Rick's Free Auto Repair Advice

Engine cranks but won’t completely fire up

Engine cranks but won’t fire up

When an engine cranks and starts to fire up but can’t quite keep running, the problem is usually caused by a air/fuel fuel issue rather than an electrical problem like worn spark plugs.

The proper air/fuel mixture is especially critical on a cold start. If there’s too little fuel it can’t keep the engine running. If there’s too much fuel, the engine will flood.

Here are the most common suspects:

• Dirty throttle body
• Dirty idle air bypass valve
• Vacuum leak
• Inaccurate air/fuel mixture due to dirty MAF sensor or bad engine coolant temperature sensor (ECT)
• Clogged fuel injectors
• Clogged engine air filter
• Dirty throttle body or Dirty idle air bypass valve

Carbon buildup in the throttle body can restrict airflow during startup. Especially on electronic throttle bodies, the computer commands a certain opening of the throttle plate and expects to see a certain amount of air enter the engine. In older vehicles with manual throttle bodies, startup airflow is controlled by the idle air bypass valve. If it’s carboned up, it won’t allow enough air into the engine.

Cleaning the throttle body or IAC with throttle body cleaner can correct the problem.

Vacuum leak

A vacuum leak introduces too much un-metered air into the engine, causing a lean mixture that’s unable to state the engine. Vacuum leaks are caused by cracked or broken vacuum lines or cracks in the air duct between the filter box and the throttle body. If you or a service facility has checked the air filter lately and you’re now experiencing starting issues, check for a disconnect vacuum line, improperly connected air duct or cracks in the air duct.

Dirty/contaminated MAF or bad ECT

Oiled engine air filters can contaminate the hot wire elements in MAF sensors, causing them to send inaccurate air density data to the ECM/PCM. If you have an oiled air filter and are experiencing startup issues, clean the MAF sensor with MAF sensor cleaner. If that corrects the problem, clean the air filter and apply less oil.

If you use a regular air filter and it’s in good condition, check the MAF sensor for debris and then clean

Bad engine coolant temp sensor

Your car’s computer determines the proper air/fuel mixture based on ambient temperature, engine coolant temperature, and barometric pressure. The most common failure in this system is the engine coolant temperature sensor. If it’s off, the computer will think the engine is warmer than it really is and it won’t provide enough fuel to start a cold engine.

To test the engine coolant temperature sensor, partially depress the gas pedal and hold it there while starting. A partial gas pedal will override the computer and provide more fuel to the engine. If the engine starts, have a shop test and replace the engine coolant temperature sensor.

Clogged fuel injectors

If the fuel pressure is correct but the injector spray nozzles are partially clogged your engine will have a hard time starting and will run rough. Clogged fuel injectors will be worse on a cold start than a warm restart because they can’t deliver enough fuel. Try depressing the gas pedal to override the computer and force it to add more fuel. If that works, try adding a fuel injector cleaner like Chevron Techron or RedLine Fuel System cleaner

Clogged air filter

Late model vehicles measure the density and volume of air coming into the engine and meter the proper fuel based on the incoming air. But a clogged air filter can screw up the air/fuel calculation, especially on a cold start. If the computer sees less air coming into the engine, it will cut back on fuel accordingly. In a severely clogged air filter situation, the air restriction may be so great that the computer can’t compensate. It will cut fuel to the point where the engine may not start. So check your air filter. If it’s clogged with debris, replace it.

©, 2020 Rick Muscoplat



Posted on by Rick Muscoplat


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