I took my 95 Previa (300,000 km) to the dealer 3 weeks ago because of rough idle and white smoke coming out exhaust.

My trusted mechanic of 15 years diagnosed a head gasket (white smoke was coolant fluid) and was confident that a new head gasket would solve both problems. After new head gasket, ground head and valve seats, new distributor cap, rotor, plugs & plug wires, PCV, thermostat, oil & filter, the white smoke has disappeared but the rough idle problem persists. The dealer pulled head off again and returned it to the head grinder only to learn that head was cracked! The dealer installed a re-conditioned head but the rough idle problem persists. Vacuum checks are all OK. I drove the Previa for 40 km in case that would help, but to no avail. I have spent C$2600 so far. The dealer is now suggesting an IAC valve replacement for C$700. If it doesn't work, he will charge me for labor only, and take the new valve back and re-install the old one.

How can I resolve the problem for which I brought it to the garage in the first place, that is, rough idle!

It sounds to me that you had two problems going on there. The "white smoke", is actually steam...yes from your coolant but white smoke designates one of three things, a blown head gasket, a cracked head or a cracked block. When a vehicle comes in for a "blown head gasket", normally they send the cylinder head to a machine shop to have the valves re-worked and the head "shaved" due to the warp age that blew the head gasket in the 1st place. A visual inspection of the cylinder head (once it's pulled), will usually pick up cracks in between the valves (usually), so they can replace the entire cylinder head (cost effective to you).

The second problem, insofar as the rough idle could be because of the cracked head but the IAC, which I call the AIS Motor or Automatic Idle Speed motor keeps the throttle open just enough when your foots off the gas pedal to keep it idling properly. I feel the O2 sensor(s) (Oxygen) sensor(s) have to be fouled with the anti-freeze going through them as the steam went out of the exhaust. Your oil should have been changed due to the coolant that leaked into the cylinders and mixed with the oil or the engine will seize up. Spark plugs need to be replaced due to the contaminants from the non-diagnosed cracked head for they will become fouled too. Your catalytic converter may be fouled or inoperative but since you have an OBD I vehicle, it wouldn't cause it to run rough. I would suggest getting your mechanic or you to put a "Scanner" to pull all the Codes out of your computer or PCM for the IAC motor will definitely store a code in the Computer of your vehicle. I'm from the old days and I like to look at a live reading with an oscilloscope of all the cylinders. Have someone pull the codes or fault codes (known as DTC's now, for Diagnostic Trouble Codes), and see what comes up before replacing the IAC which seldom goes bad because it's located on the side of your throttle body to control AIR into the engine not fuel or spark.

Good Luck,

~Stillin~

Thank you for the info and the rapid response. I will review with the technician tomorrow and keep you posted.

Latest news, block is OK according to the technician. Service manager said error codes are non-existent unless "check engine" light is on, which it isn't, BUT I just remembered that I had the check engine bulb removed 5 years ago because the backup oil reservoir was pumping oil for no reason (level indicator switch was sending false reading to pump) and when I had the pump disconnected, the chk engine light was on so I had the bulb removed! (cheaper than having the level indicator switch repaired).

IAC will be replaced this week, and removed no charge if it doesn't solve the problem.

Stay tuned (sorry for the pun)

New IAC valve was installed but problem persists. What next?

New IAC valve was installed but problem persists. What next?

The o2 sensors should be checked.

But first replace the engine fault light......Check for fault codes.

Check all vacuums EGR too.
I would also check the MAF sensor (mass flow)
I would check and clean the throttle body first.

What engine is it? 2tz-fe or 2tz-fze?
The resistances are different depending on the engine for the IAC.

Measure the IAC for resistance between terminals +b and the other terminals isc1 and isc2 or iscc and issc depending on year for 2tz-fe.
The resistance should be between 18.8 and 22.8 Ohms.

for the 2tz-fze the terminals are +b and rsc , rso
Resistance should be between 17.0 and 24.5 Ohms cold and 21.5 - 28.5 hot.

Check the ECM is receiving proper voltages from sensors.

Check that big air hose going to the throttle body area. About 3 inches around, and has a smaller line or two off it. Mine was dryrotted when I replaced the ISC valve. If you are in an area of salt and rust, check filler neck off fuel tank, for pinholes. Or, listen for that hiss when you remove fuel cap.

About that oil sensor.........If you do not change oil regularly, these floats get gunked up. I bought on that had this problem. If you take the switch out of the pan, soak it in some solvent, or gas. Use a fine sandpaper to clean the rod the float rides on. Then soak it in some oil for a day or two. Put a little white lithium grease on the rod, and run the float over it a bit. This should straighten things out, without replacing the switch.