Fuel pump test

WANKEL

MEMBER
1987 MAZDA RX7
2WD
MANUAL
84,000 MILES

Hello 2carpros:
I ask for expert advice specific to to car make model and year in question.

I couldn't get the car started. I only started it occasionally since the summer. But now after being parked for some time, I couldn't get it started. I had the battery recharged and tried again but no cigar. I read some post in which they said there was an electrical plug under the hood near the passenger side spring tower. I think I located that electrical plug and placed a piece of wire to short out the connector but I didn't hear the sound of what would be the fuel pump engaging. The next method, I unplugged the rubber tube that goes from the fuel filter to where it is supposed to go under the hood, then I turned the key to "on" but didn't start it, and I observed that fuel didn't flow from that unplugged tube. So I suspected that the fuel pump need testing. By the way the fuel filter was replaced some time ago and has little miles on it and the car started after the replacement. I also had tested the fuel pump at the time the fuel filter was replaced and the car started, but the test was done with the fuel pump removed from the fuel tank and applying battery voltage and ground to it. At the time the fuel pump spun. But now I want to try to test the fuel pump while it is not removed from the tank with the battery disconnected from under the hood. My plan: with a stand alone battery, apply 12 volts to the blue wire (correct me if I'm wrong but I think the blue wire is for positive voltage) going to the fuel pump on the tank side and ground the negative side of the battery and listen for any sound from the pump. I think it should be safe but I wanted to make sure. It could be an issue other than the pump, don't want to unnecessarily remove the fuel pump from the tank and find out that the pump spins. So would like to know if I could do my test without removing the fuel pump from the tank. Safety is my concern before embarking on this test. If you think the problem lies somewhere other than the fuel pump, advice is welcomed.

5 Replies

HARRY P

MECHANIC
2,292 POSTS

Always start with the simple things first. Personally, I'd check to make sure that that test connector is receiving power. Just probe it with a multi-meter and see. Also, check your fuses. Particularly #15. A blown fuse there and your car will be dead in the water. Then, just because you have power to the fuse doesn't mean that you have power at the pump. So you'll have to check for 12 volts at the connector while someone turns the key. If there isn't 12 volts consistent power, then the problem isn't your pump, it's in the wiring somewhere.

As for testing at the pump, the good news is that you don't have to drop the fuel tank to test it out. I've attached all of the pertinent information in the images below. Let us know what you find and we'll move on with troubleshooting.

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Thursday, January 10th, 2019 AT 9:01 PM

WANKEL

MEMBER
5 POSTS

Hello Heyman:

This is what I did: I disconnected the hose at the fuel filter located near spark plugs and inserted the hose into a empty bottle in order to observe any fuel flow. I turned the ignition key to "on" position. At the passenger side strut tower, I placed a piece of wire into the electrical connector (thereby shorting the two prongs), and I was able to see the fuel flow into the bottle. Is the fuel pump the culprit? By the way, the car is a 1988 model, I don't know if it is the same setup as a 1987.
Thank you in advance.

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Saturday, January 12th, 2019 AT 2:35 PM

HARRY P

MECHANIC
2,292 POSTS

1987 and 1988 are both S4 cars, so they should be the same setup. If you're able to jump the prongs and get fuel flow from the pump, then that means that the pump is working on some level. If memory serves, these pumps should put out somewhere near 80 psi. So, in short, it wouldn't just be flowing into a bottle/bucket/catch pan. It would be spraying in there with real force. On my old 2000 Chevrolet Blazer (which puts out about 60 psi), when the fuel line popped off (previous owner didn't get it clipped in right), that sucker sprayed gas a good ten feet forward. So 80 psi would really spray with force.

Now that I looked it up, the fuel pressure should be between 64 and 85 psi for a non-turbo car, and between 71 and 92 psi for a turbo car. So basically, it would spray like a fire hydrant.

If it didn't spray like that, then I'd highly suggest borrowing a fuel pressure test gauge from the local auto parts shop and running the proper fuel pressure test. I can't seem to find any pictures of the fuel rail, but if there's a valve on there, then what you'll do is relieve the system of it's fuel pressure. Start the car and then pull the fuse for the fuel pump. The car will die. Then crank the engine for a couple seconds, at least five times. Next, hook up the fuel pressure gauge to the valve. Turn the key on for two seconds, then off. Turn it on again and observe the pressure reading. If it's not high enough, then it's time to replace the pump and filter.

Check it out and let me know. We'll go from there. Good luck.

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Saturday, January 12th, 2019 AT 9:36 PM

WANKEL

MEMBER
5 POSTS

Heyman:

I wanted to give you an update of what I found. Under the hood, near the coolant reservoir, a relay with its cap bolted in that area often comes out of its cap. I reinserted the relay back into its cap (but it is still susceptible to come off its cap, need to secure it tightly later). After I recapped the relay, I was able to get the car started. Is that relay related to the function of the fuel pump? Assuming it is: why would the cap off prevent the relay from doing its job? Ground maybe?

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Sunday, January 13th, 2019 AT 2:10 PM

HARRY P