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??

Dude, where have you been? I thought you were in Austin.

Also sounds like bad timing.

More in lower gears than higher ones? Check your motor mounts and trans mounts.

--DD

I always suspect points/fuel pump when this happens. Assuming the electrical stuff checks out...

There was a beautiful 356 outlaw I worked on whose fuel pump appeared to pump the right volume while idling, but would not keep up under load. Simply had to rig an electric pump to find the problem. The engine would burp and backfire from a lean mixture until it got a new pump.

Good luck

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.

Fuel filter is available at the bug places, like bughaus, and at napa. Its a Napa gold filter.
Bughaus has the Israeli made one.

My NAPA filters are made in Israel too. NAPA part number 3274.

Napa 3274 looks like the right part - will pick one up this afternoon. Thanks JawjaPorsche and Cabinetmaker.

I still don't think the issue is fuel starvation - the loss of power is fairly minor - feels like one cylinder going out, not all 4.

How many miles n the motor?
What break in procedure did you use?

r_towle - Not sure of the mileage or breakin, this motor was a hand-me-down from Mark. I've probably put 15k on it since I got it.

The good news - I replaced the spark plugs, cap and rotor and it seems to be running much better. The cap was particularly nasty. The weird sound I was hearing must have been a misfire under load. Very strange. Will see if it comes back.

The bad news - leakdown on #4 was two times higher than #3, so there is still something going on with that cylinder. Valves seem ok, and leakdown was to the crankcase, so it must be rings. Will keep driving it and see what happens.