Will Bad O2 Sensor Cause Rough Idle .

[claydoctor]

I did a search on the "Check Engine ", "VSC off" and "VSC Trac" lights being on . Consensus was bad O2 sensor . My problem began 3 days ago with a rough idle that would last about 20 seconds, when starting the car and then clear up . Two days ago , the rough idle persisted and so we shut it off and took the other car .
Today, it started fine , no rough idle but unlike before, the Check Engine light came on as well as the VSC Off light and the VSC Trac light . I checked the gas cap and it was tight . I drove the car several miles and it drove fine but the lights remain on . Is the rough idle a separate issue or more evidence that the problem is a bad O2 sensor ?

 

[BlackWorksInc]

What makes you think its a O2 sensor? Just because the lights are on does not indicate anything more than an active emissions related DTC stored in the PCM.

The MIL, VSC OFF, and VSC TRAC lights ALWAYS light up at the same time when there is a DTC in the system. Not all Toyota's do this, but the Vans, trucks, and SUVs seem to do this more than the cars.

You need a code reader to pull the codes and figure out what direction you need to go in your diagnostics. Most likely there are some P0300-P0306's stored along with some other codes. A faulty AFR or O2 sensor will not directly cause poor idle but skew the fuel trims enough to cause a problem, but unlikely without setting lean or rich codes.

 

[97hb]

Rough start up then runs smooth I would do a leak down. Like black said don't assume o2 get it read at auto zone for free and come back with pxxxx.

 

[claydoctor]

Dealer suspects blown head gasket. Says they have been seeing this on some high mileage (180,000 in my case) 4-Runners . Computer suggests misfiring in cylinder #4 . I have the V6 . Apparently water leaks in overnight and then engine runs rough until it clears out the water . Need to take it back and leave it overnight so they can check it with a borelight in the morning .
Right now, I have no warning lights on. If problem is not fixed , why have lights not come back on ?

 

[BlackWorksInc]

When you erase the DTCs in an OBDII car, it resets all the monitors and collected data. So the car has to run its monitors again, those monitors are what determine whether or not the MIL will be turned on. It can take a short time or a long time for a DTC to trip again, it can vary a bit. If you have a severe misfire it'll set pretty fast, but if its an occasional misfire it might take a couple drive cyles. O2 sensor codes won't set until the cat efficency monitor has run and that can vary a lot from vehicle to vehicle. Anywhere from 30-100+miles

 

[claydoctor]

Now it's getting interesting. Prior to Thursday's dealer visit, it would run rough with every start up and eventually the lights came on prompting me to take it in. Since bringing it home, the lights have remained off and car has started up fine the past two mornings. All the dealer did to my knowledge was put in on computer and pull the plug on the offending cylinder to look at it. I have an appointment to take it back in Monday but if it starts up OK the next two mornings, I think I'm sitting tight. If it were a blown or leaking head gasket (my worst fear) would it behave this way? I don't want to risk taking it back and being out big bucks because it's "most likely" a blown head gasket. Another idea was it might be the spark plug coil.

 

[Calimander]

Now it's getting interesting. Prior to Thursday's dealer visit, it would run rough with every start up and eventually the lights came on prompting me to take it in. Since bringing it home, the lights have remained off and car has started up fine the past two mornings. All the dealer did to my knowledge was put in on computer and pull the plug on the offending cylinder to look at it. I have an appointment to take it back in Monday but if it starts up OK the next two mornings, I think I'm sitting tight. If it were a blown or leaking head gasket (my worst fear) would it behave this way? I don't want to risk taking it back and being out big bucks because it's "most likely" a blown head gasket. Another idea was it might be the spark plug coil.

If you are thinking that it could be a coil pack, swap two around and see if the problem moves. I think you need to have a look at some of the threads here relating to blown head gaskets. I also think you should take your car back to the dealer and ask that they look for coolant leaking in the cylinder that you are blowing a code in.My 05 head gasket went with under 100k on it Best of luck to you.

 

[Chrisygirl17]

I am having some what of the same problem, We swaped out the fuel injector and it has seemed to fix the issue. We had tried the coil pack and new spark plugs first but that did not help. I had a code of cyl 4 misfire. if you have yet to fix the problem i would try this.

 

[claydoctor]

If you are thinking that it could be a coil pack, swap two around and see if the problem moves. I think you need to have a look at some of threads here relating to blown head gaskets. I think you should take your car back to the dealer and ask that they look for coolant leaking in the cylinder that you are blowing a code in. Best of luck to you.

They changed the coils around at the dealership . Problem is the rough idle only occurred early in the morning , so they could not see if switching coils at the dealership made the problem move . That's why they wanted to keep it overnight to observe the miss and check cylinder for coolant . I guess the codes can show a problem with a particular cylinder even if it is not misfiring at the time . I was to take it in today (Monday) so they could check it tomorrow morning , however, today is the fourth morning in a row that it has started up fine and run like a top with no warning lights on . I'm beginning to think that messing with the plugs and coils somehow fixed it . Only time will tell if it is a permanent fix . Thanks to all .

 

[Calimander]

It only occurs after it has sat awhile(like overnight), and the rough idle is the fact that there is coolant in the cylinder and it takes a second or two to burn it off.Keep a eye on your coolant level in the overflow tank. If it is low or slowly disappearing ---bad news awaits.The rough idle came and went with me too.

 

[claydoctor]

It only occurs after it has sat awhile(like overnight), and the rough idle is the fact that there is coolant in the cylinder and it takes a second or two to burn it off.Keep a eye on your coolant level in the overflow tank. If it is low or slowly disappearing ---bad news awaits.The rough idle came and went with me too.

Isn't there also something about checking your dipstick to see if the oil looks frothy ?

 

[Sungod]

Isn't there also something about checking your dipstick to see if the oil looks frothy ?

frothy oil would indicate that you have coolant leaking into the crank case. If it is leaking into the combustion chamber, it will go out the exhaust.

 

[claydoctor]

frothy oil would indicate that you have coolant leaking into the crank case. If it is leaking into the combustion chamber, it will go out the exhaust.

Good point! I can say that for the fifth morning in a row, it has started up without a problem and the coolant level is in the normal range.