'90 LS400 fuel pump system problem
09-19-13, 06:55 PM
#16
Moderator
Suppling the 12V directly to the motor can distinguish the problem.
It can be done as is indicated by a red line.
Last edited by Yamae; 09-19-13 at 07:10 PM. Reason: to add the schematic diagram
09-20-13, 06:13 AM
#17
Pole Position
Thank you Yamae.
After 30 minutes in traffic this morning, I checked the fuel temperature resistor and it felt really hott!!
Please advise me regarding my next step, should I inspect the resistor, the relay or the pump first?
After 30 minutes in traffic this morning, I checked the fuel temperature resistor and it felt really hott!!
Please advise me regarding my next step, should I inspect the resistor, the relay or the pump first?
09-20-13, 06:40 AM
#18
Moderator
I think you mean to say the fuel pump resistor.
It generates about 8 to 15 Watts of heat and it is very warm. In summer, it is too hot to keep touching. But If it is really hot and you only can touch just a few seconds, some extra current might be consumed by the fuel pump motor.
Try bypassing the relay as I showed by the red line when the engine is not running well.
If the situation is improved by that it probably be caused by the under power of the fuel pump motor and it needs extra DC voltage to work well. You need a new pump in that case.
09-24-13, 01:21 PM
#20
Pole Position
Yamae, please advise me accordingly. From the attached picture of the fuel pump relay, which slot should I run the 12V direct from; slot 1,2,3 or 4. Also, im running this directly to the fuel pump resistor. If the car runs well after this bypass, then the problem would be the fuel pump relay not so?
09-24-13, 02:34 PM
#21
Lexus Champion
Yamae, please advise me accordingly. From the attached picture of the fuel pump relay, which slot should I run the 12V direct from; slot 1,2,3 or 4. Also, im running this directly to the fuel pump resistor. If the car runs well after this bypass, then the problem would be the fuel pump relay not so?
you can answer all of your own questions about what to jumper just studying the schematic Yamae provided and the diagram on the relay and a multimeter!
if we just tell you what to jump, then you don't really learn anything, so pretend it is a puzzle!
09-24-13, 04:39 PM
#22
Moderator
Yamae, please advise me accordingly. From the attached picture of the fuel pump relay, which slot should I run the 12V direct from; slot 1,2,3 or 4. Also, im running this directly to the fuel pump resistor. If the car runs well after this bypass, then the problem would be the fuel pump relay not so?
Usually the relay only works when the engine is needed more fuel quickly such as at the timing of the cold start up, going up hills, high speed run and etc.
09-24-13, 04:48 PM
#23
Lexus Champion
The red line I added below is the way to do. If the situation is improved by the jumper wire, it simply means that the fuel pressure is not enough. The jumper wire gives the DC 12V to the fuel pump motor to work it harder. The low pressure is caused by the worn fuel pump or the increased resistance of the fuel pump resistor.
Usually the relay only works when the engine is needed more fuel quickly such as at the timing of the cold start up, going up hills, high speed run and etc.
Usually the relay only works when the engine is needed more fuel quickly such as at the timing of the cold start up, going up hills, high speed run and etc.
09-25-13, 11:12 AM
#24
Pole Position
Thanks Yamae.
LScowboy, my guess to the reason why he picked these two points are primarily because there is an open circuit between these 2 points, therefore connecting a jumper wire where he has the red line drawn would serve as a direct. Correct me if Im wrong.
LScowboy, my guess to the reason why he picked these two points are primarily because there is an open circuit between these 2 points, therefore connecting a jumper wire where he has the red line drawn would serve as a direct. Correct me if Im wrong.
09-30-13, 07:52 PM
#26
Pole Position
After running my 1996 LS400 for approximately 15 minutes afternoon, which ran like a beast might I add...But was enough time to let the temperature reach the half-mark level, and then its back to my common nightmare these days....Rough Idle, followed by a shutting off of my 1UZ...
(See Post #1 of my thread https://www.clublexus.com/forums/ls4...s-to-half.html )
As advised ( and thankfully I am as a matter of fact, by Yamae and LSCowboy )
I have replaced the Fuel Pump Relay with a "direct" or "jumper wire" as indicated in the picture to the further below. However, to my dismay the situation is not improved and the rough idling still persists. Meaning the Jumper wire did not solve the problem. Therefore I guess my Fuel Pump ( which I replaced with a genuine new Toyota Fuel pump last week) and my Fuel Pump Resistor are in good working condition ?
Point To Note !
After replacing the fuel pump relay with the Jumper Wire, I took my 96 LS400 for a drive around town for about 20 minutes, but the rough idling persisted. However, after I was finished and parked up in the garage (Jumper Wire have not been removed yet ) I decided to switch the ignition ON ( Not starting though) just to see if I hear the Fuel Pump "ON" or making the noise that it does when the vehicle is started (back-seats and cushion have been off since fuel pump change hence I can hear the fuel pump) BUT no noise was heard.....Im puzzled, if the "jumper wire" is in place isnt it supposed to be giving the DC 12V to the fuel pump motor to work it right through, once the ignition is on ?
(See Post #1 of my thread https://www.clublexus.com/forums/ls4...s-to-half.html )
As advised ( and thankfully I am as a matter of fact, by Yamae and LSCowboy )
The red line I added below is the way to do. If the situation is improved by the jumper wire, it simply means that the fuel pressure is not enough. The jumper wire gives the DC 12V to the fuel pump motor to work it harder. The low pressure is caused by the worn fuel pump or the increased resistance of the fuel pump resistor.
