lexus gs300 rpm surges at cruising speed

redf1

I have a 1998 GS300 with only 16,000 miles. It has started to have oscillating 200 RPM changes at cruising speed. This occurs with or without cruise control engaged. The dealer has had the car for 5 days and can not tell me what the problem is. Any help?

Mr_Shiftright

Well, kinda hard to isolate, but two general things come to mind. If it is actually surging, that is, if the car actually accelerates without you moving the gas pedal, then extra fuel is going in, or the throttle plate is either fluttering or sending a false signal (fooling the computer into thinking you are stepping on the gas a little) or....or.....perhaps the engine timing is changing slightly...this could also do it.

SOMETIMES people mistake an unauthorized downshift for a surge, when actually the rpm counter is going up but the car is not actually accelerating...this could happen if the car is dropping out of overdrive.

200 rpm is pretty subtle. The dealer may have a tough time with this one.

redf1

If the vehicle is in cruise, any load on the engine (eg slight rise in road slope) will cause an abrupt 200 RPM increase. Likewise without cruise engaged, a miniscule pressure on the gas pedal will also cause an abrupt 200 RPM increase. The engine will then abruptly have a 200 RPM decrease. This happens repeatedly while at cruising speed. This is not a downshift into 4th gear. It is also not a slow acceleration of 200 RPM. Could it be a problem with the lockup torque converter?

alcan

It sounds exactly like a torque converter clutch cycling on and off. Try pressing the brake pedal enough to turn on the brake lights with your left foot while holding a steady road speed. This will disable the tcc. If you notice the same rpm increase, then no surging, you've confirmed tcc cycling.

Mr_Shiftright

So what exactly causes the rpm increase with the tc? Is it just the 'release' and therefore less drivetrain resistance?

redf1

The dealer is telling me that this is normal functioning of the vehicle. When I tell him that it never did this prior to 2 weeks ago, he states that I just did not notice it. I have never had a car that had this variation at cruising speed. With only 10 months to go in the factory warranty, I think he is just stalling. Should this not result in extra wear on the engine, transmission, and lockup torgue converter and also adversly affect fuel economy?

alcan

When the converter's unlocked the turbine's slipping, allowing the engine rev higher than the turbine and trans input shaft. When the clutch locks it eliminates turbine slippage, bringing the engine down to trans input shaft speed. Typical speed change is around 300 rpm @ 60 mph, typical fuel economy increase is 2.2 - 3.5 mpg.

redf1: converter clutch apply is controlled by the powertrain control module, which monitors various sensor inputs to determine engine/vehicle operating conditions and decides when to apply/release the tcc. Inputs may include vehicle speed, throttle position, engine vacuum, mass air flow, engine rpm, coolant temp, trans oil temp, gear selector position, brake pedal status, etc. If any sensor input is outside specified parameters the PCM will disable the tcc. When it releases, engine rpm increases as does manifold vacuum. I'd be checking TPS adjustment and manifold absolute pressure/mass air flow sensor inputs. Most tcc hunting concerns are directly engine driveability related.