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Discussion Starter · #1 · (Edited)
Question for you fox guys.


What is the voltage supposed to be at the pump when key is first turned to prime the pump?


I have a 70 Mustang with a 89 5.0 and 5 speed installed along with a Painless EFI harness. When I first turn the key to run the voltage is around 12 volts, tried starting it many times with no luck, then drops down and sits around 6.87 volts.



The fuel pump does kick on and run then shuts off like it's supposed to.



I did a fuel pressure check today and at first I got around 8 PSI. A few more prime cycles and it sat around 20 but then quickly dropped off. I ran straight 12 volts to the pump and it held around 40psi but the car still wouldn't start. Removal of the 12 volts the pressure dropped to almost nothing. I have a new pump and pressure regulator on their way.



Any who, voltage at the pump when key first turned??
 

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Discussion Starter · #3 ·
The voltage (12 from relay) primes then drops off if the ecm doesn't see ignition pulse. Sounds like ignition(TFI?)



That's the thing tho, I'm not trying to start it. I'm just testing things by myself at the moment and don't have the ability to crank the car over and test the wires at the pump at the same time.



I had my 12 year old son helping earlier and he was affraid of starting the car, even tho it wouldn't start because of the low fuel pressure.
 

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If you don't crank it, you will lose voltage in a few seconds. I'm not familiar with the painless setup, but my fuel pump relay is situated in the trunk. I used bungee cords to hold in view the relay and test light in the cavity that feeds the pump while I cranked. My issue was a poorly grounded TFI. Some parts stores still have the machine to test the ignition module off the car. I'd setup a spark tester where you could see it while cranking and check spark. These things are great when they run, frustrating when they don't!
 

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Discussion Starter · #5 ·
If you don't crank it, you will lose voltage in a few seconds. I'm not familiar with the painless setup, but my fuel pump relay is situated in the trunk. I used bungee cords to hold in view the relay and test light in the cavity that feeds the pump while I cranked. My issue was a poorly grounded TFI. Some parts stores still have the machine to test the ignition module off the car. I'd setup a spark tester where you could see it while cranking and check spark. These things are great when they run, frustrating when they don't!

My relays and the fuses for the Painless harness are going to be mounted in the glove box. The ECM is mounted behind the dash as well. I agree about being frustrating. At one time I got the car to run, rather rough, and eventually found out the ECM was bad. Sent it out and got it fixed and now I can't get it to run at all, even like crap. Today we tested the fuel pressure and it was really low so a new fuel pump will be going in once it arrives. Fingers crossed that's the problem. If not, I'll look into the TFI and see if Autozone or Oreillys can test it.
 

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My relays and the fuses for the Painless harness are going to be mounted in the glove box. The ECM is mounted behind the dash as well. I agree about being frustrating. At one time I got the car to run, rather rough, and eventually found out the ECM was bad. Sent it out and got it fixed and now I can't get it to run at all, even like crap. Today we tested the fuel pressure and it was really low so a new fuel pump will be going in once it arrives. Fingers crossed that's the problem. If not, I'll look into the TFI and see if Autozone or Oreillys can test it.
It sounds as if, when energized, your pump ran 40psi. Don't know about the rest of your setup, but if it consists of used components, anything can get ya'. My donor car sat 20 years. Pinfit and bad injectors drove me nuts. Keep pounding it out, it's worth it. BTW call around before you run all over with this module. Only one Advanced auto had the machine AND the adapter to test. These fox bodies are classics now; I saw one yesterday with an antique tag!
 

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65 Fastback 289 4 spd, 65 convertible 5.0L 5 spd. 3.73 8.8
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Question for you fox guys.


What is the voltage supposed to be at the pump when key is first turned to prime the pump?


I have a 70 Mustang with a 89 5.0 and 5 speed installed along with a Painless EFI harness. When I first turn the key to run the voltage is around 12 volts, tried starting it many times with no luck, then drops down and sits around 6.87 volts.



The fuel pump does kick on and run then shuts off like it's supposed to.



I did a fuel pressure check today and at first I got around 8 PSI. A few more prime cycles and it sat around 20 but then quickly dropped off. I ran straight 12 volts to the pump and it held around 40psi but the car still wouldn't start. Removal of the 12 volts the pressure dropped to almost nothing. I have a new pump and pressure regulator on their way.



Any who, voltage at the pump when key first turned??
You have a voltage drop somewhere. With a volt meter ck at the fuse, then the relay, then at the pump. Also ck the ground side. So to ck a voltage drop for instance if you take a volt meter across the fuse one prong on one side and the other on the other sid, set to volts you should have very small amount of drop (.oo1) would be good. across any connector, fuse, etc. if you show 5 volts that is your problem.
 

