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Discussion Starter #1
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 :wrench: 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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Discussion Starter #2
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...
 

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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.
 

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Discussion Starter #6
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!
 

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Discussion Starter #7
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!
 

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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.
 

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Discussion Starter #10
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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Discussion Starter #11
Update: I haven't forgotten about this! Life just sometimes gets in the way.

Anyway about a week ago I cleaned the TB connectors, but still got the idle fluctuations a few days later.

Cleaned MAF sensor contacts (and the MAF again because why not) yesterday, so I'll report back if it starts acting up again.
 

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I had similar issues with cool starts several months ago. Rough idle when 1st started, felt like and sounded like a soft misfire (no CEL) but it cleared once I revved the engine. After a few months, I finally got a CEL and it was the MAF. New MAF solved all of the issues and the car is running great.
 

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I would try watching block 032 idle fuel trim to see if it is changing significantly and is perhaps causing these idle events. If ecu is taking away fuel causing this you may have a bad sensor somewhere or bad FPR. If adding fuel you may have missed a vacuum leak somewhere but I can't imagine where as you seem to have checked everything.

Edit: for this sort of diagnosis you should probably be watching short term fuel trim. I think block 034.
 

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Discussion Starter #14
I had similar issues with cool starts several months ago. Rough idle when 1st started, felt like and sounded like a soft misfire (no CEL) but it cleared once I revved the engine. After a few months, I finally got a CEL and it was the MAF. New MAF solved all of the issues and the car is running great.
I'm starting to think it's the MAF sensor. Cleaning the contacts for the MAF didn't do any good either. Gas mileage has been down as well--only averaging 22mpg combined city/highway. I know we are both spirited drivers, but I recall a few years back averaging closer to 25mpg with similar driving conditions. Gonna order a new MAF when I get paid later this week and see if that solves it.

Uberbeagle, I'll check on that. What are the numbers I should be looking for in the fuel trims? It's a 1.8t AWM with APR Stage 1, to recap.
 

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Normally the long term idle trim should be close to zero.
The short term fuel trim should also be zero oscillating slightly +/-.
Also you could watch the upstream lambda sensor for spikes at idle.
Since you said you had eliminated most possibilities of vac leak, I was guessing that maybe the ecu is intermittently cutting fuel due to a perceived rich condition perhaps due to a leaky FPR diaphragm or leaky N80 purge valve.
I had the former causing idle problems a couple of years back.
You can use a mighty-vac or similar to test out either of these components.
 

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Discussion Starter #16
Okay so...new MAF sensor did not solve the problem. Back to square 1.

Going to look into FPR and the N80 valve, also closely inspect fuel injectors for any signs of damage since those were out recently for the vacuum system job. Gonna look into fuel trims as well and see if anything is off there.

Any other suggestions? These kind of problems drive me batty.
 

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Discussion Starter #17
Car finally threw a CEL today while my BF was driving to work. So far no new symptoms, so hopefully this code is related to the issue we've been having and can point me in the right direction. I'll post up what I find when I scan it tonight.
 

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Discussion Starter #18 (Edited)
Pulled the following codes:

P0456-001 - EVAP System - Very Small Leak Detected - Intermittent

P0411-002 - Secondary Air Injection System - Incorrect Flow Detected - MIL on

Is there any way these codes could be related to each other? The car was throwing no codes at all until the CEL came on and presented me with these two.

I'm gonna do some poking around on the forums to see where the problem could lie, but if there's any good starting points I'm all ears!
 

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I can't think of any way these 2 codes would be related but maybe someone else will come up with something.

For the SAIP most common cause is probably the 373 relay (in the ECM box). If that is OK, you can check the piping from the fan to the combi valve, the combi valve itself, the vacuum switch/solenoid under the air intake manifold, the motor in the air pump and all the associated vacuum lines. I think the SAIP systems uses feedback from the O2 sensor to detect correct operation but if the O2 sensor was bad, I assume you would get other codes.

Can't help much with the EVAP stuff but I don't think it shares any components with the SAIP.
 

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The Evap system uses the Leak Detection Pump to see if the system has any issues. Since the code is for a "very small leak", it could be something as simple as a sticky valve on the airbox or a loose gas cap. The SAIP does use the O2 sensor to confirm ops. On the 1.8T, the most common issue I have seen (after a failed pump) is a cracked rubber elbow right above the turbo.
 
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