Lean condition on cylinder #1

Question

Hi Scotty and forum members!

I have a 2008 Subaru forester 2.0 L variable valve timing 158 hp. I know Scotty's thoughts on subaru but this is a 2 liter engine. They have thicker cylinder walls so they don't blow up head gaskets.

I'm trying to diagnose a lean condition.

Around 7% at idle going to around 4% at higher rpm. At idle I can feel a slight misfire and also in acceleration, in the low rpm range. Especially when it is hot outside.

Cylinder 1 spark plug looks clean like in the picture below. From my understanding it's lean afr and it's burning hot.

Checked for vacuum leaks for ages and I can't find any. Replaced spark plugs and coil. I have 2 sets of injectors because the car is converted to run on lpg. Same behavior on lpg. So the injector is ruled out.

Also when I unplug one of the healthy cylinder's injector the fuel trim goes to 29%. When I unplug the cylinder 1 injector the fuel trim goes to 26%.

I have pulled the valve cover on that head to find the valve lash almost in specs. 0.2 mm for intake and 0.35 for exhaust valves. Only one exhaust valve on the cylinder 1 is slightly out of specs. It's around 0.27mm when it should be 0.35mm. Can this make such a big difference?

Compression it's in specs for all cylinders.

I have checked and reseated the timing belt.

I have pulled and moved around the wiring harness for the injector and coil pack, nothing.

It's too consistent. It's a permanent condition. Like a vacuum, leaking valve or ecu fault.

One more clue - it seems that the misfire is better when the engine is cold. And gets worse when it's in closed loop. But I think it's not that relevant, because then the fuel is dumped and not really metered. So it tends to run better because it's a rich afr.

Yet I guess it gets a bit worse when it's properly warmed up. Sounds a lot like a vacuum leak or leaking valve.

I didn't find a cylinder leak down tester here (i know some mechanics and they didn't even hear about it) but i did put some pressure in the cylinder at top dead center through the spark plug hole. And apart from a bit of hissing in the oil filler neck I could not hear anything. I had a healthy cylinder to compare. Also I put some pressure with my mouth in the tube and I felt similar amounts of air escaping. But with the engine cold.

Please give me some advice!

Thank you!

spark-plug.jpg

HillBilly · Answer

Definitely different afr between those plugs. Yes on adjusting exhaust valve...after you check that cam.

BTW, the issue with head gaskets on non turbo suby's is the design of the HG, it's laminated and if you park your suby on a slant, oil sits in the cylinder and weeps by the rings and delaminates the gasket over time. The turbo gasket is different and doesn't delaminate so if you ever need to replace one, use the turbo gasket. However it doesn't stop that oil weep/ usage that is unfortunately normal for suby's boxer.