kirby747

My 91 LS400 has the dreaded engine surging problem, we have all read about it.
One of the solutions on this site was that a part from the fuel pump was bad. There is a click when the car idiles really low. I was looking at this while it was dark and I could see light from inside this resistor. I tried to get video but was unable to. I guess my wuestion is should I be seeing the flash while this is happening. I will post a picture soon as I can.

kirby747

Here is a




This is where I saw the light comming from.

Yamae

I'd check the resistance of the fuel pump resistor first. When at the idle, the relay should keep opening the contacts because the engine needs the fuel just a little. The relay should only work when the engine needs the gas a lot.

dicer

Where do you read all it? I did a search and tons of unrelated stuff comes up.
The flash would be the relay opening and closing.
And I had no idea the relay would be opening and closing so much Yamae. I thought fuel pressure would be regulated, with a pressure regulator I need to study the books on this one.

Yamae

Sorry to say dicer, but you should need to know the basic some more before you advise people. I know you always try to help people but without knowing the basic, it often confuses the person who is in need of a real help. It would be nicer if you'd study the basic more and post.

The ECU does not have any fuel pressure sensor. The ECU monitors the amount of the fuel getting the length of the injecting time and the swing voltage from the pre-cat O2 sensor. When the time is longer and the lower voltage is continued, it judges that the fuel supply is not enough and enables the fuel pump relay on to bypass the the fuel pump resistor. This increases the current to the fuel pump and the additional fuel is supplied.

Remember that the fuel pump is given the current through the fuel pump resistor whenever the engine is running. Without the fuel pump relay, the fuel pump still works but not fully. The full work is only done at a start up and a heavy engine load such as an acceleration or a high speed run.

kirby747's fuel pump relay is driven on and off while idling. This simply means that the fuel supply is not enough or the ECU is malfunctioning. This often is caused by the increased resistance of the fuel pump resistor or the fuel pump is weak or the fuel line is clogged.

I have seen several increased fuel pump resistors among old Celsiors. High wattage resistors tend to increase the resistance due to the heat and the oxidization when aged. This reduces the current and the fuel supply becomes less. Also old cars tend to have clogged fuel lines or a weak fuel pump. This is why we need to check the resistance first. If it's OK, the next step will be the rest.

dicer

Yamae, I do know the basics very well I've been doing the engine related stuff for close to 40 years now, and yes the electronic stuff too. Not on the LS though that all is still a learning deal. Most other auto makers have a fuel pressure regulator and do not turn the fuel pump on and off or through a resister. I never said the ECU had a fuel regulator as they are usually mechanical like this example.
And thank you for explaining the operation of the relay and the resistor for the LS.

RA40

iTrader: (6)

Yamae, would this be a similar issue as mentioned in this older thread?

(Drives us crazy when we don't get follow-up. There are similar threads that we'd like to merge because we've seen them with some frequency. As well sticky material.)

https://www.clublexus.com/forums/ls4...y-problem.html

dicer

I read up on it a bit. Yamae is right on with the 2 speeds of the fuel pump. And there is also a fuel pressure regulator.

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When the fuel pump relay coil is energized the contacts connect to the resister and that lowers the pump speed.
So then that means with relay static that the pump is on high speed.

The ecu controls a vsv to control the fuel pressure regulator.