3vze: voltage & amperage across +b and fp
02-03-2008, 07:20 AM
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3vze: voltage & amperage across +b and fp
In the process of trying to figure out why my engine looses fuel pressure I have either replaced or tested just about every thing in the fuel system. Pump, filter, efi relay and vafm have been replaced. COR and fuses check out.
I have 12 volts across the +b - fp connection with the ignition on and not running. I also have 12 volts at the fuel pump connection. Jumpering the connection will cause the fuel pump to turn on. When I engage the starter the voltage drops to zero at the +b - fp but is fine at the fuel pump connection. I assume the voltage drop at the +b - fp is due to the starter motor engaging and voltage is maintained by the COR/vafm.
I have 0.4 amps across the +b - fp connection when the engine is running. Some where I saw a post that listed the amperage across +b and fp as 4.5 +/- 0.5 amps. Does anyone know if that is correct. If so, do you have any ideas what could cause the low amperage.
The engine will idle perfectly but stepping on the acceletor causes the engine to rev up slightly and then misfire. It will usually return to idle if you release the accelerator but it sometimes dies. The fuel pressure drops as the rpms increase until the misfires occur. When idling I have 38 lbs of pressure. Reving the engine drops that to 20lbs or less. Moving the vane in the vafm causes the engine to stall almost immediately. I can play with the throttle and the vafm vane and get the engine to rev up a little bit better. The pump holds 38 lbs for at least an hour so I assume no leak back or serious injector leaks. The fuel pressure regulator action appears to be normal. Clamping off the return hose only raises fuel pressure slightly and doesn't improve running.
Any ideas on what is going on would be appreciated. I am guessing possibly a bad ground. If so it doesn't appear to affect any thing else.
Thanks,
Ike
I have 12 volts across the +b - fp connection with the ignition on and not running. I also have 12 volts at the fuel pump connection. Jumpering the connection will cause the fuel pump to turn on. When I engage the starter the voltage drops to zero at the +b - fp but is fine at the fuel pump connection. I assume the voltage drop at the +b - fp is due to the starter motor engaging and voltage is maintained by the COR/vafm.
I have 0.4 amps across the +b - fp connection when the engine is running. Some where I saw a post that listed the amperage across +b and fp as 4.5 +/- 0.5 amps. Does anyone know if that is correct. If so, do you have any ideas what could cause the low amperage.
The engine will idle perfectly but stepping on the acceletor causes the engine to rev up slightly and then misfire. It will usually return to idle if you release the accelerator but it sometimes dies. The fuel pressure drops as the rpms increase until the misfires occur. When idling I have 38 lbs of pressure. Reving the engine drops that to 20lbs or less. Moving the vane in the vafm causes the engine to stall almost immediately. I can play with the throttle and the vafm vane and get the engine to rev up a little bit better. The pump holds 38 lbs for at least an hour so I assume no leak back or serious injector leaks. The fuel pressure regulator action appears to be normal. Clamping off the return hose only raises fuel pressure slightly and doesn't improve running.
Any ideas on what is going on would be appreciated. I am guessing possibly a bad ground. If so it doesn't appear to affect any thing else.
Thanks,
Ike
02-03-2008, 08:40 AM
#2
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I recently changed the fuel pump on my truck and found the connector in side the fuel tank at the pump was fried. you may be loosing your power there.
Here's pictures of the problem I found:
The connector at Fp and +B serve to bypass the circuit opening relay (fuel pump relay).
Jump the connector and check the current through the circuit with the key on engine off I think it should be 5-6A or so If it is significantly less look for resistance in the circuit using voltage drop testing. if you can't find anything outside of the tank I'm afraid you'll be having to look inside it.
Hope this helps.
Here's pictures of the problem I found:
The connector at Fp and +B serve to bypass the circuit opening relay (fuel pump relay).
Jump the connector and check the current through the circuit with the key on engine off I think it should be 5-6A or so If it is significantly less look for resistance in the circuit using voltage drop testing. if you can't find anything outside of the tank I'm afraid you'll be having to look inside it.
Hope this helps.
02-03-2008, 01:05 PM
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Targetnut:
That sounds like a possibility. I have had the fuel pump out twice and bench checked it and it appeared to be fine.
When I do as you suggested and check the amperage with the key on and not running I get 4.86 amps. Do you know if that is upposed to remain high or drop off to the 0.4 volts I see when the engine is ideling. Jumpering the +b - fp causes the fuel pump to run but doesn't help the fuel pressure problem.
Another thought I have had is that it may be an ignition switch problem. I suppose that a COR could be the problem but it check's out fine.
Ike
That sounds like a possibility. I have had the fuel pump out twice and bench checked it and it appeared to be fine.
When I do as you suggested and check the amperage with the key on and not running I get 4.86 amps. Do you know if that is upposed to remain high or drop off to the 0.4 volts I see when the engine is ideling. Jumpering the +b - fp causes the fuel pump to run but doesn't help the fuel pressure problem.
Another thought I have had is that it may be an ignition switch problem. I suppose that a COR could be the problem but it check's out fine.
Ike
02-03-2008, 08:42 PM
#4
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when you check the voltage across Fp-+B when the engine is running you are only checking the voltage drop across the circuit opening relay. It should be low, and really it should be a bit lower than the 0.4V you measured.
does it stay there or does it increase after it runs a while.
do you have a new screen on the pump? They can get clogged by very fine sediment that may not appear to be a problem on visual inspection.
Also I have a friend who got his truck running after sitting for a few years. The pump was dead, he replaced it with a new factory part and that pump died in less than a month because of rust and sediment in his tank.
I need to check my notes at work tomorrow and see what the current is supposed to be. I think it is 5-6A but I'll double check.
does it stay there or does it increase after it runs a while.
do you have a new screen on the pump? They can get clogged by very fine sediment that may not appear to be a problem on visual inspection.
Also I have a friend who got his truck running after sitting for a few years. The pump was dead, he replaced it with a new factory part and that pump died in less than a month because of rust and sediment in his tank.
I need to check my notes at work tomorrow and see what the current is supposed to be. I think it is 5-6A but I'll double check.
01-26-2010, 02:08 PM
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Man I've got the same problem on an 89 Pickup with 3VZE. Idles perfect with 37 PSI. Stomp on the gas and it bogs down at 3500 RPM and fuel pressure drops to 18 PSI. Have new pump w/ sock, filter, regulator, mas air, & TPS. haven't tried O2 or any other sensors yet but they wouln't make pressure drop? I wish the fix or at least a follow up was posted on this thread?
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