Diagnosing an old school mechanical SBC fuel pump.

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  #1  
Old 04-12-11, 07:48 PM
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Diagnosing an old school mechanical SBC fuel pump.

A good friend recently purchase a 62 Chevy Impala 2dr. Prior to the purchase he asked me to go to the garage where it was being parked and to give my opinion on how the engine ran. They had to pour gas down the carb, plus still use ether and keep a battery charger connected. After a few minutes of cranking, pouring and spraying the engine finally started. And in my opinion it sounded good. No miss, no tapping, no smoking, no knocking, no nothing. Besides the obvious body repairs that it needed, the engine for now was mechanically good. So I gave it the ok to buy it.

Fast forward a week later and now the car is in my home garage. After a closer inspection, you can tell someone has been trying to correct this hard start condition. The fuel tank has new hanger straps, there are new hoses from the filler tube to the tank and from the tank to the fuel tube that goes to the engine compartment. The fuel pump and internal (in the carb) fuel filter has been changed.

After checking the compression on each cylinder (6 cylinders were at 160psi, 1 was at 130psi and the last one at 110psi) and regapping the plugs to .035 (the plug gaps ranged from .065 to .040) I attemped to start the engine with the aid of starter fluid. After a few minutes of spraying and cranking, it still wouldn't start. So I disconnected the fuel line going into the carb and crank the engine. No gas was coming out the line. Then I disconnected the inlet line going into the pump and applied compress air to the filler tube. With the compress air applied to the filler tube, gas was coming out the disconnected fuel pump inlet line. So I assumed the line between the tank and the pump was ok. Not crimp, bent or broken apart. I aslo removed and inspect the fuel pump. The pump lever was intact and had good spring tension. The rod that actuates the fuel pump lever is still there and moves when I crank the engine.

Since I can tell the fuel pump has been changed, I'm a little cautious of saying the pump is bad. I'm a fuel injected wrencher, but the same basic concept should be the same of a mechanical carb vehicle.

Now that you have the background, here's my question. With the carb inlet line disconnected, gas should be coming out that line with the engine cranking, correct? If not, what could cause gas not to come out the carb inlet hose while being cranked? Is it same to assume the pump is bad?
 
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  #2  
Old 04-13-11, 06:08 AM
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I think that those fuel pumps would normally pump gas with something like 8lbs. pressure. When you crank the engine do you feel any suction with your finger over the inlet? Does the pump look new or one that was just cleaned up? Did you try putting some gas through it and letting it sit a while just in case things got dried up inside. I think all they are is a diaphram and spring basically. If there is no suction you might be able to take it apart depending, and have a look for any tears in the diaphram or anything else obvious. How much fuel is in the tank also?
 
  #3  
Old 04-13-11, 08:22 AM
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Even if the fuel delivery system is not functioning,if you pour a little gas in the carb it should start and run for a few seconds. You may be fighting two problems. How does the spark look? With the line disconnected at the carb you should show around 6 lbs pressure and cranking it over should pump a pint of gas in 30 sec. I have heard of the lobe wearing off that operates the fuel pump. but this is rare. With the pump off you can operate it manually and feel suction on one side and press on the other.
 
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Old 04-13-11, 01:47 PM
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Originally Posted by equinox View Post
I think that those fuel pumps would normally pump gas with something like 8lbs. pressure. When you crank the engine do you feel any suction with your finger over the inlet? Does the pump look new or one that was just cleaned up? Did you try putting some gas through it and letting it sit a while just in case things got dried up inside. I think all they are is a diaphram and spring basically. If there is no suction you might be able to take it apart depending, and have a look for any tears in the diaphram or anything else obvious. How much fuel is in the tank also?
- I didn't try feeling for suction on the inlet, but I did try feeling for pressure on the outlet. No pressure was felt.
- The pump looks new and not cleaned up.
- Remember the engine was running at the garage where it was being sold and that was less than a week ago.

To eliminate the possibility of the problem being the pick-up tube in the tank, I plan on running a line from a gas can to the inlet side of the pump.
 
