Harryerwin

I have a 1997 LS 400 with 200000 miles and it starts and idles fine but if you
Speed up the engine in neutral to about 2000 RPM it begins to run rough.
Above and below 2000 it smooths out.
While driving you can feel the roughness at about 45 mph.
It seems to go away when you get to about 55.
Any ideas?

LScowboyLS

this thread has the solution for a great majority of LS400 not running right issues

PureDrifter

iTrader: (10)

please read the stickies, '96+ LS400 didn't have many ECU issues though opening yours up and checking for leaky caps isn't a bad idea.

I would check out the condition of the ignition system.

Yamae

I'd rather think it's a transmission mount which cause the problem.

Yamae

So you say below is the rare case. It's a 97.

LScowboyLS

well perhaps not back in the day, but now all LS400 model years are having issues, in fact, suddenly it seems that a ton of unresolved strange LS400 issues on all years are being traced to this problem!

this could exhibit itself as some issue seemingly unrelated to the ECM, such as the A/C compressor not engaging, air bag light flickering, or even transmission not shifting at all - my 96 had all of those issues and several more until, in a MacGyver-Steve Jobs moment of insight, I replaced all 6 of the electrolytic capacitors a few weeks ago

through our research, yamae and I have had a breakthrough discovery on the prevalence of this and the correct fix, he explains why this is happening here

Harryerwin

Would the transmission mount cause the rough running while in neutral?

phlorida

Having just had the motor and transmission mounts replaced on my 1998 LS my guess would be either or both of those.

Yamae

Believe it or not, I have experienced rough running in the neutral as if 1 or 2 cylinders were not firing and also the sound of the engine was just like that. Replacing the transmission mount fixed the problem completely. The symptom happened only at certain range of the revolution which was a bit higher than 2000rpm.

Why don't you read the thread below "vibration on my 99".
https://www.clublexus.com/forums/ls4...n-my-99-a.html