# Sticky  FUEL PUMP - OPERATION & TESTING IT



## PontiacJim (Dec 29, 2012)

*THE MECHANICAL FUEL PUMP OPERATION & TESTING IT*

This article covers part of our fuel system, mainly the fuel pump and fuel delivery to the carb as questions like "no gas in my carb," "is my fuel pump bad," "what is a vapor/gas return line," seem to pop up often enough. So a little info and how to test a fuel pump.

Pic #1 shows how to correctly install your hose over the fuel fitting barb and affix your hose clamp. The question of "where do I put the clamp" has come up. You don't want the hose clamp on top of the barb, or somewhere before the barb. You also don't want to tighten your clamp so tight that the rubber squeezes out of the small slots on your screw-type hose clamp which most of us use. Make it tight, but not killer tight using a ratchet/socket and 5-foot breaker bar.

Pic #2 shows the needed hose that joins the steel fuel line coming up from your gas tank that runs along the frame up into the engine compartment. The rubber hose then fits over the steel line barb/end and connects to the engine fuel pump's inlet barb/end. This rubber line needs a little play in it to compensate for the engine torquing over, a busted engine mount, or the engine vibration from a Big .900" lift cam, LOL. If you make it too tight with no play in it, the hose could be torn loose, the engine die out on you due to no gas to the carb, or worse - fire.

Pic #3 shows the effect of the gasoline flow when air enters the fuel line, but not enough to have a severe leak that fuel is cut off to the fuel pump - your engine may just peak in RPM and go no more. In the diagram, it shows a rupture in the steel line. This can come from a crack due to steel line fatigue, the line gets hit/damage (you did a 6 foot wheelie and came down hard), or rust through the line. Air can also enter through a bad rubber hose connection either at the rear at the gas tank connection or at the front at the fuel pump connection (possibly your carb as well if you use a rubber line) due to a faulty hose clamp connection, or dry/cracked/rotted rubber. Don't assume it is good because it is not leaking. Best way to check is to physically grab the hose and flex/bend it in a 90 degree or better shape and look for cracking. If the line is hardened and does not flex or cracks along the outer skin - it is bad, so just replace it and be done with it. (FYI - If you have an electric pusher fuel pump and the line cracks, you could be spraying gas all over and this can lead to a nifty fire). I always purchase, and recommend, the high pressure rated rubber hose for fuel injection cars - but my experience is you may have to ask for it so you know the parts counter guy gives you the correct rubber line.

Pic #4 is the typical fuel filter used in a Quadra-jet carb. Factory filters were typically a porous bronze filter, but a paper is more often than not used as a replacement. Get the correct length for your carb and the large nut housing/holding the filter assembly - some are long, some are short. NOTE how the filter is installed. If installed backwards or without the spring holding the filter off the opening going into the carb body, no or almost no gas will get into the carb - some have learned this when the carb was not getting gas but the line at the carb would pump plenty of gas out of it. The guy before may have lost the spring, or put the filter in backwards. Use care when disassembling/re-tightening the large filter/carb nut as it can easily strip out the threads in the aluminum body of the carb - make it tight and leak free, but not killer tight, and it should use a large O-ring flat gasket to seal the nut/carb body. And do use 2 wrenches - one to hold the large nut and the other to loosen/tighten your fuel line. NEVER tighten the large nut, remove your wrench, then use the other wrench to tighten the fuel line into the large nut as you may apply too much wrenching to get the fuel line tight and then unknowingly be tightening the large nut even more and.......strip out the threads on the large nut. ALWAYS use 2 wrenches together and tighten against the wrench, not your NUTS!.


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## PontiacJim (Dec 29, 2012)

PontiacJim said:


> *THE MECHANICAL FUEL PUMP OPERATION & TESTING IT*
> 
> This article covers part of our fuel system, mainly the fuel pump and fuel delivery to the carb as questions like "no gas in my carb," "is my fuel pump bad," "what is a vapor/gas return line," seem to pop up often enough. So a little info and how to test a fuel pump.
> 
> ...



