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Carburetted Piper Cherokees: aux fuel pump for cold starts?

Since I had a bit of a hard time starting the Cherokee 180 yesterday, I started to wonder about the aux fuel pump.

According to the (50 year old) owners handbook:

No mention of engine starting.

In the checklist section, it however shows aux fuel pump “on” before engine start.

Yet, in that manual, they never advise to switch it off after starting, and then back on before takeoff (as they do in later POHs, say for the Archer II). I actually dislike that, since if followed strictly, it means not checking at all whether the engine will run on the engine driven fuel pump alone.

Anyway, that’s what the “book” says, and leads me to believe that the only reason for switching it on before starting is to not have to do it before takeoff.

Got me wondering: does the aux fuel pump on these carburetted Cherokees actually do anything to aid in cold starting? (I obviously has a manual primer for priming the engine.) Does someone know?

Last Edited by boscomantico at 17 Jan 13:30
Mainz (EDFZ) & Egelsbach (EDFE), Germany

boscomantico wrote:

Anyway, that’s what the “book” says, and leads me to believe that the only reason for switching it on before starting is to not have to do it before takeoff.

I have never tried to start a carburetted PA28 with the auxiliary pump off, but I don’t think it will work. Since the PA28 is a low-wing aircraft there must be a pump to feed fuel to the engine. The engine-driven pump obviously doesn’t work before the engine starts, so…

ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

The engine-driven pump obviously doesn’t work before the engine starts, so…

Shouldn’t the EDP run as soon as the starter turns over?

always learning
LO__, Austria

Our Archer III POH wants the electric fuel pump on for start, only turning it off briefly during run up before turning it back on again before takeoff, however our checklist has electric fuel pump off for start (honestly, I don’t know why the difference, I’ve never questioned it), so we only turn it on before takeoff. We have never had any issues starting with it off.

United Kingdom

Snoopy wrote:

Shouldn’t the EDP run as soon as the starter turns over?

Yes, but will you get enough fuel pressure at such low speeds? Or maybe the engine will run long enough on the priming fuel to build up pressure. I must try someday.

ESKC (Uppsala/Sundbro), Sweden

I have never tried to start a carburetted PA28 with the auxiliary pump off, but I don’t think it will work. Since the PA28 is a low-wing aircraft there must be a pump to feed fuel to the engine. The engine-driven pump obviously doesn’t work before the engine starts, so…

They often will start even without the pump on.

Also, why would the book (snippet in first post) not mention that the pump also aids in starting, if they go the length to mention the other purposes?

Interesting that the Archer III manual calls for starting without the pump on.

Mainz (EDFZ) & Egelsbach (EDFE), Germany

The highwings otoh have a fuel pump only for starting.

always learning
LO__, Austria

boscomantico wrote:

Also, why would the book (snippet in first post) not mention that the pump also aids in starting, if they go the length to mention the other purposes?

Interesting that the Archer III manual calls for starting without the pump on.

The Archer II manual does, too.

ESKC (Uppsala/Sundbro), Sweden

boscomantico wrote:

Got me wondering: does the aux fuel pump on these carburetted Cherokees actually do anything to aid in cold starting? (I obviously has a manual primer for priming the engine.) Does someone know?

Yes, all the electric fuel pump does before starting is fill the carb float bowl, which is partially empty due to evaporation. Once that is done it can be shut off without effect and the engine would run on the fuel in the carb for a minute or so without any additional fuel supply. By that time the engine driven mechanical pump will be supplying fuel and keeping the float bowl full. There is no difference in any of the engines and low wing fuel systems described, they all work the same way.

Old cars that use the same type of cam-driven mechanical pump don’t have the electric pump and so when starting an engine that hasn’t run for a while they have to be cranked on the starter until the mechanical pump supplies some fuel to the carb. This will also work on the plane engine, but it provides more immediate startup to crank the engine with the carb already filled by the electric pump. Sometimes on a car you might pour a little fuel into the open mouth of the carb instead, which is in effect like starting an aircraft engine on the primer while cranking with the starter motor.

Last Edited by Silvaire at 17 Jan 16:30

Airborne_Again wrote:

The Archer II manual does, too.

I just checked. Their Archer III (mentioned above) actually doesn’t.

But it does have the same text in the “Sytems” section as the Cherokee 180 manual.

Mainz (EDFZ) & Egelsbach (EDFE), Germany
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