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Fuel pressure loss???

 
Old 06-11-2019, 02:07 PM
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larryhobman
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Default Fuel pressure loss???

I have been working on getting my 1986 running after 10 years of sitting. I bought the car not running miles are 79k. I drained and cleaned the tank, new gasket and fuel filter. My pressure with the engine off is 39psi. Running pressure is about 34psi. When I open the throttle the tire will spin then a huge bog and will stall. When it is about to stall the pressure drops to 20 psi or less if I do not let up on the pedal. If I take it easy I can build up speed. I ohm tested the injectors and they are all about 19 ohms and checked pulse with a noid light. If I pull the vac line off of the regulator the pressure goes up as it should. I cleaned all of the grounds too. No engine codes, all clear. I need to know what to look at next, is my regulator still bad from sitting so many years, or do I have an injector problem? I needs some help before I order parts and do the work and still runs like crap. Let know your thoughts please.
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Old 06-11-2019, 02:10 PM
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dr_gallup
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Sounds like the pump to me. Pressure should not drop like that.
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Old 06-11-2019, 02:18 PM
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larryhobman
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Sorry, I missed that I did put in a new fuel pump too. The fuel pump was sized up when I got the car.
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Old 06-11-2019, 04:10 PM
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aklim
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I can recommend you replace everything seeing as how the stock injectors might be Multecs all the way to the pump but if it were me, find out where the pressure is going first.
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Old 06-12-2019, 12:36 AM
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JackDidley
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Fuel filter.
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Old 06-12-2019, 10:22 AM
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KyleF
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Originally Posted by larryhobman View Post
Sorry, I missed that I did put in a new fuel pump too. The fuel pump was sized up when I got the car.
Unfortunately, that means nothing. New parts do not mean verified good parts.

Regulator is not the issue because you tested if it can control the pressure and it did when you pulled the vacuum line.

Have you done a leak down test? KOEO to bring up pressure and then turn the key back off. Leave the gage hooked up and and track how it falls off. Should be a bit quick at first, but after 30 minute you should still see about 18-20PSI. If not, you have a leaking injector or a bad check valve. The only thing I can think here is that when you go WOT if an injector is hanging open you may not have enough volume to support the others and pressure drops.

Have you pulled the plugs? Is one or more look different than the rest? This could also help verify an injector hanging open.


All that are good to know test, but I bet your pump is either not getting enough voltage or is failing. It appears it can make pressure against a low flow (idle) and can provide fuel to slowly accelerate the car because the volume demand isn't that great, but when you got WOT it can't keep up the volume and pressure drops.

Other thing it could be is an obstruction in flow limiting volume as well. I know you said you cleaned the tank, but it could have had gunk in the lines that have choked flow somewhere.
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Old 06-12-2019, 10:54 AM
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aklim
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Why not crimp off the lines and test
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Old 06-12-2019, 12:10 PM
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Originally Posted by aklim View Post
Why not crimp off the lines and test
Test what? What is that suppose to tell him? - Honest question since you didn't provide detail

Dead Head Pressure? He is not having trouble making pressure, it is maintaining it when the engine needs fuel. Something is obviously causing a lack of volume delivery. He already verified the regulator is doing it's thing but still has pressure drop when he goes deep in the throttle.

What should be max dead head pressure? Never had to test it and can't say I have every seen that stated.

There is also to take the feed line off and jump the fuel pump on and fill a container to check for flow. I don't remember off the top of my head but I know there is a Volume that should be supplied in a unit of time that you can also check. I think I saw that in a Haynes?

Last edited by KyleF; 06-12-2019 at 12:17 PM.
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Old 06-12-2019, 12:19 PM
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Originally Posted by KyleF View Post
Test what? What is that suppose to tell him? - Honest question since you didn't provide detail

Dead Head Pressure? He is not having trouble making pressure, it is maintaining it when the engine needs fuel. Something is obviously causing a lack of volume delivery. He already verified the regulator is doing it's thing but still has pressure drop when he goes deep in the throttle.

What should be max dead head pressure? Never had to test it and can't say I have every seen that stated.

There is also to take the feed line off and jump the fuel pump on and fill a container to check for flow. I don't remember off the top of my head but I know there is a Volume that should be supplied in a unit of time that you can also check. I think I saw that in a Haynes?
Why is he having trouble maintaining it? Could the injectors or the check valve in the pump or maybe the pulsator be leaking pressure? He might be maintaining it in spite of the leaks but at WOT it might be more pronounced?
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Old 06-12-2019, 12:27 PM
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KyleF
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Originally Posted by aklim View Post
Why is he having trouble maintaining it? Could the injectors or the check valve in the pump or maybe the pulsator be leaking pressure? He might be maintaining it in spite of the leaks but at WOT it might be more pronounced?
So, is crimping the lines a test done when running or crimp line and shut the engine off like a leak down test?

I have never had a check valve go bad, but should it cause an issue while the pump is running.? Shouldn't that cause a difficult to start issue where it can't maintain pressure at prime to be ready for the crank cycle? Seems as long as the pump is running the valve should be open.

Could be cruded up and not able to open all the way.

Last edited by KyleF; 06-12-2019 at 12:28 PM.
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Old 06-12-2019, 01:09 PM
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larryhobman
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How do I check the fuel pump pulsator? That is the piece that the pump goes into on the line in, right???
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Old 06-12-2019, 02:24 PM
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Originally Posted by KyleF View Post
So, is crimping the lines a test done when running or crimp line and shut the engine off like a leak down test?

I have never had a check valve go bad, but should it cause an issue while the pump is running.? Shouldn't that cause a difficult to start issue where it can't maintain pressure at prime to be ready for the crank cycle? Seems as long as the pump is running the valve should be open.

Could be cruded up and not able to open all the way.
I crimp the feed line. You turn on the key and have someone immediately crimp off the feed line. If it holds pressure it is something in the tank. If it doesn't, I'd repeat the test after waiting a few minutes and crimp off the return line and see.
If it holds pressure, probably regulator. If not I suspect injectors.

Don't know but there may be more than one issue. Might as well analyze it and rule it out before we proceed? I think.

Last edited by aklim; 06-12-2019 at 03:04 PM.
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Old 06-12-2019, 03:05 PM
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Originally Posted by larryhobman View Post
How do I check the fuel pump pulsator? That is the piece that the pump goes into on the line in, right???
If the fuel pressure holds when you crimp off the supply line, I would think that it is anything from the crimp to the pickup area in the tank?
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Old 06-12-2019, 04:55 PM
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Originally Posted by aklim View Post
I crimp the feed line. You turn on the key and have someone immediately crimp off the feed line. If it holds pressure it is something in the tank. If it doesn't, I'd repeat the test after waiting a few minutes and crimp off the return line and see.
If it holds pressure, probably regulator. If not I suspect injectors.

Don't know but there may be more than one issue. Might as well analyze it and rule it out before we proceed? I think.

I see now, Isolating feed from the fuel rail.
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Old 06-12-2019, 06:16 PM
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Originally Posted by KyleF View Post
I see now, Isolating feed from the fuel rail.
Just wanted to make sure we are not chasing a problem AND a leak in the system.
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Old 06-13-2019, 07:07 PM
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Had this exact same problem couple weeks ago. I had replaced my entire pump assembly with a new one. The pulsator had a crack in it I couldn't see. I replaced it with submersible hose and all good. You can test it first with regular fuel injection hose but don't leave that in there. Good thing it's easy to get the pump out
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