Mtnmblmech Posted October 28, 2012 Share Posted October 28, 2012 Hello BMT!!-I am in need of some technical help on a 79 Mack 350. I am a mechanic who mostly works on agricultural equipment and smaller diesel trucks. I was recently somewhat conned into helping my old timer neighbor into putting a 79 350 Mack engine into a 73 Mack Dump truck. Needless to say it was a bitch of a project. Considering I'm just doing this for beer, hunting rights, and use of his machine shop..it nearly made me reconsider our deal. Well the engine is in and hooked up the way it should be. Primed it and bled the injector lines and she fired right up and purred like kitten being petted by a 400lb gorilla. When I try to rev it though, it is sluggish and will die if I keep in the throttle. I can feather it back down to idle and keep it from dying when I do it by hand. I have simplified the fuel system to one filter and even put a high flow electric pump from the tank to push some fuel. I know that this should not be needed as it is a siphon feed fuel system, but the beer told me it couldn't hurt to try. My old buddy says that this engine ran fine in the truck it came out of, but I'm hesitant to believe him as he has so many old trucks, engines, parts sitting around, and is getting pretty damned senile; I could see how he could maybe have mixed this engine up with another. I'm thinking that the pump is out of time, the pump is fubar, or I'm missing something simple. The model no. on the pump is 313GC 5135P4. I have rebuilt some old john deere pumps in the past and have a pretty good understanding of how things work, so is there a chance I could most likely handle rebuilding this pump. He does not have another pump that will work from my research, so that is looking like our next option. He does not want to put much more money into this project either. I love the old fart and I would like to see the smile on his crusty old face when I drive it out of the shop. Any help would be very appreciated and thank you for your time. Quote Link to comment Share on other sites More sharing options...
staxx Posted October 28, 2012 Share Posted October 28, 2012 Need to double check your fuel lines should be going into primary fuel filter out of primary to lift pump in correct direction out of lift pump to secondary filter out and to injection pump then return out of pump to tank. At fuel return fitting on pump should be a checkvalve with a ball and small spring this is what makes the fuel psi . This sounds like a fuel psi issue to me welcome to BMT I too lived in MT at one time little town called Colstrip Quote Link to comment Share on other sites More sharing options...
Mtnmblmech Posted October 28, 2012 Author Share Posted October 28, 2012 Thanks for the reply staxx! As far as I can tell this system has no lift pump and is a siphon feed system. This engine is not the original engine that was in the truck and I had thought that it needed to have a lift pump of some kind, but my old buddy said that it did not and was siphon fed. I had mentioned to him that it seems to be a pressure problem. We put a high flow pump on it, but I explained to him that high flow is not the same as high pressure. I imagine that if we put a high pressure pump on it we will see different results. He assures me that this engine ran fine before, but I imagine that unit had a lift pump and was not a siphon feed system. I also suspected that there may be a check valve in the system that would require a higher pressure for fuel system to operate . I will try talking him into springing for a high pressure electrical pump as there is not a mechanical on this engine. I should probably find out what max required pressure is for this pump is first. Any info on this spec would be very appreciated. I'm originally from Miles City staxx, which is not too far from Costrip. Thanks again for all the help it is very appreciated. Quote Link to comment Share on other sites More sharing options...
theakerstwo Posted October 28, 2012 Share Posted October 28, 2012 Like Staxx has said but when done checking i would check fuel pressure to the injection pump inlet fitting and you need some were around 30 psi. I made a tee out of a # 6 swivel and male fitting and taped a thread in the side of the fitting to connect a hose and gauge. I done this 45 years ago and have still got the same fitting and it has been loaned out to coustomers many times.The check valve can be check easy with out the pressure gauge by using some vise grips on the fuel return hose to the tank. Mash it almost closed and see if it runs any better.But who knows it may have a pluged line like mine did a few weeks ago. I found a hard shell bug in the supply like next to the primary fuel filter that had fell in the fuel tank when fueling. glenn Quote glenn akers Link to comment Share on other sites More sharing options...
theakerstwo Posted October 28, 2012 Share Posted October 28, 2012 If it is a inline fuel pump it will have a lift pump on the out side lower towards the front operarted from the injection pump cam. If it is a rostamaster round rotor pump that was not used much then yes you need a electrial pump to push fuel to the injection pump.But again you need a bout 30 psi to the inline pump and that is under power. glenn Quote glenn akers Link to comment Share on other sites More sharing options...
staxx Posted October 28, 2012 Share Posted October 28, 2012 Lift pump is mounted on the injection pump 3 small bolts are holding it on , other option is that it has a pump on tach drive which is below injection pump in engine block this is a low pressure fuel system 7-15 psi as i remember . Have been to the roundup in milescity many times remember the days when they run cattle up the interstate . Quote Link to comment Share on other sites More sharing options...
Mtnmblmech Posted October 28, 2012 Author Share Posted October 28, 2012 I was planning on rigging up some test fittings. I have everything I need somewhere. Just bought a house and seemed to have buried everything I have needed lately. Pump is and inline made by United technologies diesel systems for Mack Model no. is in original post. I did not hook up the pump just removed secondary filter temporily and added high flow pump to try to get it running. I will look to see if it has lift pump under the pump and was not ran through it. As i is the fuel supply line just goes straight into the pump and t's out to the injector line. Which didn't make any sense to me, but the old man says that's how it goes. Thanks for all the help fellas, I will go check her over again and do some testing. Enjoy your Sunday. Quote Link to comment Share on other sites More sharing options...
staxx Posted October 28, 2012 Share Posted October 28, 2012 Should look like thissorry for the sideways pics 1 Quote Link to comment Share on other sites More sharing options...
Mtnmblmech Posted October 28, 2012 Author Share Posted October 28, 2012 Awesome thanks for the pics. I thought it should have a mechanical pump on it, but did not look hard enough and it appears that it is hidden under the pump. This should be easy now..given the pump works. Thanks again staxx. Makes a lot more sense now, as I have never seen a setup that was plumbed the way he had it, but the old guy is hard not to believe as he has grease under his fingernails that is older than me. I will get back to you on how it goes and take care. Quote Link to comment Share on other sites More sharing options...
rsb502 Posted October 29, 2012 Share Posted October 29, 2012 Staxx quit takin pics of your truck till you get the engine all clean and painted, Napa had that paint in stock! Quote "Any Society that would give up a little LIBERTY to gain a little SECURITY will Deserve Neither and LOSE BOTH" -Benjamin Franklin"If your gonna be STUPID, you gotta be TOUGH" "You cant always get what you want, but if you try sometimes you get what you need" Link to comment Share on other sites More sharing options...
staxx Posted October 29, 2012 Share Posted October 29, 2012 Sometimes pictures can explain the problem better then words and I will paint the engine soon still need a few parts , striped and cleaned the inside of super cab today. Quote Link to comment Share on other sites More sharing options...
theakerstwo Posted October 29, 2012 Share Posted October 29, 2012 Should look like thissorry for the sideways pics I dont remenber ever seeing the tach drive pump or have forgot. I remenber two different rotery style pumps used in the pass and one was rostmaster and one was the PSJ so that may be were the tach drive pump was used. The ones i remenber used a stewart warner electric gas pump to push fuel at lest to the engine. Quote glenn akers Link to comment Share on other sites More sharing options...
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