Usually the relay only works when the engine is needed more fuel quickly such as at the timing of the cold start up, going up hills, high speed run and etc.
Usually the relay only works when the engine is needed more fuel quickly such as at the timing of the cold start up, going up hills, high speed run and etc.
I have replaced the Fuel Pump Relay with a "direct" or "jumper wire" as indicated in the picture to the further below. However, to my dismay the situation is not improved and the rough idling still persists. Meaning the Jumper wire did not solve the problem. Therefore I guess my Fuel Pump ( which I replaced with a genuine new Toyota Fuel pump last week) and my Fuel Pump Resistor are in good working condition ?
Point To Note !
After replacing the fuel pump relay with the Jumper Wire, I took my 96 LS400 for a drive around town for about 20 minutes, but the rough idling persisted. However, after I was finished and parked up in the garage (Jumper Wire have not been removed yet ) I decided to switch the ignition ON ( Not starting though) just to see if I hear the Fuel Pump "ON" or making the noise that it does when the vehicle is started (back-seats and cushion have been off since fuel pump change hence I can hear the fuel pump) BUT no noise was heard.....Im puzzled, if the "jumper wire" is in place isnt it supposed to be giving the DC 12V to the fuel pump motor to work it right through, once the ignition is on ?
09-30-13, 08:44 PM
#27
Moderator
After replacing the fuel pump relay with the Jumper Wire, I took my 96 LS400 for a drive around town for about 20 minutes, but the rough idling persisted. However, after I was finished and parked up in the garage (Jumper Wire have not been removed yet ) I decided to switch the ignition ON ( Not starting though) just to see if I hear the Fuel Pump "ON" or making the noise that it does when the vehicle is started (back-seats and cushion have been off since fuel pump change hence I can hear the fuel pump) BUT no noise was heard.....Im puzzled, if the "jumper wire" is in place isnt it supposed to be giving the DC 12V to the fuel pump motor to work it right through, once the ignition is on ?
but your car is not a 90 LS400 but a 96 JDM Celsior
. The correct circuit diagram is as below. I picked this up from a JDM UCF20/21 manual and put English names on it for your better understanding since every name was in Japanese originally. Will you confirm the voltage seeing my explanation under the diagram?
As you see above, the terminal 3 is connected to the fuel pump. When the jumper wire is not there and the engine is ON, the voltage at the terminal 3 should be lower than the voltage of the terminal 4 which is about 12V. When the jumper wire is inserted, it should have about 12V as is the terminal 3. What you need to do is as follows.
1. Remove the jumper wire and check the voltage of the terminal 3 and 4.
2. Insert the jumper wire and check the voltage of the terminal 3.
Above 2 should be done when the ignition key is at the run position.
From your report, the rough engine problem may not be related to the fuel pump. Did you replace those ECU capacitors already?
10-01-13, 08:19 AM
#28
Since your problem is consistent, why don't you just get the fuel pressure checked at the rail, both before and during the problem? If it turns out to be low, THEN you can start looking for related causes.
10-01-13, 10:45 AM
#29
Lexus Champion
and here is a kickin nice tool to do it with
and yes, it fits the Toyota/Lexus banjo bolts, which many do not
10-01-13, 11:34 AM
#30
Pole Position
That is strange but your car is not a 90 LS400 but a 96 JDM Celsior. The correct circuit diagram is as below. I picked this up from a JDM UCF20/21 manual and put English names on it for your better understanding since every name was in Japanese originally.
Will you confirm the voltage seeing my explanation under the diagram?
As you see above, the terminal 3 is connected to the fuel pump. When the jumper wire is not there and the engine is ON, the voltage at the terminal 3 should be lower than the voltage of the terminal 4 which is about 12V. When the jumper wire is inserted, it should have about 12V as is the terminal 3. What you need to do is as follows.
1. Remove the jumper wire and check the voltage of the terminal 3 and 4.
2. Insert the jumper wire and check the voltage of the terminal 3.
Above 2 should be done when the ignition key is at the run position.
From your report, the rough engine problem may not be related to the fuel pump. Did you replace those ECU capacitors already?
but your car is not a 90 LS400 but a 96 JDM Celsior. The correct circuit diagram is as below. I picked this up from a JDM UCF20/21 manual and put English names on it for your better understanding since every name was in Japanese originally. Will you confirm the voltage seeing my explanation under the diagram?
As you see above, the terminal 3 is connected to the fuel pump. When the jumper wire is not there and the engine is ON, the voltage at the terminal 3 should be lower than the voltage of the terminal 4 which is about 12V. When the jumper wire is inserted, it should have about 12V as is the terminal 3. What you need to do is as follows.
1. Remove the jumper wire and check the voltage of the terminal 3 and 4.
2. Insert the jumper wire and check the voltage of the terminal 3.
Above 2 should be done when the ignition key is at the run position.
From your report, the rough engine problem may not be related to the fuel pump. Did you replace those ECU capacitors already?
I will confirm the above asap. However, just to clarify ;
#1 is to be done when the engine is on?
#2 is to be done with the engine off but the ignition key at run position
Also, I am confused as to why my fuel pump is not running (with jumper wire in place) when the ignition is ON the RUN position but engine off...I therefore believe I should check the resistance of my resistor before I start in the morning, where the resistance should be between 0.7 and 0.76 ?
http://www.ls400.ru/lexus-89-2000/il.../fpr2/insp.pdf