Tahoe power loss and backfires

[99 Two Door Tahoe]

Okay guys I’ve just purchased a 99 two door tahoe 4x4 with the 5.7 vortec. I knew it was going to need some love under the hood but don’t know exactly what. For starters it has after market valve covers witch tells me someone’s broke her open befor. I was told the exhaust that’s on it is a magna flow I believe it is leaking. The main issue is I’m losing power if I get into the gas to hard, it makes a real deep sound that just sounds clogged sluggish and weak. If I come off the gas it goes away and I have to work my way up to speed in order to prevent that from happening. When shifting from first to second I feels like it goes into neutral if I get into the gas as well followed by backfires. I usually get it right at the shifting point and back off so it will shift out. My check engine light is on and I went to hook a code scanner to it today but the scanner wouldn’t come on. Went to a friends truck and plugged it in and it came right on. I’m guessing it’s my little receiver port? Anyhow I’m seeking advice on where to start everything I’ve seen people go through injectors, plugs wires, throttle body sensors, catalytic converters, fuel pumps and filters etc. I plan to replace all of that in time but would like to fix my problem first wich is the power loss and backfires. Then proceed to restore the rest any help would be appreciated thanks

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[Hipster]

Verify timing. It sounds like a timing chain that's jumped a tooth or way off. Sluggish to rpm, no power, spitting etc. As far as the port it can be a nightmare figuring out who did what as to why it's not working

 

[99 Two Door Tahoe]

Verify timing. It sounds like a timing chain that's jumped a tooth or way off. Sluggish to rpm, no power, spitting etc. As far as the port it can be a nightmare figuring out who did what as to why it's not working

Verify timing. It sounds like a timing chain that's jumped a tooth or way off. Sluggish to rpm, no power, spitting etc. As far as the port it can be a nightmare figuring out who did what as to why it's not working

How can I check timing without having to tear it down? I’m currently working out of town but will be going back home for Xmas and have about a 500 mike drive. I just got the truck havnt had it a day but I don’t want to get that far into the motor not being home. Any suggestions other than that that aren’t to tough to handle?