I've had this issue for quite a while now. The engine stumbles badly at around the 1,800 RPM mark - but ONLY when decreasing speed or if I'm keeping speed constant at around that RPM - NEVER during acceleration.

Has anybody experienced this before?

Could it be a clogged fuel filter?

 

There are some good checks in this thread regarding the 400 https://www.clublexus.com/forums/sc-...on-issues.html

 

thanks a ton.

 

I have the same symptoms occuring on my 92 sc300. They only showed up after a tune up. I don't know if the car has ever been tuned up in its 141k mile life. The old plugs were roasted and the old distributor rotor was done.
I feel like either the old battery can't power the plugs or the computer is confused with the plugs burning the fuel more completely.

 

The problem ended up being my ignition (plug) wires. One was split right past where it plugs into the distributor cap. The stumbling used to only be when driving, but in the past week had started happening the moment I started the engine and got noticeably worse each day. I also found a couple of pinched ignition wires in the set.

I was almost all set to swap out the fuel pump ECU, but didn't notice any change in performance upon doing the Fp +b bypass. Then when I mentioned this to the head service tech up at the Lexus dealership, he told me to just come in for a complimentary diagnoses. He said he just didn't want to see me replacing a part that wasn't bad. In his experience (since 1993) he says he's only seen maybe 2 or 3 fuel pump ECUs go bad.

Nothing showed up on the diagnostic, but as soon as we removed the cover on the drivers side bank of plugs there it was - the broken ignition wire. DISCO! Problem solved.

Yesterday, I'd check your wires. If you just got a tune-up, there's a chance the guy doing it didn't get each of those wires tucked properly underneath the spark plug / distibutor cap covers and one or more got pinched badly enough to reduce the current to the plugs.

 

Thanks man I'm gonna check it right now!