2003 Kia Pregio Diesel Van: engine starter problem (video included)

Question

Included video is here: https://www.youtube.com/watch?v=iorvSoMjvaA&feature=youtu.be

The reason for pausing to start is for the glow plugs (diesel van).

Battery is OK! I tested myself and at an auto parts store. No problems at all with the battery.

Description: When turning the key, I can hear a click from the engine bay (I believe this is the solenoid, but the starter motor either does not engage, or just barely turns the motor over).

This is an intermittent problem! It first occurred over a month ago. It can go weeks without having any problem, then it happens all the time.

The trick I used to finally get it to start: I turned the key 90% to full, then released to 30% of turn, an turned back to 100%. This "double-turn-without-release" trick, whatever it does, seems to usually be successful in starting the car--I have no idea why, though!

Edit: I forgot to mention this: The problem seems to occur only on the first attempt to start the car for the day. After I've got it started and everything warmed up, starting it again has had a higher success rate. It is still intermittent, though, sometimes it still shows this problem when warm, but the likelihood appears to be much less.

Second edit: Tested starting the motor with a voltmeter. The starter motor is getting 12volts on successful starts, but getting less than that when it is not starting.

Answer 1

This sounds like the starter core is sticking. Use a long metal stick (screwdriver, piece of tubing, etc) and hit the side of the starter with it multiple times before turning it on. Try and start it. If you notice a difference then you can go ahead and remove it and have it tested further.

Answer 2

Could just be a poor connection.. Check the battery terminals are secure and making proper contact, also check main battery connection terminals at the starter end too. You may just have a bad connection. Also check the engine earth cable for a good contact between engine and body.

Answer 3

I am assuming it is a ford E-series diesel van with dual batteries.
While you can read proper voltage on the batteries in the vehicle (they are hooked in parallel) they have to be disconnected from the heavy cables to fully check. They also have to be externally charged fully and then load tested for current and voltage checks at the same time.
I had tried when my batteries failed, which here is about every two years, on my F-series to test in the truck while hooked up. But you just can not check for proper current loading while in the truck. Very likely one battery is not putting out good current and under load drags the system down (battery number two). I have seen both in my truck and my sons truck where the battery will indicate good voltages and with a battery charger show a FULL charge and even will run the headlights. But just dies on the heavy current load of glow plugs and starter. I have even seen a failed battery load so bad that a jumped battery will not start the truck. Myself when it get to the state your video shows first thing I do is put in two new batteries.
I have taken the pulled batteries and been able to put one that is still good in a car and start the engine. Even have used the good battery out of the two my wife's car or my land crusier for another year. But if I only replace one battery leaving the "good" one in the truck it usually fails in a few months so if you find a need to replace one do both of them.