LS400 Fuel Pump Voltage

Craig_1974 · April 12

Hello Everyone,

My 1991 LS400 runs fine when engine and air temps are cold. But when warmed up, the voltage to the fuel pump will all of a sudden drop and go down below 5V (~4.7V). Of course the engine barely runs as fuel pressure is compromised, and either surges or stalls. If the gas pedal is pumped at just the right time, the engine will come to life and the voltage to the pump will shoot up above 12V (I hooked a meter up to watch voltage to pump as I drove). If the engine rpms remain high long enough (over 2500-3000 rpm) the vehicle will accelerate and run fine until the gas pedal is released. Then the voltage drops down below 5V and engine idles sporadically and/or dies.

Most of the time the it would restart but now that outside temps are higher, engine must cool off before doing so.

When running voltage to pump is around 8.8 to 9.6V at idle and low acceleration or at low engine load, but as engine warms to normal running temperature, voltage going to pump slowly and steadily drops down to about 7V before dropping off instantaneously to below below 5V.

Any information regarding this issue will be greatly appreciated. Thank you in advance.

Sincerely, Craig

P.S. - Bypassing the fuel pump resistor did not correct this issue. I had to wait for engine to cool down before car would restart so i could make it home. No cel codes stored or reported by ecu.

Malc1 · April 13

Graig hi

this sounds overly complicated for the quite often Ls400 problem where the revs drop to zero at certain times when running, then braking, slowing down,..................... and long back Lexus changed the ECUs in the USA cars under warranty .......... not in the UK tho' sadly

NOW all I do to overcome the issue is to just keep my foot on the " gas " pedal a little, even when I'm braking, to stop her cutting out ...... it works for me ok

been doing it for years this way on my 1995 car but haven't found that needed yet on my 1991 car ..... yet !

Do you have this same problem ?

Malc

steve2006 · April 13

Hi Craig and welcome to the LOC.

The fuel pump should be fed with a constant 12 Volts supply when running under load.

I would check the B+ incoming supply to the fuel pump relay and then check the output voltage to the pump both of which should be at full Battery voltage.

I would suspect either a faulty relay or a bad connection to it, the pins are known for corrosion issues.

The basic diagram below shows the power routing to the pump.

I’m not sure if there is another relay the same in the fusebox which you could try swopping over to see if it cures the fault or you could strap the B+ connection to the FP connection in the diagnostic socket to TEMPORARILY bypass the relay.

If you need further help please ask

BigBoomer · April 15

I agree with Steve. If you see any voltage drop at all to the fuel pump, then there is something in the wiring between the Battery and the fuel pump that is introducing resistance like a broken wire or a corroded connection or relay contact.
The only exception to this would be a starter motor and that causes voltage drop because it draws a large current which means that the tiny resistance of the wires and the internal resistance of the Battery have a large effect. Even then you should only see a drop of 2-3 volts when using the starter motor.

Jon Dee · April 15

Here is a bit of generic information on the operation of old school Toyota 2-speed fuel pumps. Sounds like the speed control is trying to do its stuff but you have an extra resistance somewhere. Bad connection or relay contacts are in need of a clean. Maybe even a dry solder joint as temperature is involved. Does that FP relay get REAL hot ?

Cheers...

Malc1 · April 15

Craig the OP hasn’t looked back on here at these replies since his original post on Friday 🤔

Malc

Graham Mc · April 16

Hi Craig.

I had a 92 400LS since new and sold it a couple of years ago with 22k on the clock.

I had the issue you describe and eventually tracked it down to a failing denso supplied unit located on the o/s front strut inside the engine compartment and underneath the fuel pump relay. They are only fitted to the 89 to 92 models and their power values changed after this time.

They have not been available from Toyota world wide for quite some time.

I bought three s/h units from the USA as I did not wish to scrap a virtually as new car for the sake of a virtually unobtainable spare resistor.

I sold the car three years ago and still have two spare resistors on a shelf.

The numbers on your old unit in your 91 should have the part numbers below.

TOYOTA. 23080-50011.

196170-0150

073 061 15.

Denso. MADE IN JAPAN.

You could give me a call at 07850-400100 if you think I can help further.?

Graham Mc

Malc1 · April 16

Graham this is brilliant news that you have some unobtainable bits for our early cars ……. do you have more ?

Malc

Graham Mc · April 16

Hi Malc.

I handed all items that I had gathered over more than twenty seven or more years of ownership on to the trader who purchased my car for resale.

It was only reading the post today from your contributor that reminded me of the two items laying on a shelf in my office.

I had looked for an answer to the described issues for many months before eventually getting lucky and solved my cutting out problem.

I hope it may help him out.

kind regards,

Graham Mc

I have advertised the two items on eBay in case there is some interest out there.

Malc1 · April 16

I have no idea of their value but do know that should an owner of an early Ls400 have a future need then it would be good to know how to find them …….. there’s so few of us really early car owners left in the uk I surmise ……..

Ive had my 91, just short of 100k miles, car since August, my 95 car for some 13 years too ….. covered 125k of it’s 248k miles 😄

Knowing availability and source of rare parts is good 🤔

Malc

steve2006 · April 17

Still no response from the OP, perhaps fixed now then?

Malc1 · April 18

Amazing people write queries and then don't look back for the answers .. Craig hasn't been back on here since the day he posted the Q

BUT it's good to know there's a real quirk with this for the 89 - 91/92 Ls400s and maybe this Post could be transferred to the Ls400 Section specifically for future reference

Steve ...... is this possible ?

Thanks

Malc

Craig_1974 · April 22

Sorry for my delay and thank you all for your responses. The risistor gets extremely hot >200°F (~98°C). I was thinking it may be the issue but at low temperature the resister checks out okay. Before changing the fuel pump, I bypassed the resistor and nothing changed. Now that I have changed the pump, the car stalls shortly after reaching normal running temperature, and WILL NOT restart until temperature drops all the way down.

Jon Dee · April 23

OK... based on the facts as disclosed, the resistor is not the problem. Plus, bypassing the resistor should have applied 12V to the pump and the car would run at all times normally albeit with a little more noise from the pump as it would be running constantly at high speed.

Therefore you must have a problem with your 12V supply to the pump. There is nothing in the simple on/off fuel pump control that can change the pump speed/supply voltage gradually. It should be either 12V or a fixed lower voltage as determined by the value of the resistor. If you check the temperature of the FP relay after you have been driving for a while, it should not be any more than hand warm. If it is too hot to hold your hand on then it is likely that the relay contacts have arcing corrosion and the relay should replaced.

Replacing the relay will also eliminate the possibility of a dry solder joint inside the relay. I mention that as dry joints can be responsible for faults that require a "cool down" period before the engine can be re-started. It is a feature of DC motors driving a fixed load, that if the supply voltage is reduced the current drawn will increase. That accounts for the resistor getting hot as it is having to dissipate greater than its design wattage.

I suggest you first check the power supply to the FP relay to make sure that it is getting a full 12V at all times. If so, the problem is with the relay or one of its wiring connections.

Cheers...

Craig_1974 · April 27

Thank you Jon,

I replaced the FP relay today and have driven about 20 miles and so far so good (knock on wood). The weather was a bit cooler than normal so I am going to test the vehicle out good in a few days once warmer. Should the problem show itself then, I will post back. Thanks again Lexus Owners Club 🇬🇧!!!

Sincerely,

Craig Rogers