Engine surge at low speed

Hi,

Does this only happen when the brakes are applied? If so, my first suspect is there is a vacuum issue at the vacuum brake booster. If there is a leak, it will draw more air into the intake manifold and can cause the idle speed to change.

The vacuum brake booster is mounted under the hood, behind the brake master cylinder.

Do me a favor. I attached a pic below showing the booster and components. Check the parts I highlighted. Make sure they are in good condition, not damaged or dry rotted, and secure.

Next, remove the check valve. The valve only should allow air to pass in one direction. Confirm that for me.

Let me know what you find. Also, start the engine and listen for any hissing sounds that could indicate a leak.

Take care,

Joe

See pic below.

Joe,

When I removed the check valve there was a release of air. The vehicle had been sitting for about 48 hours. I would think that it would indicate that both the check valve and vacuum hose are good. Air will only pass through the valve one way and the hose shows no sign of breaks or hardness. Good idea but did not solve the problem.
Thanks,
Chuck

You mentioned that it doesn't happen all the time. I (and this is a theory) have a feeling it is something within the booster that leaks out when it happens that causes the issue.

Have you checked if there are other vacuum leaks? If the intake vacuum is already low, it could be causing the issue when the booster needs additional vacuum.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Is there anything other you can do to cause it?

Let me know.

Joe