wan8dan

96 Lexus ES300 sometimes (especially when car left un-driven more than two days) get low idle speed (less than 100 rpm) at cold engine start, have to step on gas pedal slightly to keep it running. But after a minute or so, idle goes back to normal. Sometimes I also get high idle speed at cold start (ie: more than 1500 rpm), after a minute or so back to normal. I've cleaned intake throttle, idle air control valve, and MAF sensor about 9k miles ago, but problem still persisted. Not sure if this is indication that idle air control valve or throttle position sensor is going bad. Any advice would be appreciated.

mmarshall

One other possibility........

Have the coolant-temperature sensor checked. If it is not working properly, it may fool the engine computer into thinking that a stone-cold engine is already partly warmed up, or vice-versa (hence the yo-yo idle speeds with warmup). Because of fuel-vaporization characteristics, a cold engine will normally need a richer-than-normal mixture (more fuel, less air), and a higher idle speed, gradually leaning out and slowing down as it warms up.

Is the gauge panel's CHECK ENGINE light on steady (or blinking)? It is usually a yellow or orange color, and, when lit, indicates a problem somewhere in the computer/ignition/sensor/fuel-mixture systems that regulate emissions. If lit, a qualified technician will have to do a series of tests for that specific code to isolate and correct the problem. The light could also be something as simple as a loose or worn gas cap-seal.....but probably not in your situation, which appears to be ignition/sensor/fuel-mixture related. And a good fuel-system cleaning with a good, well-proven product like BG-44K cleaner couldn't hurt, especially if you have plugged injectors.

wan8dan

Thanks for the information.

The CHECK ENGINE light never lit. I also replaced the coolant temp sensor 9k miles ago. I've been using Lucas fuel treatment. I will try to use BG-44K that you recommended.

Could this symptom have anything to do with weakening alternator such that it is not outputting enough current at the first minute of cold start?

mmarshall

If you are already using a good fuel-system cleaner you probably won't need BG-44K (though it wouldn't hurt). It is generally not available in auto-parts stores.....you may have to get it at a service-shop.

Maybe, but I doubt it. A weak alternator will usually show itself by the interior dash and other lights dimming at idle, then getting brighter again as you hit the gas....A/C current output varies with engine speed. It usually doesn't take very much current to operate the electronics associated with idle speed. A more common situation are weak ignition coils which prevent spark plugs from firing properly (many newer cars have individual mini-coils for each plug)...but that would have engine roughness (and a possible CHECK ENGINE light) rather than simply yo-yo idle speeds.

bitkahuna

iTrader: (20)

moved. best of luck.

Hayk

iTrader: (1)

I would check to make sure you have good fuel pressure. No fuel - lumpy idle.

ES300NZ

A TPS sensor is pretty basic, I'm not sure one actually 'can' go bad. It's just a variable resistor, anyway, it doesn't form part of the idle calculations. The ECU does idle from the NE and G + THC. so that's crank, cam and water temp (two pin). It also uses the MAF and other air/fuel sensors, but it's NE, G and THC which set engine speed resolution. If crank, cam sensors were bad the engine would not run, at all. So those will be fine.

The idle is a two fold control system, the ecu advances the timing and the idle control to set the engine speed. If you block the idle control, the engine will 'seek', pulsing up and down looking for idle speed, sending a signal to the idle speed control valve, to open / or/ close to stop the accent of idle speed. The idle control valve does not, is not used to stop 'decent', as far as I can tell.

My guess, .. the timing wheel for the crank sensor might be dirty, or the idle control valve might be sticky, or the wiring to it damaged.

pauloil

do these motors have an idle air motor? if so, clean or replace it and this may help