Uneven Idle - Only when warmed up

[ykobayashi]

Hey guys,

I know that there have been several posts on uneven or lumpy idle. I looked at them and none seem to have the situation I do. The car runs smoothly when its cold but as soon as the engine gets warmed up, the idle is super lumpy and uneven.

I am not throwing a CEL but when I pull away from a light there is some hesitation at times but then I give it a bit more gas and its smooths out.

I am thinking, EGR? PCV? O2 Sensor? Vacuum Hose? Like to take a shot at it before I take it to the dealer.

Any constructive feedback will be appreciated!

Yoshi

[SoCal LS-L]

Id try resistance checking the coolant temp sensor first....its the easiest and could be giving the computer false temp readings, which affect timing, mixture, etc.

[ykobayashi]

I just replaced the Coolant Temp Sensor less than a year ago, think it would be that?

[SoCal LS-L]

Im actually dealing with the same thing...... sounds like you may have some misfiring going on. With no MIL, id try running a good injector cleaner through your system first..... Then the hard part...... tracing through vacuum hoses to listen for a leak. My idle wanders for a short time after hitting normal operating temp, then after it gets driven and fully hot, the idle stabilizes at just about 500 rpm, which is low.

In your case with the rough take off it could even be the throttle position sensor...... do you have a multimeter you can resistance check it with?

The computer controls idle with the "bypass air control solenoid valve" which is conveniently buried under the manifold assy..... but its pretty much listed as the first thing to check in the service manual flow chart. Have you downloaded it yet?

[ykobayashi]

Hey, thanks for the great feedback! Yeah, I already ran 2 bottles of Chevron Techron through 2 tanks thus far with no change in the idle. (My injectors must be squeaky clean!)

I'll try and take a look at the TPS see what I can find. Could it be a coil pack or spark plug? I recently changed the spark plugs, all but #6, because its a real b**ch to get to. Otherwise I may have to take her to the dealer (with a large tub of vaseline.)

Thanks again!

Yoshi

[SoCal LS-L]

Originally Posted by ykobayashi

Hey, thanks for the great feedback! Yeah, I already ran 2 bottles of Chevron Techron through 2 tanks thus far with no change in the idle. (My injectors must be squeaky clean!)

I'll try and take a look at the TPS see what I can find. Could it be a coil pack or spark plug? I recently changed the spark plugs, all but #6, because its a real b**ch to get to. Otherwise I may have to take her to the dealer (with a large tub of vaseline.)

Thanks again!

Yoshi

Well since it only does it in closed loop, I wouldnt think it would be any plug or coil..... I was checking on the test procedure from the online service manual for the bypass air control solenoid, the only problem is the manifold has to be yanked to test it. That valve is also $334 . I have heard of coolant temp sensors being bad or out of range when newer like that.... and its one of the easier sensors to resistance check on the car. As far as the dealer they should have a set diag price....at Reno Subaru it was $109.

[ykobayashi]

Hey So Cal LS-L,

Thanks so much for all the feedback. I gave up and tried to take it to my dealer. They didn't have the OBD1 connector so I had to leave her overnight at the dealer, they of course left her outside

Anyways, I came back the next day and they gave me a laundry list of issues that "could" be wrong. They said that pulled many fault codes from the car. The list went from dirty MAF, O2 sensors, TPS, Knock sensors, etc. Everything that I could have guessed the problem was. They wanted to charge me the $109 you mentioned. I questioned what exactly I was paying for if they were not able to pinpoint what the problem was. Service Advisor dropped it down to $50 stating that he would appreciate if I could at least pay for some of the time. I agreed and $50 later I still don't know what the cause of the problem is!

I cleaned the MAF sensor this evening hoping that it might be as simple as that using CRC MAF sensor cleaner. Tried it out, same as usual, fine when it was cold, when it warmed up, started going into rough idle once again.

One thing that the dealer did mention is that they were concerned that the engine cooling fan was on for an inordinate amount of time. They thought that perhaps the thermostat on the fans was out as well. This makes me suspicious that you may be right, it still may be the TPS that I replaced last summer that is out of range. I bought mine at Autozone and it seems like others have been having issues with them as well. Do you or anyone else recommend that I swap it out for a TPS from the dealer?

Thanks again for all your help!

Yoshi

[redlightningsvx]

I had a rough idle and jerky throttle a week or so ago with NO codes. I hooked up the Subaru select monitor and reset the ECT and TCU, let it idle and drive for 10 min and runs perfect now.

[SoCal LS-L]

Originally Posted by ykobayashi

Hey So Cal LS-L,

Thanks so much for all the feedback. I gave up and tried to take it to my dealer. They didn't have the OBD1 connector so I had to leave her overnight at the dealer, they of course left her outside

Anyways, I came back the next day and they gave me a laundry list of issues that "could" be wrong. They said that pulled many fault codes from the car. The list went from dirty MAF, O2 sensors, TPS, Knock sensors, etc. Everything that I could have guessed the problem was. They wanted to charge me the $109 you mentioned. I questioned what exactly I was paying for if they were not able to pinpoint what the problem was. Service Advisor dropped it down to $50 stating that he would appreciate if I could at least pay for some of the time. I agreed and $50 later I still don't know what the cause of the problem is!

I cleaned the MAF sensor this evening hoping that it might be as simple as that using CRC MAF sensor cleaner. Tried it out, same as usual, fine when it was cold, when it warmed up, started going into rough idle once again.

One thing that the dealer did mention is that they were concerned that the engine cooling fan was on for an inordinate amount of time. They thought that perhaps the thermostat on the fans was out as well. This makes me suspicious that you may be right, it still may be the TPS that I replaced last summer that is out of range. I bought mine at Autozone and it seems like others have been having issues with them as well. Do you or anyone else recommend that I swap it out for a TPS from the dealer?

Thanks again for all your help!

Yoshi

They seriously told you that?? The fans are controlled by the ECU, which gets its signal to turn on from the Coolant temp sensor!! Also they had to wait for an OBD-I code reader to check the codes???? They didnt even know about the self-test?? And hopefully they were at least smart enough to make sure the fans werent on because of the A/C...... So it would seem the CTS isnt functioning correctly, I previously mentioned that, and you said it was replaced within a year or so? Maybe you got a bad one.

Another thing you should do is replace that 6th spark plug, its not really that hard to do... I did all 6 of mine in about an 90 minutes, taking extra time to clean out the plug areas. My #6 had oil all over the coil from a bad plug gasket which was causing a misfire, my idle smoothed out considerably after I cleaned it up.

Finally what else you can do is run throttle body cleaner through your intake, start by removing the air ducting to the throttle body and then spray the heck out of the butterfly valves, cleaning them up nicely. Then reinstall the intake ducting, start the car, and spray it into one of the vacuum ports while slightly revving the engine so it doesnt stall. This might free up a sticky Bypass Air Control valve (IAC).

A good mechanics mirror is also helpful for getting a look under the manifold and checking for a vacuum hose loose/cracked etc.