pp000830

I see this as something done from time to time.
I now own my 2nd 911, the last one ran great at 150K miles and have never contemplated this.
Why do it?
What are the objective vs.subjective reasons to do such an expensive and invasive thing?

Andy

race911

"Running great" and being internally within spec are two wholly different subjects.

Of course the most common reason those of us here have cars which have had a factory assembled engined opened up at some point was because of a failed smog test, which led to valve guide replacement.

But with the final version of the air cooled engine in these cars (or 964s, actually), we have not had to deal with: holed pistons, failed chain tensioners, pulled engine studs, broken engine studs, and failed rod bolts. Among other less common failures. Oh, and crap valve guides all along the way........... And there's your progression of what's kept Porsche shops in business since maybe 1967 when the first of the 911s came out of warranty.

Yet again I'll say that if valve guides were as DIY as brake pads they'd all get done every 30K. Even if the cost was not minimal.

So to sum it up, the 3.6 engine is extremely robust and reliable as a street engine. No reason to think, with proper maintenance from Day One, that 250-300K is not the expected lifespan with one aftermarket valve guide (and related) replacement along the way. Can we count on more than one hand how many here have that many miles on one?

NP993

There are no subjective reasons to do a top end rebuild. The objective reason is because of worn valve guides.

Steve Weiner-Rennsport Systems

What Ken said,....

Spot-on!!!

Remember,....everything is always great up until the moment when it isn't.

T0M993

^^^^^ +993

Mike J

One of the deals with valve guide wear is heat transfer. The valve stem glides inside the valve guide when the engine is running, and the valve, especially the exhaust, transfers heat that it absorbes through the oily gap between the valve and the guide, and obviously to the mass of the head. When the valve guide wears, the gap increases, and can get so wide as to really inhibit that heat transfer. As a result, the valves can run much hotter than intended since they cannot transfer the heat away as needed. This can lead to a number of issues including total valve failure where pieces of the valve breaks away or the stem breaks. Nasty stuff, and there is no warning other than indications that the valve guides are worn.

The most obvious indicator is excessive oil consumption.

Cheers,

Mike

race911

But even that didn't deter most in the 3.2L era. While I wasn't running an especially large "shop" (out of a home garage for Shop 2.0, when I was going to law school), I had enough 3-7 year old 3.2L cars coming in with the high oil consumption complaint to figure out something was off. Quick consultation with the machinist I used indicated there was a problem. Which was masked in the pre-3.2L days, as most of the cars couldn't get past 50K without puking a tensioner. I'd say only a couple of the cars I saw this on had owners willing to properly fix the problem. Rest just added oil, or sold the car. Sample size of maybe 15 cars.

Actually, worst on the 3.2L cars was changing oil for a first-time customer, and only draining 5-6 quarts............ At least I could educate and monitor from that point on.

Bottom line is that the inability to register a 993 from a check engine light that can't be reset to sufficiently pass a smog check is the strongest deterrent to neglecting valve guide replacement.

T0M993

What is "excessive"? I have a subtle ticking (after lifter replacement) and burn about a liter every 1000 miles which I'm told is nothing to worry about. Do I just monitor the loudness of the ticking going forward?

Steve Weiner-Rennsport Systems

Tom,

I tell people that 800 miles/qt signifies time to fix things properly.

Excessive oil consumption also creates lots of engine deposits which can trigger detonation when hot. The octane rating of oil is rather low so such conditions breed detonation-related issues that include reduced performance.

One can view this indirectly by watching knock counts with a scan tool in hot weather to see if they are excessive for the conditions.

solomonschris

The valve guide issue is a real mystery to me. When I sold my last 993na to a fellow RLer it passed leakdown with 97% and above on each cylinder. I can't recall the exact mileage but it was well north of 50K. When I found the tt I had been looking for I wasn't going to do a leakdown since it had barely 24K miles. Members of this and the tt forum disabused me of that notion. Sure enough, one cylinder was in the 70's. The compression test had all cyclinders between 140-145. The tear down revealed one badly worn exhaust valve guide with a slightly burnt valve. Nothing was bent but my guess is that something would have been soon. There were no signs of excessive oil consumption either. The PPI ws done by Schneiders Auto Haus, the best shop for our cars I have seen yet. So some of our cars go many tens of thousands of miles with no guide failure, and some fail very quickly. I dunno, but I would have enough in the car fund to have a top end job if your car has the original valve guides. I believe once that is done, several hundred thousand miles are possible, if not likely.

ABCar

This is good info...thanks

Jeff U

I used to have a '86 3.2L and this is a spot on description of why I had to rebuild the top end. I did have some above average oil consumption but it did not seem too bad. In my case it was a failed leak down test that was the big sign something was amiss.

Valve guide wear, loss of heat transfer, over heating of the cylinder # 3's exhaust valve; a "burnt" valve.

Burnt valve does not seal and the motor failed a leak down test on that cylinder.

mstolo

I have a '96 993 with 112k miles and SAI/CEL. I've got several leaks that need to be taken care of and the longgg list of other items. I was about to get a leak down test done to see what the numbers look like so that maybe I didn't need to open up the top end and could defer for several years. I was going to do the job myself as a father/son "bonding experience".

Based on what the experts say above, sounds like the leak down test is a waste of $ and I just need to go ahead and get my hands dirty with the valve guides and then do the rings as well. Clearly will help the resell value.

You guys agree from the arm chair?

Mike J

Nope.

Leakdown may or many not indicate valve guide wear, but if the leakdown is bad, its means something is really bad.

What is your oil consumption like? Your guides are likely worn, but its hard to tell if they are worn out yet. Some cars go for very high mileage without issues, some have major valve guide wear issues a very low mileage. There are a number of factors, including how the car was driven, where it was driven (hot/cold), as well as even what the factory tech was doing when he fit the valves to the guides.

What I am saying is its not a foregone conclusion to do a top end based on just mileage. If you have a good leakdown and above tolerance oil consumption, then drive it, no worries.

About 1 quart/600 miles is in rebuild territory, but i also know of a few who have been driving that way for years.

I also comment a while ago about heat transfer and cooling of the valves. See post #6 in this thread.

Cheers,

Mike

mstolo

Thanks so much Mike! I haven't been tracking my oil consumption that closely, probably in the 1 qt 1,000 miles would be my estimate. Ok, i'll go ahead and get the test done...not that much money and i'm sure i'll learn something in the process. Just some more $ in my p-education. I'll report back.