Car runs good but idles rough/stalls once warmed up | Easy fix, hard diagnosis

Your car runs perfectly, but when the engine warms up, it starts to shake, idle rough, and eventually stall? 

First thing first, there are many different causes for this problem. It needs proper diagnoses to identify the specific issue. Don’t read a few sentences on the internet, think that’s your car’s problem, and blindly replace that part. 

You may fix it, but it’s going to take a lot of time and money until you do. 

So, in this article, I’ll show you: 

• How to diagnose the right way.
• How much it costs. 

Let’s dive in!

Get an OBD2 scanner – DON’T be a part changer! 

An OBD2 scanner is an inexpensive, easy-to-use tool. It can tell you the Diagnostic Trouble Codes (DTCs), which let you know where your car gets hurt, and the internet will tell you what to do next. 

For this case, the problem lies within the engine. So, a cheap scanner is good enough for you. 

Here’s my recommendation: 

Note: Make sure your vehicle is 1996 or newer to use any OBD2 scanners. 

When you have got a scan tool, let’s see what the problems are! 

Rich condition 

A cold engine needs a rich mixture (too much fuel, not enough air) and a high RPM idle to function correctly. When things get to the normal operating temperature, this fuel-air mixture must lean out (less fuel) to the perfect air/fuel ratio: 14.7:1 

For some reason, the rich mixture keeps being rich, and a warmed engine can’t burn it efficiently, causing your car to lose power, idle rough, hesitate during acceleration, or even stall. 

In short, a cold engine likes running rich, but a hot engine doesn’t. 

P0172 and P0175 are the DTCs that generally tell you the system is too rich. 

The below are the causes that make your car run rich.

Faulty MAP/MAF sensors

MAP and MAF sensors are responsible for calculating how much air the engine is taking in, which determines how much fuel should be injected. The idea is that no matter what happens, the engine control unit (ECU) always wants to achieve the perfect air/fuel ratio. 

A faulty MAP or MAF will send the wrong data to the ECU. In this case, they’re saying that “so much air is coming.” In fact, it’s not that much!

But the stupid ECU believes it anyway and sends the signal to pump more fuel, causing the engine to run rich. 

Again, when you start the engine cold, a rich air-fuel mixture is good enough. But when the engine gets hotter, it really doesn’t need that much fuel! 

If you get any of the codes below, the MAP or (and) MAF sensors can be bad. In that case, you should clean them first. If that doesn’t work, replace them. Don’t worry, replacing these sensors is very easy and cheap!

Faulty O2 sensor

The O2 sensor’s function is to measure the oxygen percentage in the exhaust gas. The final purpose is (again) to help the ECU to balance the air-fuel ratio. 

To check for a bad O2 sensor, go to the “Live Data” on your OBD2 scanner. Choose “O2 sensor output voltage.” If you see a flat line with a voltage value always higher than 0.6V, you’ve found your problem!

There are a few different sensors in the two banks. Make sure you check them all. 

Faulty coolant temperature sensor

The coolant temperature sensor tells how hot your engine is. In case it’s bad, it will keep saying to the ECU that the engine is still cold even though it has already warmed up. 

As I said before, a cold engine needs a rich air-fuel mixture to run properly. In this case, the ECU thinks that the engine is not hot enough, so it continues to pump the rich mixture to the combustion room, causing your car to run rich. 

To know if the coolant temp sensor is bad or not, use the “Live Data” function in your OBD2 scanner. You should be able to see the coolant temperature live data. 

Compare it with the actual number you have using an infrared thermometer. If the number you got on the scanner is much lower than the actual number, it’s time to replace a new coolant temp sensor.

Stuck open EGR valve

An exhaust gas recirculation (EGR) valve basically takes a little bit of exhaust gas and puts it back in the combustion chamber. 

The idea is to reduce the engine temperature and control NOx emissions. 

Over time, the carbon will build up inside the valve and make it stuck. When the EGR valve is stuck open, it will allow a constant exhaust gas stream to enter the combustion chamber. 

This gas will take the place of the air, making the air-fuel mixture too rich (due to lack of air). 

One more symptom, this valve is not supposed to open during idle. But in case the valve is stuck open, it will cause rough idle or even stalling (too much exhaust gas makes the combustion reaction less efficient). 

When you receive these DTCs – P0400-P0408, it’s time to clean the EGR valve. If it doesn’t work, replace it with a new one (actually very easy to do at home). 

Bad fuel pressure regulator

Simply speaking, a fuel pressure regulator (FPR) helps to keep the fuel pressure stable. When your car has a bad FPR, it can cause either lean or rich conditions. 

However, a lean condition will cause a lot of trouble anyway, regardless of the engine temperature. 

In case your car runs good until it warms up, the opposite scenario makes more sense. A faulty FPR can increase the fuel pressure inside the fuel rail, making your car run rich. 

