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91 Ls 400 Suddenly Looses Power-no Cel-no Codes


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I have a 91 LS400 with 164,000 miles. It has been a great automobile until recently, but currently it has me baffled. The vehicle, without warning, will suddenly lose power and drop rpm. It doesen't die, it will stumble and miss and it just can't get the rpm back up. This may last a few or several minutes, then just as suddenly the malfunction corrects itself and everything is back to normal. The car is fine again for maybe 50, 100 or 200 miles and then it will re-occur. Strangely, the CEL never comes on and I can never find any stored error codes.

At first I concluded that the fuel filter was blocked and replaced it. No change. Then I suspected water in the tank, so I drained and refilled. No change. I can't believe that the fuel pump can be the culprit, since the malfunction recovers. In my experience fuel pumps fail terminally. I still suspect a fuel problem. Everytime I open the fuel filler cap I hear a lot of in-gassing. Does anyone know how the fuel tank vents? Could conditions in fuel tank create a vacumn that the fuel pump is struggling to overcome? Does the tank vent through the filler cap?

Could I be completely wrong about the fuel issue and have some other obvious problem? If you get no CEL or error codes where would you start trouble shooting?

Any insights or suggestions you might have are appreciated.


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Could it be your spark plugs? Timing belt? Just a thought as I had the same issue and when I changed the sparks, all was better. Haven't changed my belt yet though...

Glad to hear that you solved your problem. I find it odd that a spark plug could malfunction and then consistently recover. Perhaps in the process of replacement you solved a different problem. I replaced the timing belt at 100,00 so I am not suspicious of that component. Again I don't know how a timing belt problem could recover. I am just trying to avoid the "replace everything till it goes away syndrome". That is an expensive approach for solving a problem.

Thanks for the reply.

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I had the same problem on my 90 LS, and 93 SC. Check engine light did not come on for weeks and the car would occasionally lose power and stumble for a minuite or so. When the code did finally appear, it was the oxygen sensor on the exhaust. there are two on each side. I think it was the one that is more accessible. Anyway the dealer knew it was a common problem and pointed out which one to change. I did it myself. Make sure you have the perfect wrench or socket to pull it out because it is stuck in there very tight. Avoid stripping the thing. Good luck.


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