80SC Engine Noise Question

markf · 02-04-2008, 06:56 PM

I have searched posts on exhaust noise, broken head stud noise, etc., but would appreciate any comments/suggestions with my current situation.

I drive my Porsche on the weekends (typically less than 100 miles). This past week I took the vehicle on a 400-mile trip and drove it hard. On the way back I noticed what I would describe as an exhaust leak sound. The sound happens 3500+ rpms and under load. The harder the acceleration the more predominate the sound. The sound does not appear below ~3500 rpms, or with the clutch in even over 3500 rpms. The sound is now always there under the conditions described.

After searching some posts, I was concerned I may have broken a head stud. Although I believe this would be unlikely as the engine was overhauled about 8 years ago at ~122,000 miles. Car now has ~125,000 miles. However, I reviewed the parts list from the overhaul and did not see head studs. The overhaul was performed by what I believe to be a reputable shop so I have to think they were left off the invoice as an oversight. The car had the tensioner upgrade when it was overhauled.

I took the lower valves covers off, and the studs were intact. Also, the studs did look relatively new, but there is no way for me to verify their age for sure.

Questions:
Should I take the upper valve covers off as a precaution? My understanding from the posts is the lower studs are most susceptible to failure.

Do exhaust leaks appear only under load? I would think you would typically hear them under all conditions. If it is an exhaust leak, how would you ever find if it only occurs under load? Are there any areas of the exhaust more prone to failure – exhaust is stock and appears original.

Thanks in advance for any comments/suggestions.

FL911 · 02-04-2008, 07:23 PM

markf,
I had the same noise under load around the same RPM on my 81 SC and thought I had found the problem...crack in the cat pipe. So I was in the process of the valve adj. and to replace the pipe when I removed the driver side lower valve cover and found 2 broken head studs and 1 with the allen nut totally gone. I am in the process of repairing this mess.
I would think that if you had the lower valve covers off of the engine and found no broken head studs then you more than likely do have an exhaust leak. Probably a hairline crack in one of the pipes (crossover or cat). Especially if the studs and nuts were in tact and not lose. If you had broken studs this is where they would be.
I would check the pipes for a light greyish discoloration at the welds for any cracks and go from there. Or to reasure yourself call the shop and inquire about the work they did.

Gogar · 02-04-2008, 07:45 PM

There's a possibility your timing could be advanced too far, and you could be getting some pinging, especially when hot.

Check your timing?

cab83_750 · 02-04-2008, 10:22 PM

Out of curiosity, what does the sound like?

jtfreels · 02-05-2008, 06:57 AM

Mark,

Very interesting. I've got the same symptoms you describe... under load I hear a pronouced exhaust sound. As if a hole in the exhaust, but no sounds at a constant speed.
I'm dreading the broken head stud but I've not taken the valve covers off. Gave a brief look at the cat but I'm going to double check that.

Please keep us posted, I'll check back with any results I get.

Thanks

Jay

markf · 02-05-2008, 07:42 AM

cab83_750:
The best way I can describe the noise is an exhaust leak. It does not sound like pinging or metal.

Has anyone had an exhaust leak only under load?

Do the head studs under the upper valve covers typically fail?

Thanks.

Guard Bum · 02-05-2008, 07:58 AM

Mark,

My car is in the shop right now and had similar symptoms. I heard what I was sure was an exhaust leak when I was driving it to my mechanic for some electrical work. It was certainly more noticeable at higher RPMs. I'm thinking a new exhaust system is in the near future. Pulled into my mechanics place and he immediately said that doesn't sound good. A trained ear suspected broken head studs immediately. Unfortunately he was right. I had 4 broken studs (broken off clean), some cam and rocker damage as well. If your bottom studs are intact, sounds like you more than likely have an exhaust problem. Wish that had been my problem. Good luck.

w21055 · w21055's Garage

Is it a tinging or popping sound?

Zacko · 02-05-2008, 08:43 AM

Check the gaskets heads/heat exchangers. I had one that was bad and could only hear it under load. Found the leak with a thin tube/hose, one side in my ear and searched with the other side around the gaskets while engine was on idle.

/Hans
911 3.0 SC -79

markf · 02-05-2008, 02:24 PM

w21055:
I would describe it more as a popping sound.

Superman · 02-05-2008, 02:51 PM

In my experience, exhaust leaks nowhere near the heads often sound like hissing or spitting noises. Exhaust leaks at the head sound like valve tappet tics.

ianc · 02-05-2008, 06:35 PM

Quote:

I would describe it more as a popping sound.

Typically the sound a loose head will make is like a paper bag popping...

ianc

jtfreels · 02-07-2008, 05:39 PM

An update on my problem. I discovered a hole in the muffler, looks like a rivet has fallen out.
See my thread at http://forums.pelicanparts.com/porsche-911-technical-forum/390527-odd-noise-engine-post3755343.html#post3755343

I've got some photos.

Jay

markf · 02-07-2008, 05:55 PM

Jay:
Thanks for getting back. I'll inspect my exhaust more closely over the weekend. I may even try and take my upper valve covers off - I hate to deal with all the oil again though!

It sounds like your noise was more random than mine - appeared in some gears and not others and at different rpms. My noise always appears around 3500+ rpms in all gears, and the harder you accelerate, the more pronounced the noise.

I'll let you know what I find.