Fuel pressure 1986
Thread Starter
Instructor
Joined: Jan 2010
Posts: 123
Likes: 1
From: Lakeland Florida
Fuel pressure 1986
Car is hard starting when cold. I need to crank it 3 or 4 times then push pedel to the floor to start it. So i do a fuel pressure test.turn key on for 2-3 sec. Pressure goes to 42 lbs than drops to 20 in about 10 sec. Couple min. Later. To 10 lbs and pretty much stays there.
Now what??
Now what??
Melting Slicks
Joined: Nov 2008
Posts: 2,765
Likes: 5
Car is hard starting when cold. I need to crank it 3 or 4 times then push pedel to the floor to start it. So i do a fuel pressure test.turn key on for 2-3 sec. Pressure goes to 42 lbs than drops to 20 in about 10 sec. Couple min. Later. To 10 lbs and pretty much stays there.
Now what??
Now what??
A fuel injector that is leaking down.
Or a fuel pump pulsator ***. that is leaking fuel. That part is located within the gas tank on the fuel pump hanger assembly.
Do You have a 1986 GM Factory service manual ? It would make You life easier troubleshooting your problem with your 86 vette.
Its all outlined in the GM service manual troubleshooting this particular problem.
BR
Melting Slicks
Joined: Dec 2009
Posts: 2,235
Likes: 286
Prime the system & pinch off the return line, watch for pressure drop.
Pull the vacuum hose on the regulator, prime the system, no fuel should be present.
You can prime the system & pinch off the feed line too.
Pull the vacuum hose on the regulator, prime the system, no fuel should be present.
You can prime the system & pinch off the feed line too.
Team Owner
Joined: Jan 2003
Posts: 20,315
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From: Summerland B.C. Canada
Car is hard starting when cold. I need to crank it 3 or 4 times then push pedel to the floor to start it. So i do a fuel pressure test.turn key on for 2-3 sec. Pressure goes to 42 lbs than drops to 20 in about 10 sec. Couple min. Later. To 10 lbs and pretty much stays there.
Now what??
Now what??
http://members.shaw.ca/agent86/Fuel%...20Delivery.pdf
http://members.shaw.ca/corvette86/Co...0View%2086.pdf
Time to pinch the fuel lines under gas door. I had a similar drop, it was the pulsator(replaced it with injection hose and clamps). Didn't cause me any starting problems.
You may have a problem at the fuel pump relay and are now relying on the oil pressure switch. It may take long cranking to close the oil pressure switch.
Team Owner
Joined: Jan 2009
Posts: 73,568
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From: Somewhere in Mo
St. Jude Donor '09-'10, '14
Does the car smoke at start up? If so it more than likely your injectors leaking down. If it does not it points more to the pulsator or fuel pump check valve.
Regulator maybe, but they usually go to zero if they are bad.
Pinch off the lines to be sure.
Are you flooring it to correct a flood situation?
Also what is the reading after about 15 minutes, does it stay there as you suggest?
Regulator maybe, but they usually go to zero if they are bad.
Pinch off the lines to be sure.
Are you flooring it to correct a flood situation?
Also what is the reading after about 15 minutes, does it stay there as you suggest?
Last edited by RetiredSFC 97; Feb 22, 2010 at 11:29 AM.
Thread Starter
Instructor
Joined: Jan 2010
Posts: 123
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From: Lakeland Florida
agent 86, i took off the fuel feed line and fuel return line from gas tank blocked both. took pressure test nothing. i hope i did it wright. gas did come out of feed line from tank.
dont know how to pm?
dont know how to pm?
Thread Starter
Instructor
Joined: Jan 2010
Posts: 123
Likes: 1
From: Lakeland Florida
Does the car smoke at start up? If so it more than likely your injectors leaking down. If it does not it points more to the pulsator or fuel pump check valve.
Regulator maybe, but they usually go to zero if they are bad.
Pinch off the lines to be sure.
Are you flooring it to correct a flood situation?
Also what is the reading after about 15 minutes, does it stay there as you suggest?
Regulator maybe, but they usually go to zero if they are bad.
Pinch off the lines to be sure.
Are you flooring it to correct a flood situation?
Also what is the reading after about 15 minutes, does it stay there as you suggest?
Safety Car
Joined: Sep 2001
Posts: 4,157
Likes: 554
From: Glenbeulah Wi
Car is hard starting when cold. I need to crank it 3 or 4 times then push pedel to the floor to start it. So i do a fuel pressure test.turn key on for 2-3 sec. Pressure goes to 42 lbs than drops to 20 in about 10 sec. Couple min. Later. To 10 lbs and pretty much stays there.
Now what??
Now what??
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Le Mans Master
Joined: Sep 2006
Posts: 6,050
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From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
to PM, click on someone's name, there will be a drop down menu. Click on send a private message, and a message screen appears. type your message, then send.
To check for starting problems, you started in the right place. Fuel delivery pressure is of the utmost importance in a computer controlled engine.
Fuel pressure is important for injector nozzle atomization.
