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Recently had my junk 5.3 go out due to lifters. It was already cammed so I went back with a 6.2 drop out. Had a stage 3 cam kit push rods headers high pressure oil pump and all the goodies done. Tune was set and ran good for the first few days. Then it started not cranking without throttle. This is only on initial start up. Can shut off and will start right back up. So it only does it in the morning going to work and afternoon leaving work. Assumed it needed some fine tuning for more fuel or air. Or like it has a bleed back fuel system. As I am no engine guru but know my way around vehicle. Took it back to the shop. Man said he missed a setting in the map sensor should be good. Great. Get there starts fine get home runs better. Next morning back to same thing. Tired of going back to the shop as it’s over an hour away so I’m looking for advice on the culprit. Is it the tune? Something bad part wise to check that may not be new? Again truck runs great once running only problem I can tell other wise is vibrating air filter box I believe. And again issue is only on initial startup. Give it some gas and it runs fine. Thanks for any advice in advance.
 

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I take it that your 6.2 has factory port fuel injection. On fi cars, when you push down on accelerator your not adding extra gas. Your adding extra air. So you need to do several tests. 1st put a fuel pressure tester on fuel rail. Do a key on prime. See what pressure is. Ls based engines should have around 50 psi @ key on prime.

See if pressure holds for several minutes after key on prime. If pressure leaks down quickly, you can have a problem with fuel pump, pressure regulator, fuel line leakage, fuel injector leakage. Cold starts require a richer mixture.

You should also hook an obd 2 scanner that can do real time sensor data. with key on engine not running, check that throttle is opening & closing as it should 6.2 engine has a drive by wire accelerator that uses actuators to control throtte body opening.

Also make sure that coolant temp that PCM is seeing is correct. Read the temp on the scanner, not the temp gauge.

Get car started and read the real time sensor data being shown by scanner. Readings like maf, map, O2, fuel trims, etc are important for correct starting & running.

If you haven't a clue to what sensor data means, PM me with your email address. I'll send you the pdf file called "scanner readings and what they mean."
 
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