fuel return line
Thread Starter
Racer
Joined: Sep 2002
Posts: 420
Likes: 1
From: SAN DIEGO CA
fuel return line
86 vette. Im trying to pinch off the return to diagnos a rapid drop in fuel pressure. Is the return line the top right hose I see coming from the left to right (if you were standing in front of the rear bumper). Of the two lines on the bottom which is the feed line.
Burning Brakes
Joined: Nov 2005
Posts: 848
Likes: 1
From: 1992 LT1 6 speed Cayman Islands
Not sure about a 86, however if you pinch off one of the lines only ( put a bolt on each side and clamp a visegrip on these to make sure it is blocked off properly) and the preassure goes up higher than normal it is the return line. If the preassure does not come up at all it is the preassure line. If nothing different happens it is the vapor return line.
Hope this helps
jerris
Hope this helps
jerris
Thread Starter
Racer
Joined: Sep 2002
Posts: 420
Likes: 1
From: SAN DIEGO CA
next step
ok i replaced the AFPR with a holly unit because I was getting fuel in the vacuum line connected to it. The pressure would build to 42 Psi and then drop down to 30 in about 2 seconds. I also pulled up the Injectors out of the manifold while attached to the fuel rail. I then powered up the Fuel Pump via the "G" terminal and a 12 volt battery. I pressurized it again and it went to 42 psi. I looked for leaks for about 4 minutes and they were dry as can be. I also had the injectors cleaned and pressure checked about a year ago. The injectors are about 2 years old. They are Fords. I did the pinch test and the pressure went down fairly quickly. Is the last thing the Fuel Pump? What should be my next step? 
Race Director
Joined: Sep 2000
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From: The Top of Utah
Originally Posted by PMARTINEZ92126
I did the pinch test and the pressure went down fairly quickly. Is the last thing the Fuel Pump? What should be my next step?
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Instructor
Joined: Jun 2002
Posts: 231
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From: 80/680 junction Ca.
Thread Hi-Jack!
This is an 87
Lets say the cars been sitting for an hour. Hook up fuel pressure guage, turn on ignition, zero fuel pressure, don't hear the pump turn on. Crank the engine, after a few seconds, sometime more, pressure rises and engine instantly fires up. Shut off ign. pressure returnes to zero, wait 10 seconds, turn on ignition, pump runs for 2 seconds pressure climbs and drops as soon as pump turns off. Turn off ignition and wait 5 minutes, cars right back to ignition on, no pump.
I pressurized the system with the G term. Pinched off the pressure line and de-energized the G term and it holds pressure, release clamp, pressure instantly returns to zero.
Questions.
This is an 87
Lets say the cars been sitting for an hour. Hook up fuel pressure guage, turn on ignition, zero fuel pressure, don't hear the pump turn on. Crank the engine, after a few seconds, sometime more, pressure rises and engine instantly fires up. Shut off ign. pressure returnes to zero, wait 10 seconds, turn on ignition, pump runs for 2 seconds pressure climbs and drops as soon as pump turns off. Turn off ignition and wait 5 minutes, cars right back to ignition on, no pump.
I pressurized the system with the G term. Pinched off the pressure line and de-energized the G term and it holds pressure, release clamp, pressure instantly returns to zero.
Questions.
- Can the check valve be fixed or replaced, or is the only way to fix it by replacing fuel pump?
- Even if the check valve was good, how is 40 lbs. of fuel pressure kept from leaking past the connections at the pump, pulsator and sending unit? I'm suprised all the fuel doesn't just leak at these locations right back into the tank when the pump is running.
- Why when I wait past a certain period of time does the ECM not turn on the pump when the ignition is turned on? I thought the pump ran for 2 seconds any time the ignition was turned on.
Le Mans Master
Joined: Oct 2003
Posts: 6,130
Likes: 2
From: seattle WA
Maximus-if you're not hearing the fuel pump come on when key is turned on then your FP relay is capute! and it will start only after oil pressure reaches 4 lbs. I don't know if the check valve can be replaced-but if I'm going in I'm replacing entire FP and sock.
Last edited by rick lambert; Feb 14, 2006 at 10:03 AM.
Race Director
Joined: Sep 2000
Posts: 17,298
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From: The Top of Utah
Originally Posted by rick lambert
Maximus-if you're not hearing the fuel pump come on when key is turned on then your FP relay is capute! and it will start only after oil pressure reaches 4 lbs.
Originally Posted by Maximus9000
Shut off ign. pressure returnes to zero, wait 10 seconds, turn on ignition, pump runs for 2 seconds pressure climbs and drops as soon as pump turns off.
