Rick's Free Auto Repair Advice

Car won’t start, cranks but won’t fire up

Diagnose a car won’t start issue

If you turn the key and the starter engages and rotates the engine but the car won’t start,  this post is for you. Let’s divide the topic into these sections:

• Car cranks but won’t fire up when cold or freezing temperatures

• Car cranks but won’t fire up when warm or hot

Diagnose a cold car won’t start condition

This is really tough to diagnose with no diagnostic tools. However, let’s take a look at the basics. To start a cold engine you need:

• The right cranking speed

• The right air/fuel mixture with fuel at the right pressure

• Spark plugs that are in good condition

What factors affect cranking speed?

Cranking speed is directly related to the battery charge, battery temperature, the quality of the electrical connections and the proper viscosity oil. Some car makers recommend using a thinner oil for winter. Check your owner’s manual for the recommendation. If your car requires thinner oil and you didn’t change over before winter that could be part of the problem.

Do you change your oil on time? Old oil thickens as you put on the miles and that thicker oil can slow down engine rotation to the point where the engine won’t start.

Cold car batteries have less power

Car batteries produce power through a chemical reaction. That reaction slows down as temperatures plummet. Cold temperatures also increase a car battery’s internal resistance. Both of these cause a reduction in cranking power as temperatures drop. Batteries left in a discharged state are also susceptible to freezing, which damages internal components and containers.

dead car battery

Battery power available when cold or freezing outside

Cold engines require more power to crank

At the same time, cars require an increased amount of cranking power in cold weather because the motor oil is thicker and makes engines harder to crank.

How long your battery has been sitting without starting?

All maintenance-free (most common type sold and installed today) lead acid batteries self discharge at the rate of 0.5% per day at room temperature. However, car computers constantly draw a small amount of current to keep memory alive and to be at the ready state when you operate your keyless entry fob. That small current draw, combined with self discharge and long idle times can kill a car battery.

Car battery acid stratifies when sitting idle for long periods

Acid stratification is also a factor in the state of charge of your battery. Battery electrolyte is composed of approximately 35% sulfuric acid and 65% water. When a battery isn’t used regularly, the acid, which is heavier than water, falls to the bottom of the battery. That leaves a very low concentration of acid near the top of the battery. This condition is referred to as acid stratification. It’s the sulfuric acid that prevents a battery from freezing in cold weather. When the battery experiences acid stratification, it is more prone to freezing. In fact, the top acid/water layer can freeze at or near 32°F. That permanently damages your battery.

car battery acid stratification

In summary, the longer you let your battery sit unused, the more likely it will be low on charge. If you leave it discharge in cold weather, you dramatically increase the chance of it freezing.

The right fuel at the right pressure

Winter gasoline is more volatile than summer blend gasoline. If your tank is filled with summer blend gasoline, you may have a hard time starting it in very cold weather. Adding “Heat” moisture control additive won’t change the fact that the summer gasoline isn’t as volatile. If you have a tank of summer gas and your car won’t start, getting it towed and stored in a warm shop may help. Once you get it started, add fresh winter fuel and drive a long distance to use up the old fuel.

Is it gas line freeze-up?

Naw. Modern fuel with ethanol and tightly closed fuel tanks have pretty much eliminated gas line freeze-up. Ethanol is water loving, so it absorbs any moisture in your tank. Modern fuels contain 10%-15% ethanol, so you really don’t have to worry about gas line freeze up.

Not convinced? Think about this: Many DIYers think they can add a 16-oz. bottle of “Heat” to fix the problem. If you have a 20-gal tank and the gas is 10% ethanol, you’ve already got 2 GALLONS of alcohol in your tank. Adding another pint won’t make ANY difference. Trust me, you don’t have that much water in your tank. The fuel tanks in modern don’t let in that much moisture.

However, ethanol fuels can experience “phase separation,” where the ethanol and water separate out from the gas. Since water is heavier than gas, the ethanol/water settle to the bottom of your tank. In other words, your engine is sucking water instead of gas. But even then, the ethanol will prevent freeze up. If the gas in your tank is old, that may cause a no start problem. But adding alcohol won’t fix. it.

Low fuel pressure or fuel leakdown pressure can prevent starting

When you shut off your engine, a check valve in the fuel pump prevents the fuel at the engine from draining back into the tank. Fuel pressure should remain within 5-10 psi after sitting all night. Unfortunately, check valves can fail in extremely cold weather. When that happens, you get into your car and try to start the engine when it has NO fuel pressure. Since the battery is low on charge at a time when the engine needs even more power, you drain the battery trying to start with no fuel. By the time the fuel pressure builds, the battery may be so low that it can’t crank the engine fast enough to fire.

What to do? If you have any indication that it’s taking longer for your engine to fire up, try this:

Turn the key to RUN, but don’t try to start the engine. Turning the key to the RUN position commands the fuel pump to perform a 2-second “prime” cycle. Turn the key to OFF. Repeat this procedure three times. That should fully pressurize the fuel rail and fuel injectors. Then try to start the engine. If it starts right up, that’s an indication your check valve is failing. Get the fuel pump replaced immediately.

fuel pump cutaway showing turbine, motor and check valve

Turbine style fuel pump showing the turbine, motor and check valve

Wrong air/fuel mixture

Cold engines need a much richer mixture before it can ignite. A faulty or dirty Mass Airflow Sensor (MAF), manifold absolute pressure sensor (MAP) or engine coolant temperature sensor (ECT) can cause the computer to calculate the wrong air/fuel mixture. These sensors are hard to diagnose without a professional grade scan tool. But there are three things you can try if your car won’t start:

1) Try depressing the gas pedal halfway and then cranking the engine. The computer is supposed to calculate the right air/fuel mixture with your foot off the gas pedal. If it’s getting bad information from a bad or failing sensor, depressing the gas pedal will OVERRIDE the factory programming and force the computer to add more gas.

2) If you’ve cranked the engine for long periods without starting, you may have flooded the engine. Depress the gas pedal to the floor and HOLD IT THERE while cranking. This procedure tells the computer to stop adding fuel and opens the throttle plate all the way to let more air into the engine. If you get any indication that the engine wants to fire up, immediately release the gas pedal.

3) Clean the MAF sensor with MAF sensor cleaner. See this post on how to do the cleaning.

4) Use starting fluid. Unhook the air intake tube from the throttle body and spray starting fluid directly into the throttle body. Then try starting the engine. Don’t overdo it on the starting fluid. You can flood an engine by spraying in too much.

Spark plugs must be in good condition or your car won’t start when cold

Even winter gas is hard to ignite with a weak spark. Worn spark plugs produce a weak spark that’s not hot enough to ignite cold fuel. If you haven’t changed your spark plugs according to the car makers recommendations, they can leave you with a car won’t start when cold condition. If you have the facilities and tools, install new properly gaped spark plugs.

Diagnose a hot car won’t start condition

Many of the same factors listed above apply to a hot car won’t start condition.

Worn spark plugs, wrong air/fuel mixture, low or leaking fuel pressure, faulty MAF, MAP, ECT can also cause a hot no start condition. Try all the tips listed above.

©, 2018 Rick Muscoplat

Posted on by Rick Muscoplat


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