My Car Is Hard to Start When Warm

Updated on October 15, 2019
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Several faults can cause your car not to start when warm.
Several faults can cause your car not to start when warm. | Source

If your car is hard to start when warm, you can trace the issue to one or more several potential faulty components, for example:

  • Clogged air filter
  • Corroded or loose battery terminal or cable
  • Corroded or loose engine ground connection
  • Vapor lock in the fuel system
  • Faulty fuel system
  • Engine control system problems

Sometimes, diagnosing a hard-to-start warmed engine can take you a few minutes; othertimes, it may take some probing around different potential components or systems. To make it easier, the following sections outline the most common sources of trouble and what you can do to check them for potential problems.

Index
I. Check for a Clogged Air Filter
II. Check for Corroded or Loose Battery Terminal or Cable
III. Corroded or Loose Engine Ground Connection
IV. Check for Vapor Lock
V. Check for a Leaking Fuel Pressure Regulator or Injector
VI. Check the Starter Motor and Circuit
VII. Other Potential Problems
A clogged air filter can lead to a hard-to-start condition after the engine has warmed up.
A clogged air filter can lead to a hard-to-start condition after the engine has warmed up. | Source

I. Check for a Clogged Air Filter

Over time, dust, debris, and other contaminants will block the flow of air through the filter. This can also lead to one or more performance issues.

Clogged air filter symptoms may include:

  • Hard to start
  • Poor acceleration
  • reduced engine power
  • misfires
  • increased emissions

What to check:

  • filter element for damage
  • oil or water contamination
  • Make sure a flash light can shine through the filter media
  • replace the filter according to the service schedule

Consult your vehicle repair manual for more information, if necessary. If you don't have the manual yet, you can buy a relatively inexpensive copy through Amazon. Haynes manuals include step-by-step procedures for many troubleshooting, maintenance, and component replacement projects. So you can recoup your small investment soon.

Check inside the battery terminals for corrosion.
Check inside the battery terminals for corrosion. | Source

II. Check for Corroded or Loose Battery Terminal or Cable

From time to time, it is necessary to check battery condition, including the terminals.

Symptoms of corroded battery terminals may include:

  • hard to start
  • no start
  • undercharged battery
  • faulty electrical systems
  • bad starter motor

Corrosion in the electrical system, terminals or connectors, act like resistance against electrical current. A little corrosion may not prevent you from starting the engine, but as the electrical system temperature rises due to resistance in the circuit, it will make it more difficult for electrical components in the circuit to operate. This can make it harder to restart your engine until it cools.

However, corrosion is hard to detect when it hides inside connectors. You may need to remove the battery terminal.

What to check:

  • Connect a computer memory saver into the cigarette lighter of your vehicle.
  • Disconnect the battery negative terminal.
  • Disconnect the battery positive terminal.
  • Closely examine both terminals for white or green substance around the inside and around the battery posts.

Removing Corrosion:

  • Mix a spoon of baking soda into 8 oz of warm water in a disposable cup.
  • Use a soft brush to apply the solution to the terminals and battery posts.
  • Use a shop rag to wipe clean the terminals and battery posts.

For a more detailed procedure, check this other post on cleaning battery terminals.

Check engine for missing grounds, loose or corroded ground connections.
Check engine for missing grounds, loose or corroded ground connections. | Source

III. Corroded or Loose Engine Ground Connections

The conditions in which components and circuits work under the hood can cause electrical connectors to become loose, disconnected or corroded.

Loose or corroded engine grounds, specially, will create all kinds of electrical issues since circuit resistance effectively increases. This can affect several electrical systems, including the starting system.

High temperatures will make a loose or corroded terminal worse. And just like a loose or corroded terminal, a bad engine ground is hard to diagnose just by looking at it.

Some of the symptoms of bad engine grounds may include:

  • erratic behavior of one or more electrical components
  • hard to start issues
  • no-start condition
  • automatic transmission problems
  • increased voltage around sensors and actuators

But you can use your digital multimeter to do a simple voltage drop test of your engine grounds. You may need the vehicle repair manual for your particular model to locate your engine grounds.

Fuel can boil if lines are too close to hot engine components.
Fuel can boil if lines are too close to hot engine components. | Source

IV. Check for Vapor Lock

One of the main characteristics of gasoline fuel is its volatility. This is what allows it to mix with air for a good combustion. But allowing fuel to vaporize before it reaches the combustion chamber will create a performance issue called vapor lock.

