Fuel pump problem (Mallory 140)?
 

Good evening gentlemen. We've been running our car on jack stands to break in the new tremec and have come across a fuel pump problem we are trying to solve. We are running a new Mallory 140 pump and return regulator, Mallory 140 series fuel filter between the pump and tank, 1/2" lines and 6 psi of fuel pressure. When the car is cold, the pressure hovers around 6 psi +/- .5 or so. Then as the car is run for a while the fuel pressure slowly begins to drop. At about 180 degrees oil temp, the fuel pressure drops to around 3 psi and at 210 degrees, the pressure drops off to nothing. Once it's hot, if I shut the pump off and turn it back on, the pressure 'jumps' to about 2 psi and then it quickly falls off to nothing again. The pump is wired with a relay and it's getting 12.2V.

This is the second pump we have had on the car. The original grenaded when it was first run and Mallory replaced it, recognizing they had a problem with QC on a recent batch of their pumps (this was back in 2000). This one has been sitting under the car for 2 years.

At first I thought the loss of pressure would be due to a drop in voltage, but that checks out okay. The pump is mounted by the tank and it's staying fairly cool, so excessive heat shouldn't be the cause either. Has anyone else had a problem with this with an electric pump? Tomorrow I will pull the filter out and check it, as I haven't done that yet and it's probably the first thing I should check.

Any opinions will be appreciated. Thanks!

Mike

I had a similar problem with my holley blue pump holding steady pressure. It would be at 7psi then it would get erratic and sometimes dropped to zero. I noticed that the sound of the pump would change with the pressure changes. I started looking at how the fuel line was run from the fuel tank to the pump.

Here is what I found: the fuel tank had a ridgid aluminum pickup tube extending out of the tank about 5" and I had attached a braded line that looped up another 5" or so before it started down to the Fram canister fuel filter which was mounted just below the top of the fuel tank. I had mounted the fuel pump as low as possible and it was below the fuel filter. These electric fuel pumps make great pushers but are horrible at creating suction so they need some gravity feed and with my fuel line routing there was no gravity feed so the fuel would cavitate with air. This make the pump very loud(it is still loud but at least I know it is working) and hot as the fuel acts as a cooling agent for the pump.

I corrected by cutting the pickup tube so it only extended up about 1" then eliminated the loop up in the braded line by making a 90 degree turn(do not kink the hose) straight forward to the fuel filter. Now the pump keeps steady pressure and is a little less noisey.

Hope this helps.

I don't think it has much to do with the oil temp, probubly just a time frame issue. Do you have foam in the gas tank/fuel cell? If so, it could be getting picked up and clogging the fuel path. Turn the pump off and it all settles back down. Also, the voltage should be 13.5-14.0 volts, not 12. You had also mentioned a return regulator, if that is malfunctioning it could very well cause the problems you described.

Mr.Fixit,

The tank has had a liner installed (professionally) but no foam. If I turn the pump off for 5 minutes and back on, the pressure is still low, so I don't think there is any problem with debris getting stirred up. Besides, wouldn't it get trapped in the fuel filter and the pressure would be lower all the time?

I have 13.8V until I turn on my electric fan (a big Spal unit), at which point the voltage drops to around 12.2V. What would cause a regulator to function like this? It's just an adjustable check valve, correct? If so, it should either work or not unless I am missing something. Is there a way to check the regulator? I have spoken to some others who say they have the same problem when idling for a long time but they have never had any problems when driving. And they're using the same liquid filled gage as I am (by coincidence), so maybe it's a gage prob??

I didn't have any time to work on it yesterday, but hopefully tonight. I'll let you all know what I find out.

Thanks again Aumore. The pickup and lines are all horizontal and gravity fed, so that shouldn't be a problem...

Mike

gauge trouble
 

Check your gage,when I dynoed my engine my gage read 4.2 and dyno gage 6.5psi,regards,Bernie

Many race style fuel tanks with foam and bladder have a screen on the pickup which can catch debris before it makes it to the filter. The regulator is just a spring loaded bypass, but it can malfunction. Gages can be wrong as well. Have you noticed any fuel delivery problems while driving? The regulator has a bypass, and depending on design, if the bypass line cannot dump it's pressure, that can cause the regulator to not open until input pressure exceeds the static pressure in the bypass line (depending on design) thus no outlet pressure to carb. Just theorizing Check your lines for: kinks, sharp bends, and collapsing on the inlet to pump line. Also check filter cleanliness. If you can, verify the gage's reading with another. Check pressure before and after the regulator at the same time to verify the regulator is getting pressure. You can also disconnect the fuel line from the outlet of the pump and have it pump gas into a 5 gallon gas can, see if it starts losing flow as indicated by the gage far upstream. The electric motor could be getting hot and losing efficiancy.