Low power when low fuel - multiple choice.

So I've been chasing a low power situation forever, and it seems that I've finally figured that it happens more often (maybe always?) when the fuel is low, like less than 1/3 tank.

I've done some research and have a multiple choice question for y'all.

Which one of these is the cause?

1. When the fuel is low, the pump heats up and runs less efficiently since it's not submerged in gas that will cool it. OR, the fuel heats up because some of it is cycled to the fuel rail, warmed by the engine, and recirculated by the FPR back to the fuel tank. The engine runs less efficiently on this warmer fuel. Either way, nothing I can do to fix it.

2. The fuel pump is old and weak, and maybe runs less efficiently when it gets hot due to low fuel. I should change my fuel pump.

3. There is water in my 20 year old gas tank, and when the tank gets low there is a higher percentage of that water in the fuel, and that makes the truck lose power. OR, there is crap in the tank or a clogged sock or a rotted pickup hose, and that problem is worse when the fuel is low. Clean out my tank and it will fix it.

4. Something else is wrong that is totally unrelated to fuel, like low compression in cylinders 2 & 3 (a fact). Only why would this make it run worse when the fuel is low?

 

the loss of liquid in a closed system is creating a vacuum that is not being vented by a bad gas cap? and the vacuum is causing the pump to have a hard time pulling fuel out. open your gas cap and drive again. see what happens.

 

That won't work. All that will do is turn on your check engine light. If you got a bad gas cap, the light will already be on.

Still thinking on the loss of power on low fuel level. Interesting.

 

if taking off the fuel cap turned on the check engine light, everyone would have a check engine light after refueling. and a bad gas cap can go both ways. it could be allowing too much air to escape. or it's not allowing any air to pass either direction.

 

If it was me....I would go ahead and change out the fuel pump and filter and go from there. Are you getting any P300 codes like if it had low fuel pressure? Check the fuel pressure.

 

I think finding out if the fuel pressure is in fact falling off first. Going to need to tee in a pressure gauge or just change the fuel pressure regulator. They are cheap.

 

$83 is cheap to you?

*edit*

oh wait, he has a 2.7. theyre only 40 bucks. mine for the 3.4 is 83

 

I'm curious what you mean by low power? How low? Anyway, I say fuel pump. The gas in the tank lubricates the bearings of the pump. It's not the pump heating up you gotta worry about but those bearings. Maybe some of them are slightly warped?

I've noticed some vehicles seem to have more power immediately after filling up. This sounds like bs because how could added weight make a vehicle faster? I dunno but I've noticed it with several vehicles. So it doesn't seem that odd that your having these issues.

Oh yeah I almost forgot but sometimes when fuel pumps go bad you can physically hear them. My old Lincoln had this problem. I dunno if this applies to Toyotas but check it out.

 

Try not letting the tank go below 1/2 tank see if that eliminates the issue .

I always fuel around 1/2 tank on my vehicles

Then without fuel pressure numbers your just guessing

Bearings /Fuel pump are pretty much the same thing to me all the same package

 

Thanks for the input, guys. No codes. Fuel filter is relatively new. I do not hear a big hissing from the gas cap when I remove it, but I will try running with it off for a bit to see if that helps. Filling the tank when it gets 1/2 empty is not really an option - I drive a lot and would spend wayyy to much time at the gas pump if I only ran it down 1/2 way.

I have tested the fuel pressure when the tank was full. Pressure was fine. I don't think it's a matter of pressure, I think it's the volume of fuel that the pump is able to deliver is lower than it needs to be in some situations.

The symptom is loss of power. For example, having to gear down one more gear to get up the same hills at the same speeds. So a hill that I can cruise up at 65 in 4th gear on a full tank I am having to drive up at 50 in 3rd gear on 1/4 tank.

 

Just throwing this out there.....but you said your fuel filter is relatively new. Maybe that is your problem.

 

The problem existed both before and after the new fuel filter, so I'm pretty sure it's not the cause.

 

Good luck finding the issue. You seem to have a handle on it.

 

I'd also go with the gas cap as the first step, it sounds like a venting problem to me. The fuel pump will still be cooled even on an almost empty tank, as long as there is fuel you should be fine in that aspect. But excess vacuum may cause an issue. And it's easy enough to do a quick pop of the cap and see if it resolves the issue. Or just leave the cap loose and see if the problem happens at all. The CEL for a bad (loose) cap won't put you in limp mode, you'll just have the light. My wife's tracker throws an emissions code from time to time, 100% fix on tightening her fuel cap.

If it works you save a lot of time and money on a new fuel pump. If it doesn't, you're not out anything and move to the next step of troubleshooting.

*That doesn't* mean that your fuel pump isn't getting weak at the same time. It may just lack the oomph to overcome the excess vacuum, if indeed the vacuum is a problem. A little circular logic

 

Meh, worth the shot. Very cost effective. I am curious how much fuel you had when you did this test? Oh and I shoulda pointed out that you're not looking for a hissing sound but a kind of buzzing/whiring sound. When mine went bad (in the Lincoln) you could clearly hear it walking up to the car. A buddies van was the exact same.

Wow! Yeah that's a pretty big loss. Is this power loss just as bad on flat ground as on hills? If hills are worse then that's more damning evidence of the pump. That being said, I never change parts until an issue is completely diagnosed (ironically this can be costly!). I especially encourage proper diagnosis for this because obviously swapping fuel pumps ain't like changing oil.

That being said I think your assessment of vapor or whatever in the tank may be just as close to the mark as the pump. I just don't know, never ran into this problem before. Again, I wouldn't got out and buy anything but if you have a can of Seafoam or something similar hanging out maybe dump that in the tank. I'm not suggesting the full Seafoam procedure.

I have several books from when I took auto repair at a local community college, I'll look up fuel pumps later tonight. I gotta work on my Tacoma today and I'll check out my fuel pump. I think they do buzz even when in working order it's just quiet.

Being a new guy here I wanna point out that I'm not a mechanic. That being said I've never gone to the mechanic (in the past 10 or so years, I'm 30 now) unless it's tire mounting/balancing/aligning or something like that. And after doing the repairs I did on that Lincoln the Toyota is a real pleasure to work on. Sooo much easier.

 

Here's more data, all for the same hill:

-Yesterday morning, 100% gasoline, 2/3 full tank, 65mph in 4th.
-Later that day, 100% gas, 1/3 full tank with gas cap on tight, 50mph - 3rd gear.
-Today, 100% gas, about 1/8 tank, gas cap loose, 50mph in 4th gear.
-Today, gas with ethanol, full tank, gas cap tight, 54 mph in 4th gear.