Engine surging in Park

[elspectro29]

Hello again everyone. It's been awhile since I've been here, but I've just come across a new problem. Yesterday when I took the Lude to the the auto shop to replace the brake master cylinder, it started acting a bit strangely. Sometimes when I start it, it will be perfectly normal, but other times, the engine will run at a slightly higher RPM (idling at about 1100 instead of its normal 700 or so). In the case of the latter, when I put it in Park after driving, the engine will surge a bit, going from about 1000 RPM to about 1800 then repeating. I took it back to the shop, since it may have happened while they worked on it, but they couldn't figure it out, as it wouldn't surge for them when they started to look at it (however, it did surge a bit when I first pulled up and showed one of them). They moved some hoses and wires around near the brake master cylinder and it seemed to be fine until today, when I took a short drive. After I got out of the car and got back in, the same issue happened when I put it in Park, but it didn't do it any time before I turned the car off. I know surging can be caused by several things, but has anyone had a situation similar to this happen to them? Thanks.

Edit: The Check Engine light has not come on at any point. Not really sure if that's helpful or not, but I figured I should throw it in there just in case.

[spiffyguido]

Since there isn't an engine code, I'm going to suspect you're experiencing the classic problem with the IAC valve. This generally causes idle surging when the car is in park until the engine warms up. Does your problem go away once the engine is warm?

If it happens when the engine is warm, it might be a vacuum line. Something might just have been split or pulled during the work that was done on the brake master cylinder.

[elspectro29]

It's mostly doing it when it's warm and I've driven it around for awhile, then turn the car off and back on. However, it's been starting to do it as I drive in the past day or so. The shop said if it kept doing it to bring it back, which I will tomorrow. I'll have them check both the vacuum lines and the IAC valve. I'll let you know how it goes.

[elspectro29]

I dropped the car back off at the shop today, and they looked at it for awhile but it wouldn't surge, even after the engine was warm, so they gave it back to me. I then proceeded to an auto parts store and bought some fuel injector cleaner, then drove the car on a short trip (about 40 miles total) mostly running constantly at 45-55 MPH, and it never had a problem. Even when I got home and restarted the car a few minutes later, it didn't surge at all. I'm going to drive it some again tomorrow, when it's supposed to be a little warmer than it was today, to see if the problem has cleared.

[spiffyguido]

It's weird that the problem is so intermittent. It's possible that the problem has fixed itself. Sometimes the IAC valve will get an air bubble in it causing problems, which then subsequently gets worked out while the car drives. Your car might be fine again.