[john cross]

Just want you opinion on something. For years now my car ('78 stock) has been hard to start. My fuel pump has also made that characteristic loud whine which tells me it is about to go. So, last year I bought a new one and have it stowed safely in my car, waiting for th day wehen it finally goes. In the meantime, I have a fleling it is just not up to snuff. THe last time a mechanic looked at it, he did a pressure test, said it was fine, and that was it.

However, this is my theory. I just started the car, it was very difficult, and I noticed the pump sound get louder just after it finally caught. People have noted before, how come my car is so hard to start and runs liek crap for a minute or so? It has pressure, so why does it not just run righ from the get-go. This is also referred to as the vapor-lock theory, etc.

How about this: I know that the starter pulls alot of current, by far more than anything else on the car. lights will dim when it is on. What is the failure mode of the fuel pump is to be unable to function well under starting conditions, so that it cannot maintain pressure while the starter is cranking, hence making the car difficult to start??

THis would explain alot. I swear I heard it noticeably recover AFTER the car started..

Thoughts? Should I go ahead and replace my fuel pump?

 

[MeloPark]

You own the fuel pump take an hour put it in and post the answer to your question so we all will know.
Thanks
MEL

 

[Caezar75]

put the new pump on...no reason to chase ghosts if it doesnt fix the problem then it is something else but you have the part so you arent wasting money.

good luck..jon

 

[79-280-ZX]

More Theory and Rambling Fuel Pumps

If I had the luxury of having a spare new fuel pump here is what I would do.

Collect up the following:

Flexible fuel line.
Build a fuel pressure gauge.
Find a Fuel Pressure Regulator.
Find a vacuum pump.
Two containers to hold fuel.
A 12 volt power supply.
An on / off switch.
Some wire.

Cobble together as follows:

Enough fuel line to go from fuel container # 1 to the input of the pump.

Connect the output of the fuel pump to the Fuel Pressure Gauge.

Connect the output of the Fuel Pressure Gauge to the input of the Fuel Pressure Regulator.

Connect the output of the Fuel Pressure Regulator to enough hose to go to the fuel container # 2.

Hook the on / off switch to the positive side of the pump and the power supply.

Hook the negative side of the pump up to the power supply.

Hook the vacuum pump to the Fuel Pressure regulator.

Now, when you turn on the switch the pressure should rise to 42 PSI once all the air has been pushed out of the lines.

Pump the vacuum down to 16 inches and the pressure should drop to 32 PSI.

Now you can get a listen to the pump when there is no restrictions in the lines and what it should sound like when it is on the car.

And you can also measure the flow by timing how long it takes to fill a known size canister at both working pressures.

If you hook all of this "plumbing" up to your car the results should be the same. If the volume per second is lower and the "loudness" stays the same then the filter on your pump inlet is probably plugged.

If you hook the fuel line from your tank back up, and repeat the test, and the volume per second is lower, then the pick-up tube in the tank is probably plugged.

If you hook the output of the pump back up to the original fuel line you can measure the volume coming out at the return line of the fuel rail. All you need to do is remove the small gauge wire from the solenoid, so the engine will not turn over, and hold the key in the "start" position to make the fuel pump run.

Out of all the FSM's I have seen I have never seen a specification for the volume these pumps are supposed to produce.

On my 79 the pump is noticeably "louder" when starting because there is no vacuum being applied to the FPR and the pump is producing 42 PSI. When it goes down to 32 PSI it is quieter but I have never heard a loud noise coming from it unless I pinch of the return line and that "dead heads" the pump at 60 PSI.

I have already collected all the bits and pieces needed with the exception of the fuel pump. My goal is to be able to test the flow rate and pattern of fuel injectors.

Wayne Monteath
Masham, Quebec.

 

[helowrench]

John,
from some experience with this type of problem I can relay a few things to you.
what you are seeing is a voltage drop of about 3-4v as seen at the pump terminals when the starter is engaged. Actually your starter is probably closer to failure than your fuel pump, as is evidenced by the greater current draw and thus voltage drop.
This is also why you see such a change in the intensity of the lights as soon as the engine catches and alternator comes on line. Fuel pump will run down to about 6 or 7 volts. will develop enough pressure at 7 volts BUT ECU will not function below 9 or 10 ( I do not have exact figures in front of me but I think it is 9 ).
Also contributing to all of this is bonding strap decay. Chack with a meter all of the bonding straps that interconnect the engine-battery-body-trans. In all of my bosch type FI cars I installed a few extra jumpers to ensure extremely good current paths. Ohm out from batt neg term to eng block to body etc for checking.
Lastly as to hard start when cold check for operation of the cold start injector.If it does not work or you would like more info post again and I'll get out ref material for procedures K good luck.

 

[john cross]

Thanks for the input. fyi, my starter was rebuilt about 10K kilometers ago, so it is probably fine. The fuel pump is ancient. Never been changed and I have had the car for 200K. I was making the comment about lights dimming on cars in general, not on my car.

how do you know thee is a drop of 3-4v? Are you guessing?

If the ecu cannot function below 9 or 10 volts, then how can the car start?? because if there is a voltage drop when the starter is operating, the ecu would be below that voltage.

I am not sure what a bonding strap is. Is this something the layman can check? I assume that you mean the grounds.

thanks for the help.

 

[helowrench]

bonding straps= ground straps
unbolt one side of the strap and use an ohm meter
I stated that incorrectly sorry for the confusion let me try again.
the reason you see the lights get brighter, and hear the pump get louder as your engine passes through about 600-650 rpm is you pass the cut-in speed of the alternator which begins to put out 14.5 or so volts. until then the BEST a battery can do is 12.6 ( 2.1 volts per cell at 6 cells for a lead acid )
When you activate the starter you will see a voltage drop from battery nominal of 12.6 to probably around10.5 then as alternator reaches cut-in speed and starter shuts off voltage will go up to 14.5
good starter = less current draw
good battery = more available ampere hours
good grounds = less resistance to current draw
any one of these effects the whole system
easy ground check / with multimeter in ohm setting put one probe on negative battery terminal and put other one on clean spot on the engine block or frame
if you get more than 1 or 2 ohms remove battery ground from both body and frame and use a wire brush to clean both term and attach point. be sure to caot these with something to protect them. Everyone cleans the terminals at the battery but they are only one point in the system.

 

[helowrench]

BTW Bosch L-Jetronic book says you are correct that a noisier pump is usually closer to failure but also states that there is no way to predict time of or until failure.
see y'all tomorrow

 

[SleeperZ]

Funny you mention this

I am in process of making a fuel bench. I fired it up last night, but I have a blocked regulator (no return line flow), so I need to hit the scrapyard and get another. I got lots of pumps and injectors to test

 

[Lee Smith]

I have posted several times my experence with loud fuel pump.

On my 78 it was a blocked intake screen, making the fuel pump very loud, not the pump going bad. I cleaned the screen twice in 2 weeks, then relined the tank.

If you want more details on relinning E me. [email protected]