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TimberJack

2008 MP8 hard to start.

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Truck is losing its prime within 5 mins of cutting it off. I ran a clear hose from the back of the head to the filter stand. I didn’t see any bubbles while run. I shut the truck off and you can watch air bubbles come up from the filter housing. Im not sure if it could be leaking from cups. Some times you can be in a pull and it will start to skip then clear up, it will also do it just sitting at an idle. Any pointers would be great. Thanks 

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After idling for about 5-10 min check in each fuel tank where the fuel returns inside the tank. If there is bubbles in one or both tanks then the cause is most likely injector cups. To make sure, check the small plastic cap at the fuel filter base (next to the primer pump) is not wet from fuel leaking around it. If so, the problem is the cap---leaking o-ring sucking air into system. You can get a replacement cap from Mack.

If ok make sure 7th injector air purge "t" check valve isn't leaking air back to fuel system. Remove the fuel line from the "t" fitting, turn key on and there should be no air coming out "t" fitting.   Some '07 emission engines had the air purge line, some didn't .

If both above items look ok then most likely you have injector cup issues. 

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what he said!😡

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Ok thanks for the input. Headed to check it out now. 

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looks like cup problems, ran clear hose on the return side and lots of bubbles. 

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On 2/2/2019 at 4:44 PM, TimberJack said:

looks like cup problems, ran clear hose on the return side and lots of bubbles. 

Yup, that's cups. 

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I have run into this twice recently and both times it was a bad injector. One is sticking open I believe therefore leaking down quickly. Both of the trucks I did, both ran great no drivability issues and no air bubbles indicating cups. There were no injector codes . Cylinder balance test will show it being off some but not really enough to condemn it on that alone.

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