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No Power At Fuel Pump On 91 Ls400


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#1 mpeterson

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Posted 25 August 2010 - 02:45 PM

i just bought the car it doesn't run... the way he described it dying on him was a rough idle.. if he pushed a the gas the miss would get worse then would have to keep on the gas to keep it started. Then it finally died. i have found that there is spark, timing "should" be on, but there is no power at the pump the wires seem good all the way back to the front of the vehicle then they disappear behind the ecu. no power at the fuel pump relay either.... im stuck is there a security issue or vacuum issue? Half tempted to take it to THE MAN and pay for it but trying to avoid the bill. any ideas would be helpful Thanks!!!

btw i can use a jumper to turn the fuel pump on but still doesn't start even using ether still no kick

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#2 fsuguy

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Posted 25 August 2010 - 03:04 PM

i just bought the car it doesn't run... the way he described it dying on him was a rough idle.. if he pushed a the gas the miss would get worse then would have to keep on the gas to keep it started. Then it finally died. i have found that there is spark, timing "should" be on, but there is no power at the pump the wires seem good all the way back to the front of the vehicle then they disappear behind the ecu. no power at the fuel pump relay either.... im stuck is there a security issue or vacuum issue? Half tempted to take it to THE MAN and pay for it but trying to avoid the bill. any ideas would be helpful Thanks!!!

btw i can use a jumper to turn the fuel pump on but still doesn't start even using ether still no kick


Not sure whether this may help, but have you checked your fuse block inside the car? Under the dash by the pedals, there is a fuse block. In my '92 there is a plastic cover, and yours may be similar.

HTH

mike

#3 mpeterson

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Posted 25 August 2010 - 03:15 PM


i just bought the car it doesn't run... the way he described it dying on him was a rough idle.. if he pushed a the gas the miss would get worse then would have to keep on the gas to keep it started. Then it finally died. i have found that there is spark, timing "should" be on, but there is no power at the pump the wires seem good all the way back to the front of the vehicle then they disappear behind the ecu. no power at the fuel pump relay either.... im stuck is there a security issue or vacuum issue? Half tempted to take it to THE MAN and pay for it but trying to avoid the bill. any ideas would be helpful Thanks!!!

btw i can use a jumper to turn the fuel pump on but still doesn't start even using ether still no kick


Not sure whether this may help, but have you checked your fuse block inside the car? Under the dash by the pedals, there is a fuse block. In my '92 there is a plastic cover, and yours may be similar.

HTH

mike



yeahp and the fuses are all good!

#4 AndyLowe

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Posted 26 August 2010 - 07:10 AM

First confirm you have spark on BOTH banks of the engine. (I suspect you have spark on one side only).

Next, buy a subscription to www.alldatadiy.com and extract the Diagnostic Trouble Codes, via a paper clip and the Diagnostic Connector on the top of the engine. Research what these codes are and how they relate to your symptoms.

The wiring diagrams on your alldatadiy.com subscription will prove very valuable to solving the Fuel Pump Relay having no voltage.

Your subscription will also provide you which two connectors on the Diagnostic Connector on top of the engine to jumper, so that you BYPASS the Fuel Pump Relay entirely and give the Fuel Pump +12v.

When you do this, use your ears to hear the fuel rail. Do you hear fuel flowing through it?

If you have fuel, and you have spark on BOTH Banks, you have probably skipped time on your Timing Belt.

When any engine jumps its timing belt a few teeth, it will not start, but will have spark and fuel pressure.

Let us know!

#5 mpeterson

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Posted 26 August 2010 - 08:52 PM

i do have spark on both sides im thinking its timing but havent got to it yet. one thing that still boggles my mind is why there isnt power to the fuel pump it seems like the car shut it off..

#6 curiousB

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Posted 26 August 2010 - 10:30 PM

i do have spark on both sides im thinking its timing but havent got to it yet. one thing that still boggles my mind is why there isnt power to the fuel pump it seems like the car shut it off..



Well if timing was good you should have gotten some reaction to the ether down the intake manifold test.

