New battery, Higher idle

Black GT

Active Member
Sep 8, 2018
148
9
28
65
Jacksonville, Florida
I installed a 700 cca battery in my GT yesterday and now my car idles at around 1100 rpm's. When I give it gas while idling it wants to hang at around 2000 rpm's for a couple of seconds. Anyone have any idea why this is happening? My old battery that was in the car died and had this brand new NAPA legend laying around so I used it as a replacement. The battery is taller and longer than the battery I took out, but I don't think that would have anything to do with the problem.
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Black GT

Active Member
Sep 8, 2018
148
9
28
65
Jacksonville, Florida
No, check engine light is not on. I replaced the IAC valve with a new motorcraft one about 3k miles ago so I do not think it is the problem. The idle would do the same thing it is doing now every once in awhile before I changed the battery. Turning the engine off and restarting it always fixed the problem though. Since the battery swap it does it all the time. When I push in the clutch or put the car in neutral idle stays at around 2 grand for about 20 seconds and slowly drops down. Before it would drop to around 700 rpm's but now idles at around 1100. I have made no adjustments to anything.
 

Admiral_peck

Member
Feb 3, 2020
7
1
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20
Midland, tx.
I'd guess the bad battery might've been creating a high load on your alternator, and now it's not creating much of a load at all, meaning a much weaker magnetic field in the alt. My 96 does idle around 1100 in N or park if I let it sit. Drops to 700 in gear. Before when my alt was shot it would idle around 800 putting out 11.9 v from said alt. Likely battery didn't want to charge to 12v and pcm was looking for 12v and only ever getting 11.5 or so and is therefore loading the alt as hard as it can to try to charge said battery.
 
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Black GT

Active Member
Sep 8, 2018
148
9
28
65
Jacksonville, Florida
Since I have been driving it the idle is getting close to more normal. The old battery I think was going bad for awhile. It used to turn over slower and take a second to start. Now when I turn the key it spins over faster and starts instantly. I am hoping its just a matter of the PCM relearning the extra charge and it will go back to normal soon. I hope anyway.