911 engine knocking noise

speedy914 · 01-02-2021, 03:15 PM

I just fired up my fresh 2.7 mechanical injection engine today. Fired right up and ran great, but, I have a light knock/tap noise noticeable at idle but seems to go away under light load/light throttle, comes back off throttle or no load. Any suggestions before I start to cry.

ahh911 · 01-04-2021, 12:54 PM

Don't cry yet, right after the first start gremlins are always in our mind playing tricks.

Sound from above or below? Long screwdriver against bottom case to listen for noises? Placed against upper cylinders where possible? I would think anything serious could be heard. Oil pressure and volume assumed reasonable during these tests. Sound bite for the experts would help if still concerned.

Phil

dannobee · 01-04-2021, 01:55 PM

Can you make us a video with the noise? That would help.

911pcars · 01-08-2021, 01:57 PM

At that idle speed, short out each cylinder in turn (R&R spark plug wire). It the knocking goes away, usually indicates a con rod bearing issue (excessive clearance). If knock is constant and louder, could be main bearing issues.

Localized hearing with screwdriver, hose, stethoscope, etc. as Phil suggests is also a good idea.

Sherwood

Eagledriver · 01-08-2021, 05:43 PM

Knock is different than tick. If you’re not familiar with 911 engine sounds, valve ticking is very common and not really an issue.

-Andy

stownsen914 · 01-09-2021, 09:42 AM

For what it's worth, rod knocks don't usually go away with increasing revs. But as suggested, a video would help.

911pcars · 01-09-2021, 12:33 PM

Quote:

Originally Posted by stownsen914

For what it's worth, rod knocks don't usually go away with increasing revs. But as suggested, a video would help.

They typical do when the cylinder is unloaded (spark suspended). Try it at idle/off idle/cruise RPM. Between the rod and main bearings, knocking at all engine speeds is indicative of main bearing issues despite eliminating a cylinder from the firing order.

Yes, some knocking EFX would be helpful. Hope it's from loose rocker/valve clearance.

dannobee · 01-09-2021, 01:57 PM

If you just rebuilt it, and I assume that you checked and double checked everything on reassembly (yes, I know, but I'll give you the benefit of the doubt), the most likely cause of any noise is likely a loose flywheel or clutch cover. But again, please post a video of the noise so that we can make a (more) proper diagnosis.

John Barron · 01-09-2021, 04:27 PM

Quote:

Originally Posted by 911pcars

At that idle speed, short out each cylinder in turn (R&R spark plug wire). It the knocking goes away, usually indicates a con rod bearing issue (excessive clearance). If knock is constant and louder, could be main bearing issues.

Localized hearing with screwdriver, hose, stethoscope, etc. as Phil suggests is also a good idea.

Sherwood

But . . .be careful not to get zapped. Ask me how I know.

speedy914 · 01-10-2021, 02:13 PM

Thanks for all the suggestions. Everything is fresh. Triple checked valve adjustments this weekend, maybe a hair loose, but not really. Very familiar with 911 engine noises. Something is not right. Did the screwdriver/engine ear thing. Cant really pinpoint a specific cylinder or area. Only ran for about 15 mins then 2 miles. Oil pressure seemed fine, did not get oil up to temp by any means. I'm gonna check valve/piston clearance next (but that wouldn't be at low RPM) I'm gonna replace the tensioners (they are old) just to rule them out but gut doesn't think that's It (hoping gut is wrong) If still noisy then I'll try do to a video/sound. No misfires short drive ran fantastic good throttle response

speedy914 · 01-10-2021, 02:18 PM

flywheel and clutch (71 911 aluminum reverse pull on 914-6) all torqued correctly

faapgar · 01-11-2021, 07:14 AM

Was this a 2.4 to start built to a 2.7 spec?Fred

speedy914 · 02-24-2021, 04:37 PM

Quote:

Originally Posted by faapgar

Was this a 2.4 to start built to a 2.7 spec?Fred

No. Yes. Kinda. Late 2.7 US case. 2.4 Euro Mechanical heads (opened to 36mm duplicating 2.7) 2.7 RS mechanical injection. Basically a 2.7RS clone but with late 2.7 case

speedy914 · 02-24-2021, 04:57 PM

https://youtu.be/vftBCfqd9hQ

Link to youtube video of engine running. Using an engine ear noise seems the loudest around the oil pump area. Does not change with clutch activation.

Does seem to change with load, maybe. I really haven't driven it yet other then around the block. Cold weather and job delaying my posts sorry. New tensioners changed nothing. Seems to get louder as oil get warmer?

Eagledriver · 02-24-2021, 05:44 PM

Sounds like a single valve clack. One valve extremely loose. Tenstoners make more of a machine gun sound.

-Andy

speedy914 · 02-25-2021, 07:51 PM

Quote:

Originally Posted by Eagledriver

Sounds like a single valve clack. One valve extremely loose. Tenstoners make more of a machine gun sound.

-Andy

Thanks, another friend said similar to check all rocker shafts. Fingers crossed, exhaust down and valves covers off again this weekend. Such a pain in a 914-6. triple checking this weekend...

BURN-BROS · 08:20 PM

Possible sources of a knock at idle is;
Excessive crank endplay at #1 bearing

Excessive gear lash between the intermediate gear and the crank gear.

Porsche made a batch of 3.0 liters that had excessive crank endplay and exhibited a knock below 900 rpm

They were replacing engines under warranty until they figure if they increased idle rpm above this, the knock was not present and “fixed” the complaint.

The likelihood of either of these conditions has increased as age and the subsequent wear on both of these parts iirc, spec is .004” for endplay. The cranks were out of spec and had .006-.008 of endplay.

If you are positive that you do not have any tolerance issues and your oil is clean of debris, increase idle speed and enjoy.

911pcars · 02-25-2021, 10:03 PM

Here's another. Rocker arm shaft only partially supported = valve noise.

BURN-BROS · 02-26-2021, 06:00 AM

I just noticed the video, That sounds like Valve lash.

I would look at the rockers carefully and make sure the shafts are in the correct positions.

mikedsilva · 02-26-2021, 04:53 PM

I went through something similar a while ago although my noise was nowhere as prominent. At start up, it was silent.. 2 minutes after it got louder.. as engine got up to full temp, it went silent. So my symptoms were definately different.

Someone suggested to me it could be the large steel gear on the nose of the crankshaft that drives the ims gear. Apparently in rare cases, it can actually be loose and the noise is the sound of the gear hitting against the woodruff key.

I pulled my engine down, had the gear inspected by machine shop.. no such problem. But I did find that my aluminium ims gear had a lot of lash against the steel gear. So I bought new, new sprockets for the chains, etc etc.. put it all back together.. and the noise is still there.....

I'm now wondering whether it could be worn elephants feet (valve screw adjusters) or this rocking from side to side of the rocker itself...???