Trany or Engine

[shortline1]

I'll lay it out

74 K20 full time 4x4
383 GM crate motor 450 HP, 465 ft torque
350 Turbo 3 speed auto transmission, rebuilt.
411 gears ( a grunt)
The motor has 100 miles on it.

I'm pulling a 6000 lb RV trailer (attempting)

It feels like I'm at 14000 feet pulling it up a steep mountain pass. both in reverse and forward... Has absolutely no power.
Feels like under powered as well with out trailer. But, I don't know what to expect from this motor on a 3/4 ton full time 4x4.
Checked out the engine with the app, all comes back green, good to go.
GM techs say it the torque converter (my thoughts)
Trany guys say it's the motor.

ANY THOUGHTS???

 

[Bextreme04]

I'll lay it out

74 K20 full time 4x4
383 GM crate motor 450 HP, 465 ft torque
350 Turbo 3 speed auto transmission, rebuilt.
411 gears ( a grunt)
The motor has 100 miles on it.

I'm pulling a 6000 lb RV trailer (attempting)

It feels like I'm at 14000 feet pulling it up a steep mountain pass. both in reverse and forward... Has absolutely no power.
Feels like under powered as well with out trailer. But, I don't know what to expect from this motor on a 3/4 ton full time 4x4.
Checked out the engine with the app, all comes back green, good to go.
GM techs say it the torque converter (my thoughts)
Trany guys say it's the motor.

ANY THOUGHTS???

I think that's a ton of power to put through a TH350 and NP203.... especially towing a 6k lb load. I've used my K25 to pull my 6k lb travel trailer and it only had a problem with the fuel pump not being able to push enough fuel into the carb. It was a defective mechanical pump that finally gave out on me. Mine has a TH400 and NP205 though.

What kind of fuel delivery is this running? What "app" did you use to check out the engine? Is this fuel injected? Carbureted? Has it been dyno tuned so you know it actually puts out that kind of power? Do you have a big enough external trans cooler to not burn up that transmission pulling a load with that kind of power?

 

[dvdswan]

How is it without the load? Does the engine rev just as easily or struggle. Does the trans shift harder or softer?

Key things;
motor has 100 miles on it. IT'S NOT EVEN BROKEN IN YET, IIRC, GM said to break engine in 500 or 1000 miles before towing.

how many miles are on the trans? when was the filter last changed or flushed? Does it smell burnt?

The point is just because it's new or rebuilt doesn't mean it's not broken.

 

[RecklessWOT]

motor has 100 miles on it. IT'S NOT EVEN BROKEN IN YET, IIRC, GM said to break engine in 500 or 1000 miles before towing.

how many miles are on the trans? when was the filter last changed or flushed? Does it smell burnt?

The point is just because it's new or rebuilt doesn't mean it's not broken.

my thoughts exactly

 

[80BrownK10]

I'll lay it out

74 K20 full time 4x4
383 GM crate motor 450 HP, 465 ft torque
350 Turbo 3 speed auto transmission, rebuilt.
411 gears ( a grunt)
The motor has 100 miles on it.

I'm pulling a 6000 lb RV trailer (attempting)

It feels like I'm at 14000 feet pulling it up a steep mountain pass. both in reverse and forward... Has absolutely no power.
Feels like under powered as well with out trailer. But, I don't know what to expect from this motor on a 3/4 ton full time 4x4.
Checked out the engine with the app, all comes back green, good to go.
GM techs say it the torque converter (my thoughts)
Trany guys say it's the motor.

ANY THOUGHTS???

I bet it's the tranny. That trans should easy start out that light if a trailer. A 383 with 4:11 should be able to burn out with that trailer sitting on that truck.

I can pull 9000+ pounds of firewood on my trailer with .h 350/350/205 combo with 3.08 gears and 33" tires!! It's not fast to speed but can do it. Done it several times.

 

[fast 99]

Suggest going back and recheck the tune up, timing and fuel. Although not ideal a new motor should run decent under load. First thing to check is what was worked on or changed. Yes, a torque convertor could cause that but not the first place to look.

 

[AyWoSch Motors]

I'd say its either old worn out trans, burnt fluid and a clogged filter, its low on fluid, or old torque converter. Could be the timing is off pretty heavy.
I suspect though that between the torque of that new 383 and the high gearing of that 4:11, that the trans can't handle it. .

 

[shortline1]

Rebuilt Trany, new TC, all fluids new and topped off. timing right on spec at 12 degrees BTDC. Fuel pressure was high (78 PSI) due to bad (new) regulator, but now on spec at 58 PSI

 

[shortline1]

How is it without the load? Does the engine rev just as easily or struggle. Does the trans shift harder or softer?

Key things;
motor has 100 miles on it. IT'S NOT EVEN BROKEN IN YET, IIRC, GM said to break engine in 500 or 1000 miles before towing.

how many miles are on the trans? when was the filter last changed or flushed? Does it smell burnt?

The point is just because it's new or rebuilt doesn't mean it's not broken.

Totally agree.. I had to move the RV, so i took it for a 3 mile test ride. (my jeep pulls it better) The trans was rebuilt and beefed up. he put in a stock Torque converter with a lower stall RPM, VS. what i asked for per GM specs. (3000 - 3500 RPM stall) but I trusted his advice, as he's been at this for 35 years. He's willing to check it out, but he's 65 miles away, and its tough to get there. thanks for the advice