strange engine problem, loss of power under load, strange noise Options

[mightyohm]

Hey all,

I'm having a strange engine problem that I'm hoping someone can help me diagnose.

When I give the car heavy throttle, I hear an intermittent noise that seems to be coming from the engine compartment. It's like a muffled banging or exhaust sound. When I hear the noise I feel a loss of power. When the car is warm, the sound is always accompanied by a whif of burning oil.

Aside from this problem the car is running fine - idle hasn't changed, no obvious misfiring. It's only under load that I have this problem.

It seems to happen the most when the engine is in its peak HP range - upwards of 3000 rpm.

This started happening about 6 months ago. I drive the car to work every day, so I have been noticing that it is slowly getting worse over time. Initially, it only happened when the car was cold, and completely went away after the car warmed up. Now it happens all the time, but it's still worst before the car has had a chance to warm up.

Any ideas? I'm fearing the worst - cyl to head leak? Cracked head??

[mightyohm]

It doesn't feel like fuel starvation or a lean-misfire. The noise is the biggest difference. It's really a very strange noise - like an intermittent burbling exhaust noise in the engine compartment.

I have an AFR meter on the car and I haven't seen any odd readings under load, so I'm pretty sure it's not fuel starvation.

I did a compresson check last night.

Cyls #1-3 are 155-165, but #4 tested at 110. I added some oil to the cylinder and got 125psi.

The plug for #4 looked a bit different than the others.

#1-3 were light grey/brown. #4 plug had shiny black deposits on one half of the insulator, and noticably more carbon around the end of the plug.

So it seems like something is going on with #4. I'm going to try a leakdown test and adjust the valves on that cylinder.

Edit: Dave - it's definitely a combustion sound, not a mechanical noise. So I don't think motor mounts are to blame. I think it happens in all gears, though it's not as apparent in 1st because I can't keep the pedal down long enough.

kg6dxn - I moved to Redmond about 1.5 yrs ago, have been driving the 914 every day since!

pcar916 - are fuel filters available for the later style fuel pump configuration? (I believe they are small inlet, big outlet?) I checked a couple years ago and they were NLA.