Pic #5 - using my factory experimental Pontiac X-ray machine, this is the inside view of a typical fuel pump. No, it is not an actual Pontiac fuel pump, but it represents the various parts inside a fuel pump (I think it's a Hudson V-16 flanigan fuel pump used on an 1898 blimp). The pump has an Inlet and Outlet. There are 2 valves that work opposite of each other allowing gas to be pulled in (Inlet Valve) or pushed out (Outlet Valve). The Diaphragm is a metal like sandwich having a flexible rubber center/gasket in the middle. The lower half of the sandwich has a Pull Rod secured to it and a return Spring. The action of the Diaphragm is what draws a slug of gas from the tank through the Inlet Valve into the body of the fuel pump and then pushes (pressurizes) that slug of gas right back out the Outlet Valve up to the carb.

The fuel pump cycle begins with the tension of the Spring below the Diaphragm pushing up on the Diaphragm which also raises the attached Pull Rod. The Pontiac fuel pump eccentric pushes against the Rocker Arm as it begins to move to its highest point and causes the Pull Rod to pull down against the tension of Diaphragm Spring which causes the Diaphragm to draw in the gas from the gas tank line through the Inlet Valve. As the Rocker Arm follows the Pontiac eccentric/cam past its high point and back down to its low side, the tension of the Diaphragm Spring then expands/pushes back up against the Diaphragm to pressurize the fresh slug of gas out through the Outlet Valve and up to the carb. Then the cycle repeats. The movement of the Pull Rod going up and down (called stroking it, Baaad65) causes the pumping action of the Diaphragm and is what opens and closes the Inlet or Outlet Valves directing the flow of gas and maintains pressurization in the Outlet side of the fuel line that goes to the carb. There is no pressurization of the gas coming in from the gas tank line, only pressurization of the gas going out to the carb.

Pic #6 shows gas entering the fuel pump Inlet. In this diagram, the Pontiac fuel pump eccentric rotates to its high side and pushes on the Rocker Arm which pulls down on the Pull Rod and stretches the Diaphragm as it simultaneously compresses the Diaphragm Spring. This increases the volume of the fuel pump's chamber above the Diaphragm and produces a low pressure area, or vacuum. This causes the Inlet (Check) Valve to open and the Outlet Valve to close. As a result, fuel flows because it is being pushed by the outside atmospheric pressure due to the differential of the low pressure area (vacuum) causing the fuel pump's fuel chamber to fill with gas and making it ready for the output stroke that pushes gas out the Outlet side.

Pic #7 shows the gas exiting the fuel pump Outlet port. In this diagram, the Pontiac fuel pump eccentric rotates away from the Rocker Arm to its low side and releases the pressure on the pump arm. The fuel pump's output stroke begins as the compressed Diaphragm Spring pushes up on the Diaphragm and the fuel in the fuel pump's chamber (like the compression stroke in the engine). The upward pressure applied to the fuel in the chamber closes off the Inlet (Check) Valve and opens up the Outlet Valve and the gas in the fuel chamber is forced into the fuel line that goes to the carb. The volume/quantity of gas being pumped out depends on the stroke & diameter of the diaphragm. Fuel pump pressure depends on the stiffness or strength of the diaphragm spring.

Pic #8 shows the fuel pump "idling." A fuel pump can normally provide more fuel than an engine needs. Most of the time a fuel pump's output does not have to be at its maximum output, especially when an engine is idling or cruising along at a lower RPM. The Rocker Arm is designed so that it can slide up and down on the Pull Rod. When the carb is full of fuel and the needle & seat close off, and the fuel pump's fuel chamber is at full capacity, the fuel pump's output needs to be reduced other wise the continued output would be more than the needle & seat could hold back and flooding of the carb would occur - same as high pressure/high volume aftermarket fuel pumps without a pressure regulator.

