Far better it is to dare mighty things, to win glorious triumphs, even though checkered by failure, than to take rank with those poor spirits who neither enjoy much nor suffer much, because they live in the gray twilight that knows neither victory nor defeat.

Theodore Roosevelt, The Strenuous Life

By , on August 3, 2010

Cars and Trucks


I am one of those folks who is usually greatly impressed with the amount of information on the web. My search to understand the purpose of the IMRC system was not such an experience. You can find information on where it is on your engine, how to disconnect it, likely symptoms when it fails to operate, what it physically looks like and how it operates. But I found nothing on why it’s there.

However, I concluded why it exists by a description of how it is controlled. There are two inlet ports cast in the manifold for each cylinder. The IMRC is a vacuum controlled actuator that opens butterfly valves in one of the two inlet ports. A V6 engine has two IMRCs, one operating each bank of cylinders.  The actuators open the butterfly valves at high rpm, allowing plenty of air to enter the cylinders. The IMRC system keeps the airflow even to all the cylinders when the engine is operating at low speed by restricting airflow through the valved port. This is done by closing the butterfly valves, limiting each cylinder’s ability to get air by increasing the inlet resistance. With greater air velocity and pressure drop, inlet air will not take the easiest path to the nearest combustion chamber. The result is a reduced tendency for most of the air to enter the cylinders with slightly lower pressure losses associated with them. There is not a path of substantially lower resistance into one cylinder at low air use rates, which would otherwise occur at low engine speed.

An IMRC that failed to close at low speed would lead to poor fuel economy, uneven engine operation, and higher emissions. Failure to open at high speed would limit power and performance.

This was one of the big technological puzzles I studied last night before writing my first F-150 post. A week ago I didn’t know IMRCs existed.

NOTE: This post is one of my most popular. I edited it on November 12, 2010 and February 2012 to make it more clear. If this post helped you or you think it could be improved, please comment. I moderate your comments – that means I approve them before they post. But additional information in your comments could be helpful to subsequent readers, and I will definitely post such comments. Having good information on the web for the do-it-yourself crowd is good for all consumers. We save a ton of money and learn a lot in the process. And there is plenty of work remaining for the professional mechanics to make a good living.



2 Comments to “What Does the IMRC (Intake Manifold Rail Control) Do?”

  1. [New Post] What Does the IMRC (Intake Manifold Rail Control) Do? http://lawyermusings.com/?p=416

  2. Frank G says:

    This article really helped me understand the dynamics of the IMRC. I just had mine go out on my car and I finally got it fixed after a check engine light came on. I paid 755.00 to the dealer. I was unsure at first if I should even spend the money on such a case. I am really grateful I had it repaired and thanks for the article.

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