08 Nissan Titan cranks fine, but won't start without a jump? New battery, and new relays. Any good ideas welcomed.?

There may be a high resistance due to corrosion at the battery terminal on the positive side. The thinner wire, I.e., not the one going to the starter, may not be making a good connection.

Normal battery voltage is 12.7 VDC, during cranking it drops below 10 VDC. Jumping from another car, with it's engine running, will keep the voltage above 14 VDC. This is a solid 40% increase in voltage. It can make all the distance in the world when there is a high resistance where it should not be.

You can test this theory with a volt meter at one of the power points. See if there is a difference during cranking with and without the jumper cables.

If you don't have a volt meter an incandescent 12 VDC bulb will do. There is a marked difference in brightness between 10 VDC AND 14VDC. Halogen headlights won't do, nor will LEDs, they are not sufficiently voltage sensitive.

Those models had some issues with faulty or corroded IPDM modules over time or anti-theft NATS system issues that may cause a no-start but could be other issues as well. They also had a recall for defective fuel tank sending unit which may allow the tank to run low on gas which can also cause starting or dying/stalling issues.

Any trouble codes? Does the anti-theft light come on when cranking? If it's off, then that may not be the issue but if it's on, then you can try to reset the NATS system. See video link below for more details on replacing the IPDM or resetting NATS. You can also check the terminals on the IPDM for any corrosion and clean as needed with DEOXit, or similar cleaners.

From your symptoms, I'm leaning to a bad IPDM but I'd also do a starter draw test and see if battery voltage is dropping too low during cranking which can affect other electrical systems/PCM to cause a no-start. Some relays may not operate correctly if voltage is too low.That may be why it starts with a jump.

https://caig.com/deoxit-d-series/

http://ww2.justanswer.co.uk/uploads/InfinitiExpert...

A little long winded but he touches on various issues...

https://www.youtube.com/watch?v=HHDU2KDqSOA

That's pretty confusing. How can the starter motor crank the motor over fine if you have to jump start the motor?!

When's the last time the spark plugs were replaced?

What's the fuel pressure?

could be a faulty alternater

Get the battery charged and load tested. Battery could be a lemon. Make sure it is the correct battery for your vehicle. Battery is good, replace the battery terminals. Check for a bad block ground.

check the cable clamps on the battery terminals & make sure they are clean.