Piper Cherokee 140 low fuel pressure with full tanks

Patrickjmars

Patrickjmars

Filing Flight Plan
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Mars83R
Has anyone experienced low fuel pressure after topping off the tanks in PA-28-140? My aircraft generally sits at 5-6 PSI on the fuel pressure. After topping of the tanks I have often seen the pressure gauge read 1-2 PSI. Turning on the electric pump will cause it to rise to normal levels. I have never seen the low fuel pressure unless the tanks were full, but does happen on both left and right tanks.

This never caused me much issue since the engine ran well and the pressure was always “in the green”. Until today, (80 degrees) climbing to 4,500. Pressure dropped to almost zero and engine ran rough. We turned the boost pump on and landed at the closest airport. We got out opened all the fuel caps, inspected the fuel pump and fuel vents. No leaks or or clogs were found.

We started back up and continued home with no issue. I’m curious is this is a venting issue with full tanks and changing air pressure at higher altitudes or if it’s a possible bad engine driven fuel pump?

1967 PA-28-140 N8483R.
 
caveat: I'm not an A&P, just a (sadly, former) 140 owner.

inadequate venting coupled with a engine fuel pump that isn't quite up to snuff?

(or maybe it's simply a bad fuel pump)
 
Hard to tell. On my Cherokee, the fuel pressure indicates low some days and higher on others on the ground. I couldn't really find a correlation for when it indicates low vs. higher. The checklists says you can turn off the electric fuel pump at 1,000 AGL. When I do that, fuel pressure looks kind of low although the engine keeps running fine. Since I don't like seeing the pressure indicator that low on climb out, I now simply leave the electric pump on until I reach cruising altitude. Once in cruise, fuel pressure indicates normal.

When you turned the boost pump on today, did the engine run smooth again?
 
Not a mechanic. But pretty sure there are two vents in each wing on a Cherokee. One under the wing, little tube. The other in the cap itself. If both of those are plugged, you have a problem. I've talked w/ pilots who were unaware of the cap vent. But...if it's a vent problem, the tanks are independent as far as I know for venting. So if the low pressure thing happens on both tanks, it would lead me to believe it's unlikely to be a vent problem. But possible. All that said, I wouldn't be comfortable flying a Cherokee that didn't give good pressure on just the mechanical pump.

I don't know how to check those vents, but that's what I'd do first. Maybe they're all partially plugged.
 
I doubt that the vents are blocked since it does it on both tanks like what was mentioned above. Easy way to check the vents is to slip a 4-5' hose over the vent tube and blow with your mouth and only your mouth. Take the fill cap off and listen also.
Atmosphere pressure and temp affect fuel pressure so your fuel pressure will vary some. But it does sound like the motor driven fuel pump is having a problem and needs to be inspected before next flight. I would guess your AP is servicing the fuel filter?

Typically the vents in the fill caps are emergency vents, they act as a back up. The caps are usually pressure vacuum caps meaning they need a little pressure or vacuum to open that way they don't leak gas or vapor under normal conditions.
 
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I’ve been in a 140 with a similar issue. Seemed to be a hot day / mogas issue.
It’s possible it may have been “ Winter Blend” mo -gas in the tanks in my case. Winter Blend has a higher RVP ( Reid Vapor Pressure) which is the ability to evaporate or vapor lock.

My method of checking vents.

Blow in the underwing line . A piece of tubing may help here. Note there is a small
hole drilled in the vent line you will want to cover. As you blow in you can hear and feel the tank expand. If you can’t blow in the tank the line is likely plugged.
When you stop blowing in in the Tank you will hear the air leaving the Tank and smell fuel vapor. This tells you all is well. If you leave the Caps off you or a helper can see vapors leaving the Tank. Will not have vapors leave vent line in this case.

DO NOT USE COMPRESSED AIR TO DO THIS.
 
If you notice times for #6 and #7 there is clear evidence that I don’t type fast enough!
 
Thank you for all the advice thus far. I checked all the vents today using the “blow” technique, and found that they were all clear. The gascolator has been cleaned at annual (April 2022) but I have been flying a lot since then.

I’m leaning more and more towards the engine driven fuel pump, which is 25 or more years old is the culprit. I found one in town and my mechanic and I will be replacing it and cleaning the gascolator next week.

I’m still a little suspicious since this only happens with full tanks, and it was indeed very hot and high DA. I will keep the thread updated with what happens after the fuel pump replacement.
 
Hotter it is the harder it is to suck gas into a fuel pump without it turning into bubbles which is commonly known as vapor lock. Fuel pumps can’t suck bubbles also know as vapor. That happens when there is a restriction or marginal size line on the suction side of the pump. Hot temperatures tax fuel delivery systems and may perform marginal in hot weather.
The fuel pump cam or cam lever could be worn reducing capacity and you only noticed it when the fuel pump has to work near 100% in hot weather.
25 years old is nuts, you know that right?

Check your hoses, can’t have any leak at all on the suction side of your fuel pump.
I noticed on my buddies Cherokee the electric pump has it own intake air duct to keep it cool.
 
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Blast tubes connected?

Left lower forward side of Firewall is the Fuel Strainer and Electric Pump.
They are enclosed in a Shroud Fed from a 2 “ Cooling Blast tube. This SCAT line must be disconnected to drop the lower Cowl. Disconnected?

As Challenger said; the Engine driven pump also has a Blast Tube.
Some early Cherokees did not have this feature. Methinks Piper added this for a reason. Vapor Lock?

I have seen small fuel stains on the run of tubing on the left side of cabin ; under the upholstery panel. The stains were nowhere near any connection. When these were addressed the fuel pressure was more stable. Sucking air? It seems it would be worse with LOW fuel level.
 
Magman said:
Blast tubes connected?

Left lower forward side of Firewall is the Fuel Strainer and Electric Pump.
They are enclosed in a Shroud Fed from a 2 “ Cooling Blast tube. This SCAT line must be disconnected to drop the lower Cowl. Disconnected?

As Challenger said; the Engine driven pump also has a Blast Tube.
Some early Cherokees did not have this feature. Methinks Piper added this for a reason. Vapor Lock?

I have seen small fuel stains on the run of tubing on the left side of cabin ; under the upholstery panel. The stains were nowhere near any connection. When these were addressed the fuel pressure was more stable. Sucking air? It seems it would be worse with LOW fuel level.
Click to expand...

SCAT hose seems to still be attached. Not seeing any fuel stains on inside near fuel selector either. My engine driven fuel pump does have the blast tube as well.
 
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