexpensive Haynes aftermarket manual through Amazon. The manual is just a small investment compared with the hundreds or even thousands of dollars you can save in expensive repairs later on.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
© 2017 Dan Ferrell
skodabeast on July 07, 2020:
Thanks for the reply Dan,turned out timing was a bit off,had belt done over two yrs ago,not sure how it went a bit off but that was the story :)
Dan Ferrell (author) on July 06, 2020:
The ECT sensor can only 'command' to add fuel to the mixture when working properly. When it is unplugged, the PCM goes into a preprogrammed value for the engine to operate.
When the engine warms up, try to start it and spray some starting fuel down the throttle. If it seems like it wants to go, the problem is in the fuel. Check the coolant sensor circuit. A connection or wire along the circuit could be dirty, loose or damaged, sending the wrong signal to the computer. You may want to check fuel pressure at this point as well. Otherwise, there could be an air or vacuum leak. Make sure the air cleaner assembly is properly connected too and there are no tears or holes. Hope this helps.
skodabeast on July 06, 2020:
skoda octavia 1.9tdi 2009,i am having a hot start problem,i have been with 3 mechanics now and none can find the problem,it will start cold first turn of key,after engine gets hot and i switch off it will not restart,it cranks but wont start/fire.I have changed fuel pump,coolant temp sensor,crank shaft sensor,and reconditioned starter just been installed but still same problem,i have tested battery aswell.to make it more of a puzzle it will start with the coolant temp sensor disconnected hot or cold,alot of research pointed to things i replaced above but none worked,alot of the forums people have giving up trying and installed an on/off switch to bypass the coolant temp sensor into tricking icu to think it is cold starting when warm,i have spent so much money trying to fix this issue :( i would rather find the underlying problem than install a makeshift on/off switch!!! apart from hot starts the car runs perfect,no stalls,starting cold first turn and if coolant temp sensor disconnected will start first turn,i owned this car now 2.5 yrs and never had a problem starting before this and problem literally came into it overnight,any suggestions? no faults coming up on diagnostic machine
Dan Ferrell (author) on April 16, 2018:
Make sure you got spark and the correct fuel pressure. Check that you connected everything back as it was.
zinga on April 16, 2018:
my car i took out the injectors and clean them and put them back but it wont start