Here is how this work - The pressure in the fuel pump's chamber holds the Diaphragm Spring compressed, and the Pull Rod extended. The Rocker Arm end only pulls down on the Pull Rod - it does not push up on the Pull Rod to raise the Diaphragm to pressurize gas out the Outlet side because that is the role of the Diaphragm Spring. So the Pull Rod simply slides (idles) up & down on the Pull Rod as long as there is back pressure in the fuel line leading up to the carb which in turn keeps the Diaphragm Spring compressed. As soon as the fuel bowl in the carb starts to become emptied, the needle & seat in the carb open, and the back pressure holding the Diaphragm Spring compressed is lessened and the compressed spring allows the Diaphragm to raise up and push fuel back out the Outlet Valve and into the gas line going to the carb. As it does this, the Pull Rod (which is part of the Diaphragm), also raises up until it reaches a point where the slide movement of Rocker Arm (ie "like" removing the slack from the slide movement) can once again pull down on the Pull Rod to draw gas in from the fuel tank line to fill the fuel pump's chamber again. In a nutshell, the slide function of the Pull Rod allows the Pontiac's fuel pump eccentric to rotate and and the fuel pump's Rocker Arm to move/function normally up & down with the eccentric without any Diaphragm pumping action sending gas out the Outlet side when the carb/engine doesn't need the output of the fuel pump - it is simply "idling" doing nothing until needed when the float level drops in the carb bowl and opens the needle & seat to let more gas in to fill the bowl.


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## PontiacJim (Dec 29, 2012)

PontiacJim said:


> Pic #5 - using my factory experimental Pontiac X-ray machine, this is the inside view of a typical fuel pump. No, it is not an actual Pontiac fuel pump, but it represents the various parts inside a fuel pump (I think it's a Hudson V-16 flanigan fuel pump used on an 1898 blimp). The pump has an Inlet and Outlet. There are 2 valves that work opposite of each other allowing gas to be pulled in (Inlet Valve) or pushed out (Outlet Valve). The Diaphragm is a metal like sandwich having a flexible rubber center/gasket in the middle. The lower half of the sandwich has a Pull Rod secured to it and a return Spring. The action of the Diaphragm is what draws a slug of gas from the tank through the Inlet Valve into the body of the fuel pump and then pushes (pressurizes) that slug of gas right back out the Outlet Valve up to the carb.
> 
> The fuel pump cycle begins with the tension of the Spring below the Diaphragm pushing up on the Diaphragm which also raises the attached Pull Rod. The Pontiac fuel pump eccentric pushes against the Rocker Arm as it begins to move to its highest point and causes the Pull Rod to pull down against the tension of Diaphragm Spring which causes the Diaphragm to draw in the gas from the gas tank line through the Inlet Valve. As the Rocker Arm follows the Pontiac eccentric/cam past its high point and back down to its low side, the tension of the Diaphragm Spring then expands/pushes back up against the Diaphragm to pressurize the fresh slug of gas out through the Outlet Valve and up to the carb. Then the cycle repeats. The movement of the Pull Rod going up and down (called stroking it, Baaad65) causes the pumping action of the Diaphragm and is what opens and closes the Inlet or Outlet Valves directing the flow of gas and maintains pressurization in the Outlet side of the fuel line that goes to the carb. There is no pressurization of the gas coming in from the gas tank line, only pressurization of the gas going out to the carb.
> 
> ...



Pic #9 shows where fuel pump problems & failures can be found. Looking at the diagram, you can see a seal under the Diaphragm Spring and around the Pull Rod. If the Diaphragm develops a tear, crack, split, or wears out, the gas can find its way past the seal. If the pump is failing or has failed, most mechanical fuel pumps have a weep hole on the bottom side of the unit. When the internal diaphragm leaks, fuel escapes through the weep hole to notify the vehicle owner of a malfunction. The gas can also go into the fuel pump body below the seal and out the opening where the Rocker Arm passes through and sits on the Pontiac fuel pump eccentric. The raw gas is now able to find its way into the engine oil/pan. Another sign of a failing diaphragm is that you can usually smell a heavy odor of gas in the oil - which you can also get from worn out rings (but you will also know by all the oil you are burning).

