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Phantom and Random Misfire Codes - What Next?

4K views 15 replies 5 participants last post by  md1twal3 
#1 · (Edited)
Been chasing misfires on my 2007 Passat 2.0T for several weeks. Has 135K on it with the 6 speed. I've owned it for about 6 months and replaced some parts relative to the misfire codes. I clear the codes and sometimes it will be cylinder 1 consistently. I clear again and it may be 3 & 4 but not 1. Sometimes, it's all 4 cylinders. It may or may not trigger the CEL and sometimes causes flashing CEL (which I understand why) It is most predominant on /moderate to hard acceleration and is pretty smooth if you eased through the throttle. The strange thing is it seems sporadic....sometimes it will miss, sometimes it will not. I can't associate a bunch of common variables like weather, temp, city/highway, etc. except it seems to show up more upon driving from a cold start. It idles fine, no roughness or misfires.

I started with plugs and coils. The plugs were visibly bad so was a noticeable improvement but the missing continued. The clips for the coil harness connectors were broken so I soldered in new connector pigtails and used heat shrink to seal up the welds. Replaced the air filter as well. I also had a code for the VVT solenoid and found the connector broke on it so replaced it too. It was going through a lot of oil and found the PCV valve failing so replaced it. That issue is resolved. I removed the HPFP and replaced the cam follower. There was no visible wear on the original so assumed the cam was fine.

I had significant cam chain noise so figured maybe the slack in the chain was causing valve timing to change slightly under hard acceleration causing the misfire. I bought a new chain, adjuster gear, seals and tensioner and performed that repair. Noise is gone but still has missing issue.

I thought maybe a fuel pressure issue but it seems there would likely be CEL codes associated with rail pressure. I thought diverter valve but also appears low/no boost codes would likely set, not misfire codes..

The random and sporadic nature here has be baffled. It is most noticed cold under moderate/hard throttle but that's all I've got. I thought about carbon on the valves but might expect idling issues too. Clogged fuel injectors are also consideration but I haven't conceded to pulling it apart to check them. That said, has anyone had luck running an injector/valve carbon cleaner through the engine and if so, what is the most effective cleaner and method that yields the best results?

I did have it at the VW dealership for the fuel pump module recall and asked them to investigate the misfire. $160 later, they couldn't find anything. They too noticed the cam chain noise and suggested they repair it as a first step. Saved myself about $1200 and did it myself. Needed it for sure, but didn't resolve the misfire.

I'm not opposed to throwing a diverter valve at it just because it probably doesn't have the updated one in it but curios to see if anyone has experienced misfires and misfire codes due to a failed diverter valve.

Appreciate any expertise anyone can offer!

Edit: The misfire/surge/chug seems to occur at the time boost is starting to apply. It will miss or buck a couple of times and then come out of it as boost increases. At a certain point the miss/buck goes away and boost comes on hard. No whirring or whistling that I can hear. Diverter valve cause this? Or a vacuum leak somewhere? ANy common spots for a vacuum leak?
 
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#3 ·
#4 ·
I have a similar issue but on idle and have changed a lot of stuff, but it doesnt seem anyone that actually knows anything about these cars use forums any more. I am trying a couple of FB groups and I will let you know how it goes because the best part about the internet is collaborating with others to try and solve issues.. I thought anyway.
 
#6 · (Edited)
What codes are you getting? Anything besides the P300"s for the misfire? I had an issue a while back where I was getting misfires from a vacuum leak. A leak will cause random misfires due to a lean condition. Check the vacuum line going to the master cylinder first and report back. Also, the random misfires on cold start is a symptom of carbon buildup as mentioned....
 
#7 ·
I am only getting misfire codes for all 4 cylinders. After some thought, I don't think it's carbon build up. That generally causes cold start misses due to the fuel being soaked into the carbon, which takes away from the need for additional fuel on cold start. With carbon build up, the misfires will generally subside once it's warmed up.

After driving it different ways, the misfires are more evident the harder you accelerate. Light throttle with gradual throttle = minimal misfire, Moderate throttle = moderate misfire. Hard throttle = violent misfires. It will do this when the engine is hot or cold.

I really feel like it's a fuel pressure issue but everything I've read says the HPFP will usually throw a fuel pressure related code, which I am not getting. Maybe the primary pump is weak? What should my fuel pressure be at the rail at idle? What should it be under acceleration?

BTW - Apologies for being absent. Other forums I belong to email me when there is a reply to my post. Didn't happen here so thought I hit a dead end.

Thanks in advance!
 
#8 ·
Just checked for vacuum leaks with propane. Nothing. Removed oil cap while running, nearly died.

Driving it today, I got into the throttle hard, it bucked hard and then the boost kicked in hard. Other times it will flutter a bit under light or moderate throttle and boost isn't as predominate.

Fuel pressure hits around 1800 PSI under hard load.

What does the boost control solenoid do? Can it cause this issue and not throw a code?

Really stuck here folks...
 
#9 ·
Fuel rail should be ~50 BAR (~725 PSI) at idle. The OEM FPRV (fuel pressure relief valve) is calibrated for 130 BAR (~1,885 PSI) which means rail pressure should not be able to exceed that. What's the pressure and duty cycle look like on your in-tank pump?

On your boost control solenoid, if you're referring to the N75 valve it opens/closes the wastegate to control boost.

Would recommend carbon cleaning every 30k miles.

Any mods to the engine?
 
#10 ·
When I test drove it with my scanner on it, my rail pressure under load was about 1750 so I think it's good there. Can you help me understand how to check pressure and duty cycle of the in-tank?

I am referring to the N75. It really acts like the wastegate is cycling when this issue occurs because the few times it doesn't surge, the turbo applies hard. I just drove it on a 950 round trip and no other issues, got great fuel economy. Only issue was if I put my foot into it to pass or switch lanes...then the bucking would occur. Makes me think turbo/wastegate but not sure. Can a bad N75 cause this violent bucking?

Carbon cleaning is in the back of my mind as the carbon could be soaking up fuel before it gets past the valve making it a lean condition. Just seems odd that there are no issues on cold start/cold idle since there is more fuel demand.

No mods or upgrades at all.

Appreciate the response!
 
#13 ·
Cleaned the valves and ports and blades this weekend. They were carboned up but not really that bad for 138K. Anyway, another possibility checked off but unfortunately, the issue still exists.

I was inspecting the turbo while I had the car up and noticed a crack from the wastgate flap downward about 4". Reading other posts here, sounds like it isn't uncommon. However, I can't find any that speak of the crack affecting wastegate function and how. In my case, could it be causing the wastegate flap to flutter,/stick causing the bucking somehow?
 
#14 ·
As of tonight, the problem is solved. A new set of OEM NGKs and it runs better than it ever has. I may also have not had the coils fully seated. Took a rubber mallet and gave them a gentle tap. Seemed to drop down a little when I did that. I have to say, I wouldn't have thought this 2.0T with a manual trans could pull so hard! It's pretty impressive.

as I look through my threads it's easy to consider that I threw a bunch of parts at it to fix an issue. Partly true. The things that were fixed/cleaned/replaced could have caused the problem but didn't fix it were all still necessary and overdue, even though it didn't fix the problem. And....I learned A LOT.

Next thought may be that I should have tried new plugs in the first place. I did. Twice, so I dismissed them as a possible contributor. Is what I should have done in the first place is use the factory recommended NGKs.

Last mention, on the turbo housing being cracked...something I will eventually fix but it's on the exhaust side of the housing so it doesn't impact turbo performance.

Thanks for the views and input.
 
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