However, sometimes the problem is not the FPR. It’s the vacuum hose attached to it. If there is any crack on the hose, air will leak in. Then the FPR thinks that you’re revving the engine, and it will close to increase the fuel pressure, causing the engine to burn more fuel than necessary. 

P0089 is the common DTCs for a bad fuel pressure regulator. 

Whether the problem is the FPR or the vacuum hose attached to it, fixing this is easy and inexpensive. You can totally do it at home and save some money. 

Leaking fuel injectors

Leaking fuel injectors will leak fuel right into the combustion chamber. It will cause no problems when you first start your cold engine as all the leaked fuel has evaporated from the last shut down. And a cold engine can burn liquid fuel easier (but not so efficiently) than a hot one. 

But when your car runs for a while, the leaked fuel will become a problem. It can’t be burned because it’s a liquid, not vapor. As a result, the engine will lose power, idle rough, and stall. 

You must fix this problem asap. Otherwise, the fuel can leak into the oil pan. Over time, it will make the engine oil thinner. This can lead to severe engine damage or even an explosion inside the engine. 

There is no specific DTC for leaking fuel injectors. To verify if that’s a bad fuel injector or not, check the engine oil dipstick. If you can sense a fuel smell, then yes, the fuel injectors are leaking.  

Lean condition

Let me make it clear. When your car runs good but idle rough or stalls when it gets warmed up, a lean condition (too much air, not enough fuel) is NOT always the case. 

The lean air-fuel mixture will cause problems disregarding whether the engine is hot or cold. However, some “lean” cases actually can be the causes of the problem.  

P0171 and P0174 are the general DTCs indicating that your engine is running too lean. When you have these codes, there are two things you need to check: 

Bad fuel pump

If you have a gasoline car, this is very likely to be your case!

The fuel pump is located in the fuel tank. Its job is to pump fuel to the engine. Obviously, right?

When the engine gets hot, the fuel in the fuel tank also gets hot. A bad fuel pump will be very sensitive to the temperature. So, it starts to perform poorly, resulting in a fuel pressure decrease. And the engine can’t get enough fuel, causing it to run too lean. 

In other words, when the engine is cold, a bad fuel pump may function properly. When it warms up, the fuel pump can’t deliver enough fuel for the system, making the engine shake and rattle. And when there is too little fuel in the mixture, the engine will shut down. 

P0087 is a common DTC when you have a bad fuel pump. If you have this code, bad news, the repair cost can be up to $1000.   

Vacuum leak

A vacuum leak will let the air enter the engine without the ECU knowing it, leading to the lean air-fuel mixture. 

If you have a large vacuum leak, you will see the problem immediately when the car starts. Hot or cold, it doesn’t matter. 

In case your car has a small vacuum leak, a little unwanted air can get inside the engine. However, as I mentioned above, when you start a cold engine, the ECU will command a mixture with more fuel. And the air leaked in won’t make any significant impact on the combustion reactions because the added fuel has made up for it.  

But when the car warms up, the ECU will cut back the fuel and balance the air/fuel ratio. Now, the impact of the air leaked in is more noticeable. The engine will lose power due to a lack of fuel. 

Also, some vacuum leaks are sensitive to heat. When things are cold, the crack is hard and tight. When the engine bay gets hotter, the gaps open, and the air leaks in. 

If your OBD2 scanner throws the code P0171 or P0174 (system too lean) and no other related codes are found, a vacuum leak is likely to be a problem.  

Ignition system

Another common cause for this problem is the ignition system. Some faulty components in this system can not function properly when the engine gets hot. 

And when they go bad, just replace them. They are cheap and easy to replace, so don’t worry!

But first, let’s see what they are. 

Ignition coil 

A healthy battery can only deliver very low voltage (12.6+ volts). An ignition coil helps to increase that number to thousands of Volts. And then, the spark plugs will use that energy to ignite the fuel. 

A faulty ignition coil may short some windings, making the inductance lower. When the engine gets hotter, the coil resistance will increase significantly, causing the voltage to drop.

Without the proper amount of voltage, the spark plugs can cause the engine to misfire, lose power, and eventually stall. 

Spark plug

A bad spark plug will find it’s much easier to fire in rich conditions and high engine RPMs. 

But when the ECU leans out the air-fuel mixture, that spark plug will cause all kinds of problems:

• Rough idle
• Hesitation during acceleration
• High fuel consumption

Car runs good until warmed up – How much to fix?

Now, you know how to properly diagnose your car and the possible issues it may have. Good news, most of the problems are easy and cheap to fix by yourself.

The table below is the estimated repair cost for all the causes I mentioned above.

Finally, don’t forget to leave a comment below if you find any trouble. I’m here to help!