Bucking, knocking, shaking, and dying, your engine might seem like something seriously wrong, but it might just need a little time. Here’s why.

Earlier this week I bought and installed a new 12V battery for our family’s 2008 Toyota Corolla. But, when I first started the little 1.8L four-cylinder, the Corolla was idling super, low, like less than 500 RPM at times.

When I tried to put it in drive, the little Corolla would just die.

In a panic, I honestly thought the new battery was no good. After all, I got the cheapest value battery from Wal-Mart so, I just assumed that was the problem.

But, even with a higher capacity, more expensive battery, the same thing happened, a low and concerningly rough idle.

Then, I did what I should’ve done from the start and googled the symptoms, and found several threads on ToyotaNation.com from other Corolla owners who went through the same issue (I’ve linked the ones I looked at below.)

Turns out a rough, low idle is completely normal and is to be expected with a new battery or a check engine light code clear, especially if your car is older and high mileage (our Corolla has 180,000+ miles.)

Basically, carbon’s built up on your throttle body (the device that varies the amount of air entering the engine.)

Enough carbon’s built up that when you start your car and idle, your engine’s ECU’s “learned” what value/percentage to hold open your throttle body to maintain a normal idle.

A new battery or clearing the check engine light effectively wipes the ECU clean.

With no memory, it reverts to its factory specs which do not take the built-up carbon into account.

ToyotaNation.com user Fibber2 explains what’s going on really well.

“Around 2004 Toyota went to an electronic gas pedal/throttle control and eliminated the physical link and the IAC (idle air control) valve. With that change, the single throttle plate controls everything from idle on up.”

“Toyota issued a TSB advising dealers to always clean the crud off the tip of the throttle plate any time the battery is disconnected long enough to reset the learning on the ECU.”

“Net is that when you changed batteries, your ECU lost track of the adjusted throttle gap needed to ensure a good idle, and reverted back to brand-new factory settings. If you cleaned the crud from the plate and body, you’d have the right gap. If not, you need to give the system time to relearn the gap that compensates for the crud.”

As Fibber 2 mentioned, you can either clean the throttle body or…just drive for a bit.

With no throttle body cleaner, I did the second thing and went for a drive around my neighborhood.

It does not take long.

I essentially found a clear and traffic-free stretch of road and drove up and down it twice, making sure the engine shifted through a couple of gears (your transmission’s got to learn its shift points all over again, too.)

The Corolla died once, which I expected but I just put it in park, restarted the engine, and made sure to not let it completely roll to a stop (if I could help it.)

I only drove around for less than five minutes and, when I returned to my driveway when I put it in park, the Corolla was idling juuust fine.

If you decide to drive around to fix this issue and can wait ’til early morning or late at night when there’s less traffic, the better.

If you DO decide to clean the throttle body, these two videos below should help.

Note, if you go out to buy throttle body cleaner, do not buy cheaper carb cleaner, they are two totally different cleaners with different amounts of chemicals that can wreak havoc on your engine if you use the wrong one.

Make sure you specifically buy Throttle Body & Air Intake Cleaner (like this one) or just plain Throttle Body Cleaner.

Note, that these symptoms after a fresh battery change or CEL clear can happen on all makes and models of cars, particularly relatively new cars with electronic throttle bodies, not just Corollas.

The solution is the same, drive your car a little or clean the throttle body or both.

1) https://www.toyotanation.com/threads/low-idle-after-battery-replacement.1657288/
2) https://www.toyotanation.com/threads/low-idle-after-battery-replacement.1123090/


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