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How to sample fuel in a car

[I was surprised that my search of the forums turned up no prior discussion of this matter. If I missed such, please point to the prior discussion.]

How can I get a sample of the gasoline in my car?

In older cars, I could simply siphon a sample from the gas tank through a hose inserted at the fuel port.

AFAIK, anti-theft devices and fuel evaporative control systems prevent that method working in modern cars.

I have access to the schrader valve on the fuel rail above the engine in the engine bay.

Though I am tool proficient (especially with electronics), my automobile knowledge is spotty.

With difficulty due to nearby obstructions, I can place a collection cup of ~1oz under the schrader valve, but cannot easily insert a separate stick to open the schrader valve.

Is there a normal way to sample fuel from modern stock cars?


Is there fitting/tool to get a

sample of fuel from the schrader valve?

Where can I get one?

Does the engine have to be running to get fuel from the schrader valve?

I understand that pressure is maintained in the fuel lines for hours after the engine is shut off, so that a sample can be taken from the shrader valve after the engine has been shut down and cooled off.

Is this correct?

What safety measures should I be aware of?


Why do you want to sample the fuel, are you worried there is water in it or what? The less contact you have with the fuel the better, that is part of why it is so hard to get to. If you must get to it, wear double rubber gloves of the dish washing type. Crack the fule line under the car at the filter or gas tank, and drain few drops into a glass container, and close everything back up tight as soon as possible. I dont know why you want to do this. In 40 years of working on all my cars I have never once had to take a sample of the fuel.

Race cars are equipped with quick release valves to test fuel mix for performance and legitimacy.

Aviation fuel is routinely sampled and examined for water, dust, … before every flight
from standard quick release valves.

Is there a fitting/technique for such sampling from a stock automobile?
Given the standard nature of the Schrader valve, is there a standard quick release fitting available or standard sampling technique?

No. There is no such ‘quick release’ system to sample the gas in any factory made cars.

You can sample the fuel from the Schraeder valve with a Fuel Injection Pressure Tester equipped with a pressure release valve. My Actron tester has a pressure release valve and a plastic tube that allows the released fuel to be collected away from the engine. Here’s the one I have available at Autozone:

You can sample the fuel from the Schraeder valve
with a Fuel Injection Pressure Tester equipped with a pressure release valve.

Though I’ve not yet had opportunity to use it,
that seems to be the ‘fitting’ required to safely sample fuel.

Thank you,

You can either use a short hose with a Schrader valve depressor or use a tire valve stem tool to remove the Schrader valve.
In the case of the latter simply fit a rubber hose with clamp over the valve fitting.

Route the hose into a container and use a jumper wire on a couple of fuel pump relay terminals after the relay is removed or by powering up the pump test lead. (depending on make of car)

To clear the fuel lines remove the pump relay and start the car. After a few seconds it will die due to the fuel pressure going away.
And as a precautionary note, keep everything coverer with a few rags and do not allow gasoline or gas fumes near any relay jumper wire.