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Chris Clark

Fuel Filter, Maybe?

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I've been looking around the website looking for anyone else having the same issue and I guess i'm just not realizing my situation is the same as anyone else as I'd hope I'm not the only one with this issue. So, I apologize if this has already been gone over.

I have a '92 LS400 with 270k miles on it and I have had zero problems with it other than the AC died on me. When we bought the car a few months ago my stepdad drove it home and mentioned to me an issue where all of a sudden the accelerating just stopped. It was almost as if pressing on the gas was slowing the car down, literally. I never noticed it and so we never had it checked out (doh!). That's been a while back and the problem never resurfaced until recently. I did an oil and oil filter change not long ago and the car was running great - as was expected after the oil change. However, a few days ago the problem came up. What's happening to the best that I can describe it is this: I'll get in the car, drive, no issue, get on the interstate, no issue, get up to speeds of 85, no issue until at any given moment my RPMs and speed significantly decrease. There is absolutely nothing consistent with when it happens. I've tried to pay attention if it's when I'm going fast, going slow, turning, accelerating, decelerating - there is no rhyme or reason that I can see. Also the car NEVER has an issue starting, doesn't overheat, and it idles perfectly fine until the problem takes place and then it idles very rough revving from 1.5k rpms to 3k rpms.

Any help would be greatly appreciated. Thank you.

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The fact it idles rough once the problem sets in makes me think it isn't a clogged fuel filter. A clogged fuel filter would restrict fuel. At idle fuel demand is low so it doesn't seem to fit the symptoms.

It could be any of the following:

1) Ignition (one bank of cylinders dead) due to failed coil, problem distributor cap and broken or arcing wire to chassis.

2) Intermittent fuel pump. Once the fuel pump warms up it doesn't pump reliably.

3) Bad tank of gas

4) Dirty throttle body and/or sticky IACV. This would only show up as an idle problem not a highway speed problem though so I doubt this one.

I would bet it's ignition related but filling up with some fresh gas from a reputable station is easiest to try first. If you buy a fuel pressure guage (about $20) you can check to see fuel pressure on injector rails is within spec. Search this forum about ignition issues its very well documented and many things you can try there.

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Just one thing I might add. I had a very similar problem, just this month, with A BMW. After reading a lot on here, plus some in the BMW Forums, I figured it was probably the coolant temperature sensor. This did turn out to be the problem. Once it was replaced, all went back to normal. That sensor is totally different to the sensor for the water temperature gauge. Good luck.

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It's your fuel pump. This happened to me about a year and a half ago on my '92. All symptoms were intermittent-rough idle, sluggish rpms and acceleration. I would bet dollars to doughnuts that it's your fuel pump.

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Today has been my first chance I've had to be able to check this and I thank each of you for your help. I'm going to replace the fuel pump and see where that leaves me and then check the other things as needed.

I feel a little silly having not mentioned this: I don't use 93 grade gas EVERY time - could that also be causing a problem?

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I feel a little silly having not mentioned this: I don't use 93 grade gas EVERY time - could that also be causing a problem?

Doubtful. I haven't used premium in my car in >2 years. Not side effects or performance hits.

You might want to get a $20 fuel pressure guage and measure fuel pressure before forking out the dollars for a new fuel pump. Try and replace can be an expensive way to wittle down a problem. I agree fuel pump is a likely culprit but they are fairly costly.

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