The Stovebolt Site Forums The Shop Area Engine Shop Fuel pump test

I’m having a problem with my 67 c10 cutting out & dying when driving it. I’m pretty sure it’s fuel related. I would like to check the fuel pump to see if it’s pumping enough go juice. I know you disconnect the fuel line and have something to catch the fuel as you crank it over. My question is how long should I crank & how much fuel should I collect in that time frame. Oh yeah it’s a 250 inline 6 with mechanical pump.

I have wasted literally billions of electrons explaining how to test a fuel pump properly. If you actually follow the procedure I'm about to repeat for probably the 15th. time, you will be the only one who will admit to having done so in the past 20 years or so that I've been hanging out here.

Disconnect the fuel line at the carburetor and attach a length of rubber fuel line long enough to reach over to the fender, or somewhere similar. Start the engine and let it idle- - - - -it will run on the fuel in the float bowl for a while. In 30 seconds at idle, it should pump at least a pint of gas. Now you have done the first of three tests - - -don't stop now. Attach a pressure gauge to the line you removed from the carburetor- - -clamp it if necessary. Run the engine for a few seconds. You should see somewhere between 2 and 4 pounds of pressure, and this is important- - - -the pressure should stay there for several seconds after you shut the engine off. There is a one-way valve in the pump that prevents back flow. Now, attach a vacuum gauge to the inlet side of the pump. Run the engine. You should see 10 inches of vacuum or so, and it should also stay put for a few seconds after shutdown. If the pump can pass the pressure and vacuum tests, but is low on volume, start looking for a restriction between the pump and the fuel tank, or a clogged pickup tube in the tank. It's also possible to have an air leak in the flex line between the frame and the pump that will suck air but won't leak gas. Good luck!
Jerry

Thank you, again, Jerry

And if your mechanical pump is broken, you can get what you need to rebuild it (safe for ethanol fuel) from these people: http://www.then-now-auto.com/fuel-pump-repair-kits/

But if you want to give up on the mechanical pump (as many have done), here is one I've had good luck with (output is the right psi...about 2.5):
https://www.amazon.com/gp/product/B00496RE82/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1

Be certain if you switch to an electric fuel pump to mount it back close to the fuel tank. Those were made to push...not to pull. Good luck.

Hot rod Lincoln, thanks for the information. I have a vacuum gauge but no pressure gauge. I will probably just take the money one would cost and just buy a pump. If the one that’s on it isn’t from 1967 it’s still over 10 years old which is how long I’ve had the truck.

Jon, didn’t know the manual pump could be rebuilt. I like the idea of the electric pump, is under the cab close to where the fuel line comes out of the cab the best location?
I’m assuming you still run the fuel through the manual fuel pump with the electric?

If the carb turns out to be part of the problem what’s a good source for a rebuilt one or a good kit?

I can't imagine any shop not having a pressure/vacuum gauge- - - -it's something every tool box should have.

https://www.amazon.com/DuraChoice-U...nid=16310161&s=industrial&sr=1-9

Finding a rebuildable fuel pump these days is going to be difficult - - - -most of them are crimped together without any screws.

Jerry

If your pump is rebuildable (there will be screws around the perimeter) and you want to keep the mechanical pump, the folks at "then and now" are who I'd suggest dealing with. I wouldn't waste even a dime on a new fuel pump made in China, Argentina, Egypt, India, etc. They won't last for long. For a high quality rebuilding kit, please call the other Jon (CarbKing) on Monday or Tuesday. (573) 392-7378

Yes, back under the cab close to the fuel tank is the best place. I'm certain you know this already, but one of the advantages to an electric pump is quicker starting (don't have to crank the engine while it fills the float bowl). And they are not expensive. Just be certain not to get one of the high pressure output pumps. I keep a spare under the seat in case mine fails...but the one I replaced not long ago had been working since 1996. No, I did away with my mechanical pump for another reason. I have a 235, and I wanted to install a more modern oil filter, so I ran the plumbing under the crank and out that hole. You can see below:

I didn't know a fuel pump could be rebuilt until a small AD school bus, which had been converted into an RV, pooped out in front of our house. The old timer got out and pulled his new fuel pump. A check valve had been put in backwards.

I wish I had a picture of that AD RV. It was cool.

Hot Rod Lincoln, looked at my gauge this evening. It is a vac/pressure gauge. Don’t use it very often and only as a vacuum gauge. Got it several years ago to track down problems with the vacuum system on an old Mercedes diesel I used to own.

Here is another way to test without gauges. Tells you the same thing in 1/2 the time and you only need hose, a gas can, a water bottle and a screw driver.
If fuel pump is mechanical:
Take center wire off of distributor so it won't start. Run a hose from a good, clean gas can near front tire, on the ground, into pump inlet. Run the pump outlet into a container, like a water bottle. Crank motor. Should see pulsing good flow. If you get a good flow, hook tank hose back up and leave outlet into a bottle. Crank motor. If both test show good flow, it's the carb.
If one test fails and the other does not, you identify a specific direction to follow.

Good flow in both tests eliminates:
1. Pump
2. Lines
3. Filter
4. Clogged tank pick up
5. Tank not venting
6. Tank gas level too low

You've got the right equipment to do a proper test with that vac/press gauge. The most important result you're looking for is the amount of time the pressure and vacuum remain after stopping the engine. Those check valves used to be the most common failure items- - - -now the rubber parts in older pumps are failing due to "deathanol" additives to gasoline that create delamination and leaks, either into the crankcase or to the outside. A damaged diaphragm can fill the crankcase with explosive vapors, and/or dilute the oil enough to get rod and main bearing damage.

Good luck!

Jerry