Symptoms Of Wrong Power Valve

Tim Miller

The power valve is a small but crucial component of a vehicle’s exhaust system. To function properly, it must be matched to the vacuum level of your engine. Let’s explore the symptoms of a wrong power valve or blown-out power valve, so you can figure out if this is something you’ll need to repair or replace in your vehicle.

What Is A Power Valve?

A 2-stroke power valve is a piece of metal in the exhaust port of your engine. This lets the engine control the exhaust port’s size and better control the power that is delivered. You can adjust the spring’s tension on the power valve to alter how much it opens at various engine RPMs.

When the exhaust port is smaller, the airflow is more restricted. This reduces the potential power the engine can produce. A larger exhaust port allows for more airflow, in turn increasing the power potential. Because of this, a power valve enables a two-stroke engine to produce more power throughout the RPM range than it could without it. 

Power bands are traditionally tuned to optimize the power in certain engine performance ranges. As an example, many utility power bands are tuned to put out more power in the low and mid RPM range. Conversely, power bands in racing vehicles are typically tuned to provide more power in mid- to high RPMs (or may be removed entirely and replaced with a plug, for vehicles that are only used for racing).

That said, you will find a wide variety of power band options on today’s market. These can be mechanical, controlled by the engine’s speed, or electrical, which provide more precise tuning and control over the exhaust port’s opening size.

Deciding which power valve your engine needs can be tricky. A good first step is to identify what kind of valve it is using now. You can find your car’s power valve in the metering block. If you are not sure where that is, consult your repair manual. It will look like a small, round rubber diaphragm with a spring on the bottom.

Every power valve has a number stamped on it that indicates at what level of vacuum it will open. You can compare this to the ideal specifications for your vehicle. Ideally, the power valve’s number should be half the vacuum reading. If your engine vacuum is 13-14, a 65 (or 6.5”) power valve is recommended, while a 45 (4.5”) valve is best for engines with an idle vacuum of 8-9.

In this article, you’ll learn about the symptoms of a wrong power valve

Symptoms Of Wrong Power Valve

How will you know if you have installed the wrong power valve in your engine? There are a few common symptoms to look out for. A power valve that is the wrong size will produce very similar symptoms to one that has been blown-out.

If the power valve value is too low, it will open later than it should for your vehicle. This leads to symptoms, including:

  • Air/fuel mixture is too lean
  • Popping and backfires, especially when accelerating
  • Poor acceleration or hesitation to accelerate
A few common symptoms of a wrong power valve in your engine
Some common symptoms that you’ve installed the wrong power valve

A power valve value that is too high will open sooner than it should, allowing more fuel into the air/fuel mixture than should be there. This will produce symptoms like:

  • Reduced gas mileage
  • Black smoke in the exhaust
  • Residue or fouling of spark plugs
  • Rough or poor idling

A blown or malfunctioning power valve’s symptoms can include any of the above, depending on the conditions and how it is damaged.

If you suspect you have a blown power valve, there is an easy way you can test it. You will just need to have a vacuum gauge, which you can get at an auto parts store if you do not already have one. Once you do, follow these steps:

  1. Start your vehicle’s engine and allow it to be idle until the engine reaches the average operating temperature.
  2. Turn the idle mixture screws in. 
  3. If the engine dies, the power valve is operating as it should be. If it continues to run, the power valve is blown and should be replaced.
  4. Hook up the vacuum gauge to a vacuum port of the intake manifold. If your vehicle is an automatic, put it in the drive. If it is a manual transmission, keep it in neutral.
  5. Note the vacuum reading during idling after the engine is warm.
  6. Divide that number in half, then remove the decimal point. This will be the correct power valve number for your engine. Compare this to the number on the power valve in your engine to determine if you need a different one.
How to test your power valve when you suspect you have a blown one
Six steps to test your power valve


It can be more challenging to diagnose a wrong power valve if that valve is functioning correctly and simply not a good fit for your system. Taking the vacuum reading is the best way to figure out what power valve you need. We hope the advice in this article helps identify and fix issues with your power valve!

Read More: Autel MaxiScan MS300: Cheap & Easy-To-Use OBD2 Scanner

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