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Car not starting after sitting for several days. New battery and alternator

My car: 2002 Mustang GT, manual transmission. Runs great once I get it started with a jump or my boyfriend uses one of those special charger things that plugs into an electrical outlet. Replaced the alternator a couple months ago, replaced the battery a month ago right before going on a month long trip to visit my mother. I splurged and got the best battery money can buy - Optima yellow top, it barely fits in the engine compartment. What the heck else can it be??? It also killed the previous new (much cheaper) battery I put in it.

Has anyone checked for a parasitic current draw?

https://www.batterystuff.com/kb/articles/charging-articles/testing-your-battery-for-parasitic-load.html

Tester

I presume you mean by “not start” that it doesn’t crank, you don’t hear that rrr rr rr sound w/the key in “start”. If the battery is fully charged and good (based on a load test) and it still won’t crank reliably, next on the list of culprits is the starter motor. A faulty starter motor can definitely cause this symptom. Shops generally wouldn’t replace stuff and hope to fix the problem though, they’d do a proper diagnosis, which generally means they’d measure the voltages at the starter motor during attempted cranking. If the voltages are ok and it doesn’t crank then they’d replace the starter motor. If you feel lucky you could just replace the starter motor without the tests. Might do the job.

Correctamundo. There is no noise, nothing at all - it’s dead, utterly and completely. The remote won’t even unlock or lock the doors (the anti-theft light still flashes in the dash though, that is the only sign of life.) But after a jump, it runs just fine, as long as I don’t let it sit for too many days without starting it again.

I thought I read somewhere that a bad starter would cause the car to be unable to start even with a jump? Was that misinformation?

Nope. This does seem logical, though. I sorta wondered, with having some extra stuff put on like HID lights, nitrous, (not my idea! It was already installed when I got the car!)(but I had a nitrous bottle opener installed) and music touch screen system (still has factory sound system).

With several different additional electronic devices installed, it would stand to reason that one of those installers could have done something wrong, right?

Bingo , you are going to need a shop to look at this.

So, do I just tell them I think have an electronic parasite? Describe the symptoms like I did here?

Do I need to look for a specialty shop for this type of work? Would a performance shop be my best option? This car has been such a good performer, I haven’t had to have very much work done on it, and I don’t have a really strong relationship built up with a specific shop yet.

Your first step, and it should be done right away, is to find out if your nitrous system has a bottle heater, and if so, make sure the bottle heater isn’t running when you don’t want it to be. Not only will it drain the battery, but it can also explode if the safety vent is compromised:

Honestly, if the NOS wasn’t your idea, and you don’t use it, I’d disconnect the bottle anyway, and cap off the lines.

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Frankly, I’d love to just have the whole nitrous system uninstalled and sell it anyway. It does have a bottle heater, I didn’t think it would work without one? I will definitely tell whoever I take it to, to look there first for the source of any electrical problems.

A nitrous bottle opener? Are people huffing nitrous while driving?

LOL! No, it’s installed in the dash so you don’t have to get out and open the trunk to open the damn bottle every time!

Right. With emphasis.
And, as shadow said, have a knowledgeable shop remove that nitrous system immediately.
If that bottle has any N2O in it, it’s dangerous. That stuff is nothing to play with.

An “iffy” starter might only work when the battery is 95-100% charged. But not when it is less than 90% charged. And it’s like a roulette wheel; how sensitive it is to the battery charge state depends on what commutator segments the brushes stopped on last time you started the engine. Add to that, temperature has an effect. A good starter motor isn’t nearly as sensitive to battery charge state, the roulette wheel syndrome, and temperature.

Another factor, when your car is being jumped by another car, if the engine is running in the other car the voltage is higher than just the battery voltage. That extra voltage can be enough to crank an iffy starter motor.

Like I mention above, for no-cranks a shop will measure the voltage at the starter motor. If it is getting the correct voltage and isn’t cranking the engine, the starter motor needs to be replaced. If either of the tow voltage aren’t up to spec, then they’ll work backwards along the wiring toward the battery to figure out why.

What do you do with it after you open it?

That’s what we did with it in college.

well, you say the car came with nitrous and it has a bottle heater and you than had a bottle opener installed and now say you wish it was all gone? how are these items powered? by a fused line? remove the fuse. i have to believe the heater uses a few watts when working

It sounds like you have a slow drain coming from somewhere( inside light staying on,dashlight,trunk light,etc.) A hot wire grounded out/ short in wiring,or electrical components badly corroded. Also a Alternator that is undercharging or overcharging could kill a battery

Since you are a first time poster, I will point out that this post is 3 years old. The Dec '17 in the upper right corner shows when the post was made. I suspect the original poster has either fixed it or sold it at this point.

But welcome to the forum.