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ENDT864 PSM Rotary Pump advice needed, please

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Wonder if anyone can help (or suggest someone who might know the answer) with my injection pump ?

1970 Mack with the ENDT864 engine (am aware not one of Mack’s finest engines..)


AMBAC Rotary PSM pump (I know, not a brilliant pump as well…)


Problem: When cranking engine, it goes from ‘not firing’ to ‘running away’ at 1400rpm with the lightest of throttle movement.


Recent work done:

1) Pump is overhauled (in the UK by person familiar with the PSM pump), is refitted and all works fine.


2) Engine then just stops running after about 15 mins of running on tickover.

However, the injection pump’s ‘lift pump’ was a known problem as not able to find overhaul parts and the one fitted was the ‘best of two’, so suspected that as the issue.


3) Bypassed lift pump with electrical and all running again straight away, just fine.


4) Found overhaul parts in the States for lift pump and had one of them overhauled and tested here in the UK.


5) Refitted lift pump (now works fine) and, at same time, overhauled the throttle linkages / bushings and also ‘reworked’ the throttle rod to avoid a new fuel line fitting.


6) Have removed the governor cover to checked that the throttle linkage ‘movement’ is in sync with the governor, and all appears correct.

NB: Mack used to have a PTO, so we have a dash ‘fast idle’ pull lever which allows me to fine tune the tickover.  This pull lever is NOT at issue here – indeed it is useful to incrementally adjust the tickover


My questions are;


1) Could I have refitted the lift pump incorrectly that would cause the engine to ‘race’ ?

(or inadvertently moved something)


2) Viewing inside the pump once the governor cover is removed, I can see the movement inside the pump actuated by the throttle lever – which looks fine.

But can I view / check  this with movement with engine running and the governor cover removed ?

Or will I get oil squirting out ?


3) Or could it just be coincidence that something else in the pump may have failed to make it ‘race’ ?


Any thoughts / comment welcome.





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just found a posting on you tube from 5 months ago Mack END 864 injection pump from jobbers inc. Philadelphia Pa. they have 2 rebuilt pumps that are ready to go a PSM and a PSJ there may be someone there that knows what your trouble is

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