The check valve is integral with the fuel pump. Since a bad check valve only allows the fuel system to depressurize back into the tank, and doesn't leak fuel to anywhere it doesn't belong, the check valve (fuel pump) CAN be retained, with the only draw back being extended cranking times. It goes against my grain to replace a pump that is pumping perfectly well.
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Burning Brakes
Joined: Nov 2005
Posts: 848
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From: 1992 LT1 6 speed Cayman Islands
Check your fuel pump relay and your FP fuse. Change the FP relay and you should hear the 2 sec pump run and have preassure without cranking, if not check the wireing for current supply when key is fifst turned on.
Would not worry too muck about the check valve although yours is definately defective. If you fix the relay problem just torn on the key for 2 secs before cranking and you will have preassure for instant starting.
Jerris
Would not worry too muck about the check valve although yours is definately defective. If you fix the relay problem just torn on the key for 2 secs before cranking and you will have preassure for instant starting.
Jerris
Race Director
Joined: Sep 2000
Posts: 17,298
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From: The Top of Utah
Originally Posted by Jerris
Check your fuel pump relay and your FP fuse. Change the FP relay and you should hear the 2 sec pump run and have preassure without cranking, if not check the wireing for current supply when key is fifst turned on.
DON'T replace the fuel pump relay without further trouble shooting. The pump runs for the 2 second period, some of the time. Now it's time to figure out why not all of the time. Is the ECM not always sending the signal? Is there a problem with the wiring between the ECM and the relay? Or is the relay working intermittently? Don't throw money at the problem replacing possibilities. Determine the cause and FIX it.
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Thread Starter
Racer
Joined: Sep 2002
Posts: 420
Likes: 1
From: SAN DIEGO CA
I just received my FSM and I am a little confused about where the Fuel pump relay is. On page 6c-5 its says the Fuel pump relay is above the battery. Im assuming its near the pillar post where the hood latches down. Now on page 6d5-5 it say the Fuel relay is near the window wiper motor. Is there two of them. I drive an 86 automatic.
Safety Car
Joined: Apr 2004
Posts: 4,532
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From: Lahaina Hi
I can't remember ,and I was just playing with yesterday.
On yours it should be right near the wiper motor almost on the fire wall.
It has a bunch of wires going to it.
Its a black box almost on the firewall next to the break booster.about 11/2 inch by 2 inches.
Looks like the burn off relays near the battery.
On yours it should be right near the wiper motor almost on the fire wall.
It has a bunch of wires going to it.
Its a black box almost on the firewall next to the break booster.about 11/2 inch by 2 inches.
Looks like the burn off relays near the battery.
Drifting
Joined: May 1999
Posts: 1,478
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From: Sacramento, CA Money can't buy happiness - but it's more comfortable to cry in a Corvette than a Yugo.
On an 87 with a manual tranny you have 2 relays on the firewall near the wiper motor. One is for the fuel pump and one is for the manual tranny OD unit. They are both the same part number GM # 14078915. If your 87 is a manual tranny, you can swap the OD relay with the fuel pump relay to see if it is the relay.
According to GM the 87 MAF power and burn-off relays and the 88 MAF power and burn-off relays are the same, part # 14089936. On the 88 the OD relay is the same as the 88 MAF relays, so even if you don't have a manual 87, you should be able to swap the fuel pump relay with the MAF relay to test. The 87 MAF burn-off relay is the one on the left just under the battery against the firewall. If you use the MAF relay, I'd just substitute it in for the fuel pump relay and leave the FP relay out of the system rather than putting it in the burn-off spot. You may set an MAF code, but that is easy to clear.
If you swap relays and that doesn't fix the problem, it still could be an intermittant issue in the wiring from the relay to the ECM. If the relay is at fault, take it to NAPA and get a good quality replacement for about 75% of the cost of a GM part.
According to GM the 87 MAF power and burn-off relays and the 88 MAF power and burn-off relays are the same, part # 14089936. On the 88 the OD relay is the same as the 88 MAF relays, so even if you don't have a manual 87, you should be able to swap the fuel pump relay with the MAF relay to test. The 87 MAF burn-off relay is the one on the left just under the battery against the firewall. If you use the MAF relay, I'd just substitute it in for the fuel pump relay and leave the FP relay out of the system rather than putting it in the burn-off spot. You may set an MAF code, but that is easy to clear.
If you swap relays and that doesn't fix the problem, it still could be an intermittant issue in the wiring from the relay to the ECM. If the relay is at fault, take it to NAPA and get a good quality replacement for about 75% of the cost of a GM part.
