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Low power low boost pressure


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I have  a 1994 rd688s with 400 HP with mechanical pump and computer on the end of it I am not sure what they call that system,  but I am not getting HP I feel it should have and just put boost pressure gauge on it and 21 PDS is it, Mack hooks it up no codes come up, it runs fine starts good, uses no oil the motor is fine, I feel the motor needs something, short of replacing parts an a pump being a little high just to replace it, I looking if anyone has had something like this?

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Glenn in my opinion It  is a little low I believe 25 to 30 wouldn't be out of the way for a 400! First  IS the gauge correct? Next 

are you trailing smoke while pulling? If so!

If you have a a pyro it should be staying around  or under 1100 deg max  when pulling if not check the air filter and check for boost leaks !

If you have a 18 sp Mack trans  unplug the torque limit switch at the rear upper range cyl see if this helps the situation!

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I just looked in three tune up manuals and one was for a 91 and one 93 and the other was for a 95 year.The boost is posted from 20 psi to 32 psi depending on hich year so 21 would be on the low side.A friend of mine has a 94 400 which was running about 21 or 22 psi.We traded turbos from a 427 HP and gained I think to 29 psi but it had more smoke at low end than the 400 turbo.It did not pull any different.

glenn akers

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I was wondering what difference a bigger turbo would make, it sounds like not a lot, thanks for looking. So maybe I am expecting to much, it seems to struggle with loads, this is a truck I just got been setting a while, the previous owner had changed motors, the drivers didn't like the truck so it set, so I don't know a lot about it, or what to expect, I have an 04 370 HP Rd model, that will out pull this truck fairly easy, it's electronic over the older systems, that why I am looking for any good advice from anyone

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Use a air regulatoe and connect to the tube that comes off of the turbo going to air air cooler.You will have to turn the engine slow until you find a place that enough valves are closed to hold air pressure and this way you air check more places for leaks.Some times you find intake manifold gaskets leaking as well as the hose going to the air compressor.Also remove exhaust and intake hoses from the yurbo and with a go strong light look at the fins on tge wheels for damage.Any damage of the fines can drop the boost plus its can do damage to the engine.

glenn akers

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11 hours ago, finkexcavating@gmail.com said:

I will check it out using 25 psi, I had not done it that way, thanks

Good advice Glenn. Good explanation too ! most Guys just check the  cooler and that aint  enough .Check the whole engine and get the whole story !!!  A common leak is the the compressor inlet on some of the engines  depending on how its plumbed  !That wont get picked up by testing just the cooler! And put 30-35  psi to it more is better!

So What trans do you have? 

Edited by fjh
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We alway bump the pump timing up 1  to 1.5 degree's over what it say on the valve cover. Most of the older VMAC 1's and VMAC 2's we check, the pump timing is always set lower that whats on the valve cover.  21 to 30 psi  is the Spec on the boost pressure , so your way on the low side of spec.

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Never thought of doing that way,  that's a good idea, thanks. The company I got it from changed the motor, from a 350 to the 400 HP, and left the 350 HP valve covers on it, could they have used the 350 injection pump also, I don't know if possible and if fuel delivery would  be a problem too. I don't know what the timing should be either with wrong covers

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Transmission is maxitorque extended range overdrive, everyone calls them something different, to me it's like a super ten, it five gears with air splitter, this one has a short shifter lever, it's a super low only works in low gear,  I never saw a another one like it, Mack dealer mechanic never saw one before, 

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Did you Try what Glenn posted? That will show up any leaks you have even if it was a compressor I have seen em dump boost out the air drier under certain conditions ,not often but have seen it!

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No haven't tried anything yet, were  busy as can be right now, I have to wait til weekends then it seems always something going on, I am going to try to get it checked out, it's running OK just doesn't have the power it should,  I want to thank you guys for your advice it's been very helpful, I am hoping to find something that is a easy fix, but it's always going to be one step forward two steps back, 

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1 hour ago, finkexcavating@gmail.com said:

No haven't tried anything yet, were  busy as can be right now, I have to wait til weekends then it seems always something going on, I am going to try to get it checked out, it's running OK just doesn't have the power it should,  I want to thank you guys for your advice it's been very helpful, I am hoping to find something that is a easy fix, but it's always going to be one step forward two steps back, 

That's the way it works.No one can tell you until you do some test.I know some people claim to know it all but there is more than one thing that could be going on.You check one thing and go on to something else before you start buying parts.

glenn akers

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Thanks guys I will keep you updated, I got a 1999 ch612 really nice truck I converted to a work truck, we added 3 ft to frame, I had custom built bed and headache rack built for it, low miles nice truck, then fuse panel relay burnt up, they totalled the truck I brought it back, but now having the darnest time finding a cab , have any suggestions? So to rewire this truck,  

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9 hours ago, finkexcavating@gmail.com said:

Did you find out the reason why it was weak? Or did you just run it

I bought it with 300 and some thousand miles on it and sold it with almost a million on it and it was always that way. Everywhere I ever took it said that's the way they were. It never let me down though and it's still running.

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