911sc: head studs / carbon buildup question

SchnellBoy · 12-27-2001, 12:26 PM

I too am on the hunt for a nice 911sc. I've noticed there are some board members in the Bay Area looking for the same type of cars; chances are, I've looked at the same ones as you guys.

I'll get straight to the point: Head studs. I've witnessed 2 PPIs, and both mechanics did not check for broken head studs (pulling valve covers). However, they told me that since the engine had solid compression (150-160), inspection for head studs (and leakdown test) was not necessary. If an engine has solid compression #s all around, would that mean the head studs are ok? I realize that the head stud issues in these cars were a corrosion issue...are there warning signs, or does "it" just happen? It just seems like the mechanics were stating the "obvious", and they made me feel like I was a complete idiot for even asking the question about the head studs, especially since the compression checked out fine.

Second, I followed a thread by Hopkinskid regarding a 911sc purchase last month. He noted that originally, there was a "problem" with the #6 cylinder, with compression at 135 and leakdown at 25%. This turned out to be a carbon buildup issue. Is that right? Would carbon buildup lead to a leakdown of 25%? He also noted that a little was leaking from the intake and exhaust during the 1st PPI (when leakdown showed 25%). Would carbon buildup cause all this?

thanks

Superman · 12-27-2001, 01:06 PM

Yes, carbon buildup can cause low compression test numbers. No, a good compressoin test does NOT ensure intact head studs. I have had a broken head stud for many months (years?) and my cylinders range from 190 psi to 194. Valve covers MUST COME OFF during a PPI. Period.

I'd suggest you continue to shop for a 'professional' inspector.

TCGGrafix · 12-27-2001, 01:18 PM

From what I have heard on this board, using Techron will help remove the carbon deposits. Also, I believe you can have good compression readings even if only one stud is holding the bottom of the heads / cylinders. If the other stud comes loose then that will probably change. I just pulled the passenger side valve cover only to find 3 broken studs, (1 broke stud on 1 cylinder and 2 broken on 1 cylinder.) I did this after I started to hear noise. Compression was fine until the last stud broke on the cylinder that I found the 2 broken on. Like Superman said, get them checked. Good luck!

emcon5 · emcon5's Garage

Quote:

Originally posted by Superman
I'd suggest you continue to shop for a 'professional' inspector.

I don't necessarily agree that he needs to change mechanics, I think it is just a matter of making clear in advance that you want the head studs checked, and are willing to pay the extra labor/parts costs for this. (I know if I was selling a car and a buyer wanted to pull my valve covers I wouldn't agree unless I got new gaskets)

When did the SC head stud problem start to show up?

I know I hadn't heard of it when I bought my car this time last year, and I did a lot of reading, both books and on the net.

Tom

TCGGrafix · 12-27-2001, 01:50 PM

Quote:

When did the SC head stud problem start to show up?

I think I read somewhere that the broken studs started showing up around 95 - 96. It's said that it is caused by corrosion where the head and cylinder meet. (Exposed area?)

Early_S_Man · 12-27-2001, 02:00 PM

Tom,

It was discussed in Bruce Anderson's technical column in Excellence on more than one occasion in '94-'95, and is documented in the 2nd edition of his Porsche 911 Performance Handbook, which was published in 1996.

john walker's workshop · 12-27-2001, 02:12 PM

head studs have been breaking since the mid 80s. just tell the tech that you need to have the covers pulled, and to do a leakdown whether he thinks so or not. you're paying for it, so what's it matter to him?

Don Wohlfarth · 12-27-2001, 02:27 PM

I can remember hearing about broken studs in the late 80's and the fix was Raceware studs, same as today. The big culprit is corrison which helps to start stress cracks.
Don't know if most mechanics would remove valve covers on a PPI, usually just a leakdown. If they didn't like the leakdown might go to a compression check and then maybe pull the valve covers. Think it would depend on how far the person paying for the inspection wants to go.

911pcars · 12-27-2001, 03:58 PM

"This turned out to be a carbon buildup issue. Is that right? Would carbon buildup lead to a leakdown of 25%?

No. Carbon buildup removes volume from the combustion chamber which raises the compression ratio (and thus compression pressure). I'm still thinking of some way to build up carbon evenly on my engine.

Yes. Loose carbon can lodge between a valve and valve seat to create a slight compression leak. This might happen while unscrewing a spark plug for a leakdown test.

Sherwood Lee
http://members.rennlist.org/911pcars

Early_S_Man · 12-27-2001, 04:59 PM

There is also carbon buildup in the intake and exhaust ports, sometimes worse than in the combustion chamber! The cases of leakage at both intake and exhaust valves will not be taken care of by adding Techron to a fuel tank! The problem usually occurs in cars that have been stored for a year or more without use.

