M934a2 sluggish low rpm performance

[brazengoat06]

Just got this 8.3 6cta back together. As I went to drive it around the block it seemed to run great. Made one lap and began having poor low rpm performance. Very very slow off the line until you reach 1700 rpm. Also getting a turbo chirping noise under boost. I'm not sure what's normal and what's not as this is my first 8.3. Thanks for assistance.

 

[red]

8.3 relies heavily on the turbo for power, so it will be gutless until the turbo is spooled up.

Check all the air intake tubing, might has a loose connection where the compressed air is venting.

 

[snowtrac nome]

or a boost tube leaking

 

[brazengoat06]

I have a small amount of black smoke but not as much as I would expect if I had poor boost. It does run better in the higher rpm so I have to assume it's a boost leak. The chirping noise when I build boost also makes me think that. But I haven't been able to locate a leak.

 

[74M35A2]

red and snow are both right on. There are 2 aspects to boost. One is the turbo piping that blows the compressed air into the engine. The other is a small tube that goes to a diaphragm on the back of the injection pump. Either could be leaking. The small injection one allows increased fueling once boost pressure is up. The fuel rack is held back in restricted position until so.

The 8.3L has weak bottom end to begin with, so keep this in mind. That can be somewhat compensated by increasing your low end fueling via the top adjustment screw on the pump, but you for sure want to have it running properly before asking it to do more.

 

[snowtrac nome]

on the b series I have seen the intake cover gasket blow and make a buzz sound under boost. once you rev the engine even in neutral you should be able to feel air leaking. if you have a blown gasket or loose intake cover bolt soapy water can also be used

 

[brazengoat06]

Used soapy water and have not been able to find a boost leak. Checked the charge tube and the tube to the IP. Oh and the aftercooler gasket

 

[WillWagner]

Get a second body. Use a socket, extension and a breaker bar to hold the turbo shaft from spinning, start the engine, use starting fluid LIGHTLY at all of the intake joints, the aftercooler inlet and outlet, crossover hoses, etc. If there is a leak to atmosphere, the starting fluid will find it and the RPM will come up. Manifold and turbo mount gaskets not sealing will give the same symptoms you are describing too.

You could have two issues, check for full throttle, make sure the tab on the throttle arm touches the stop screw on the governor housing...DON'T ADJUST THE STOP SCREW!!

 

[brazengoat06]

i checked for full throttle and it checks out ok

 

[74M35A2]

Turbo will not make positive boost until you are driving it. Try Will's method.

 

[98G]

If I understand you correctly, the first lap felt "normal" and then something changed?

 

[brazengoat06]

Yes it seemed to have more low end when I first drove it. I checked for leaks using the starting fluid and no luck. Maybe I need to drive another one and compare? My nhc250 feels like it would run circles around this thing. But it also doesn't have a 10klbs box on the back.

 

[98G]

Yes it seemed to have more low end when I first drove it. I checked for leaks using the starting fluid and no luck. Maybe I need to drive another one and compare? My nhc250 feels like it would run circles around this thing. But it also doesn't have a 10klbs box on the back.

If you didn't experience a change after the first lap, this is what I'd say:

This is typical impression when someone used to an NH250 goes to an 8.3. The NH250 feels like way more power right off the line and shifts better. The 8.3 feels like it has a profoundly delayed stall converter in comparison, especially loaded.

However, if something changed after the first lap that isn't the explanation. I watch with interest to see the resolution.

 

[brazengoat06]

i had it on the highway for the first time today and it takes forever to reach 50-55 but once you do it seems to maintain unless there is a hill. Im basically at WOT 90% of the time. the only thing ive ever driven that was like that was an Isuzu NPR lol

 

[snowtrac nome]

if you have a big ass box on the back wind resistance alone is going to slow you down never mind weight

 

[red]

Agreed. Between the box and the fact that the 8.3 (in stock form) has no guts until the higher RPM's where the turbo has spooled up then this sounds normal power wise. If the first lap was the same as later driving.

 

[brazengoat06]

Ya I keep thinking that maybe I was thinking it felt better at first and I reality it wasnt. I think for now I have to assume this is normal and get used to it. I'm guessing however that if I take it offroad I will have to solely use low 6x6

 

[Buffalobwana]

I have three 923a2's (well, now two, just sold one ... my best one ) one of mine is so gutless at low rpm, I'm sometimes scared that a small rock could stop it from coming off the line! A small incline has stopped it dead in its tracks. But, we cruise along at 60-65 with the hammer down. Even hit 70 with a load going downhill once.

Since I have two others to compare it to, I know this one is different and is definitely weaker that its brothers. All three are 90-91 923A2's so we are comparing apples to apples here.