I have some problems starting my car. if the battery is charged or jumped it'll start up fairly easily with a little stumbling. However if I run the car for more than 10 minutes turn it off and try to start there's no power, it cranks at <8V and stumbles badly. It needs to be jumped to start. When it runs the voltage reads 14.2V...

I had the battery checked and it's good but I'll probably get a second check.

My best guesses are:
- bad alternator
- bad starter
- wire going to starter shorting out

any ideas?

 

Mein first guess would be a shorted diode in the alternator.

 

The first thing you should do is charge up your battery overnight with a battery charger! Jumping a dead battery and letting the engine run for 10 minutes puts very little charge into the battery! Also, dead batteries draw very large current and this heats up the alternator and shortens its life. Alternators aren't meant to charge up dead batteries. Also, car batteries take a long time to charge up completely, always charge discharged batteries with a battery charger.
When cranking the engine, battery terminal voltage should not fall below 9.0 volts or the battery is discharged (charge it up), battery cables need cleaning, or the battery is at the end of its life.
You can tell the state of charge by measuring the battery terminal voltage, 12.0 volts and below, discharged, 12.9 volts and above, fully charged and linear in between. Measurement should be done hours after any charging!
Normal battery voltage (dash voltmeter, engine running) is 14.3 v engine cold and it drops to 13.3 v engine at operating temp. Car batteries self discharge up to 1% each day even disconnected and you should'nt let them sit for long periods (4+ wks) because lead sulfate collects on their plates and they become door stops. Buy and use a battery tender if you park your car for long periods. Battery tenders measure the battery voltage and charge them when they lose 10% of their charge and then they turn off. Kept charged, car batteries will have a normal life, but left to deep discharge, they have a short life. You decide which you want!

 

thanks for the advice, I'll do some diagnosing tomorrow

 

If battery is truly charged with an external charger and it won’t start 10 minutes later it would normally sound like a starter or poor connection. Even if alternator was putting out nothing, if all (including battery) is good on car it should start 10 minutes later.

If 10 minutes later it won’t start using your installed battery and then you jump it and it starts, then it would point initially to your battery (possible starter also). Especially when you say it cranks at <8 volts 10 minutes later.

However when you say it is running, you indicate 14.2 volts, that would basically indicate the alternator is OK. But in that short period of time not much charging occurs (as stated above) and does not seem to be relevant to your basic starting problem.

Even a shorted diode (rectifier) in the alternator which could discharge the battey over a short period of time would cause a dead battery next day or so is a different set of circumstances and does not seem to be related. The alternator seems to be charging and working.

In regard to your guess, I don’t think a wire or alternator is bad. I would look to the battery or starter.

 

I think you're right, got it diagnosed, alternator and battery are fine no problems. I also cleaned up all the grounds and that didnt help the slow start.

Definitely putting my money on the starter. Seems like whenever the starter is cold she starts up with little issue but as soon as the starter warms up it turns over much slower.

As a test I let the starter cool down, cranked, had a little issue starting but she started, let her run for less than a minute turned the car off then tried starting again she cranked very slowly and didnt start.