stumped by fuel pump
 

Car sat for two weeks while I was out of town. Come back, try to start and it wouldn't. A little investigation, I found the fuel bowls were dry. Moving backwards, I found that the fuel pump wasn't pumping anything. It's a Mallory Comp 70. The motor ran, just didn't pump.

I checked the filter, wasn't bad but I replaced it. Still no fuel pulling through pump. Before I'd parked the car, I thought I'd noticed the pump noise wasn't constant, so I figured I'd just swap it out for a Holley Blue pump I have.

Got that pump in, still no fuel. For some reason, I took the gas tank cap off and ran the pump. From the noise, it sounded like it was pumping into the tank. Pulled the hose off the carb inlet and sure enough, it was drawing into the pump. I checked the wiring, looks correct. I checked to see if I had the "in" and "out" mixed up. Doesn't look like it.

What's going on? The pump worked normally when I had it in another car. It's wired to be switched through a relay. I moved the hot wire from the mallory to the holley. Same with the ground wire.

Do you have a fuel filter between the tank and fuel pump? if there is, is it dry or not. Im asking as I had similar problem before. It turned out that the pump was sucking air through a hole in the fuel filter.

Well, tell us more.

If you take the pump out, hook it to a battery and it works, put the pump in and there is no fuel flowing to the carb, then..... there is something constricting the line, or tank.

Prime the pump. Easy way to do this, install all lines except for a return hose under the hood. Tighten fuel cap, and blow compressed air into the return hose. This perssurizes the tank, and forces fuel out the fuel line, into the pump. Once fuel is flowing, hook up the remaining return line.
Fuel pumps push fuel, but will not pull fuel from the tank. They rely on the siphon effect to keep the fuel supplied to them. As they push fuel forward, more fuel is siphoned from the tank.

The pumping into the tank its the return "I'm not sure". I think you want to take off your fuel lines and clean them "if you have time". At the mechanic shop we've had to do that alot on older vehicles. They had a period of selling bad gas and everybody who bought cheap gas is getting the after effects.

That's my opinion.

p.s. I know I know, I don't have an RX-7 but I still do like to try and help on car problems.

-nick-

Leak in the suction line

The Holley is pumping. The car starts and runs. Float bowls are filling. My concern is that it's pumping in reverse, out the "in" and in the "out". That's the problem, along with a small leak. I think the leak is from the gasket.

You might just have it backwards? Is there a warranty?

Pretty sure I don't have it backwards. It worked in the other car. Warranty? Long gone.

You need to look at the Factory Service Manual.. There is a link for the FAQ. Look at the manual and make sure everything is setup correctly.

Try priming it.....like "Rogue Wullf"

Why would I need to prime it? It's pumping, and I have fuel to the carb. What's got me stumped is why it's running in reverse when it's set up and wired just as it was in the other RX-7 I had it in.

Holley pumps will pump backwards if you reverse the wiring. If the bottom of the pump has never been taken off (like when you rebuild it) then the "in" and "out" should be correctly labeled. it could be that it just needs to have the + and - reversed and the pump spun around. If the pump has been rebuilt, I would think it is very possible to get that bottom plate on backwards and cause headaches after installing it.

Edit: Also not sure which holley pump you have but I know that the "RED" and the "BLUE" pumps have a self priming feature. It will run slow for the first couple seconds and then build up speed when it starts building fuel pressure in the line.

I'll look again, but I believe I haven't reversed the wiring. I did take the bottom plate off a while ago to clean it out. I don't see how the plate can be reversed, since there's an ear on one side for one of the screws. The plate can go upside down, maybe. I'll check wiring again tonight and look for any air leaks upstream of the pump.


It's a Holley Blue. No return line. No charcoal canister for fuel evap and the orginal SA fuel evap system is long-gone.

vapor lock
 

+1 If the hose on the suction side of the pump is perforated so it sucks air you will wind up with "vapor lock" symptoms as you describe.
Replace all the hoses at the tank while you are in there and let us know what happened.