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mikee

1990 Ls400 Limp Mode Resolved

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I hope that this will help somebody who is dealing with sudden and occasional loss of power on an early model LS400. This problem seems to be called “Limp Mode” in these forums. My model is a 1990 LS400, all original equipment, 205,000 miles, original owner.

Symptoms: Sudden loss of power, as if out of fuel, while driving or waiting at a stop. Engine may seem as if it is stalling or idling rough. Applying gas does not increase idle speed, but may allow the car to move slowly, hence the term “Limp Mode’. No CEL. It can be quite scary if you are in traffic. It can also resolve itself if the car is cooled down, and may not happen again for months.

Conditions: (In hindsight, these were important in my case) A relatively hot day, 75 F or more. Car had been sitting in the sun for a while. Gas was at or below a quarter tank full, but definitely not empty. On one occasion, AAA arrived and gave me some fuel and that fixed it. This should have been a clue.

After reading the shop manual and many suggestions in this and other forums, I changed the following and other items:

New Plugs, Igniters, Rotors, Ignition harness, Temp sensor, fuel filter, PCV…….checked the gas tank cap for proper sealing.

I also checked and cleaned the throttle body and did the Seafoam thing as described elsewhere, cleaned the EGR filter. All of these items, plus new rear engine mount, helped restore the engine to much smoother operation.

It seemed like the issue was resolved, until it happened one more time. This time, I bought a new Denso fuel pump from their website which was cheaper than anywhere else I could find. I drained the fuel tank, and the gas was remarkably clean, almost zero sediment. I also confirmed that when the red light comes on, there is 3 gallons left in the tank. So the fuel gauge was operating normally. Put in the new fuel pump.

I am happy to report that the problem has been resolved since replacing the fuel pump, so you might consider doing this if you have the same problem under the same conditions. In hindsight, my explanation is that the fuel pump was failing when it got hot. Somewhere below a quarter tank, the pump motor is no longer bathed in fuel which normally helps it stay relatively cool. With hot gas on a hot day, the pump would overheat and either stop completely or go into a lower flow mode. If you take the pump out and test it, it will seem OK unless the same conditions are met. It’s been 2 years now, and the engine is still running smoothly. I’ve been through 2 summers and not seen the limp mode. Fingers crossed, it will stay fixed. Good luck in resolving your situation!

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Fuel pumps are inside the gas tank so they are expensive to replace. The best prevention you can have for your fuel pump is to never let your fuel level get near empty. The worse thing you can do for your FP is to run out of gas. The gasoline in the tank cools the fuel pump and when it is empty, it is not cooled very well. Before I trained my wife to not let the tank get low, she ran out a couple times then in the same car, the fuels pump went out on the Jersey Turnpike, on a Sunday night late. Result, $700 and that was in a Chevy Caprice.

Fill 'er up

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Fuel pump is relatively easy and quick to remove on the LS because of the access hole behind the rear seat. Best prevention against fuel pump failure is to replace the fuel filter about every 80,000 miles to reduce pumping strain on the pump and to keep the fuel level in the tank above the empty mark and preferably above the 1/4 mark.

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I'll continue to let the fuel levels in our cars run down to nearly empty before filling our tanks. Neither my wife or I have had a fuel pump failure in our combined 90+ years and 1,000,000+ miles of driving the relatively few cars we've owned in our lifetimes.

IMO, more frequent trips to gas stations to top up fuel tanks are a waste of our time. This method has worked for us but apparently doesn't work for everyone. I even suspect that the reason it works for us is that most of the cars have been purchased by us new or nearly new and the gas tanks have always been run down to nearly empty -- maybe no sediment or water ever builds up in our tanks.

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I'll continue to let the fuel levels in our cars run down to nearly empty before filling our tanks. Neither my wife or I have had a fuel pump failure in our combined 90+ years and 1,000,000+ miles of driving the relatively few cars we've owned in our lifetimes.

IMO, more frequent trips to gas stations to top up fuel tanks are a waste of our time. This method has worked for us but apparently doesn't work for everyone. I even suspect that the reason it works for us is that most of the cars have been purchased by us new or nearly new and the gas tanks always been run down to nearly empty -- maybe no sediment or water ever builds up in our tanks.

I do the same thing, and I've never had any trouble either (including my former 315k mile 95 LS).

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The point of my posting was not to discuss fuel pump reliability. That seems irrelevant in such an old car. It was to help with the limp mode problem that seems to trouble a number of owners. I have not seen many discussions where the issue was actually resolved.

I would say that spending $100 to $150 on a new fuel pump after 18 years and 200,000 miles is not a reason to change tank filling habits. Maybe other manufacturers have poor design, but this one held up very well in a hot climate and was only failing under extreme conditions. My experience may help other people who get stranded occasionally and cannot determine the root cause. The pump is easy to replace if you have reasonable mechanical skills. However, take every precaution when working around fuel. If you are not comfortable, have a mechanic change it even if you buy the pump/filter kit yourself.

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I have had the same problem with the limp mode from time to time...seems to be under your same conditions here in SoCal....hot day, low fuel....

My 1992 LS 400 with 270k miles may need a new fuel pump.....I will look into it.....makes sense to keep tank filled above 1/4

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