The problems occurs because the carbon absorbs moisture from the air over a large time period, and cracks, then falls away from the intake port or combustion chamber, and can lodge under either or both valves when the engine is turned over and started. Drastic action involving pouring relatively large amounts of solvents such as Berryman B-12 Chemtool or Techron down each affected cylinder's intake port and allowing the solvent to soften the carbon.

Techron in the gas tank won't help... because valves will burn in just a few hours of extended use, such as a 200-400 mile trip in one day!

SchnellBoy · 12-28-2001, 08:04 AM

Thanks.

So for the head stud problem, I'm assuming that if I'm looking at a 911SC with 80-110k miles, and the head studs are original, there's a great chance that I will need to replace them in the near future at a cost of about $2000, right? I thinking someone noted that these studs were "ticking time bombs".

I spoke with Hopkinskid (who is having too much fun right now to post, that lucky bastard), and he stated that his head studs were replaced in 1996 (40k miles ago), and even though they are also Dilavars, he "should" be safe for another 40k miles, or even more.

When I hear stories of SCs running 200-300k miles, I'm also assuming that the head studs (lower end) have been replaced at least once, right?

Gosh, I'm looking at original shocks/suspension, original transmissions, original head studs, possible valve job down the line...it does seem like a lot of owners are simply dumping off their 911 SCs with 80k-100k miles because of the maintenance/upgrade costs.

Leland Pate · 12-28-2001, 09:58 AM

I say Ho ho hold on there!!
SC owners don't dump their cars anywhere!

If you will look in SCWDPI 21-203, it states specifically, that "No owner of a SC will/shall dump their car off because of future repair/maintanance costs.
Well, there it is... can't argue with that!

All 911s have their quirks. It's as simple as that.
SCs are very prone to headstud failure. I would either buy a car that has had them replaced, save a bit of cash for the inevitable, or buy a 912. <--- ZERO headstud problems.

A top end job is not that difficult, and if you have the space and time, you can do it yourself and save an ass load of money.
I did a complete, left nothing to chance, rebuild for just under $6K. The estimates I was getting from wrenches were in the neighborhood of 10-12K!

SCs are great little cars either way.

Notice that Wayne drives an SC, BA speaks very highly of them, my 35yr in the business mechanic owns an SC... shall i go on???

Early_S_Man · 12-28-2001, 12:50 PM

Well, I was the originator of the 'ticking time bomb' ... prediction, and I was speaking from my view of the situation from a long-term perspective, which most SC and Carrera owners here don't have ...

Lee is correct that all 911's have quirks, and the stud issue is just one of them!

I am quite sure that many SC's have been replaced by 3.2 Carrera's in the 'hope' that a newer car would be more reliable, and not have the 'stud' issues ... but, on the contrary, Carrera's have the stud issue long-term, weak 9 mm rod bolts, AND valve guide wear issues for certain years equipped with the orange elastomer valve-stem seals! AND, of course, Motronic issues such as the DME relay, and head temp. sensors ...

Until somebody publishes long-term data on Dilivar stud failures --- which will probably NEVER happen, they are ALL SUSPECT in my mind, specifically because of the 'flying stud phenomenon' of brand new studs breaking a few minutes after being torqued in a fresh installation! Heat cycles and environmental exposure to water/salt/moisture simply isn't an issue with a new stud out of the parts bin -- there IS something else happeniong to aging studs, whether stored in a building for years or installed in an '85 Carrera after being coated with black epoxy in a factory, and them installed in an engine. Carrera studs are breraking, too, although maybe not with the frequency of SC studs, yet!

And there was the issue of counterfeit Dilivar studs on the market, too!

Al in all, a very perplexing situation! What was amazing racing technolgy in the early '70s in 917 engines ... hasn't worked out so well over the long term in production cars!

speeder · 12-29-2001, 07:44 PM

Schnellboy, I would have to say that your mechanic, if competent at all, certainly doesn't know Porsches and should not be giving professional advice on same. Telling you that headstud inspection is not necessary w/ good compression is simply wrong. My SC, like Superman's, has excellent even compression,( a lot more than 150-160), with one broken headstud. Car runs great and is beautiful, if I was a scumbag I could sell it for top $, as long as buyer uses your wrench for PPI.

I would even go so far as to say that checking studs on SC is a lot more important than compression test. Most all of them have good compression unless there is a serious problem, which is hard to hide- car would not run well/strong. Broken stud is easiest thing to hide- usually no symptoms at all until 2 go on one cylinder. Removing lower VCs is sufficient; have never heard or read of upper stud breaking. Not saying it's never happened, just that no one has ever seen one. Good luck, and welcome to this board!