Causes of Hard Starting When Engine Is Cold
When your car struggles to start when the engine is cold, check four common sources of trouble:
- Weak ignition spark
- Faulty sensors
- Poor fuel delivery
- Bad electric current flow
A cold engine needs more power to maintain proper speed at idle and to overcome internal friction. More power helps the engine overcome internal friction, and gives oil time to circulate and heat up.
The presence of any problems listed above will make it difficult for an engine to start. Eventually, the engine may not start at all.
Often, lack of proper car maintenance can be blamed for problems during a cold start. Worn out spark plugs and plug wires, clogging filters and corroded battery terminals are left unchecked, making it hard for the engine to start.
Whether you are dealing with poor maintenance or problems in a particular system, you can tackle many of these issues at home using a few common tools.
For this, it's a good idea to have the vehicle repair manual for your particular model on hand.
If you don't have this manual yet, you can buy a relatively inexpensive copy through Amazon. Haynes manuals include photographs, drawings, step-by-step procedures, maintenance schedule and procedures, component location and replacement instructions, and system specifications.
You can work on many maintenance tasks, and many of the following projects, at home and recoup your small investment in a short period of time.
In This Article
I discuss three systems that can cause hard starting when cold:
- Fuel System Malfunctions
- Ignition Faults
- Electrical Faults
1. Fuel System Malfunctions
Faulty components within the fuel system may cause a hard-starting condition. An engine requires a clean, flowing source of air and fuel to start and operate correctly, according to the engine and ambient temperature conditions. Here are some components you may want to check.
1.1 Clogged Air Filter
The air filter is a key maintenance item that is easy to forget. Car manufacturers recommend replacing the filter every year or two depending on the model, or even sooner if you frequently drive on dirt roads.
Over time, the filter element accumulates dirt and debris, gradually restricting airflow until the engine starves for air. This will make your engine hard to start.
Remove the air filter and place it against the sun and your eyes, or use a bright lamp. If the light doesn't seem to go through the filter element, replace the filter.
1.2 Fuel Pump Circuit Fault
Loose, corroded, or broken wires or connections work as resistance against proper current flow. This can interfere with relays, switches and even the fuel pump itself.
If necessary, check voltage drop at the relay(s) and, if necessary, at the pump circuit.
1.3 Bad Enrichment Device
Vehicle fuel systems provide a way to enrich the air-fuel mixture when the driver starts the engine at below operating temperature. This usually happens after the vehicle has been sitting for several hours or when a faulty thermostat sticks open.
- Carbureted engines have a choke or special injection circuit.
- Port fuel injection models include a cold-start injector, sensor and thermo-time switch to provide extra fuel to a cold engine.
- Newer models usually provide extra fuel by adding injector pulses or pulse length.
A fault in any of these devices usually leads to a hard-start or rough-idle condition.
A quick way to test for the need for fuel during a cold start is to add extra fuel yourself.
- Remove the air duct to gain access to the carburetor or throttle body.
- Spray some starting fluid directly into the throttle body.
- Try to start the engine.
If the engine starting improves, check for a faulty enrichment device in your system, including the injector, sensor, and switches. If necessary, consult your vehicle repair manual to locate and test components at home.
Checking Cold Injector Operation:
- Get a length of rubber hose, or use a mechanic's stethoscope.
- Place one end of the hose against the injector and the other end against your ear.
- Ask an assistant to crank the engine.
- If the cold injector is working, you'll hear it ticking; otherwise, check the injector, sensor, switch and corresponding circuits. Also, check the engine coolant temperature sensor and, if necessary, the intake air temperature (IAT) sensor. The computer uses information from these two sensors—and other devices—to run the cold injector.
Check your vehicle repair manual and the Help Resources at the bottom of this post for more help.
1.4 Bad Idle Air Control (IAC) Motor
You can find an IAC motor on throttle body injection (TBI) and electronic fuel injection (EFI) systems. Basically, the IAC is a computer-controlled valve to allow extra airflow into the intake manifold.
The car's computer, or powertrain control module (PCM), uses the motor to regulate idle speed. During a cold start, for example, the PCM opens the IAC valve to increase airflow, according to coolant temperature, engine speed and load.
Two common problems affect IAC operation:
- Carbon buildup inside IAC valve passages
- IAC motor malfunction
Either of these problems can lead to a hard-starting condition and a rough idle. The IAC motor is relatively easy to access on most vehicle models. Detach the motor to examine passages and remove buildup as necessary.
You can troubleshoot operation of the IAC motor at home. Check the Resources box at the bottom of this post for help on testing the motor. Also, consult your vehicle repair manual.
1.5 Faulty Sensors
Car computers use the engine coolant temperature (ECT) sensor and intake air temperature (IAT) sensor to operate a cold-start injector.
You can test the coolant temperature sensor using a digital multimeter (DMM). Look for a possible short circuit.
