Fuel pressure diff at altitude

[cumminapart]

This doesn't make sense to me does anyone have any idea why my fuel pressure was reading several psi lower in Denver and up in the mountains than here in Michigan. It's an electronic sending unit. --- I am here: http://maps.google.com/maps?ll=42.474292,-84.887892

[Mopar1973Man]

Colder the temps the thicker the fuel and lower the fuel pressure will be. Like here I se about 5 PSI drop in the winter compared to only a 2 PSI drop in the summer.

[AH64ID]

I agree with Mike, I see about 1 psi at idle different with cold fuel.

[cumminapart]

ok i can wrap my head around thatbut it was in relatively the same temperature and i noticed it as a got closer to denver and my fuel temp was at 100-110 degrees after the truck had been running for closer to 20 hrsi started getting concerned that my lift pump was getting weaker so i cranked it up a bit til it showed 19psi at idle instead of 13-14psi and it was good for awhilethen i noticed a little seeping coming from the sensor and tightened it up a little i got some teflon tape thinking id find some time during the week to take it off and retape it but never got around to it actuallywell once i got closer to michigan my pressure is back up to normali wouldnt suspect that the air pressure difference at that elevation would allow it to seep a little and lose a little fuel pressure would it? or maybe the fuel temp being higher after driving so long allowed it to start seeping through? but that wouldnt make sense because it shouldn't have been back up once i got home to michigan

[Mopar1973Man]

ok i can wrap my head around that

but it was in relatively the same temperature

and i noticed it as a got closer to denver and my fuel temp was at 100-110 degrees after the truck had been running for closer to 20 hrs

i started getting concerned that my lift pump was getting weaker so i cranked it up a bit til it showed 19psi at idle instead of 13-14psi and it was good for awhile

then i noticed a little seeping coming from the sensor and tightened it up a little

i got some teflon tape thinking id find some time during the week to take it off and retape it but never got around to it actually

well once i got closer to michigan my pressure is back up to normal

i wouldnt suspect that the air pressure difference at that elevation would allow it to seep a little and lose a little fuel pressure would it? or maybe the fuel temp being higher after driving so long allowed it to start seeping through? but that wouldnt make sense because it shouldn't have been back up once i got home to michigan

Don't use teflon tape or thread sealants on electric sensor this will foul the ground path and cause issue for the sensor operation. This might be the cause of the wild gauge swings.

Another mechanic confessed to me that he did the same thing on a fuel pressure gauge install and had issues with the gauge behaving poorly or showing zero a lot of times. After replace the gauge and sender it did it again... After we figured out the teflon tape was creating a insulation barrier for the ground path it couldn't ground properly... So low and behold cleaned the threads and tighten the sensor in without sealants and the gauge worked perfect...

[KATOOM]

Hmmm..... I'm not sure how to address this because I'm obviously not the norm here. In the winter, my fuel pressure "increases" about a pound, where as in the summer my fuel pressure "decreases" about a pound. I only attributed that to colder fuel which equals more dense fuel which would equal less pressure drop throughout the system and hot fuel is less dense so causing the fuel pressure to drop a little throughout the system. In saying that, my cold weather is only down in the teens and I have yet to drive in sub zero temps where fuel could gel either so maybe I'd get different results then. As for high altitudes affecting fuel pressure, I dont really know why it would cause your fuel pressure to drop 5-6 psi at idle. Was your WOT pressure staying the same? I've driving up to 5k-6k feet many times and my fuel pressure never changed. Maybe you got some junk in the fuel or maybe your gauge/sender was acting up. Also, I dont use Teflon tape or thread sealant on my fuel fittings but then again, my fuel fittings are all JIC fittings which dont use such sealants. But point being is that I know my fuel pressure sender also has its own ground wire so I cant imagine how many senders utilize the fuel line itself as a ground since most fuel lines are rubber or other non-conductive material.

[CUMMINSDIESELPWR]

This doesn't make sense to me does anyone have any idea why my fuel pressure was reading several psi lower in Denver and up in the mountains than here in Michigan. It's an electronic sending unit. --- I am here: http://maps.google.com/maps?ll=42.474292,-84.887892

its normal, i notice a significant drop in psi when i go from denver to ~10,000 ft or higher, i have heard the physics behind this before but id have to research again one thought it when you set the pressure to "x" at a certain sea level that air pressure is balanced with the pressure setting at that elevation. when you climb altitude the external air pressure drops and the pump now has to pull fuel from the tank harder which takes away energy used to pressurize at the same setting. i tested this and run 19 at idle in denver, got to a high elevation and the fuel psi dropped to like 15 IIRC so i readjusted to 19. when i got back to denver it was around 25 so i had to adjust it back down. think of it like this, our fuel system is a sealed one way flow, put a vacuum on the gas tank (basically what higher elevation is doing) and the lift pump has to work harder i had no leaks when i noticed this and it happens every single time i hit the mountains. when i went to arkansas my psi jumped higher so i left it knowing that it was normal. i think it affects exponentially the higher up you get pump design could be a attributing factor in this as well clear as mud?

[Mopar1973Man]

Next time I go to heavens gate I'll have to pay attention to this and see if its true. I know its not going to happen this year being the road is already snowed closed. Heaven's Gate is the highest point here in Idaho at 9K...

[AH64ID]

Heaven's Gate is the highest point here in Idaho at 9K...

I presume you mean highest road? We can get to about 9,100 at the end of the road where we hunt elk, but I have never noticed a fuel psi drop. Just talking about readings it should go up as the atmospheric pressure is lower and the gauge (liquid with isolator) will have more pressure.

