It’s a right pain in the jacksie, I’ve had this exact problem with my old Volvo. I thought it might be dampness from laying up between starts, but I was wrong. The old girl never let me down though, so I decided it was time to get it fixed, I was just killing the battery and starter.
- Low fuel level
- Weak battery
- Weak starter
- Bad coolant temperature sensor
- Glow plug fault (diesel engine)
By the end of this post, you’ll understand the common causes of hard starting, how to diagnose them and how to fix them.
What Are The Causes Of Low Fuel Pressure?
Fuel injection became popular back in the 80s and has evolved from an electric over the mechanical system to a purely electrical system, which has made them more reliable and efficient.
From the 90s onwards, fuel injectors were all fed from a common fuel rail, with electronic fuel injectors all controlled by an ECU (Computer).
Diesel and gas engines run very similar fuelling systems, the difference being diesel systems run much greater fuel pressures and have more control over the fuel quantity delivered.
The ECU computer is the brain of the operation and it determines how much fuel the engine needs by receiving readings from various sensors like oxygen (O2), coolant temperature, engine load, airflow sensor, accelerator pedal position.
The ECU in turn manages the amount of fuel the engine gets by controlling rail pressure and also the amount of time the injector is open, known as the duty cycle.
Both diesel and gas engines use the common fuel rail setup. On diesel engines, the ECU gets a fuel rail pressure reading from a pressure sensor mounted in the rail. The ECU can then control the rail pressure by opening or closing a return valve.
When the return valve is open, the fuel in the rail is sent back to the fuel tank as the engine load is low, meaning the fuel isn’t needed. Gas systems, don’t have this pressure sensor set up, instead, they normally have a simple gas pressure regulator that’s operated by engine vacuum.
The fuel flow is supplied by an electric fuel pump usually fitted at the rear of the vehicle near or submerged in the fuel tank. Diesel engines will likely have another pump at the engine, this pump provides the higher fuel pressures needed for diesel engines.
Most fuel pumps have a nonreturn valve incorporated, however, some nonreturn valves are separate stand-alone units. The function of the non-return valve is to maintain rail pressure after shut down, this provides a short crank time when you hit the key.
The fuel pump in both diesel and gas engines is controlled by a relay that is triggered by an ignition cycle.
The following is a list of likely causes of no fuel pressure:
- Low fuel/no fuel
- Blocked fuel filter
- Bad fuel pump relay
- Bad fuel pump
- Bad non return valve
- Bad fuel regulator
- Leaking fuel lines
- Leaking injectors
- Bad fuel rail pressure sensor (diesel)
Low fuel level
Low fuel levels can cause problems in some models, some ECU’s are programmed not to run the fuel pump if the fuel level is low. They do this to protect the pump from dry running. So makes sense to eliminate all the easy stuff first. Just make sure you’ve got enough fuel in the tank before going any further.
Blocked fuel filter
A blocked fuel filter will cause low fuel pressure, cold fuel is thicker than hot and so on cold starts, there may be greater restriction through the fuel filter. Engine Heat soak and the cycling of fuel through the fuel pump will warm the fuel, possibly the reason for only cold hard starting.
If your fuel filter hasn’t been changed recently, go ahead and change it out. Older cars with metal tanks can suffer from corrosion in the tank, so inspect the fuel filter for any signs of rust debris.
Some basic tests will help us eliminate a fuel pump problem. If you listen carefully, when you turn the key to ignition on, (not cranking) you should hear a steady hum for a few seconds. This indicates the fuel pump is working, it doesn’t, of course, indicate that the pump is working correctly, for that we’ll need to pressure test at the rail. (more on that below)
If you don’t hear the pump, try banging on the fuel pump or the bottom of the gas tank (if the fuel pump is in the tank). This doesn’t sound very professional, I hear you say. True, but it will often cause the pump to fire up, and if that happens you found your problem – replace the fuel pump.
If you still can’t hear the pump hum for a few seconds when you cycle the key, check your fuse box, for the fuel pump relay. It’s more likely to be in the fuse box under the hood and it should be clearly marked on the cover “Fuel Pump”.
Fuel pump relay
Remove the relay and refit it, and test the pump again. Have a helper turn the ignition on while you listen and feel the fuel pump relay click, remove it and test again by fitting with the ignition on, if the relay doesn’t click, it may be faulty.