Pic #10 is what can happen if a lot of gas gets pumped out the fuel pump's body and into the engine. The heat of the engine can create a lot of gas fumes and it is the gas fumes which become highly flammable. A fire can result, or worse, it could explode (and you were braggin' to all your buddies how that 7,000 RPM power-shift blew the bottom right out of your engine! LOL). Probably not quite as severe an issue if ONLY using a mechanical fuel pump. BUT, if you have an electric pusher pump installed at the back of the car near the gas tank to help gas reach the mechanical pump, the electric pump could be forcing gas past the diaphragm and still maintain enough line pressure going to the carb to keep your engine running - so you could be pumping raw gas into your engine while you merrily cruise down the highway. You may not notice a problem until you put the gas pedal to the floor and the engine seems to starve for fuel or won't rev very high, but let off the gas and the engine runs fine.

Pic #11 shows the best/correct way to install your fuel pump. Always try to get the Pontiac fuel pump eccentric found on the front of the camshaft on its lowest point where the fuel pump Rocker Arm will be riding upon. In this example the Arm is riding on top of the eccentric. If the fuel pump is tilted at an angle because the Arm is on the high side of the eccentric, you will have to fight the pump to get it even and level with the block mating surface and then hold it into position against the heavy tension of the Arm's spring and then insert and finger tighten the 2 bolts that hold it to the timing cover. Get the bolts cross-threaded when you insert them, then grab your ratchet/socket to tighten and draw the fuel pump into place, and you might open a can of worms that will make a lot more work for you because you did not get that eccentric to its lowest point before installing the fuel pump - like stripped out threads. There will be some slight tension on the fuel pump Arm when the eccentric is at its low point, but very managable and easy enough to hold in place and finger start/tighten the 2 bolts and then torque down to finish. 
Pic #12 shows how to perform a Volume/Fuel Flow test to find out whether the pump can move enough fuel to meet the demands of the engine. The purpose of this test is to determine the condition of the pump diaphragm, check valves, pump arm linkages, fuel pump eccentric, tank strainer (sock), and any filters. I have not found anything that speaks about those fuel pumps that have the 1/4" vapor return line or the 3-barbed gasoline fuel filters used on some AC cars. I am going to assume that these smaller lines would need to be capped/pinched off to get an accurate pressure reading otherwise the vapor return line would seem to me to act as a "leak" and lower your actual fuel pressure at the Outlet side in these tests.

Do this test in a safe area and have a fire extinguisher handy. You will need gas to be in the carb bowl to do this test as you need the engine to run at idle with the fuel line disconnected to the carb. You can run the engine up for a short time to get fuel into the carb bowl and fill it or if you know how to fill the bowl through the top of the carb you could do that as well. The output line going to the carb gets disconnected and a length of rubber hose securely connected to the metal line at the carb or even the Outlet side hose barb on the fuel pump, and then insert the hose end into a marked and graduated container, one marked for ounces- something that can hold a couple pints of liquid. Pinch the hose shut with a pair of vise grip pliers. Start and idle the engine. Then with a phone/watch, open the vise grips and let the gas flow into your graduated container. After 30-45 seconds, clamp the hose off with the vice grips to stop the gas flow. Shut off the engine. Then check the amount of fuel that is in your graduated container and compare it to the fuel pumps specification.

Generally, a 30-45 second run-up of the fuel pump at idle will put out at least 1 pint of fuel (which is spec for a Pontiac) with some high volume/high pressure or bigger fuel pumps putting out more, so knowing what the fuel pump's Gallons Per Hour rating is can be factored to give you an idea of how much your pump will put out in 30-45 seconds.

Determining how many GPH you engine requires. Using the Holley website formula, at wide-open-throttle an engine consumes 0.5 pounds of fuel per horsepower per hour. 400 HP x 0.5 = 200 pounds of fuel needed per hour. A gallon of gas weighs 6 pounds. 200 lbs/hr ÷ 6 lbs = 33.3 gallons per hour(GPH) needed to feed a 400HP engine. 