Vapor lock may happen when fuel begins to boil on its way to the fuel injection system or carburetor. Under this condition, vapor or bubbles mix with liquid fuel and prevent free flow. Hot weather and fuel lines too close to a hot engine or other hot components will contribute to vapor lock.

Another cause for vapor lock is clogged vent lines. This line or lines coming from the fuel tank, allow fuel vapor to exit the tank and collect into a charcoal canister. If the vent line clogs or the valve in the line sticks in the close position, vapor will mix with the fuel as it rushes out of the tank, causing engine performance issues.

Other symptoms of fuel vapor lock may include:

  • no starting
  • lack of engine power
  • engine stalling
  • hard starting

What to check:

  • Clogged fuel return line
  • Engine line close or touching a hot engine part
  • Clogged vent line
  • Look for any other condition that may overheat the fuel in your particular system

Sometimes, a faulty fuel pressure regulator can prevent a warm engine from starting.
Sometimes, a faulty fuel pressure regulator can prevent a warm engine from starting. | Source

V. Check for a Leaking Fuel Pressure Regulator or Injector

This problem is closely related to vapor lock. Hard warm starts can also be traced to a leaking fuel pressure regulator (FPR) or fuel injector. Sometimes, a fuel pressure dampener may cause the same symptoms.

At the beginning, a worn FPR or fuel injector may give in to pressure in the fuel line and then stop leaking. If the fuel lines are hot, under hot weather conditions and a hot engine, the fuel will have room to evaporate inside the line, creating a hard-to-start condition.

Other symptoms of a bad FPR:

  • stalling
  • misfiring
  • hesitation

Also, trying to start the engine with a flooded cylinder(s), from a leaking fuel injector, will require extra seconds of cranking.

You may check for a flooded cylinder using a simple test:

  1. Shut off the engine after it has warmed up.
  2. Wait for a minute or two.
  3. Fully depress the accelerator pedal to allow more air to enter the engine.
  4. Try starting the engine.

If engine starting improves with the accelerator depressed, you may be dealing with a leaking fuel injector. Sometimes, adding a fuel injection system cleaner to the fuel tank may help to clear a sticking injector.

To check for a leaking FPR:

  • disconnect the vacuum line from the FPR
  • if the line is wet with fuel or it smells like gasoline, the FPR is leaking.
  • replace the FPR

If necessary, test the FPR and fuel injector(s).

Worn starter motor brushes can lead to engine starter problems.
Worn starter motor brushes can lead to engine starter problems. | Source

VI. Check the Starter Motor and Circuit

Internal starter motor connections wear down over time, causing a hard-to-start condition when the engine temperature rises.

A starting motor use brushes (carbon bars) and a commutator (slip ring) to allow current flow to the armature and keep the windings spinning inside.

Brushes wear out and the springs that hold them in place weaken over time. This effectively increases resistance in the starter motor's internal electrical circuit. Under this conditions, engine heat compounds the problem by increasing circuit electrical resistance even more.

Eventually, the engine won't start.

This problem may be a little harder to diagnose but checking for voltage drop in the starting system may help you pinpoint a faulty starter motor. If necessary, have the starter motor checked. Most auto parts stores will test the starter motor for you.

A faulty coolant temperature sensor will send the wrong signal to the car's computer.
A faulty coolant temperature sensor will send the wrong signal to the car's computer. | Source

VII. Other Potential Problems

Depending on your particular model, or fuel system configuration, other faulty components may cause you trouble when starting a warmed engine.

What to check:

  • Sometimes, a faulty fuel pump check valve may cause starting problems when the engine has been warmed up.Keep this in mind when your fuel pressure regulator or fuel pressure damper test OK, but there seems to be something wrong with system pressure.

  • Also, check the throttle position sensor. If your particular sensor is adjustable, try adjusting it following the instructions in the vehicle repair manual for your particular model.

  • Check the engine coolant temperature sensor. If the sensor is not responding to coolant temperature, it may cause the computer to enrich the fuel mixture even though the engine is hot.

In the following video, you can see how a faulty check valve in the fuel pump caused a Nissan Murano problems to start when at operating temperature.

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.

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    © 2019 Dan Ferrell

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