fass fuel pump stops working at high elevation
 

my fass fuel pump stops working when i get to higher elevations but runs fine when im down in town runs like a champ could it be going out or do i need to replace the brushes in the pump need help????

How much of a difference in elevation are we talking?

from about 700 ft to about 2,000 to 3,000 ft changed all my filters and its still doing it but when it stops working when im driving i can pull over turn my key on cycle it and itll run but once i start goin down the rd like a mile it quits workin again u cant hear it in the cab

3,000 is far from "thin air" and I can't see how the slight difference in atmospheric pressure would cause your lift pump to stop working. You should probably start thinking electrical problems or an issue with the pump itself. Call FASS and describe your issues to them, they may have a workaround.

In MT I run at 5,000+ ft above sea level 24/7, and I never have issues when I drive to lower elevations (1,500 to 2,000 ft).

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To be a little more specific:

At 700 ft above absolute sea level, you are at 99 kPa (Kilo Pascals) meaning you have approximately 98% oxygen availability per atmospheric unit.

At 3,000 ft, you are at 91 kPa, with 90% oxygen availability.

Barely enough of a difference to make a carbureted engine hiccup, let alone an ECU regulated modern engine.

ya ive never had this happen before i snowmobile all winter go from elevations ranging from 1000 ft to 4500 ft with my truck and never has had a problem but it jus started acting up when i head outta the valley to the mts runs great in town

Another possibility is not the elevation change, but the angle of your truck. Does it happen on a full tank just as regularly as a half empty or quarter tank? If the FASS pump is unable to siphon fuel from the tank it may be starving the engine. Do you have any fuel rail sensors that would give you an idea of how much fuel you are getting to the injection pump in the first place?

I'm just thinking, driving around in town = fairly flat and little to no hills... driving up the mountains = 6% grades or better, here in MT anyway. So that may be another possibility.

I am at 8600 ft and my fass works great

You can always call the FASS Technical Support Department and they can walk you through several checks to narrow the problem down. The number is 1-866-769-3747.

ok guys i think i might have it narrowed down to the old relay for the stock fuel pump whoever installed my fass ran through that then to the pump i was gettn 12 V on the line comming into it but i didnt have my other meter to test continuity on the ice cube relay i messed with it a little bit went and ran it and it ran fine for couple miles then barely was runnin when i pulled back up to the house it just didnt sound like the pump was running full bore like it was when i left the house.