Altima surges at stop

I have a 2001 Nissan Altima. Recently it started to surge when I come to a stop. It doesn’t always happen, in fact it can do fine for a day or so. But every once in a while, when coming to a stop, the engine rev’s up to about 4000 rpm. It immediately returns to a normal idle, but I have to throw it into neutral to stop. Nissan said it was the idle sensor in the throttle body, so I replaced the throttle body. Still happens. Another Nissan mechanic said it could simply be that the sensor needed to be re-programmed. Did that, still happens. Today, it happened, but stayed at 4000 rpm until I took my foot off the brake. Then it returned to normal idle. It doesn’t matter if I come to a quick stop or let it drift to a slow stop. No one has a clue, so I would be very grateful for any suggestions.

@TimW you have the classic symptoms of a bad idle air control valve.

Since you already replaced the entire throttle body, you know that’s not the problem.

the only other thing it could be, besides the IAC valve, would be vacuum leaks.

Engines are designed to automatically increase idle speed for a number of reasons, like when the coolant is cold, when the headlights are on, when the rear window defogger is on, the AC is on, when the steering wheel is turned, etc. When I first start my Corolla for example I notice the engine rpm is high, then over the course of a minute or two while it idles in the driveway the rpm slowly lowers to normal. That’s because the engine coolant is warming. Likewise if I turn the steering wheel, I notice the engine rpm bump up a bit. So for you, it may be something is wrong in either the detection of the function, so the car thinks the rpm should get a bump but it really shouldn’t, or in the actuator that increases the idle speed for that function is not working. One thing you could try is see if you notice the expected increase in idle speed when you invoke any of those functions. Do you notice the rpm drop noticeably within a minute or too after a cold start for example? If you remember the idle speed did increase before when you did something, but not now, you’ll have a good clue where the mechanic should look. Best of luck.

On other thing. If you car has an idle speed adjustment, like an idle speed screw setting, and that has been used to fix some other idle problem recently, that can confuse the engine computer and cause surging. Any adjustment to the idle speed should only be done on an engine that is known to be otherwise working correctly or your risk this symptom appearing.

I agree with George, but the fact that this happens when you come to a stop leads me to question whether there is a vacuum leak in the connection between the engine vacuum source and the power brake assist, or a leak in the power brake assist unit itself. Why not inspect the hose itself, especially at the ends where it’s clamped on? You could try moving the hose around while the engine is quietly idling. If wiggling the hose changes the idle, it’s leaking. The other thing would be to step on the brake while the warm engine is idling in neutral, and see if that changes the idle a lot.

This engine has a throttle body with an idle sensor. No vacuum hoses. It’s also not a function of a slight increase due to normal driving, such as turning the steering wheel. It only happens when the car drops down into first as you slow to a stop. It rev’s up to 4000 rpm, not a “slight” increase. As soon as you let off the brake, it idles back down.

“As soon as you let off the brake, it idles back down.”

This suggests to me the brake booster. Perhaps vacuum leak or a failing check valve. Try disconnecting the hose at the booster check valve, pinching it off, and see if the problem disappears. Post back with the results. NOTE; don’t go driving…your brakes will take a lot more effort to use.