Taking this a step further to determine how much fuel should flow into a graduated container in 30-45 seconds is calculated as 33.3 GPH ÷ 60 (minutes in 1 hour) = 0.55 gallons per minute. There is 8 pints in 1 gallon. Multiply 0.55 x 8 = 4.4 pints per minute. There is 60 seconds in 1 minute. Divide 4.4 pints/minute ÷ 60 seconds = .073 pints per second (PPS). Our test will be for 30 - 45 seconds, so Multiply .073 PPS x 30 seconds = 2.19 pints or if 45 seconds =3.28 pints. So doing this test using a 33.3 GPH fuel pump and a measured time of 30-45 seconds, the graduated container will fill up between 2.19 to 3.28 pints.

QUESTION: BUT WAIT ONE STINKIN' MINUTE. The factory fuel pump is rated at 40 GPH and will support the stock GTO HP levels - I get it. But the factory test spec for fuel flow is 1 pint in 30 seconds. Your 33.3 GPH fuel pump example, which is a lower output than the factory 40 GPH, shows a higher fuel flow of 2.19 pints in 30 seconds. How can that be?
ANSWER: At idle the fuel flow is less than it will be at wide-open-throttle (WOT). The 2.19 pints in 30 seconds would be for an engine at WOT,which from what I have gathered seems to be tested at an engine speed of 1,800 RPM's where maximum flow is reached. Idle speeds will show reduced flow - so there, smart ass.

Pic #13 shows how to test the fuel pump itself, lines disconnected. Same as above, the carb needs to have fuel in the bowl so you can run the engine at idle. Remove the inlet line to the fuel pump and install a hose on the inlet barb and then into a container/gas can of gas as your source of fuel, not the car's gas tank. Start the engine at an idle and again measure the amount of fuel going into your graduated container. If you get that 1 pint of gas or so in the 30-45 second run of the engine, the pump is most likely OK. However, this time watch for any air bubbles coming through the line into your container which could indicate a tear/rip/hole in the rubber diaphram or a crack/damage to the pump housing.

If the pump tests out to be OK, reconnect the gas line coming in from the tank. This test will be an indication that the flow issue is in the fuel line, a clogged strainer/fuel sock, plugged gas tank vent, or incorrect gas cap. Get the carb bowl filled with gas and start the engine again at idle and see how much gas fill's your container in 30-45 seconds. If the flow test indicates a low flow of fuel into the graduated container with the tank line connected, start looking for a problem from the gas line back to the tank.

Pic #14 is checking the actual fuel pressure in the fuel line coming from the fuel pump going to the carb. Using a fuel pressure gauge, you can "T" into the fuel line. Many race cars will use a fuel pressure gauge to watch their fuel pressure over the span of an 1/8 or 1/4 mile drag strip to ensure the fuel pressure can keep up with the carb's demand. Pressure here may be different than test #15 if you have a vapor return line as part of the fuel system.

Pic #15 is testing the fuel pressure that exits the fuel pump. Although it does not say it, I would cap off the fuel pump's 1/4" vapor return line so no loss of fuel pressure occurs - if you fuel pump has this line.

Pic #16 is testing the inlet side of the fuel pump using a vacuum gauge to ensure it is drawing in fuel. If the diaphragm has a hole, tear, or there is an internal mechanical failure, the pump will not be creating any vacuum, or it will be very low.


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## PontiacJim (Dec 29, 2012)

PontiacJim said:


