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Bought a used 2000 F350 7.3 in 2013. Love it but now having some battery issues now.

Bought 1 new battery, 850/1000 in December and replace the second battery in April. Parts store house brand. In May, it started cranking slow.

Did I buy enough battery?

Parts store tested 3 times and said the batteries were fine. Went O'Reilly for a second opinion.
They said the batteries both showed only 25% life. He also said my regulator "didn't snap instantly but snapped fast enough he didn't think that was the problem." Alternator output was fine.

Long story short, the parts house replaced them both 2 weeks ago. Still cranking slow, better but still slow.

The thrilling part is that if the truck sits for more than 24 hours, it dang near won't start.

I am not a mechanic, don't know much about these engines however, what little I do know makes me think the following:

A) Sitting over 24 hours, the HPOP is draining down to zero pressure.
B) The engine is not turning fastest enough to build pressure and start quickly.

I drained the water off the filter, don't know if there was any there or not but it had been a while.
I changed oil because it was about due anyway.
Filled the fuel tank with fresh fuel.

Not dripping any oil, nothing on the manifold behind the HPOP.
Fittings on HPOP are dusty but nothing dripping there either.

I am ready to go to the Ford dealer and buy Motorcraft batteries, prolly should have gone there in the first place.
 

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How many miles on the truck? Could very well be the starter. There difficult to test, may show good on an out of truck check, an in truck amp load test is best. Tested cold and then hot. Cold could take up to 800 amps just to get the engine to start to turn over, 500 to 600 amps when hot. If the bearings are worn, the armature can "scrub" and amperage can exceed the above resulting in a slow crank.
 

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Agree with Trucker87. You should measure cranking amp draw.

FYI, I wouldn't worry about your HPOP. Pressure is supposed to drop to zero when the engine is not running. If you're worried about it, get a scan done and monitor ICP, IPR Duty Cycle, engine RPM and injector pulse width while cranking. IPR duty cycle should be 14% (and not more than about 35%) and ICP should fairly quickly pop up to 500 psi, engine RPM should be at least 100, and injector pulse width between 6 and 8 ms.
 
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