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hard start, poss air in system


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having trob starting A Mack, 2011 mp8 engine, we blocked off all lines to tanks, fuel seperator, and installed 2 new filters, also made sure top and bottom chk valves were working. I only guessing its the primer pump... help

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Check to see if you have air in fuel return, usually you can see this through the tank fill or run the line into a bucket. If you have air in the fuel return with the engine off but the key on then it could be a problem with the "T" fitting on top of the 7th injector. Sometimes the check valves in the "T" fitting get stuck and allow air to return through the fuel system.

Is there fuel around the hand primer pump on the filter stand? When you pump the primer pump, does fuel come out around the little plastic cap? If so, air is entering the fuel system through the little cap in front of the primer pump. Its hard to start because fuel is returning back to the tank because of that little cap. Only cure is to replace the filter stand.
If everything looks ok and you only have air in the fuel return when the engine is running then I'd have to say the injectors are most likely the issue. Just because its hard to start I wouldn't just jump in there and pull the injectors. I have removed injectors only find nothing wrong, then to find that "T" fitting bad. Ouch. Do some diagnostics, do it once and do it right.

Good luck

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  • 2 weeks later...

As Turckster said above, it could be the filter housing/hand primer pump, Tee valve on top of 7th injector (if equipped) , injector cups or something else. Years ago when I was doing somethimes 3 sets of cups a week, we got in the routine of not checking anything and just pulling the injectors out and replacing the cups. Back then about 98% of the time we were right. However those 2% made us look bad. One time I replaced the injectors and cups only to later find that the plastic elbow fitting on the fuel tank was cracked and sucking air, lucky warranty was paying for anything related to the cup/injectors no questions asked.

air in fuel.pdf

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