LS400 Cranking but not starting
03-04-19, 01:58 PM
#18
I'm surprised no one has mentioned the fuel pump as it could very well be the root cause of the problem. These cars are 19+ years old so if the fuel pump is still original & yours has high mileages then your pump could be working intermittently as it's on its last leg. Of all my 40+ years of driving, I have had 3 fuel pumps that went out on me w/ 1 of it being on a 97 Lexus LS400 at 220K miles. My symptom was exactly like yours & I was able to troubleshoot & narrowed it down to a dying fuel pump by using a dummy load (a light bulb) to simulate the pump to test out the fuel pump's relay & its control signal.
Sherl- Your thinking about the fuel pump not priming is incorrect as a working fuel pump will be activated to pressurize the fuel delivery system for a few seconds only when the key is turned to the ON position. Here's how it works: The pump is powered by a 12V hi-current via the fuel pump's relay & when the relay is energized by a control signal to provide the 12V to the pump when it should be. If you place a voltmeter right at the fuel pump then turn the key to the ON position, 12V will be present but it'll go away after a few second unless you keep on cranking and/or the car is running. This is by design to keep the pump from burning up when no fuel is being pumped/circulated.
Sherl- Your thinking about the fuel pump not priming is incorrect as a working fuel pump will be activated to pressurize the fuel delivery system for a few seconds only when the key is turned to the ON position. Here's how it works: The pump is powered by a 12V hi-current via the fuel pump's relay & when the relay is energized by a control signal to provide the 12V to the pump when it should be. If you place a voltmeter right at the fuel pump then turn the key to the ON position, 12V will be present but it'll go away after a few second unless you keep on cranking and/or the car is running. This is by design to keep the pump from burning up when no fuel is being pumped/circulated.
03-04-19, 02:11 PM
#19
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I'm surprised no one has mentioned the fuel pump as it could very well be the root cause of the problem. These cars are 19+ years old so if the fuel pump is still original & yours has high mileages then your pump could be working intermittently as it's on its last leg. Of all my 40+ years of driving, I have had 3 fuel pumps that went out on me w/ 1 of it being on a 97 Lexus LS400 at 220K miles. My symptom was exactly like yours & I was able to troubleshoot & narrowed it down to a dying fuel pump by using a dummy load (a light bulb) to simulate the pump to test out the fuel pump's relay & its control signal.
Sherl- Your thinking about the fuel pump not priming is incorrect as a working fuel pump will be activated to pressurize the fuel delivery system for a few seconds only when the key is turned to the ON position. Here's how it works: The pump is powered by a 12V hi-current via the fuel pump's relay & when the relay is energized by a control signal to provide the 12V to the pump when it should be. If you place a voltmeter right at the fuel pump then turn the key to the ON position, 12V will be present but it'll go away after a few second unless you keep on cranking and/or the car is running. This is by design to keep the pump from burning up when no fuel is being pumped/circulated.
Sherl- Your thinking about the fuel pump not priming is incorrect as a working fuel pump will be activated to pressurize the fuel delivery system for a few seconds only when the key is turned to the ON position. Here's how it works: The pump is powered by a 12V hi-current via the fuel pump's relay & when the relay is energized by a control signal to provide the 12V to the pump when it should be. If you place a voltmeter right at the fuel pump then turn the key to the ON position, 12V will be present but it'll go away after a few second unless you keep on cranking and/or the car is running. This is by design to keep the pump from burning up when no fuel is being pumped/circulated.
Any DIY for fuel pump replacement?
03-04-19, 02:25 PM
#20
For the LS400, the fuel pump is located right behind the backseat. It's fairly simple to remove/replace once the backseat is removed. There's a big round hole right in the middle of it. You'll need to remove the round cover before you can get access to the pump. Since I'm familiar w/ the process, I didn't read up on any but there s/b plenty of "How To" procedures available on the net. Just surf the net then you'll find a few. Make sure to test it out first before you condemn & replace it though. Good luck.
03-04-19, 02:59 PM
#21
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Thread Starter
I'm surprised no one has mentioned the fuel pump as it could very well be the root cause of the problem. These cars are 19+ years old so if the fuel pump is still original & yours has high mileages then your pump could be working intermittently as it's on its last leg. Of all my 40+ years of driving, I have had 3 fuel pumps that went out on me w/ 1 of it being on a 97 Lexus LS400 at 220K miles. My symptom was exactly like yours & I was able to troubleshoot & narrowed it down to a dying fuel pump by using a dummy load (a light bulb) to simulate the pump to test out the fuel pump's relay & its control signal.
Sherl- Your thinking about the fuel pump not priming is incorrect as a working fuel pump will be activated to pressurize the fuel delivery system for a few seconds only when the key is turned to the ON position. Here's how it works: The pump is powered by a 12V hi-current via the fuel pump's relay & when the relay is energized by a control signal to provide the 12V to the pump when it should be. If you place a voltmeter right at the fuel pump then turn the key to the ON position, 12V will be present but it'll go away after a few second unless you keep on cranking and/or the car is running. This is by design to keep the pump from burning up when no fuel is being pumped/circulated.
Sherl- Your thinking about the fuel pump not priming is incorrect as a working fuel pump will be activated to pressurize the fuel delivery system for a few seconds only when the key is turned to the ON position. Here's how it works: The pump is powered by a 12V hi-current via the fuel pump's relay & when the relay is energized by a control signal to provide the 12V to the pump when it should be. If you place a voltmeter right at the fuel pump then turn the key to the ON position, 12V will be present but it'll go away after a few second unless you keep on cranking and/or the car is running. This is by design to keep the pump from burning up when no fuel is being pumped/circulated.
For the LS400, the fuel pump is located right behind the backseat. It's fairly simple to remove/replace once the backseat is removed. There's a big round hole right in the middle of it. You'll need to remove the round cover before you can get access to the pump. Since I'm familiar w/ the process, I didn't read up on any but there s/b plenty of "How To" procedures available on the net. Just surf the net then you'll find a few. Make sure to test it out first before you condemn & replace it though. Good luck.
Where did you get your pump though? Part number?
03-07-19, 09:50 AM
#24
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