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You have a voltage drop somewhere. With a volt meter ck at the fuse, then the relay, then at the pump. Also ck the ground side. So to ck a voltage drop for instance if you take a volt meter across the fuse one prong on one side and the other on the other sid, set to volts you should have very small amount of drop (.oo1) would be good. across any connector, fuse, etc. if you show 5 volts that is your problem.
5 month old post.....
 

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Just some guy
67 coupe, 69 Sportsroof, 86 hatchback
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You have to get in the have to get in the habit of checking the thread origination dates. At least if you are browsing the forum in the traditional way. I don't use the new/trending posts feature myself.
You have to admit kakn never did come back to close this out. In fact, now that I look, he basically didn't come back at all.
 
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Discussion Starter · #11 ·
Sorry for the bump. I never got notifications that anyone had responded.

Welp, here I am a year later and the car's still not running right. I installed a new fuel pump. If the car sits for a few days or weeks and I turn the key to prime the pump it doesn't read on my fuel pressure gauge. Went out today after getting the car back from a mechanic on Wednesday and it took three turns of the key to fully prime the pump. I get 40PSI at the gauge when the car is running and when I shut the car off it drops down to zero in about 15 minutes. Not sure if the new pump is weak or what's going on. Mechanic mentioned that the PSI drop is faster than normal but not a whole lot to worry about.

That about sums it all up. I have pretty much gotten no where with this car in a years time lol. i'll be sure to close this out if I can figure out how to so you fellas don't get another couple month bump.
 

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Just some guy
67 coupe, 69 Sportsroof, 86 hatchback
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Well, we heard you were dead. :)

OK, not really. A little notice is all it takes. I've been working on a car over 20 years now, I can certainly cut a guy some slack for a few months. Appreciate the note, now you can take as long as you like to come back by with an update and we won't talk about you. Probably.

I've had some similar issues on an RV, whose pump is pretty darn expensive. People are telling me the only for sure solution is to spend the money for the genuine Ford pump. I'm kind of thinking maybe I can fit a performance Fox pump instead. Maybe. Not a huge priority right now.

I've seen Ford fuel pressures not build all the way up in the two second "prime" function where you turn the key on when they've been sitting a long time but bump right up to 40 psi if they have not been sitting long. So I figure that to be normal. The leakdown of around 15 minutes bothers me though. It should take longer than that. Did you mechanic check for a leaky injector? They sometimes dribble and bleed off fuel pressure.
 

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Discussion Starter · #13 · (Edited)
Not dead, but my brother is. That's how I inherited the two Mustangs I own.

I treid out a new fuel pump today from Oreillys and it decided to work when it wanted. The new pump would turn on but not prime the system. I did get it a few times to do so and got 40 PSI at the gauge tho. I was able to start the car once and that was it. Every other turn of the key turned on the pump but not prime. I put the old pump back in it and not once did it have any issues NOT running. So I took the new pump back.

On a side note.....the new pump, when it actually worked, lost pressure quicker than the old one. I might have to try a different external fuel pump. My late brother actually grafted on part of a Fox Body gas tank to the stock 70 so he could use the stock fox fuel pump assembly in tank. He was a metal fabricator and even moved over the fox baffle in the tank. I have a ton of photos of him doing the mod. The only bad part is the floor of the 70's trunk isn't flat and the darn fuel lines are in the trunk. Heck, he even managed to retain the fox body fuel lines and swapped them onto the 70. Those lines tho will eventually be replaced.

Edit: forgot to answer your question. No. He did not mention testing the injectors. I had thought the same thing a while ago but I'm not mechanically inclined to test for such a thing. The injectors and everything engine related is all comprised of used parts so I wouldn't be surprised if that wasn't a possibility.
765078
 

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Just some guy
67 coupe, 69 Sportsroof, 86 hatchback
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Hmmm. I had another one that acted flaky something like that. Worked, then didn't. Turned out to be an intermittently bad connection in the "socket" the fuel pump relay plugged into.
 

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That tank looks like a good start to a fiery death in a 1970 Mustang. There are aftermarket drop-in EFI tanks for these cars that don't have JB Weld holding the flammable stuff in.
 

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Just some guy
67 coupe, 69 Sportsroof, 86 hatchback
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It may have already happened. He hasn't been here since this thread was current.:oops:
 

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I knew it was from five months ago. Being familiar with project car timelines myself, it was a calculated risk that maybe the tank hadn't been installed yet. 90% of this forum's front page is older than five months anyway. You can't post in any thread without resurrecting something.
 

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Just some guy
67 coupe, 69 Sportsroof, 86 hatchback
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I meant the "fiery death" bit. :whistle:
 

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I caught it, but though I was getting elbowed. 😐
 

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Just some guy
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Not this time it wasn't. Though poking people who revive old and actually dead posts is known to happen...
 
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