Xtom73G

I have seen this behavior with a stuck gas cap vent.

Xtom73G

The gas cap vent allows air into tank, and if stuck, fuel pump can create a vacuum condition, stalling fuel delivery to engine. I have seen this cause tank to dent in, and then go "bong" when enough air finally lets the dent relax.

oldskewel

iTrader: (1)

Any update on this? Did you just leave the fuel cap off? Does the problem come back when the cap goes back on?

pacerdonfl

As I had previously said, my LS400, with 140k miles, would start and always run beautifully - except when stalling intermittently.

It turns out that I finally got the stalling problem resolved. Someone in a post mentioned that they had shorted out the fuel pump resistor.

Well, I did that and the car does not stall any more. I don't know if this is a "safe" method and it will wear more on my fuel pump or not.

The strange thing leading up to this is that I swapped out THE FUEL PUMP RESISTOR from another 91 LS400 (that runs perfectly) that I have, and it still would stall out after a few miles down the road. I am a little mystified as to why it worked/works fine on my other 91 LS 400, but would not work on the one in question*(&^(*^???

Anyway, I have left it at that, with a paper-clip shunting out the fuel pump resistor, and all is (apparently) working smoothly.

/Don

dicer

It could be a sign the fuel pump motor is taking more power than it should, so it could be on the way out.
And is why some folks have said a fuel pump replacement fixes it too. Maybe the bearings get bad and it drags some thus causing an amp increase and heats up the resistor more than it should. Just a wild guess for a reason.

Yamae

It would be perfect if you'd add the possibility of the clogged fuel filter. Sometimes it is caused by the rusts or foreign objects in the fuel tank though.

dicer

Are LS fuel pumps positive displacement ? I don't remember. An impeller pump would use less power with intake restriction.

oldskewel

iTrader: (1)

If you did the shorting-out paperclip method I described on 2/14, as I said there, I found it to be a useful troubleshooting procedure, and have "read about" others running like that for years before burning out their fuel pump.

By swapping/replacing parts you have confirmed the problem is not with the fuel pump resistor, fuel filter, and fuel pump relay. Anything else?

By shorting out the resistor, you are bypassing how Lexus designed the car to run. You are telling the fuel pump to run at full power all the time. Since that has proven successful in making the car run OK again, I'd think that you're either

1 - getting the most out of a weak fuel pump, which may therefore be about to die completely
2 - using the increased fuel pressure to overcome some other problem in your fuel delivery system

Whether or not you fix the actual problem now, you might want to take this opportunity to isolate the problem, since once the total failure happens, it may become harder to find.

Have you done tests with the paperclip vs. loosening the gas cap to fix it? If either one of the two will fix the problem, that does suggest some sort of clogged vent line. The clog may result in a vapor pressure (either too high or too low) in the tank that causes poor fuel delivery and bad idle, etc. Opening the gas cap equalizes pressure and all is good. Or, leaving the cap on and the tank at pressure/vacuum, the paperclip tells the fuel pump to run at full power, overpowering the problem.

I have been having a few very intermittent problems that may be related to this, so I'm interested in anything you can find out on your car.

dicer

So back to this fun thread.
I finally got some time to fool with mine, I installed another airmeter since I did get a code for that last time I worked on it. Also a better looking wrecking yard idle air control valve, a new engine coolant temp sensor, and started it today, it immediately went up to 1800 rpms and slowly climbed to 2000 rpms, and stayed there for awhile, then it started surging from 2000 rpms to 1800 rpms if it was throttled up a bit it would drop to 1200 rpms and back to its surging thing.
So kinda rules out a fuel supply problem. And its just funny it went from dying out to high rpms and a surge. And yeah I need a know good ecu to try in it. Next time I try to run it I might start blocking the air supply to the IAC valve and see what that does. Its just such a hassle to change those things.