98 Legacy GT 5MT. 191K mi. on the car, now on the second motor, a JDM EJ25D with somewhere around 120K mi on it. Timing belt/idlers/tensioner/water pump and clutch/flywheel/pilot bearing/throwout bearing/clutch fork replaced 1300 miles ago. Oil is Rotella T 10w30.

 

What I have is a rapping or knocking noise that’s engine speed dependent. It builds in volume after a cold start (cold engine, not freezing temps around here), with the noise starting 10 or 15 seconds after the car starts. You can hear it clearly sitting in the car, but it’s not obvious to me over other noise in the engine bay. Does not get louder with engine load, like I would expect big end knock to do. It’s not there (or not noticeable) when the car is up to temp, but the motor is generally noisier with more vibration than it used to have. Some of that is valvetrain noise, which is the most obvious in the engine bay along with injectors. I haven’t been able to get a decent recording of it or I would post that up. Does that sound like piston slap, rod knock, or something else?

 

Head gasket on the left bank has gone, and I’m sorting out wether to rehab this motor, put in yet another motor, or move along to something different. Car is pretty clean and is rust free, but also needs suspension work, shocks and bushings all around.

 

Any help is much appreciated.

Blackstone or maybe Polaris Labs can do an oil analysis. I think increased aluminum would be piston slap, increased iron? yeah, probably from a bearing that's getting pounded.

 

a bad/loose tensioner can let the timing belt slap around - it might mimic a knock.

Blackstone or maybe Polaris Labs can do an oil analysis. I think increased aluminum would be piston slap, increased iron? yeah, probably from a bearing that's getting pounded.

 

a bad/loose tensioner can let the timing belt slap around - it might mimic a knock.

+1 on checking tensioner. That could certainly be your problem.

I agree on the tensioner, its the most likely cause IMHO

Thanks for the replies. Tensioner (and belt, etc) was replaced 1300 miles ago. Doesn't mean it couldn't be bad now, but I'd be surprised. The belt felt and looked good when I pulled the side covers off last weekend but I'll pull the center cover this weekend to be sure.

 

Through the stethoscope, the left block half is noisier than the right, but my lack of experience means I wouldn't say anything conclusively.

you can run the engine with the covers off - if the belt is running true, tensioner is OK.

Edited December 11, 201510 yr by 1 Lucky Texan

If it goes away or gets less noisy when its warm, probably just piston slap.

Rod knock tends to get louder once the engine is warm.

If it goes away or gets less noisy when its warm, probably just piston slap.

Rod knock tends to get louder once the engine is warm.

 

Thanks for that. You say just piston slap. Does that mean you think it's generally not a big deal whereas rod knock would be?

Piston slap is annoying but generally benign. It eventually wears the piston skirts and the cylinder walls (eventually meaning like 200k miles), and may cause higher than normal oil consumption.

 

Rod knock will blast a hole in the engine if the rod breaks. Then you're stuck wherever it happens. Rod knock is very loud, and will drown out any other noise from the engine, and it doesn't go away with the engine warm. It usually gets louder.

My 98 Forester with the same 2.5 DOHC, it slaps pretty loud in the morning, especially when around 0* F. Warms up and goes away pretty much entirely in about 2-5 minutes.

Could be a cracked flexplate as well. Those will definately sound like rod knock, and wont change with engine temp. Also it will be louder in the car, as the engine muffles it.

Edited December 13, 201510 yr by crazyhorse001

Piston slap is annoying but generally benign. It eventually wears the piston skirts and the cylinder walls (eventually meaning like 200k miles), and may cause higher than normal oil consumption.

 

Rod knock will blast a hole in the engine if the rod breaks. Then you're stuck wherever it happens. Rod knock is very loud, and will drown out any other noise from the engine, and it doesn't go away with the engine warm. It usually gets louder.

 

Piston slap makes sense based on this car burning oil when I ran Mobil 1 -- about 1/2 Qt in 1000 miles -- but not on dino oil (Castrol GTX, now Rotella). That oil was going somewhere before and you know the piston/cylinder isn't getting un-worn.

 

 

My 98 Forester with the same 2.5 DOHC, it slaps pretty loud in the morning, especially when around 0* F. Warms up and goes away pretty much entirely in about 2-5 minutes.

 

Same, the noise fades away after a little driving, and unless I start driving just at startup (try not to), I can't hear it while under way.

 

I've been meaning to pick up a compression tester, sounds like this is a good time.