Okay, so as some of you know from my previous post. I recent picked up
a 1986 Pontiac Trans Am, originally equipped with a 5.0L T.P.I. small block and a 700R4 auto trans.

The guy I bought the car from took off the T.P.I. stuff (left the wiring & computer in the car) and dropped on an intake, Holley fuel regulator, and distributor. The car ran although not very well.

So my original idea was to drop the tank, pull the pump and put in a line, hook-up an in-line electric pump and call it a day. I was then made aware I didn't need to do that, I could just run a Mallory #4309 regulator and I'd be fine.

So, today I was pulling the T.P.I. wiring out (careful not to brake anything and definately not cutting any wires. May be for sale soon!), after it was removed I put the key in and turned the accessories on and the fuel pump is no longer working. This got me think, a quick inspection of the block I noticed there is a block-off panel for a mechanical fuel pump.

This brings me to my question. Can I use a mechanical pump without removing the in-tank pump? Basically using the in-tank pump as a "pick-up tube". Basically pulling the fuel through the old pump.

Thanks for any and all opinions and insight!

 

you won't be able to pull fuel through the non-operating elec pump(or enough at least) the in tank pump is turned on by the ecm sending signal to a relay. you should be able to directly apply switched power to the relay to bypass the removed ecm

 

its been done before, it works, buts it's normally recommened to just drop the tank and replace the pump with a sender/pickup for a carbed car.

 

Why not just wire the electric to come on with the ignition in run and use a bypass style regulator like the mallory?

 

Mallory Regulator = $77.89

Stock Style Mechanical Pump = $18.99

Just thought I'd try to save myself a few dollars.
I wouldn't even know where to start on wiring
the stock pump. lol

All I know is it worked when I had the computer
in, worked when I took the computer out, then
stopped working when I took the T.P.I. harness out.

Thanks for the help!

 

when i did my tbi swap to carb, I ran into the same problem you are describing. Once I pulled out the harness it wouldnt run. I cant remember which wire it was but all I did was locate that wire and I hooked it to a source that was hot when I turned the ignition on. Havent had a problem since. And I run the BG 3 way regulator.

 

The fuel pump gets it's power one of two ways:

1. ECM turns on the fuel pump relay up under the hood. It's one of the relays near the power brake booster. The ECM does this when you first go key-on for a 2 second "priming" of the lines. After that the ECM needs to see a distributor reference pulse for it to keep that relay activated and keep pumping fuel. If you don't have the computer-controlled distributor setup any more you won't get this.

2. Oil pressure sending (screwed into block above the oil filter boss) unit has a secondary feed to the fuel pump to supply 12V directly whenever there is adequate oil pressure. Doesn't take much- maybe 5-6PSI to close the switch.

When I converted from TBI to carb I left my wiring harness in place. I got the 2 second prime at key-on and after that I'm fairly sure it was the oil pressure sending unit keeping the pump running once it was started up. I'm kicking myself for never confirming that for sure when I had the car, though.

I'm sure somebody with more experience about this particular part of the electrical system will chime in.

Certainly, you could rig up a wire to the fuel pressure relay that turns it on any time the ignition is hot and it should work just fine.

 

So there's no way to just use a mechanical pump and pull through the in-tank pump?

 

Sure there's a way.

It's just not a good way.

 

if you power up the stock pump to work it's going to pump at 45psi. you need to regulate the pump down to about 7psi. if you do that you will burnout the pump in no time. the best thing to do is remove the tank,remove the old pump,stick a fuel hose on the end of the pick up tube,then run a mech pump. or cut the trunk area and make a door to your sending unit. but i would not do that.

 

Cheapest way would be to keep the stock pump, wire it to ignition and a relay and run a regulator up front.

 

Not if you use a return style regulator, like the Mallory 4309.