Car struggles to idle without stalling

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2011 Mazda6, MT, 147k miles. For the past week my car has been idling at too low of an RPM (currently idles around 500RPM but typically idles at 750RPM). The engine seems to be trying to avoid stalling out by raising the RPMs when the RPMS drop too low. The car will vibrate/gently shake like its about to stall but then the engine RPM will bump up a bit and the process repeats. Best I can describe it is that the engine sounds like it's surging, fighting stalling, surging, fighting stalling, repeat. The headlights dim when the car shakes. What really throws a loop is there is no CEL on. Car seems normal when driving, its just when slowing to a stop without gas and when the car idles.

I did replace the battery yesterday because the car failed to start and the battery had corrosion on the negative terminal. The battery was almost 5yrs old and a new battery solved that issue so my best guess is that was a fluke and not a related issue. I do wonder if a failing alternator could kill the battery and also cause the problems I'm having? I cleaned the MAF with cleaner 6 months ago to clear a CEL. I also replaced the upstream O2 sensor about 3 months ago with an OEM to clear a CEL. Spark plugs are about 6 months old. Air filter 3 months old.

Any suggestions on where to look would be much appreciated. Really trying to avoid breaking out the parts cannon on the car and wasting money replacing things that don't need replacing. Thanks.
 
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Is the throttle body clean? May need to clean the throttle body and do an idle relearn. Fact that a new battery solves the issue temporarily tells me that it may be throttle body related. Many cars will relearn the idle speed after the battery was disconnected.
I've never touched the throttle body before. The new battery only solved the no start issue, but the car was having the surging issue for days before the battery replacement so I'm not sure they are related. The battery was also leaking so I'm fairly certain it was just shot anyway. With the new battery the car starts great now, but the surging issue remains. Thanks.
 
I second the throttle body cleaning unless there is some big obvious crack in intake tubing

did the low idle happen suddenly or slowly got lower and lower.

if sudden I would lean towards a crack in tubing somewhere
if slow I would lean towards throttle body plate slowly getting gummed up
 
Maybe 10 miles so far. I've never heard of this. How long would a relearn take?


Not exactly sure but drive it for a good 30 minutes and see if that clears it. Did this happen right after the battery was replaced or was it happening beforehand?

If it happened right after the battery replacement then make sure nothing got disconnected.
 
Not exactly sure but drive it for a good 30 minutes and see if that clears it. Did this happen right after the battery was replaced or was it happening beforehand?

If it happened right after the battery replacement then make sure nothing got disconnected.
I'm somewhat confident I felt the "surging" issue a few days leading up to the battery failure and replacement. I remember distinctly one morning on my way to work that the car was idling rough when I stopped at a stop sign.
 
I'm somewhat confident I felt the "surging" issue a few days leading up to the battery failure and replacement. I remember distinctly one morning on my way to work that the car was idling rough when I stopped at a stop sign.


Hard to diagnose over the internet but I still think the lower voltage caused the computer to freak out. Also, changing batteries is not the simple task it used to be. Computers control a lot on these vehicles.
 
Hard to diagnose over the internet but I still think the lower voltage caused the computer to freak out. Also, changing batteries is not the simple task it used to be. Computers control a lot on these vehicles.
Well that would be great news if it just turns out to be the issue. I'm taking it on a 30 mile trip tonight so I will see if anything changes after. Thanks PimTac.
 
Well that would be great news if it just turns out to be the issue. I'm taking it on a 30 mile trip tonight so I will see if anything changes after. Thanks PimTac.


Let us know how it goes.

PS: We had a early 90’s Mitsubishi that got a battery replaced at a local tire and auto repair place. (Les Schwab). It did that same thing. I think this was about 1998. The guy was supposed to let it idle for a while before handing it back over but he didn’t know. Anyway we drove around for 30 minutes or so and it eventually cleared up.

These days there is a specific procedure to changing the battery. Losing the memory is a nuisance. Frying the computer would be expensive.
 
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I had almost the exact same symptoms on a 1994 Saturn with a manual transmission. It turned out to be the EGR valve sticking open just slightly. Idle was low and I could feel it fall off during shifts, but I don't remember any surging. What really made it obvious was that the interior rattled more at idle.

If it isn't a relearn issue, that might be something to check along with vacuum leaks. On the other hand, it seems like the computer on a more modern car would catch a sticking EGR. The Saturn was easy to diagnose because it had the vacuum-operated valve that could be opened and closed by hand.
 
Let us know how it goes.

PS: We had a early 90’s Mitsubishi that got a battery replaced at a local tire and auto repair place. (Les Schwab). It did that same thing. I think this was about 1998. The guy was supposed to let it idle for a while before handing it back over but he didn’t know. Anyway we drove around for 30 minutes or so and it eventually cleared up.

These days there is a specific procedure to changing the battery. Losing the memory is a nuisance. Frying the computer would be expensive.
Last night I drove about 30 miles and the idle RPM is now back to its normal 700RPM so it seems to have corrected itself. Never knew this was a thing. I had the battery replaced 5 years ago and didn’t have this issue but I think maybe the battery was changed fast enough to where the computer didn’t lose memory.

Thanks everyone for the help!
 
Sounds very similar to an old Camry I had. Was a sticking EGR valve. They are supposed to close at idle. Sticking open mimics a lean condition.
 
So unfortunately the problem remains. Symptoms:
- The RPMs continue to randomly drop at idle and then the car shakes. Then the RPMs throttle up to correct, remain for a bit and then drop back down in a perpetual cycle.
- The vehicle drives oddly. The acceleration is not smooth. The throttle seems to be all over the place meaning the RPMs don’t seem to correspond with the gas pedal like it should. This sometimes makes the RPMs go too high when I shift into 1st gear or too low.
- The car jerks a bit when I let off the gas while coasting and it slightly bucks. This happens most around 30-40mph.

Ive replaced the MAF sensor and cleaned the throttle body. I used carb cleaner to check for vacuum leaks between air intake and intake manifold and could not find any vacuum leaks. Ideas?

Would an EGR or IAC be more symptomatic of these issues?
 
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