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Rough Idle at stop

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    • May 17, 2009 9:48:37 pm
    My tC began having a rough idle a couple of days ago. I replaced the spark plugs and it helped some. It only seems to have the rough idle at a stop or crawl. However, when you push on the accelerator, the engine evens out and runs just fine. I pretty sure there's a miss fire because I can smell gas in the exhaust. There's no backfire though. I'm thinking that I've got a bad coil, or two. The two and three cylinder spark plugs that I took out were both fouled.

    I've tried researching for bad coils on here and scion life but, it's hard to find good info. I guess what I'm asking for is some sort of verification that this could be the problem (bad coils). I haven't taken the spark plugs back out yet because I've got a strut bar on and I have to be at work early.

    Has anyone else been in this situation?
    • May 18, 2009 6:16:29 am
    silversurfer897 wrote:
    My tC began having a rough idle a couple of days ago. I replaced the spark plugs and it helped some. It only seems to have the rough idle at a stop or crawl. However, when you push on the accelerator, the engine evens out and runs just fine. I pretty sure there's a miss fire because I can smell gas in the exhaust. There's no backfire though. I'm thinking that I've got a bad coil, or two. The two and three cylinder spark plugs that I took out were both fouled.

    I've tried researching for bad coils on here and scion life but, it's hard to find good info. I guess what I'm asking for is some sort of verification that this could be the problem (bad coils). I haven't taken the spark plugs back out yet because I've got a strut bar on and I have to be at work early.

    Has anyone else been in this situation?


    Any CELs? Rough idle can be a dirty intake as well. Clogged or clogging filter media can disrupt the A/F mixture at the MAF sensor. When you said the plugs were fouled, were they black and chalky-like, or black and gooey? If gooey, then you have an oil issue leaking into the cylinder. If that's the case, replace the valve cover gasket b/c your inner valve seal is possibly leaking.
    • May 18, 2009 4:32:06 pm
    I have the Alphawerks header on and I don't have an anitfouler or anything on so I've had a cel of for a couple of years.

    The plugs that fouled weren't really gooey, but the defantly were not chalky. I think the may be the reason. My brother is currently taking a auto tech class and they happen to be doing diagnostic work right now so I'm goin to take my car in ther and they will hook it up to a scanner for free.

    Thanks for the help.
    • May 18, 2009 4:57:39 pm
    Have the code read because even though you've had a CEL for a while if a coil is bad it should be throwing that code as well now too.
    • May 18, 2009 6:26:23 pm
    Mine was starting to idle a little rough after using some cheap gas before. I had a fuel/intake system cleaning done to it after and it made a big differance. If you go to wal-mart its only $20. They just hook it up to a vaccume line on the intake manifold. Its the same thing a shop or dealer charges like 150 to do.
    • May 31, 2009 2:30:15 pm
    Well my brother and I have found the problem. Blown cylinder head gasket. We're in the process of fixing it now. We hope to have it done by Tuesday. I didn't mention that I had a over heating problem because I thought it was a separate issue and thus it didn't need mentioning. I originally thought it was the thermostat, so I replaced that (it ended up being free from Auto zone with my reward points) and it seemed to solve my problem, buy then sure enough it came back. Then I thought it was the water pump, replaced that, and then we finally put it all together. Anyhoo, we did a compression test and it's between the 2 and 3 cylinders.

    So, if your losing coolant, and you have a rough idle, better get a compression test to see if your cylinder head gasket is blown.

    Also, my brother was wondering if anyone else has had this issue.

    Thanks for your help and feed back!
    • Jun 18, 2009 7:57:25 am
    Finally got the tC up an running on Monday. Here's some pics of the head gasket:





    • Jun 18, 2009 10:09:21 am
    right in the middle. just like most 4cyl head gaskets that i have seen go bad in the past.
    • Apr 01, 2012 5:43:36 pm
    i am having the same issue. i had installed new plugs, had the flower sensors cleaned and everything i can think of. No codes are being thrown. was that the same with you? does that mean its most likely my gasket?
    • Apr 01, 2012 6:54:00 pm
    Pistons look like there hurting lol
    • Apr 01, 2012 10:53:39 pm
    silversurfer897 wrote:
    I have the Alphawerks header on and I don't have an anitfouler or anything on so I've had a cel of for a couple of years.



    For the life of me I will never understand why people do that. You could have had 10 different codes come on and you would have no idea.

    Anyway, glad you figured it out. I suggest you get the antifouler. Just because you know that your light is on because of it, doesn't mean that everything is OK. When your ECU detects an issue with the sensors it's because it's trying to adjust the Air:fuel ratio and it is going beyond the parameters set as acceptable. Your car, for those couple years, has been running rich or lean at varying times. Likely never running correctly, even though it may have seemed to run smooth.
    • Jun 24, 2012 6:45:43 pm
    Eventually I'll get to how I fixed rough idle at a stop, but the story is kind of important too...

    I was having serious problems starting the engine with only 35k miles. It would take multiple consecutive cranks to initiate the engine. There was also a rough idle. I brought it to Firestone and they thought it ran fine but did a fuel injector cleaning anyway. It still had issues. I bought a new battery and installed it. Only then did I realize that the Progressive Snapshot device I had installed was setting off the fuel-cut off security system and messing up the fuel intake somehow. After I took it out, the car started fine, but the rough idle was still there...

    A year later, I realized that when I changed the battery and had cleared out the ECU (I am not car-fluent), I did not properly RESET the ECU. I was suppose to LET THE ENGINE IDLE for 5-10 MINUTES! That's all I had to do! Everything runs normal now (except for the stupid noises coming from the hatch and minor AC rattle:flame

    So, when you change or disconnect the battery, be sure that the first time you start the engine you let your car sit in park and idle for 5-10 minutes. The computer has to learn the new idle. Sorry if this is obvious to everyone else on this board, but I had no clue. I mean the manual says not to let your car idle when you warm up the engine...Before you look at your throttle body, spark plugs, engine mounts, or whatever, be sure you've done this, especially when you don't have 100k miles or more.

    Good luck. Hopefully it's as simple as my solution was. I'm going to repost this around since it was such an idiotic revelation.

    Scion TC 2006 stock.
    • Jun 24, 2012 9:09:11 pm
    Now we get the whole story. Looks like your block is slightly damaged there you may end up getting a slight miss if it is. Usually your head gasket will fail in the middle. Make sure you get the head resurfaced.
    • Jun 24, 2012 10:55:23 pm
    • Edited 1 times, last by silversurfer897 on Jun 24, 2012 11:06:29pm.
    I posted the whole story three years ago. As you can see, the head gasket went out causing the rough idle.
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