If your relay clicks, but you don’t hear the pump hum, go ahead and replace the pump, they give lots of trouble. If your pump is working, we’ll need to test how well it builds and holds pressure, and for that, you’ll need a rail pressure testing kit.
Most fuel tank pumps will have a nonreturn valve incorporated into the pump. Other models may have it as a stand-alone component, you’ll need to consult a repair manual. The function of the valve as you know is to hold fuel pressure in the rail so that the engine restarts quickly.
This immediately sounds like it’s your problem since its whole function is to prevent long hard cold starting. But in my experience, it’s not usually the return valve that’s at fault. The pressured fuel may be leaking or restricted as you know, by various other components.
The fuel regulator maintains pressure in the fuel rail. Older gas engines usually have a simple vacuum-controlled nonelectrical valve fitted, they are not ECU controlled. Instead, they have a diaphragm and return spring, these are moving parts and do wear out.
The spring keeps the valve in the closed position which blocks the return of fuel to the tank (max fuel pressure). The valve then opens reducing pressure proportionally to the amount of vacuum. Max vacuum at idle and min vacuum at full throttle.
A valve should be closed and have the ability to seal off the return pipe at shut down. If it doesn’t you can see how the rail pressure would be low and a long crank would be needed to build pressure again at startup.
Test the regulator by clamping off the return line at shut down and allow the car to sit a day. If the car then starts on a short crank, suspect a faulty regulator.
Testing with a fuel pressure test kit will obviously be better, clamp off the return in the same way and watch the gauge, fuel pressure should hold steady. The latest models have moved to a more sophisticated non-return fuel pump, it controls pressure and only delivers the amount of gas needed, all managed by the ECU.
Diesel fuel systems diverge slightly from gas systems, as diesel pressures are much greater and require greater control. The diesel has an electronic ECU-controlled return valve fitted in the rail, and when open, it returns diesel back to the tank. These sensors are very closely monitored by the ECU and if there’s a fault, it will be flagged.
Reading the codes through the data link connector is simple, and code readers are inexpensive and easy to use.
Fuel lines don’t often leak but they are worth checking, you may even smell fuel in the garage or wherever your car is parked. A leak in a diesel fuel system will be noticeable, check areas around pipe fittings for diesel, but don’t mistake it for diesel sweat.
Check fuel lines to and from your fuel filter, check that the fuel line isn’t damaged or Flexi fuel pipes kinked.
Leaking fuel injectors will cause the fuel to drain off into the cylinders and will cause a long crank as fuel pressure needs to build before the engine will fire. When injectors leak, they wash protective oil from the cylinders and dilute the engine oil, this will shorten engine life.
To test the injectors, remove them and the fuel rail from the engine, reconnect the injectors to the fuel rail and set them aside. Place tissue paper under each injector nozzle. Cycle the ignition on and off a few times to build fuel pressure, check your paper for drips.
If you find a leaker, replace them all, because if one goes the others are not too far behind. This was the problem with my Volvo, and a replacement set left it as good as new.
Fuel rail pressure sensor
Bad fuel rail pressure sensor, will only apply to diesel engines. As you already know the diesel engines are sensitive to any drop in rail pressure, the ECU will pick up any faults and flag them as fault codes.
Plugging into your DLC and reading all codes will point you in the right direction.
Weak Battery Can Cause Hard Starting
A weak or wrong size battery can cause all sorts of problems, from intermittent faults to no starts. You already know batteries don’t like laying idle and cold weather is a real challenge for them.
Using a voltmeter to test battery voltage. Set the meter to 20 volts DC, and place the red probe on the positive of the battery post and the black probe on the negative post. The following readings show the state of charge:
- 12.7 – 13.2 volts is 100% charged
- 12.4 volts is 75% charged
- 12.2 volts is 50% charged
- 12.0 volts is 25% charged
- 0 – 11.9 volts is Discharged (Flat
Your battery may read 12.65 v fully charged but could still be faulty, the best way to check it, is to load the battery. The battery should be reading 12.4v minimum, before running this test.
Battery runs flat
It’s normal for a battery to discharge, it’s the rate of discharge that causes the problem. A good battery should hold a charge for up to 3 months when disconnected from the vehicle. Connected up, it’s a different story, the amount of time it will stay charged for will depend on Amp draw.
All cars will draw some energy even after the ECU goes into sleep mode, it’s known as a parasitic draw. Things like alarm systems, clocks, and radio memories all use small amounts of energy.
