Jcamaraza Posted Thursday at 03:45 AM Share Posted Thursday at 03:45 AM I have a 1996 Mack 688 with a 300Hp motor. Truck looses prime when turned off. Once started with ether runs like a dream with no issues. Had a mechanic come over and is leaning towards a fuel injection pump. However I’m no mechanic but if the fuel injection pump is working fine when the truck is running I’m thinking the problem should be somewhere before the pump. Any information from those who know better would be appreciated. Thank you! Quote Link to comment https://www.bigmacktrucks.com/topic/74928-1996-mack-rd688-300hp-loosing-prime/ Share on other sites More sharing options...
mrsmackpaul Posted Thursday at 07:39 PM Share Posted Thursday at 07:39 PM You have a fuel leak in the return to tank side of the fuel system Now the tricky part is you most likely will not see this leak because it isn't under any pressure I have found a piece of inner tube over the fuel filler hole with a fitting to hook the shop air to is of great help Wind the regulator down as low as the compressor goes, maybe 20 psi and put some pressure on the fuel system This forces any fuel out any leaks that are there and enables you to find them Not very likely to be a injector pump problem, as you have already said Paul 1 1 Quote Link to comment https://www.bigmacktrucks.com/topic/74928-1996-mack-rd688-300hp-loosing-prime/#findComment-565208 Share on other sites More sharing options...
Joey Mack Posted Thursday at 07:44 PM Share Posted Thursday at 07:44 PM (edited) Dont go near replacing the injection pump.. that is way to far out there for this issue. Did he even try any type of diagnostics? I assume it doesnt have a hand primer. Any chance there is a Davco 382 fuel filter on the truck. MrsMackpaul is teaching you something. Put the ether in the mower shed... Edited yesterday at 12:30 AM by Joey Mack Pour sell fone sperring 2 Quote Link to comment https://www.bigmacktrucks.com/topic/74928-1996-mack-rd688-300hp-loosing-prime/#findComment-565210 Share on other sites More sharing options...
Straydog954 Posted 22 hours ago Share Posted 22 hours ago I had the same problem. The fuel return jumper between the heads was loose on mine Tightened it down and problem solved. I was told to also replace the rubber o rings in it also 1 Quote Link to comment https://www.bigmacktrucks.com/topic/74928-1996-mack-rd688-300hp-loosing-prime/#findComment-565326 Share on other sites More sharing options...
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