VR6 Intermittent misfire/loss of power running rough

[itsagolf]

My VR6 has developed an intermittent problem where it suddenly starts to run very rough, low on power, noisier than usual. Can happen any time. Have tried giving it a bit of stick, sometimes it recovers power, sometimes not and then will run rough again. Have tried some highway driving for a couple of hours, again problem comes and goes. Have not noticed any puffs of smoke from the exhaust. When the car is running well has heaps of power, goes very well.
Car has done 96k, I am the second owner, been running fine for last 12 months.
Coilpack, leads, plugs replaced about 12 months ago.
Checked engine at night and no sparks visible at night, Air filters clean, pcv valve ok

Mechanic suggested that this could be carbon build up and I need to drive the car a bit harder. It does do the drive to work in peak hour so not a lot of open highway driving. I have done some highway driving over the last 2 weekends and have put the boot in while in 2nd or 3rd, car runs great at times and then stuttering comes back.

Is carbon build up something that can cause these issues and if i keep driving hard will this help?

[JiN_KiNgs_InC]

try using a VAGCOM to check the electrics. saves you heaps of time stabbing in the dark.

[Dubba]

You might have a few airleaks and some leaks in the manifold?
I had the same problem, I replaced the coil pack, spark plugs, leads and fixed a few manifold and airbox leaks.

[Martin]

Don't give it some stick when the problem occurs in case it's a lean condition or carbon pre-ignition

Buy some Nulon fuel system cleaner and run that through one tank (don't go hard)

Check hose condition looking for air leaks

Intermittent problems are a pain to track down

[itsagolf]

Thanks for the responses, I'll try some fuel system cleaner as well as check for leaks. Won't go hard when the problems occurs.
Was happening quite a bit this morning after about 15 mins of driving but virtually nothing on the way home, just one instance of poor idling.

Read on the forum that its worth cleaning the MAF as well so might try that on the weekend too.

[itsagolf]

Quick update
1. Cleaned the MAF sensor with a MAF cleaner, went for a drive problem came back with the spluttering & misfire
2. Disconnected MAF at idle no difference, which seemed to make sense. Disconnected and went for a drive, car stuttered all the time, not really conclusive as the stutter/misfire comes and goes
3. Inspected hoses most still soft nothing seemed brittle or broken, no hissing
4. PCV valve has been repaired glued up with JB Weld & Rescue tape seems to be holding but still a potential to be replaced as it was broken an doesn't seem to have glued up perfectly
5. Running badly more of the time so booked it in.

Into vw shop today, couple of observations
- VAGCOM detected misfire on 2&5, car was running very rough this morning, also had a record of MAF failure which links up to me removing the MAF sensor cable
- plugs not good after 18 months, loose connections to the leads, plugs replaced with NGK's seems to have solved part of the problem. Plug 1 quite bad re oil and stuff may have other longer term issues in cylinder 1 but not catastrophic at this stage.

When the problems not there the car does run better, & seems the problem has changed slightly, but
- After a while sometimes idles poorly, struggles to stay around 600rpm feel like its going to stall
- When accelerating can sometimes misfire under acceleration over the 3000 rpm mark
- When stable on 3000 can also be struggling feels like misfiring, car chugs badly
- If the car is shuddering at idle problem does not go away under acceleration

Coil, plugs and leads were replaced April 2012, car has done 12000km's in 16 months.

Next thing I've been recommended to change is leads
PCV valve may be the cause of the idle issues, given its broken seems to make sense to get this replaced to eliminate that. (Shame the PCV is part of the whole air intake assembly and costs $250 ish)

Get the sense this will be a trail and error replacing one thing at a time

[HaydEn]

Tried the tps?


"I only post on Facebook if I know i am going to get likes" - M

[Martin]

I don't think the TPS would make it run rough etc (?)

Sounds like a tricky problem

I've had two cars give these symptoms, one was an air leak and the other was a fuel supply problem
(for sure there will be other potential causes)

When it's running bad/rough, spray degreaser on air lines because it shows up air leaks pretty easy (don't get it on the exhaust)
Spray a suspect line - if the engine runs better for a moment and then goes bad, it was the degresser blocking the air leak for a brief moment

Run a tank out with the stronger fuel system cleaner in case it's injectors or fuel lines

After the above (cheap) tests, a fuel pump may be a swap out candidate?
Have you had any fuel tank problems/incidents?

[HaydEn]

Tps will definitely make it run rough. Will cut power after 3000rpm too.

It's an easy one to check off the list either way. Just unplug it and see if its any better.


"I only post on Facebook if I know i am going to get likes" - M

[anthony--t]

Personally that Pcv sounds suspicious to me. You'd be taking in air after the maf if there's a leak....