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Toyota 4Runner Surging at Idle

Hi, I have a 2000 Toyota SR5 with a 6-cylinder engine. When I first start the car (cold start) in the morning it will run at high idle (1500-1800 rpm) for about 3-4 minutes. At this point, the idle will begin to drop, but then the engine starts to surge. It’s a noticeable surge that bounces from around 900-1100 rpm. It almost seems like an electrical surge, but I don’t believe that’s the problem. After running the car for a while and it’s fully warmed up to operating temperature, the surge goes away.

Any ideas? My thoughts automatically went to the IAC (idle air control) or maybe a temp sensor? The IAC is fairly expensive, so if there’s a way to rule that out, please let me know.

Otherwise, I’m stumped. HELP!

Is the ‘Check Engine’ light on? This sounds more like a vacuum leak. Unmetered air getting into the intake system can lead to the engine hunting for idle. The intake gaskets can behave this way as they may allow leaks when cold, but expand to seal those leaks as the engine warms up. Broken or disconnected vacuum lines can also do this.

As BustedKnuckles said, those are classic vacuum leak symptoms. I’d just add that a quirky IAC valve is also a possibility, but sometimes cleaning them can correct problems.

So do a search for vacuum leaks and clean the IAC valve.

Check engine light wasn’t on. It is now, but it’s not related. It’s on now due to a clogged cat.

I don’t think it’s a vacuum leak, but I could check. What’s the easiest way to check for a vacuum leak?

Another weird symptom: I seem to have to top off my coolant every so often. I have no idea where it’s going since I don’t have any leaks anywhere. But, I can add a quart or two every other week. Again, not a lot, but enough to notice.

Does that change your thoughts at all? What’s the best way to fill the coolant and ensure it’s FULL without air bubbles?