Maybe hardwire the fuel pump to 12VDC for a quick test (feed it from a fused branch circuit so you don't feed 80 Amps to a potentially shorted out fuel pump, can you say Kaboom). Be sure to get polarity correct as its a DC motor and don't backfeed the 12 VDC to the control relay (ie disconnect the connector). See if you can get the engine running knowing the fuel pump is pumping. If as you say if it has spark and the timng is in tact then with fuel it should run.

#7 mpeterson

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Posted 27 August 2010 - 09:18 PM


i do have spark on both sides im thinking its timing but havent got to it yet. one thing that still boggles my mind is why there isnt power to the fuel pump it seems like the car shut it off..



Well if timing was good you should have gotten some reaction to the ether down the intake manifold test.

Maybe hardwire the fuel pump to 12VDC for a quick test (feed it from a fused branch circuit so you don't feed 80 Amps to a potentially shorted out fuel pump, can you say Kaboom). Be sure to get polarity correct as its a DC motor and don't backfeed the 12 VDC to the control relay (ie disconnect the connector). See if you can get the engine running knowing the fuel pump is pumping. If as you say if it has spark and the timng is in tact then with fuel it should run.


exactly.... but when i do exactly that still nothing lol im about ready to light it on fire lol just kidding just kidding

#8 curiousB

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Posted 27 August 2010 - 10:21 PM

Can you hear any noise in the fuel tank when you power the fuel pump? You should hear a soft noise for the FP running. If you hear a noise but no start then maybe you need to buy a fuel pressure gauge. They aren't very expensive. Around $20. Could be FP is running but not building any pressure. Otherwise you're stuck guessing why it won't run. At some point you need to confirm or reject theories or you be stuck running in circles.

Its odd the ether test didn't get a slight stumble start from the engine but I'd complete the fuel path before jumping back to timing, ignition, and ECU theories.

#9 mpeterson

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Posted 28 August 2010 - 04:34 PM

Can you hear any noise in the fuel tank when you power the fuel pump? You should hear a soft noise for the FP running. If you hear a noise but no start then maybe you need to buy a fuel pressure gauge. They aren't very expensive. Around $20. Could be FP is running but not building any pressure. Otherwise you're stuck guessing why it won't run. At some point you need to confirm or reject theories or you be stuck running in circles.

Its odd the ether test didn't get a slight stumble start from the engine but I'd complete the fuel path before jumping back to timing, ignition, and ECU theories.


once the fuel pump is hard wired i hear the sound for it pressure is good! injectors look fine im back to ignition or something else i believe fuel is a check

#10 fsuguy

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Posted 08 September 2010 - 09:48 AM

...another thing to check is the resistor (should be on the right inner fender near the relay and igniters). From one of these related threads, it looks like the fuel pump has two circuits, one of which is for high demand situations. From what I read, juice to the FP is routed via the FP resistor, and these occasionally go south! You could check to see if these get powered up when you turn on the key.

mike

#11 VBdenny

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Posted 08 September 2010 - 04:37 PM

One other suggestion. All cars since the late 80's have a cutoff switch which shuts off the fuel flow in the event of an accident. I have heard of cars getting bumped hard in parking lots which was enough to trigger the "blowout preventor" and the result is no fuel. I believe that somewhere on the car is a reset button for this. I think I heard it is in the truck somewhere but maybe wrong. If you don't think the car was bumped (usually from the rear) perhaps there is some malfuction in the cutoff switch in which case I suspect the default would be off. Just a thought and a wild guess.

#12 Bruno Molly

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Posted 20 September 2010 - 11:10 PM

I went down this road this past winter and was able to solve the problem thanks to the assistance of Curious B. Take a test light and or VOM and see if you have B+ at your Diagnostic terminal block on the engine with the ignition on.
If you do have voltage there check for voltage at the plug on the fuel pump ECU in the trunk (93-94). 90-92 have a fuel pump relay. In either case if you lose B+ between there and the pump you've narrowed down the problem to either FP Relay or FI Relay under the fuse box under the hood. (Listen for a click as you remove the Fuel Injection Relay while the switch is on ) You can also use a 15-20 amp inline fuse and jumper FP and B+ at the diag port. If the fuel pump runs then you know you have a bad component i.e relay, or FP ecu. I think the FP wire runs directly to the pump.

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