Consult your vehicle repair manual to locate the IAT sensor in your vehicle. Some models use a mass air flow (MAF) sensor with an integrated IAT sensor.
1.6 Leaking Fuel System Pressure
This is another common cause for poor cold starts. A number of issues may affect fuel system pressure:
- Weak or leaking fuel pressure regulator
- Leaking fuel injector(s)
- Worn fuel pump
- Leaking fuel-pump check valve
- Bad fuel pump relay
- High resistance in the electrical system (check for voltage drops)
If necessary, consult your vehicle repair manual and the Resources section at the bottom for help on checking fuel system pressure and diagnosing fuel system issues.
Using a Digital Multimeter
Always use a digital multimeter with at least 10 Megohms of impedance to prevent damage to your car's computer.
2. Ignition Faults
Ignition system problems are another common source of trouble that can manifest during a cold start.
What to look for:
- Worn spark plugs: Your vehicle repair manual will give you the service interval for the spark plugs in your particular model.
- Spark plugs' gap too wide: As the spark plugs wear out, their gaps increase and make the spark difficult to jump. Check and adjust the gaps as necessary using a wire feeler gauge.
- Foul spark plugs: Too much fuel during normal engine operation or oil leaking into the combustion chamber will foul the plugs and interfere with the ignition system. Your repair manual provides images of spark plugs working under different operating conditions so you can diagnose particular problems affecting the ignition system.
- Worn spark plug wires: Just like spark plugs, wires wear out over time, preventing full voltage from reaching the plugs. If your particular model uses spark plug wires, check their resistance using an ohmmeter.
- Faulty distributor and rotor: Depending on your particular vehicle model, check any of these system components.
- Ignition coil: A faulty ignition coil can increase and make it difficult for full voltage to reach a spark plug. You can use a DMM to check coil condition.
Consult your vehicle repair manual for parts location and diagnostic; the Resources section at the bottom of this post also provides links to other posts that can help check the ignition system as well.
2.1 Failing Crankshaft Position (CKP) Sensor
A failing CKP can lead to a hard-starting condition. Although not as common, a CKP sensor that fails mechanically can also damage the reluctor wheel and make it hard for your engine to start.
Usually, a bad CKP sensor will trigger a P0336 trouble code and you'll see the check engine light (CEL) illuminate on your dashboard. But this is not always the case, especially if the sensor is starting to fail and the computer still sees it as working.
Check the sensor, if necessary. And, even if you don't see the CEL light on your dashboard come on, download possible trouble codes. A pending code can give you a clue to your engine cold-start issues.
3. Electrical Faults
Electrical faults can cause all kinds of trouble.
Loose, corroded, broken wires or connections may:
- increase resistance in the starting system
- affect engine grounds and a number of circuits
- interfere with battery power
- drain power from a perfectly charged battery
Problems in electrical systems are sometimes tricky to deal with because you don't know there's a problem with a circuit unless you are dealing with a disconnected wire or a device that refuses to work.
But a digital multimeter (DMM) can make your job much easier. You can use your DMM to check for voltage drop around the starting circuit, charging circuit, and engine grounds; you can also check for a parasitic drain emptying your car battery overnight.
If you suspect an electrical problem, check your repair manual and the Resources section at the bottom for help on voltage drops and other circuit tests you can do at home.
The following video gives you some visual pointers discussed in this post that you can also use to make sure you got the most common sources of trouble.
Video: Hard-to-start Sources of Trouble
Resources for Help With Fuel System Issues
- How to Test a Fuel Pressure Regulator
Learn how to test a fuel pressure regulator to save money and time in car repairs.
- Testing the Intake Air Temperature Sensor
To fix performance issues, check the intake air temperature (IAT) sensor in your car using a few simple tests.
- Coolant Temperature Sensor Test
This coolant temperature sensor test will tell you in just a few minutes whether you need to replace it.
Help With a Faulty Starting System
- How to Use Voltage Drop to Troubleshoot the Starter System
Testing the voltage drop can help you find trouble spots in your starter circuit before you swap components unnecessarily.
Help With Ignition System Problems
- How to Test a Crankshaft Position Sensor Using a Multimeter
Troubleshoot an inductive or Hall effect type crankshaft position (CKP) sensor using a digital multimeter.
- How to Test an Ignition Coil
Learn how to test an ignition coil to troubleshoot engine misfires, a no-start condition, and other performance issues.
- How to Test Spark Plug Wires
Testing spark plug wires to solve power loss, trouble starting, increase in gas consumption and other performance issues.
Help With Electrical Issues
- Diagnosing the Symptoms of a Bad Engine Ground
Checking for a bad engine ground can fix dimmed lights, faulty sensors, and hard starting, and it can prevent severe damage.
- Automotive Voltage Drop Testing
How to use a voltage drop test to troubleshoot common automotive problem circuits to speed up repairs and save money.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2021 Dan Ferrell