DK Audio

I have a 98' LS400 with 130K. I changed the transmission mount last summer and did timing belt/waterpump/idlers last fall. When the car is in drive and I'm at a stop it idles rough, nothing major but it bugs me. If I put the car in neutral or park the RPM's increase a tiny bit (no load) and it is perfectly smooth (this tells me it is not the engine mounts). I can then rev the car and again, perfectly smooth. As soon as I start to accelerate from the stop the vibration goes away. I am curious if my car is perhaps idling at a lower RPM than it should? In neutral I would guess it is around 700, when in drive at a stop it appears to be around 500.

jarman789

mine does that too. i'm afraid i don't have any tips on how to fix it, but it's reassuring to know that mine is not the only one doing it.

tomtheslac

Me three...any leads?

sdls

it IS the motor mounts. the fact that it is smooth at certain RPMs does not exonerate the motor mounts in any way shape or form. the motor is just naturally balanced at those RPMs. The motor mounts are there to make all the other RPM ranges smooth too.

Raddison

My thoughts exactly.

Motor mount job, added to the to-do list.

tomtheslac

That damned list never gets any shorter...does it?

DK Audio

Darn, those mounts aren't cheap either. Oh well, spring project I guess.

NugenLs400

This actually started to happen to my car more recently too I noticed it not while I was in D but wen i started it in P >< would this be any different from your situation?

DK Audio

Mine only happens when at a stop in Drive when the engine is fully warmed up (resulting in lower RPM's)

GSteg

iTrader: (15)

Put your car in park and while the engine is idling, turn on the A/C. If you can feel the compressor engaging, then it's most likely the engine mount. It's not a definite test, but it's one of the common symptoms of a worn engine mount.

I had the same thing happen on my GS400. Put in new mounts and it was like a brand new car again!

Stroock639

mine will do this too when the engine is warm, and will idle at like ~350 rpm but if i turn the light switch one click forward or with the headlights on the idle will increase to ~500 rpm and smooth out

sha4000

iTrader: (1)

You probably need to clean your throttle body or check your plugs if you have not had tune up in a while. Maybe even your IACV ( Idle Air Control Valve ) your car is running rough most likely beause the rpms are too low and it wants to stall.

Yamae

98-00 models don't have the IACV.
I suggest you Stroock639 to clean the air mix path too.

DK Audio

Yeah, 350RPM seems really low. I never did anything with my car, it is still smoother than 90% of the cars out there It is worse during winter when it is cold out. I still will probably do my motor mounts eventually, going to do my lower ball joints next weekend hopefully.

sha4000

iTrader: (1)

Yamae is right about your car not having an IACV and cleaning the throttle body is a relatively cheap fix to try.