The more sophisticated the car the higher the draw, it’s small, measured in milliamps, but obviously adds up to enough to soften the battery.
Crankshaft sensors (CKP)
A weak battery will often be strong enough to crank over the engine but not enough to actually start the engine. Since the 90’s cars have been using crankshaft sensors to time the firing of the spark plugs.
However, the crankshaft needs to turn fast enough in order for the crankshaft sensor to produce a signal (350rpm min). If the sensor doesn’t recognize crankshaft movement, it won’t send a signal to the ECU, and so spark plugs won’t fire.
Crank sensors do give trouble, but it’s usually when they get hot, or they just stop working altogether.
Ignition systems need very high voltages of 30k to fire spark plugs, they do this by amplifying the battery voltage.
Batteries have a type code, known as BCI group, this determines case dimension, pole orientation, and power output. Sizing the battery for your car is important, you’ll need to consult your manual or a parts store will tell you the correct battery for your car.
Cold weather battery strength is measured CCA, this is an important battery measurement if you live in a cold climate. The higher the CCA number the better the battery performs in cold weather.
CCA is the Amps the battery can give at 0°F for 30 seconds before dropping below 7.2volts. It’s OK to fit a bigger battery in your car than factory specs, but never fit a smaller one.
Faulty Starter Can Cause Hard Starting
A weak or failing starter motor will cause starting issues. Hot or cold starting problems may depend on how the starter is failing. You already know that cars since the 90s have a crank sensor fitted. The sensor tells the ECU how fast the engines turning and when to fire the spark plugs.
If the engine isn’t cranked over fast enough, the sensor won’t recognize the flywheel movement. A slow crank speed is usually caused by a soft battery. Using the wrong grade oil in very cold conditions can also cause a slow crank speed.
If you suspect your starter motor isn’t cranking over the engine fast enough, boost it from another vehicle as a test, see if it starts any faster with the extra power.
If you have a DVOM (voltmeter) you can do a volt drop test of the starter motor and its cables. This resistance test will check if the slow crank speed is caused by a failing starter or just bad cable connectors.
Drop volt testing simply tests that all the available power of the battery is getting to the starter motor or whatever other electrical component you want to test.
Both circuits need to be tested, the power side and the ground side. The infographic shows you how to go about it. Note the engine must be cranked over to get the measurement.
Bad Coolant Sensor Will Cause Hard Cold Starting
The coolant sensor plays a pivotal part in engine management, its reading is vital for cold starting. The ECU fuels the engine based on the coolant sensor reading.
An engine burns fuel most efficiently at a ratio of 14.7:1 air to fuel, known as AFR. Since cold air is denser than hot, cold starts will require a fuel adjustment, so that the engine has a rich fuel mixture for cold starts.
Poor readings from a bad coolant sensor or wiring will cause incorrect fuelling and poor cold starts. You can check it with a DVOM, checking its resistance at cold and verify your readings against a know good sensor, or just swap it out. They’re not expensive or hard to replace.
Bad Glow Plugs Will Cause Hard Cold Starting
This only applies to a diesel engine, if you have a diesel then you’ll have glow plugs fitted. The glow plugs are a heating element that actually glows red hot inside the cylinders. They heat the cylinder, which cold diesel needs in order to ignite.
If your diesel is having a hard time on cold starts then it’s a fair bet a glow plug fault is to blame. The plugs consume a lot of power and are controlled by a glow plug control unit or relay, they fail regularly.
A quick test with a Dvom at the glow plug will tell you if the power is getting to the glow plug or not. If you have no power at any of the glow plugs, check out your glow plug fuse, if all is and the glow plug unit is getting power, go ahead and replace it.
Replacing the glow plugs can be a bit of a mission on some cars, especially if they have been over-torqued. Soak them in wd40 the day before attempting to remove them, if they break on removal, which can happen, a special tool is available to extract them. And if it all goes horribly wrong, you’ll need to remove the cylinder head.
What can cause slow cranking? A slow cranking engine is caused by:
- Flat battery
- Faulty battery
- Dirty battery connections
- Loose battery connections
- Faulty starter motor
- Loose starter motor wiring
- Wrong oil type
- Mechanical fault
Car is hard to start but runs fine? A car that’s hard to start cold but runs fine when hot has a cold start fuelling fault. Check the following components for faults:
- Coolant temp sensor
- Choke system
- IAC system
- Fuel injectors
- Vacuum leak