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Rpms Drop Then Pick Back Up


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Hello all,

I have searched and can't quite find what I think the problem is.

I have a 92 LS400 with 159,000 miles.

It just started doing this. The rpms will all of a sudden drop to almost 0, then the engine picks back up and it goes back to wherever; depending on where I have the gas pedal. Sometimes the car will die from having the pedal pressed.

This happens in gear or in park, while driving or when stopped.

If I am in park and letting it idle, the rpms drop, and the car almost stalls, but then the rpms catch back up and it goes to the correct idle. I let it sit like this yesterday, and it just kept doing it over and over again.

I also tried pressing the gas pedal to bring the rpms up to almost 3,000, and it would do the same thing, then go back up to where I had it if my foot was still on the pedal.

The temp is where it should be and always is after it warms up.

This occurs if I have the AC on or off, and stereo as well.


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Sounds electrical but could be the MAF, or throttle position sensor. Do you have any other electrical problems? When did it start?

Have you pulled engine codes yet?

Post them here.

This same type of problem happened to me about a year ago. This smooth world class car started balking, cutting out, lurching, wild rpms, dash flickering, just seemed ready for the junk yard. It happened all of a sudden. No warning.

I thought the engine was toast.

Now it's smooth as silk w/ 255,000 miles on it.

Open the trunk. On the left side you will see a wire harness on the left hinge. After repeated openings and closings the wires inside that harness become fatigued and frayed. Then they start shorting out. Then sputtering, stalling, and everything you describe. This is a design flaw and lexus knows it.

If this describes your problem, then remove the plastic bracket on the left hinge, strip down the harness, untwist all the wires, re-connect, then re-tape.

Make sure to leave off the hinge bracket that ensures tension on the harness. Just let the whole mess swing free and loose.

In my case, problem solved, and no spending thousands of dollars in diagnostics, parts, and tech snafus.

Best of all, it's free. Takes about 30 minutes.

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  • 4 months later...

Finally decided to bite the bullet and get it checked by an indie.

Turns out it was the fuel pump; it was heating up and cutting out intermittently.

Makes sense now that it was mostly doing this in warmer weather, and when the fuel tank was at a low level. A couple times it did it on a low tank, and I filled up the tank and the problem stopped. Go figure... cool gas cools fuel pumps.

I replaced it with a pump from Schucks, but I can actually hear it running now, so I will be ordering a Japanese one later (one of a few brands that I ran across on the internet), and returning this one...

I tried checking the fuel pump myself, but I was not convinced my method was sound... pretty dumb, but I removed the rear seat and shined a trouble light against the access hole for the pump in the gas tank. The problem did not occur, and it is good that I did not rely on this method and cross that one off my list. The fuel pump sits lower than the hole, so it makes sense this did not work. I still suspected the pump to be the problem, but I figured I would have someone else do the diagnosis and just fix it myself.

Anyway, I hate it when people ask for help and go fix it, then never come back and post what they did to fix their problem, so here I am.

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