2004 Aveo, idling 1500-2K rpms. Runs fine all gears, manual trans. SOLVED: vac leak

So, I bought this car a couple weeks ago from a private seller. 123,000 miles on it. Ran and driver perfectly....until the next day. Check engine light came on and then started blinking. But car ran and driver perfectly still. Took to mechanic....they could not find anything wrong with the car at all. So I just kept driving. Yesterday I accidently almost ran it out of gas. Right after I put gas in, it started idling at 2,000 rpms. When clutch is pushed in car jumps rpms. When any gear is engaged it runs fine. Thought maybe picked up some grit, so I ran a bottle of heat through it. It returned to normal rpms almost immediatly. However, when I hit quarter of a tank of gas the next day (today) it jumped back up in rpms and is doing the same thing again. Filled tank back up and added another bottle of Heat, no change this time. Any ideas?????

What was the engine code that popped up?

The short of the story is, if your engine is running too high of idle RPMs its getting too much air. It could be a vacuum leak, or bad IAC (idle air controller). Hopefully the engine code will help diagnose it.

When I had it as shop last week it was throwing o2 sensor and random misfire. They could not find anything wrong though. Taking it to Auto zone in a couple minutes to see what it says now.

So, after eating lunch I went to start car to take to auto zone..... it made kind of a loud ish grindy noise and then rpms are back to normal. I am sure the tacos didn't fix it, but I am clueless

Ok....so I changed all the spark plugs. Gapped them all correctly. Cleaned the air filter....will buy a new on soon.... started the car and discovered there is a clank clank clank twang metal fan kind of sound when I take it up to 2,000 rpms out of gear and let it come back down on its own. That is the sound I heard when it came down from being stuck at 2,000 rpms at idle. Any ideas?

Ok, so it's been a few days and now the car is running super rich. Like your standing by the car and daaaaannnngggg. It is randomly spiking rpms when out of gear and clutch is down or up. I'm out of ideas....it codes it is showing are P0300, P0134. However, it was throwing those codes before the rpm issue began and before it was running rich like it is now.

A P0134 was set and a mechanic couldn't find anything wrong? That is just sad. First, the wiring for the upstream sensor needs to be checked, to verify that all 4 wires have connectivity and voltage coming into the power wires. If all is ok there, then either check resistance in the upstream sensor, or just replace, because it's almost certainly shot.

Yup. They had the car a full day and said they couldn't figure out what was wrong. Said there was no misfire.

Will the upstream sensor cause the random idle jumps like that?

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Originally Posted by Radiance_Komer

Will the upstream sensor cause the random idle jumps like that?

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The p0134 indicates NO response from the upstream O2. But if it's actually reading erratic (sometimes nothing, sometimes ok), then just about any engine weirdness is possible.

The usual suspects for high idle would normally be a bad IAC or vacuum leak. However, with a specific O2 code such as the P0134, you want to get that resolved first, and then see where things stand after that code has been eliminated.