2007 Scion TC TC rough idle

Tiny
SHARON1946
  • MEMBER
  • 2007 SCION TC
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 109,000 MILES
A friend put in a new battery today after my battery died. It starts fine but when I approach a stop light or put on the brake it idles very roughly like it is going to die. Then when I accelerate, it is fine. So he told me to let it run for 5 minutes or so and it seemed to be fine. After work it starts right up but after about 5 minutes it starts the rough idle again when I slow for traffic even after I drove it more than 11 miles home, with each slow down and braking the idle seems like it is going to die. How can an older woman like myself reset this without being ripped off by a shop? Help! Thank you!
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Monday, November 23rd, 2015 AT 9:31 PM

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Tiny
CARADIODOC
  • EXPERT
If you assume you're going to be ripped off, nothing anyone can do will convince you otherwise. It's funny how we hold mechanics to much higher standards than doctors. If a mechanic finds things that are going to cause you problems in the near future, you accuse him of trying to sell you parts and services that aren't needed. If he keeps his mouth shut to avoid making you feel like you're being taken advantage of, and the car breaks down weeks or months later, whose fault is it? The mechanic's, because he should have warned you. If we need a few hours to diagnose a problem with the grossly-over-complicated car full of unreliable electronics that you bought, we are considered incompetent. We charge for our knowledge, training, and experience, just like people in every other profession. Why don't people scream at doctors when they don't solve the problem on the first visit? Hospitals and dentists have to pay for their expensive equipment, and customers understand that when they get the bill. We have to update our equipment sometimes twice per year, we get soaked thousands of dollars per year for online service manuals that we never own, we have to buy new tools that cost a lot more than what you buy at a hardware store, and the shop owner has to try to cover all the government and environmentalist-imposed costs along with all the taxes and insurances. You will never be able to figure out he he can do that by charging only $100.00 per hour.

You should be blaming your friend. Had this problem occurred after a mechanic replaced the battery, what you be saying about that person? Funny how you don't have a complaint about the person who did this work. Your only complaint has to do with people who you haven't even given a chance yet to rip you off.

Now, to suggest what might be happening, when you fail to use a memory saver while the battery is disconnected, the Engine Computer will lose its memory. Fuel metering trims will be relearned as you drive without you even being aware it's taking place. It is also possible it has to relearn what Chrysler calls "minimum throttle". Until that is done, the engine may be hard to start unless the accelerator pedal is held down 1/4", you won't get the nice "idle flare-up" to 1500 rpm at start-up, and the engine will tend to stall at stop signs. That really has not been a problem with most cars, so it's hard to say, but if the rough running is due to idle speed being too low, that will go away once minimum throttle is relearned. Every manufacturer has a different procedure. I don't know what is required on your car because it hasn't been necessary to know it, but for Chrysler products, you have to drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the pedals. That's equivalent to coasting down an exit ramp, but it has to be without pressing the brake pedal. If this sounds like what is happening, someone at the dealership should be able to describe the procedure for your car.

All of this assumes the engine doesn't have a misfire. A misfire wouldn't go away when you raise engine speed a little, and it would be detected by the Engine Computer which would turn on the Check Engine light and set a diagnostic fault code indicating which cylinder is misfiring. You can figure this out yourself. When the rough running occurs, simply press the accelerator just a little to raise engine speed. If you have a tachometer on the dash, idle speed should be around 800 to 900 rpm. You can expect the engine to run rough when idle speed gets down to 500 - 600 rpm.
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Monday, November 23rd, 2015 AT 10:51 PM

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