finkexcavating@gmail.com Posted April 30, 2016 Share Posted April 30, 2016 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? Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/ Share on other sites More sharing options...
theakerstwo Posted May 1, 2016 Share Posted May 1, 2016 With 21 psi of boost I who say your engine is about right.That is a VMAC system Quote glenn akers Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332006 Share on other sites More sharing options...
fjh Posted May 1, 2016 Share Posted May 1, 2016 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! 1 Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332065 Share on other sites More sharing options...
finkexcavating@gmail.com Posted May 1, 2016 Author Share Posted May 1, 2016 I have checked for leaks I resealed at the turbo, I don't have a pyro on it, little to no smoke, if I have a leak I haven't found it yet, thought maybe having Injectors check, but thought worth asking for help here first, Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332111 Share on other sites More sharing options...
theakerstwo Posted May 2, 2016 Share Posted May 2, 2016 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. Quote glenn akers Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332118 Share on other sites More sharing options...
finkexcavating@gmail.com Posted May 2, 2016 Author Share Posted May 2, 2016 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 Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332131 Share on other sites More sharing options...
theakerstwo Posted May 2, 2016 Share Posted May 2, 2016 You say you have checked for boost leaks but again it needs checked with air pressure with all of the plumbing with it so you can look for a blown hose some were.Do that with about 25 psi. Quote glenn akers Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332133 Share on other sites More sharing options...
finkexcavating@gmail.com Posted May 2, 2016 Author Share Posted May 2, 2016 I will check it out using 25 psi, I had not done it that way, thanks Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332134 Share on other sites More sharing options...
theakerstwo Posted May 2, 2016 Share Posted May 2, 2016 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. Quote glenn akers Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332135 Share on other sites More sharing options...
fjh Posted May 2, 2016 Share Posted May 2, 2016 (edited) 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 May 2, 2016 by fjh 1 Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332145 Share on other sites More sharing options...
Mackpro Posted May 2, 2016 Share Posted May 2, 2016 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. Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332148 Share on other sites More sharing options...
finkexcavating@gmail.com Posted May 2, 2016 Author Share Posted May 2, 2016 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 Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332178 Share on other sites More sharing options...
finkexcavating@gmail.com Posted May 2, 2016 Author Share Posted May 2, 2016 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, Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332183 Share on other sites More sharing options...
finkexcavating@gmail.com Posted May 4, 2016 Author Share Posted May 4, 2016 What would happen if my air compressor was getting weak, could my boost pressure be low by going through it, instead of into the motor? Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332370 Share on other sites More sharing options...
fjh Posted May 5, 2016 Share Posted May 5, 2016 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! Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332409 Share on other sites More sharing options...
finkexcavating@gmail.com Posted May 5, 2016 Author Share Posted May 5, 2016 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, Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332432 Share on other sites More sharing options...
theakerstwo Posted May 6, 2016 Share Posted May 6, 2016 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. Quote glenn akers Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332443 Share on other sites More sharing options...
fjh Posted May 6, 2016 Share Posted May 6, 2016 yup amen on that one! Post what if anything you find When you get done then people can send you in the right direction! Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332458 Share on other sites More sharing options...
finkexcavating@gmail.com Posted May 6, 2016 Author Share Posted May 6, 2016 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, Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332477 Share on other sites More sharing options...
fjh Posted May 11, 2016 Share Posted May 11, 2016 Yup Post your fixes so others may benefit by your gained knowledge Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332949 Share on other sites More sharing options...
keg1 Posted May 12, 2016 Share Posted May 12, 2016 I had a 2000 Ch with a 400 and it was pretty sick. Very dependable but not much on a hill. My 89 R with a 350 would run circles around it. Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-332993 Share on other sites More sharing options...
finkexcavating@gmail.com Posted May 15, 2016 Author Share Posted May 15, 2016 Did you find out the reason why it was weak? Or did you just run it Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-333384 Share on other sites More sharing options...
keg1 Posted May 15, 2016 Share Posted May 15, 2016 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. Quote Link to comment https://www.bigmacktrucks.com/topic/45043-low-power-low-boost-pressure/#findComment-333468 Share on other sites More sharing options...
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