[dripley]

mine does not change goingover the blue ridge mountains. the highest elevation i see is somewhere between 5,000 and 6,000 feet though. maybe its just a rocky mountain high thing. hmmmm.....

[KATOOM]

This thread has given me reason to do some research.....I'm not discounting CUMMINSDIESELPWR theory, or possible reason, but that the more I look into this, the more it seems that there needs to be more questions asked to each of us whom have or have not ever experienced this pressure drop in high altitude. For instance, what fuel pump are you using? Do you use ANY type of fuel additive? Does this happen with the fuel you purchase at high altitude or with the same fuel you run at lower altitude? Do you have any issues with your cooling system running a bit hotter when this is happening? How much fuel is in your fuel tank when you experience this loss in fuel pressure?What I'm getting at is it seems if you do some Googleing, there is a significant similar problem people face with high altitude driving in state like Colorado. Fuel vapor lock is a big one simply because of the increase in fuel boiling points and when fuel is pressurized in the fuel pump, it can cause some vaporization and thus reduce fuel pressure. This is more predominant on oxygenated "winter" fuel.I wont go into a long drawn out post but just mull this over and maybe there is something we need to know about high altitude driving.

[CUMMINSDIESELPWR]

full tank, half tank, temps remain good i run a raptor 150 gearotor pump.

[clutch]

Hot or cold weather my pressure stays the same.

[CSM]

its normal, i notice a significant drop in psi when i go from denver to ~10,000 ft or higher, i have heard the physics behind this before but id have to research again one thought it when you set the pressure to "x" at a certain sea level that air pressure is balanced with the pressure setting at that elevation. when you climb altitude the external air pressure drops and the pump now has to pull fuel from the tank harder which takes away energy used to pressurize at the same setting. i tested this and run 19 at idle in denver, got to a high elevation and the fuel psi dropped to like 15 IIRC so i readjusted to 19. when i got back to denver it was around 25 so i had to adjust it back down. think of it like this, our fuel system is a sealed one way flow, put a vacuum on the gas tank (basically what higher elevation is doing) and the lift pump has to work harder i had no leaks when i noticed this and it happens every single time i hit the mountains. when i went to arkansas my psi jumped higher so i left it knowing that it was normal. i think it affects exponentially the higher up you get pump design could be a attributing factor in this as well clear as mud?

I have seen a lower fuel pressure as welll, however, I have also found that stopping near the continental divide and filling up really doesn't have any influence on fuel boost pressure. I saw about 1psi drop / 1000 ft of elevation change. I never found any relationship between temperature to fuel pressure. The only rationalization that I could come up with was that the fuel pressure gauge was calibrated for absolute pressure, not a relative gauge pressure. I am not sure if that is true, but it was all I could come up with.

[anoldbiker]

This thread has given me reason to do some research..... Fuel vapor lock is a big one simply because of the increase in fuel boiling points and when fuel is pressurized in the fuel pump, it can cause some vaporization and thus reduce fuel pressure. This is more predominant on oxygenated "winter" fuel.

****** I'm with you on your statements of looking into all the items including mechanical. But don't confuse diesel with gasoline. Summer time the max allowable tank pressure is different than winter, per regulations. In summer the gasoline blend is low on the high end vapors. If the pressure summer blend was the same as the winter blend, you'll have 'major' systems pressure issues. In winter the refiners are allowed to slip hydrocarbons in that will pressue the fuel higher, a major one is IC4, better known as IsoButane. Also, there are about 50 blends used throughout the USA, per federal and state laws / mandates. Diesel has very little changes in blends, summer verses winter. That's why only the diesel 1 and 2. Also vapor pressure per volume at rest for diesel is minimal compared to gasoline. A different animal.

[KATOOM]

True, true anoldbiker. There is a clear difference between gasoline and diesel with one being that gas has a much lower boiling point than diesel. So maybe thinking pressure in the fuel pump creating a vapor which could reduce overall pressure is a stupid idea...... I'm OK with that since I really have no idea why anyone would experience lower fuel pressures at altitude. As I mentioned before, I dont ever remember my fuel pressure dropping at 5000-8000 feet but that's not to say that cumminapart didn't.

[CUMMINSDIESELPWR]

i experience the pressure drops every time i go to the mountains. summer winter doesnt matter. it always comes back when i return to denver.It also could be a factor that the pressure change is exponential the higher up you go so from sea level to 5000ft in denver will have lets say 2% change but if you were to go from denver to 13,000ft the change will be %10...just a thought. but i will attest that it does happen to me.

[anoldbiker]

No doubt it can happen. But I was leaning more into the mechanics of it. Like the changes between pulling from the storage tank, the size of the lines, the amount of suction of the pump, any minimum recirculation around the pump to maintain flow and volume, the elevation changes of the lines, kinks in the line external / internal and possibly the distance between the suction line from the tank and the return line distance positioning.

We use to have issues with recirculating bypasses around a pump if the flow rate wasn't restricted to maintain it as a liquid. (these were fixed pumps and lines). If the return lines were to close to the suction inlet, this can affect the system. There are very few lines that are solid liquid, since there is most always a vapor space in the upper portion of the interior of the line.

Not to mention, yes, there can be a difference in the liquid pressure of a hydrocarbon product due to elevation.

I've never experianced this with my vehicle and it's a interesting topic.

--- Update to the previous post...

I'd like to ask those who experience this;

a) is your fuel system stock or changed ?

b) did you notice this before the changes or only after ?

c) if after, after which change ?

Might be a pattern here ?