5.3l LS Has No Low End Power

Gadametz · Apr 12, 2020

I have a buggy that I've had a ton of power/ engine issues with. It has a 5.3/ th400/ np205. When crawling it has almost 0 power. It doesnt even have the power to pull the buggy up a hill or turn the tires at low rpm from a stop. If i get a rolling start and hit above 2k rpm's it has plenty of power and will go up anything. The entire fuel system is new except injector and maintains 62psi fuel pressure. Any ideas what is causing this? It has rockwells on 43's. When I say it loses power it literally won't turn them, and the engine wont rev any higher. It'll do a hard hill climb fine

Gadametz · Apr 12, 2020

Stock th400 stall and the 5.3 has a small tsp cam

TBItoy · Apr 12, 2020

are you in high range?

Gadametz · Apr 12, 2020

TBItoy said:
are you in high range?

No lol that's what everyone i was riding with thought. More info its on rockwells and 43.s

TBItoy · Apr 12, 2020

was your th400 from a diesel truck ? and you still have the stock diesel torque converter (which is like a 900rpm stall or something silly low)

Gadametz · Apr 12, 2020

TBItoy said:
was your th400 from a diesel truck ? and you still have the stock diesel torque converter (which is like a 900rpm stall or something silly low)

It has HD on the case, but the torque converter is new. Just stock stall

TBItoy · Apr 12, 2020

if the cam is too "big" (no low end power/too much overlap) and the torque converter is stalling too low, then it will be bogging hard off the line and loading up... but I assume you know that since you're mentioning the stock stall.

but is it stock for a diesel? I think even the stock stall spec for a big block truck was like 1000-1200 rpm?

Gadametz · Apr 12, 2020

TBItoy said:
if the cam is too "big" (no low end power/too much overlap) and the torque converter is stalling too low, then it will be bogging hard off the line and loading up... but I assume you know that since you're mentioning the stock stall.

but is it stock for a diesel? I think even the stock stall spec for a big block truck was like 1000-1200 rpm?

Believe its an 1800 stall. That's about where it bogs but over that and it has tons of power

Gadametz · Apr 12, 2020

Gadametz said:
Believe its an 1800 stall. That's about where it bogs but over that and it has tons of power

It hasnt always done this

The-Boss · Apr 12, 2020

Have you checked for codes?
Unplug the MAF then start it and see if it's any better.

zukimaster · Apr 13, 2020

Is the 4 barrel kicking in??

Gadametz · Apr 13, 2020

The-Boss said:
Have you checked for codes?
Unplug the MAF then start it and see if it's any better.

Runs in a closed loop with no MAF. No codes

extremetownie · Apr 13, 2020

Is it drive by wire?

Beerj · Apr 13, 2020

Idles and revs fine when not in gear?

Sawzall · Apr 13, 2020

Watch your timing with a scan tool. If the timing is retarding while it has no power you need to replace the knock sensors and sensor harness. Water gets inside and destroys them. That was the problem on my 6.0 anyways. Make a silicone bridge to channel water around the new ones.

TBItoy · Apr 13, 2020

Gadametz said:
It hasnt always done this

Ah, well that makes a bit of a difference. The first post read like it was new (or new to you) and had always done it.

I'd look at timing and knock sensors like Sawzall said... the same-ish thing happened to my daily driver (of course it was throwing codes too)

whoever flashed the PCM may have turned off important codes (seen it happen more than once)

Gadametz · Apr 13, 2020

extremetownie said:
Is it drive by wire?

Yes

Gadametz · Apr 13, 2020

Beerj said:
Idles and revs fine when not in gear?

Yes it idles and revs fine when not in gear. Only bogs at low rpm under load

Gadametz · Apr 13, 2020

TBItoy said:
Ah, well that makes a bit of a difference. The first post read like it was new (or new to you) and had always done it.

I'd look at timing and knock sensors like Sawzall said... the same-ish thing happened to my daily driver (of course it was throwing codes too)

whoever flashed the PCM may have turned off important codes (seen it happen more than once)

What codes should I look for? I'll be back at the shop/ tuner tomorrow

Uncleboogie · Apr 13, 2020

my new build did the same thing this weekend and when we put the scanner on it it showed knock sensor codes, cleared the code and rode the rest of the day without issue. I will be ordering new sensors this afternoon