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Old 04-13-11, 01:53 PM
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Originally Posted by retired wrench View Post
Even if the fuel delivery system is not functioning,if you pour a little gas in the carb it should start and run for a few seconds. You may be fighting two problems. How does the spark look? With the line disconnected at the carb you should show around 6 lbs pressure and cranking it over should pump a pint of gas in 30 sec. I have heard of the lobe wearing off that operates the fuel pump. but this is rare. With the pump off you can operate it manually and feel suction on one side and press on the other.
The engine does run for a couple seconds with starter fluid or gas down the carb. I haven't checked the quality of the spark, but with the engine running for a second or two, we can assume spark is ther. The fuel pump lobe is a possibility. After trying the gas can to the inlet and possibly another pump, if it still doesn't start the lobe is very much a possibility.
 
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Old 04-13-11, 05:37 PM
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If you have the pump removed, you may try installing an inlet hose on the pump and put the suction line in to a can of non volatile fuel such as kerosene or mineral spirits. Press the pump lever full stroke several times an you should have fuel at the discharge. This way you have completely isolated the pump from the vehicle.
If the pump still does not work, disassemble it and examine the two check valves for trash or damage and inspect the diaphragm for tears. If the diaphragm is leaking slowly, it could be catching air, if it is worse, you will probably smell fuel in the crankcase oil.
 
  #7  
Old 04-13-11, 06:09 PM
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Problem resolved, well kinda. I attach a gas can to the fuel inlet at the pump and after a few cranks it started right up. So now this tells me the problem is either in the tank pick-up or somewhere between the tank and pump. No need to replace the pump.

Thanks all
 
  #8  
Old 04-13-11, 07:49 PM
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Originally Posted by CandiMan View Post
Problem resolved, well kinda. I attach a gas can to the fuel inlet at the pump and after a few cranks it started right up. So now this tells me the problem is either in the tank pick-up or somewhere between the tank and pump. No need to replace the pump.

Thanks all
Or you might have a crack in a metal or rubber gas line between the tank and the pump. Did you have the gas can above the pump or below it?
Mike
 
  #9  
Old 04-13-11, 08:38 PM
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With the line hooked up add a LITTLE pressure to the tank filler any breaks in the line will show up as a leak. Be careful if the tank is full you can get a kick back of gas out the filler. Be sure there are no open flames around. (water heater ect.)
But if you couldnt feel pressure from the pump its probably bad no matter how pretty it is.
 
  #10  
Old 04-17-11, 12:22 PM
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Just for future reference. We always added a "t" at inlet of fuel pump and attached a vacuum gauge to confirm proper pump pressure. Yes 5-8 psi vacuum should be enough for it to run. That way you can also monitor pressure while engine is running , revving etc. Don't know how much gas is in the tank, but it wasn't uncommon for the pick-up tube in tank to get a hole halfway up, thus would run good on a full tank, then when level got below the hole, would suck air and gas, and basically starve it for fuel.
 
  #11  
Old 04-17-11, 04:35 PM
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The same can be said for putting a pressure gauge on the outlet of the pump. I never heard of a vacuum gauge on the inlet side, but it makes sense.
 
  #12  
Old 02-24-14, 07:34 PM
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i am totally freaking out over the difference in tension between my 35 year old, 1978 Toyota Corolla fuel pump, and the new 35 year old fuel pump that I just purchased online. If I understand correctly, the metal flange rests against the cam inside the engine and the turning cam causes the metal flange to rise and the spring causes it to contract.

Now, if the old fuel pump is very easy to push in, and the new old fuel pump is next to impossible to push in, why would I would want to risk changing the cam ballistics so dramatically all at once? Would I be better off putting the old spring which still looks fine by the way, into the new old fuel pump?

One other thing, although the new old fuel pump looks new, it has a distinct burnish where the flange would make contact with the cam, like it may have once been installed, briefly tried, and then removed. The fuel pump came in what looks like a 35 year old box.

The new old fuel pump has excellent suction, but I am concerned that an original 35 year old car engine 35 does not want to have to break in anything, and that maybe breaking in a very stiff fuel pump might cause some internal engine bearings to loosen to accommodate the very stiff flange?

Am I way off base here? I am waiting before I install the new fuel pump and don't have transportation at the moment.
 
  #13  
Old 02-24-14, 07:48 PM
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Don't see it as a problem, put it in and drive it.
 
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