When the engine starts, the computer does many things, it adds start fuel, then pulls back, does a self check, and a vapor cannister purge, monitors all sensors, and then begins to regulate based on temperature, all the while keeping the already engine running..
You have to analyze what your engine is doing to discover where the fault may lie.
First, get the fuel pressure resolved, then decide if a sensor is at fault.
move on from there.
To check for starting problems, you started in the right place. Fuel delivery pressure is of the utmost importance in a computer controlled engine.
Fuel pressure is important for injector nozzle atomization.
When the engine starts, the computer does many things, it adds start fuel, then pulls back, does a self check, and a vapor cannister purge, monitors all sensors, and then begins to regulate based on temperature, all the while keeping the already engine running..
You have to analyze what your engine is doing to discover where the fault may lie.
First, get the fuel pressure resolved, then decide if a sensor is at fault.
move on from there.
Last edited by coupeguy2001; Feb 22, 2010 at 10:05 PM.
Pro
Joined: Mar 1999
Posts: 678
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From: Oslo, Norway
Hi
I read your first message here and I am not sure if I understood it right.
Did you turn the key on and measured the pressure without starting? and then measured the pressure over time?
If so the following happens. When turning on the key the fuelpump works in about 2-3 seconds then stops. It does not come back on until the engine runs so what you have measured is the leakdown in the system. You have to start the engine to see if the fuel pressure is on and have a constant value on idle That is a value some 5 psi lower than without vacuum hose connected. If you drve measure the pressure and see if there is a significant loss when loading the engine. If so there is something wrong.
Starting when cold and having difficulties may be a bad coldstart injector placed mid intake on drivers side where the runners connect at the intake. This is often a problem.
When regards to starting when hot and no difficulties indicates no injector leaks. I had a longer starting seacne when hot and changed the injectors to genIII frim FIC and now the ca starts nearly before the key is in the lock,lol.
If the fuelpressure decrease when loading the engine I would have looked after the fuel filter and changed that and see if it changed .
Good luck
I read your first message here and I am not sure if I understood it right.
Did you turn the key on and measured the pressure without starting? and then measured the pressure over time?
If so the following happens. When turning on the key the fuelpump works in about 2-3 seconds then stops. It does not come back on until the engine runs so what you have measured is the leakdown in the system. You have to start the engine to see if the fuel pressure is on and have a constant value on idle That is a value some 5 psi lower than without vacuum hose connected. If you drve measure the pressure and see if there is a significant loss when loading the engine. If so there is something wrong.
Starting when cold and having difficulties may be a bad coldstart injector placed mid intake on drivers side where the runners connect at the intake. This is often a problem.
When regards to starting when hot and no difficulties indicates no injector leaks. I had a longer starting seacne when hot and changed the injectors to genIII frim FIC and now the ca starts nearly before the key is in the lock,lol.
If the fuelpressure decrease when loading the engine I would have looked after the fuel filter and changed that and see if it changed .
Good luck
Team Owner
Joined: Jan 2003
Posts: 20,315
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From: Summerland B.C. Canada
Here is a pic that shows which is feed and which is the return line.
Le Mans Master
Joined: Sep 2006
Posts: 6,050
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From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
good for running, not good for starting. Your pump comes on to prime the system, then shuts off, and if your fuel pressure drops to 20, that's your starting fuel pressure until the distributor tells the computer that you have started, it's not allowing more fuel to get to the injectors under 40 psi because the computer didn't turn the pump back on.
So you are starting with 20 PSI.
Now, you didn't tell us what the ambient temperature is, that may make a slight difference whether or not the cold start injector works or not.
Remember, you have 9 injectors
YOU NEED A GOOD STEADY 40 psi when it's shut off. that determines if you have internal leaks.
External leaks are easy to see, but dangerous with a hot engine.
Internal leaks are hard to find, and cause operational problems. Like hard starting when cold.
If you have a leak, you need to get it fixed.
I determined a long time ago that the pulsator needs to be removed. After 25 years, it doesn't have the leak stopping ability it used to.
Use fuel injection hose, not fuel hose. Fuel hose has a burst rating of 15 PSI and fuel injection hose has a burst rating of 60 PSI.
Pump output can spike to 55, so you want the extra strength.
So you are starting with 20 PSI.
Now, you didn't tell us what the ambient temperature is, that may make a slight difference whether or not the cold start injector works or not.
Remember, you have 9 injectors
YOU NEED A GOOD STEADY 40 psi when it's shut off. that determines if you have internal leaks.
External leaks are easy to see, but dangerous with a hot engine.
Internal leaks are hard to find, and cause operational problems. Like hard starting when cold.
If you have a leak, you need to get it fixed.
I determined a long time ago that the pulsator needs to be removed. After 25 years, it doesn't have the leak stopping ability it used to.
Use fuel injection hose, not fuel hose. Fuel hose has a burst rating of 15 PSI and fuel injection hose has a burst rating of 60 PSI.
Pump output can spike to 55, so you want the extra strength.
Last edited by coupeguy2001; Feb 23, 2010 at 01:50 PM. Reason: info
Team Owner
Joined: Jan 2003
Posts: 20,315
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From: Summerland B.C. Canada