> Pic #9 shows where fuel pump problems & failures can be found. Looking at the diagram, you can see a seal under the Diaphragm Spring and around the Pull Rod. If the Diaphragm develops a tear, crack, split, or wears out, the gas can find its way past the seal. If the pump is failing or has failed, most mechanical fuel pumps have a weep hole on the bottom side of the unit. When the internal diaphragm leaks, fuel escapes through the weep hole to notify the vehicle owner of a malfunction. The gas can also go into the fuel pump body below the seal and out the opening where the Rocker Arm passes through and sits on the Pontiac fuel pump eccentric. The raw gas is now able to find its way into the engine oil/pan. Another sign of a failing diaphragm is that you can usually smell a heavy odor of gas in the oil - which you can also get from worn out rings (but you will also know by all the oil you are burning).
> 
> Pic #10 is what can happen if a lot of gas gets pumped out the fuel pump's body and into the engine. The heat of the engine can create a lot of gas fumes and it is the gas fumes which become highly flammable. A fire can result, or worse, it could explode (and you were braggin' to all your buddies how that 7,000 RPM power-shift blew the bottom right out of your engine! LOL). Probably not quite as severe an issue if ONLY using a mechanical fuel pump. BUT, if you have an electric pusher pump installed at the back of the car near the gas tank to help gas reach the mechanical pump, the electric pump could be forcing gas past the diaphragm and still maintain enough line pressure going to the carb to keep your engine running - so you could be pumping raw gas into your engine while you merrily cruise down the highway. You may not notice a problem until you put the gas pedal to the floor and the engine seems to starve for fuel or won't rev very high, but let off the gas and the engine runs fine.
> 
> ...



Pic #17 shows a vapor return line and how it attaches and works. Some Pontiacs (not all) will have this type system while some of the early cars used a fuel filter having an additional nipple that was used for a vapor return line going back to the tank - generally found on AC cars.

Pic #18 shows a Ford fuel pump with the vapor return line and you will note that the vapor return outlet has a valve that meters the amount of fuel going back to the tank. If not, fuel pressure would drop and this would reduce the fuel pressure going to the carb. Pontiac uses the smaller 1/4" return line which I am assuming is what meters the amount of fuel back to the tank. Some of the race guys use a regulator to adjust the amount of fuel that flows back to the tank. This can be used in addition to a fuel pressure regulator at the carb and can be fine tuned to raise/lower fuel pressure going to the carb when using a high volume/high pressure electric fuel pump.

Pic #19 is a fuel vapor separator which is part of the fuel delivery system. This may be the same as the early Pontiac fuel filters used on the AC cars having the vapor return line mentioned earlier.

Pic #20 are fuel flow volumes, fuel pressures, and fuel pump vacuum at the Inlet side of the pump.

Here are a few examples I gathered on fuel pump pressures of other makes/engines used for comparison. The specs come from the AMA factory specs I have for these cars.

1965 -66 Olds 442 Rochester 4GC/Rochester 4M - 7.5-9 PSI - V8
68 442 - 5.5 - 7.0 PSI Rochester 4MV
'66 Olds 6 Cyl - 3.5-4.5 PSI
'66 Buick Carter AFB/Rochester 2 GC- 4.25 - 5.75 PSI at Outlet @ 1,800 RPM
'70 Buick GS Rochester 4MV - 4.25 -5.75 PSI at Outlet @ 1,800 RPM
'70 Dodge 440 AVS/ 3 x 2 Holley - 3.5 - 5.0 PSI
'69 AMC 390 - Carter AFB - 4 - 5.5 PSI

'65 - '66 GTO AFB/3 x 2 - 5.25 - 6.50 PSI
'67 Pontiac & up Q-jet & 2 Bbl - 5.25 - 6.50 PSI


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## rockdoc (Mar 16, 2009)

Thanks for all of this information, Jim! You are an incredible source of information. Now I have some more bedtime reading.


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## pontrc (Mar 18, 2020)

X2 Jim thanks for the effort and time👍


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## PontiacJim (Dec 29, 2012)

Adding this info provided by O52:

"Pontiac fuel pumps used a *rubber coated aluminum washer* for sealing the bolt hole.

*Fuel pump bolts *were 3/8-16 x 1 1/4 and had a 12 pt head to allow for right side socket clearance
Don't over torque into the aluminum timing cover. 12-18 lb-ft."

From my research, the 1964-68 with the 8-bolt water pumps/timing chain covers used the 3/8" bolts.
The later 11-bolt water pumps/timing chain covers used the smaller 5/16" bolts. 

So if you happen to have a later fuel pump that has 5/16" drilled holes, drill them out to 3/8" to use on the 1968 and earlier timing chain covers. 

Both items, 1964-68, found in the AMK catalog.






Catalogs







www.amkproducts.com


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