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Why Does My Kohler Engine Runs Rough When Hot?

One of the most common issues with Kohler engines is that they start to run rough. There are many causes that might lead to this issue, but the most common is when the mower starts to vibrate and make loud noises.

In theory, a Kohler engine should last longer than 1500 hours without any major changes in overall performance. However, there are many factors that can contribute to this decline in performance. 

One of those factors is vibrations and noise from the mower itself. While problems should never be ignored, sometimes it can be hard to know what exactly is going on with your machine. 

This article will provide you with a few tips and suggestions that might help you find out what’s wrong with your Kohler engine that it runs rough when hot.

Kohler Engine Spark Plug
Kohler Engine

Possible causes why an engine runs rough when hot

When your Kohler engine is running rough, you may be tempted to start replacing spark plugs and fuel injectors. However, it is possible that these problems are caused by other factors. 

Before you think that spark plugs or fuel injectors are the culprits, consider the following causes of rough idle:

One of the most common reasons is when the spark plugs have worn out. A leaking fuel injector or a bad engine coolant temperature sensor can also cause a rough idle and poor fuel economy. 

A vacuum leak can cause an engine to run rough when it is warm due to a lack of air intake.

Vacuum leak

The vacuum leak will cause the engine to run rough. Vacuum leaks can be caused by a variety of issues including a clogged fuel injector, a loose or missing exhaust pipe, and an improperly attached vacuum hose.

When you start your cold engine, it takes a moment for the computer to adjust and bring on a rich mixture and high idle. 

A moment later, when the computer starts to reduce fuel at idle RPMs, a small vacuum leak doesn’t have much effect because less fuel is flowing through the engine now.

But the computer won’t notice when air is entering the engine through the vacuum hose. Because of this, your computer commands the proper air/fuel mixture.

But it doesn’t notice the leak in the vacuum hose because it hasn’t been programmed to do so.

Faulty fuel pressure regulator

The fuel pressure regulator on the Kohler engine may not show up fault when cold but at warm. When warm the air-fuel mixture will be wrong, causing the engine to run rough. 

A faulty regulator can cause the engine to run poorly and also increase fuel consumption. The issue could also be caused by a clogged air filter or dirty throttle body, so make sure to inspect these components as well.

When the gas pressure decreases in your engine, it usually causes the engine to misfire or struggle when you try to accelerate, which can result in wasted fuel and poor performance. 

If you’re experiencing issues with your mower’s rough running and decreased fuel mileage, you may need a new fuel pressure regulator. 

Dirty air filter

Air filters are one of the most important parts of an engine. They keep dirt and grime from getting in and causing your engine to run rough. If you’re running a manual transmission, you really need to get an air filter as soon as possible.

The dirty air filter in your mower can cause your engine to run rough, which can lead to decreased gas mileage. The engine will also make strange sounds that you may not notice right away.

It is important that you change your air filter often so that it doesn’t get clogged with dirt, which will cause your engine’s power output to decrease.

Fuel injectors are leaking

A leaking fuel injector can cause the engine to run rough, which might cause an extended crank and hard start when hot. 

 Kohler Engine leaking issue
Kohler Engine leaking issue

A lot of symptoms are engine stalling and backfiring, difficulty starting the vehicle, poor acceleration, and a rough idle. Fuel leaks into the combustion chamber might be shown.

It is important for vehicle owners to keep their mowers properly tuned in order to avoid any issues caused by leaking fuel injectors.

Worn spark plugs

Spark plugs may get worn out after a period of time and cause the engine to run rough. You may also feel the mower vibrating or your mower surging if these spark plugs are not replaced soon. 

A man is locating Kohler Engine Spark Plug by hand
Kohler Engine Spark Plug

The engine surges or misfires and will also have an increased fuel consumption.

Worn spark plug might not fire to perform the right blast. When the air-fuel mixture is well and the RPM might be high. And when air-fuel mixture is low the RPM will be low.

A worn spark plug causes more friction than usual, which means an increase in heat buildup and wear on components. In this case you have to replace the spark plugs with new one.

 Kohler Engine old spark plug
Replace old spark plug


A vacuum leak is a common problem on many engines. It can cause the engine to run rough when the temperature is hot and the fuel pressure regulator is not working properly. 

Fuel injectors might be leaking causing poor performance or they might be plugged causing poor fuel economy. 

The engine can also have other problems such as worn or clogged spark plugs, faulty fuel pressure regulator which causes fuel leaks, faulty fuel pressure motor, or damaged vacuum hoses leading to a loose connection.

The worst case scenario would happen if there’s a serious issue like an engine fire which could result in expensive repairs

We’ve discussed several causes why a Kohler engine runs rough when hot. We hope now you have a better understanding about the issue and you can fix the issues so that you can get the best performance out of your Kohler engine.

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