Hard cold starts, random idle stutter

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  • 3 Post By ylwagon
  • 1 Post By TheRustySuper

Thread: Hard cold starts, random idle stutter

  1. #1
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    Hard cold starts, random idle stutter

    Hello there.

    Just as a preface, I'm going to try to give y'all as much info as I can on this problem, since I've been trying to diagnose it for a while.

    The Car:

    It's a 2005 1.8t auto wagon, AWM engine code with 160k on it. Stock except for APR Stage 1, which we've had for years with no issues.

    The Problem:

    All of this started when I realized the PCV & engine vacuum system needed refreshing. Exhaust & burning oil smell on startup, occasional hard starts, occasional idle fluctuations when the engine was idling, the usual symptoms. I replaced every single valve and hose in the PCV system (034 Motorsports kit & other parts sourced from ECS), and every single check valve in the vacuum system. I spent several days on this job (mostly working evenings) making sure I got everything just right. I double and triple checked all the vacuum hoses I reused for any signs of a leak. They all looked and felt mint. No cracking, still nice & flexible. Double and triple checked all my connections, everything was tight and routed as it should be. Finally got everything back together and man what a difference! Can definitely feel more boost, and no more smells.

    But, the cold start problem and idle stutter persist. When it's cool outside, say below 50 or 55 degrees, the car seems to struggle to start. You turn the key, sounds like it's misfiring for a couple seconds, then the RPM's jump up and it idles just fine. Always starts on the first turn of the key though. This happened for a week while it was cool, but now it's warm and it's not doing it as much.

    The idle stutter happens totally at random. Sometimes it won't happen for a couple days, but it always comes back. Always seems to happen when I'm at a stoplight and the engine is warmed up. The idle drops say ~100 RPM for a second and then it's fine again.

    I should note that the car did not always do these things, it started having those issues a couple months ago which motivated me to get to work on the PCV/vacuum system.

    There are no codes. Got a couple misfire codes after I first started it after finishing the job, but cleared them and they haven't come back since. Had a brake boost fault code as well, cleared that when I put in a new 3-way check valve and has not come back.

    What I've Done & Checked:

    I've looked into this problem quite a bit, and I've found several potential problem areas that I've tried to address. I'll list any other information that's relevant here too.

    - MAF: Heard it can get clogged with oil from a K&N air filter, which I had fitted. Cleaned the MAF with dedicated cleaner, replaced the filter with a paper one.
    - Spark Plugs & Coil Packs: No recurring misfire codes so I doubt these are the problem. Plugs have less than 40k on them, coil packs have about 15k.
    - Throttle Body: Performed an alignment on it multiple times. First time VCDS was acting up. Second time worked as it should. Here's the values I got:
    Throttle Valve Sensor 1: 12.1%
    Throttle Valve Sensor 2: 87.5%
    Those values look normal? Also, is it possible to install a throttle body upside down? Mine is oriented differently than pictures I've seen online, but I don't see how that could effect anything.
    - Solenoid valves under the IM: I broke a fitting on one of these while removing the IM. Got another one from the junkyard--both P/N's matched. Could this be the culprit? Since I've got the same symptoms as I did before I started all this work, I kinda doubt it.
    - Battery is only a couple years old
    - Coolant temperature sensor: Replaced ~40k ago. Coolant gauge sits right in the middle as it should. Car also got a new thermostat 'round that time. System was recently flushed & coolant is clean & pink.
    - Vacuum Lines, hoses, etc. : I suppose the problem could lie here. But I've checked every connection so many times and they're all fine.

    Sorry for the long post, I wanted to provide as much information as possible. Little things like this always bug me, I want everything on the car to be working as it should, all the time I've seen a variety of things that could cause the symptoms I've described, but I would like some help narrowing down problems from some folks a bit more experienced with these cars than I am. What do ya think?

    Thanks in advance for your help!!!

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  3. #2
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    Drove the car to work yesterday. No problems in the morning, but on my way home there were lots of fluctuations in the idle. It happened with the AC both on and off, but more frequently with it on. Gonna scan it for codes, since I haven't seen the RPM's fluctuate this much.

    Maybe I'll try unplugging the MAF and see if that isolates the problem? Also wondering if it's the fuel filter...

  4. #3
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    Yeah, time of scanning for codes. Make think on an electrical issue. Lets see what vcds says about it.

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  6. #4
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    Just scanned it for codes, and there were none. Idle started fluctuating shortly after I started the car.

  7. #5
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    Quote Originally Posted by TheRustySuper View Post
    Maybe I'll try unplugging the MAF and see if that isolates the problem? Also wondering if it's the fuel filter...
    The fuel filter is very unlikely to affect the idle, because fuel flow is at a minimum, and therefore the pressure drop is at a minimum too. I'd use your VCDS to see what the ECU thinks the coolant temp is to begin with. Next, I would disconnect each and every engine electrical connector you can reach, grounds included, and treat them all with a contact cleaner spray, or just with WD-40, which I do. A malfunction that is so intermittent might be due to high resistance somewhere, and might not leave a code if the resistance becomes acceptable again due to vibration or whatever.

  8. #6
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    Quote Originally Posted by ylwagon View Post
    The fuel filter is very unlikely to affect the idle, because fuel flow is at a minimum, and therefore the pressure drop is at a minimum too. I'd use your VCDS to see what the ECU thinks the coolant temp is to begin with. Next, I would disconnect each and every engine electrical connector you can reach, grounds included, and treat them all with a contact cleaner spray, or just with WD-40, which I do. A malfunction that is so intermittent might be due to high resistance somewhere, and might not leave a code if the resistance becomes acceptable again due to vibration or whatever.
    I'm gonna try and check the coolant temperature today. I'll log it from a cold start, so I should get a pretty good idea of what it's telling the ECU as the engine warms up. I think I have a spare CTS laying around so if anything looks fishy should be a quick change-out. I'll post up the results from VCDS, hopefully tonight!
    VAGguy likes this.

  9. #7
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    Just tried to log it with vcds but guess I did something wrong cos I ended up not saving my log!

    That being said, at 88 degrees ambient air temperature, the readout was 31 degrees Celsius with the car off. When the engine was fully warmed up and idling, it consistently fluctuated between 96 and 97 degrees Celsius.

    Not sure if that's enough to go off of. If not I'll make sure to save my log tomorrow when the car is cooled off and I can do this again!

  10. #8
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    If they are the temps delivered to the ECM, the CTS was working correctly during the test.

  11. #9
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    Day one: Remove the wiring harness connector at the MAF sensor and treat/coat the male/female connectors with your favorite contact enhancer. Got Stabilant 22 or Caig Deoxit?

    Drive on day two.

    Day three: Repeat the day one steps, but for the connector at the throttle body.

    Have a hunch you may see improvement after cleaning the MAF connectors. If not, your MAF sensor may be on the way out as they tend to gradually get flaky not too long before failing.

  12. #10
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    Tom, those were indeed the temps being delivered to the ECM, so I suppose the CTS is fine.

    Electron Man, I don't have either of those contact enhancers, but I'll certainly go out and pick one of those up since I was planning on cleaning up the connections in the engine bay anyway. Thanks for the tip--I'll give that a go over the next couple days and see what happens.

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