2006 Jetta TDI w/ BRM engine, misfire at idle when hot and air in fuel return line

[Rezer]

First of all this engine has 330,000 miles on it and I recently did some pretty major work on it, including replacing the rods, wrist pins, rings, valves, and a few other bits here and there (different thread that I still need to get back to after I sort this out). It runs absolutely fine when cold, but when it gets up to running temperature it idles like one of the cylinders fell off the car. VCDS shows the idle stabilization on cylinder 4 maxed out at 2.99mg/s and it throws a cylinder 4 misfire code, but the torsion value is right at 0.5, sometimes hitting 1.0 now and again. At above around 2000 RPM it still runs fine, and kiiind of smooths out at idle for a few seconds after letting off the throttle before going back to being absolute garbage again.

After ruling out the cam sensor, crank sensor, injector wiring harness, and temperature sensors I put a clear line on the fuel return line. During a hot idle it's blowing a large amount of air back into the fuel filter housing, but right now I'm unsure if this is the cause or a symptom. Here's a video of what it's doing:

Would the more likely source of this be a leaking injector/injector seal, or the tandem pump blowing in so much air that it's starving the nearest injector of fuel and causing the misfire? Both are original to the car, but I understand that the tandem pump tends to give it up before the injectors do. I'm just hoping for an outside opinion before I go replacing parts without really knowing for sure which is the likely culprit.

 

[Rezer]

Well after doing a little more searching I found that the injectors were the more likely culprit, and today I changed all the injector seals. Turns out that all 4 injector hold down bolts were alarmingly loose, like they were fairly easy to remove holding the ratchet from the center using it more like a screwdriver than a ratchet. I've never touched them, so I'm unsure if they've always been that way or they just loosened up over time.

All 4 injectors had some amount of galling, but #1 and #4 were especially bad, with semi-circular bits of metal that had been formed from what had been worn off the head. I assume this is down to them moving around slightly in the bore due to the loose bolts. The bottom-most orange o-ring on #4 was also looking pretty worn from the movement, and after changed out all the seals the problem seems to have gone away.

I still have no idea why the problem only manifested when the engine warmed up, but I'm fine with it being a mystery as long as it doesn't crop up again...

 

[JETaah]

All 4 injectors had some amount of galling, but #1 and #4 were especially bad, with semi-circular bits of metal that had been formed from what had been worn off the head. I assume this is down to them moving around slightly in the bore due to the loose bolts. The bottom-most orange o-ring on #4 was also looking pretty worn from the movement, and after changed out all the seals the problem seems to have gone away.

I still have no idea why the problem only manifested when the engine warmed up, but I'm fine with it being a mystery as long as it doesn't crop up again...

You would hard pressed to find a PD injector without that wear to one side. That also leaves a burr inside of the bore that reflects where the injector wears. Seems like it is the nature of the beast along with sooty deposits being taken back to the fuel tank. I think that VW could have developed a better way to hold down the injectors (like the ALHs) to prevent this but, heck, if it made it to the end of the warranty period without problems then that is good enough, eh?

 

[dtrvler]

I have removed and installed hundreds of pd injectors. Never found one loose. To the contrary, they have to be gently worked loose and eased out with a slide hammer. If there was movement to the degree that the orings were wearing out from movement, that is failure by whatever mechanic installed them. It is not a design flaw of the hold down system. That is my opinion. It takes all new copper washers and orings with sylglide or some other proper oring lube to install them correctly. Yeah, you can anneal the copper washer/seal but as is my style, i use all new every time. Its not expensive. Yours were not properly torqued to begin with im thinking.

 

[dtrvler]

People love to hate the brm engine.
Think about this. For 600 bucks, you can replace the TBK and Camshaft kit every 100,000 miles. Yeah. Thats 6/10 of a cent per mile if you do the work yourself. And if you spend the money for a high rent cam, same cost for parts. 6/10 of a cent per mile. With the high rent cam you save the labor swapping the cam every time you change the belt. It just takes 3 or 4 hundred thousand miles to get your money outta the cam. Either way, its just not that bad.
Delete the egr, single mass the clutch/flywheel kit. Use the correct oil and the turbo will last a long time. When it goes, you can do a tune, pd140 turbo, downpipe, or whatever. But these cars just arent that bad.
Sure. You can drive an mk4 with an alh much cheaper per mile but, you have to drive a mk4! Ive seen alh cams with 750k on them that were still great. To each his own.