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Low compression on #6: options

Hey Fellas

The mechanic doing a PPI on a '66 901/05 2.0 for me says all cylinders except #6 gave 160 compression, #6 shows 95.

Leakdown confirms 10% in all cylinders except #6 which is showing 20% leakdown. He says it's not obviously leaking from one place, e.g. no clear indication it's coming from the intake, exhaust or breather.

Test was done warm, although the car was infrequently driven and was not subjected to a high-rpm blast before testing. I recognize that there MIGHT be some carbon or other crap blocking the valve- but I can't ignore the test results.

My questions:

1) This engine has not had a valve adjustment in six or seven years and a few thousand miles. I think a valve adjustment might help, but only with compression, right? Leakdown is measured with the valves closed at TDC, when there should be play between the rocker tip and the cam's base circle-- or could the valve be so out of adjustment that the screw is keeping the valve open even at TDC?

2) Suppose the valve were bent or burned. Is it possible to R&R only the #6 head? This would entail dropping the engine, puling intake and exhaust, removing the cam tower, pulling the head and taking a look, then putting everything back and retiming the cams. Is this possible, or would you advise tearing the whole engine down? Would your answer change if I told you I was going to have a shop do the work?

3) Suppose a piston ring is broken. Is this something that can be fixed without splitting the case?

THANKS in advance for any advice. I have to make a decision as to how to deal with this pretty soon, I appreciate your input.
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Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
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Old 02-24-2005, 07:07 AM
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First and foremost, if you paid for a PPI which included a leakdown test the technician should have followed through with where the leakage was occouring. The hard part (set-up) of the test was already done. If there was only 95 lbs, for sure the compressed air was going SOMEWHERE.

It doesn't take a rocket scientist at that point to figure if the air is excaping through the exhaust valve, the intake valve, the piston rings or possibly past the cylinder head and barrel. I just can't believe that he didn't do this.

Take the car back to the shop and explain to the service manager that you paid for a COMPLETE diagnosis and it was not done. It will not take long to go back to that cyl, stick the gauge in and then trace where the problem is.

The problem could be very simple such as carbon chunk / build up on either valve or it could be as serious as failed piston rings or burned valve.
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Old 02-24-2005, 07:36 AM
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Thanks fellas. This motor was rebuilt in the mid-1980's with approximately 12,000 miles since the rebuild. There are no leaks (yes, I checked to make sure it was actually FULL!) and it still has original tensioners (although I think there are safety collars).
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Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Old 02-24-2005, 09:08 AM
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Noah, I haven't made the decision to buy the car, yet. Accordingly, I haven't selected a shop to do the work.

Having said that, I'm not so sure that diagnosing this problem isn't within my skill set. I suppose it would entail the following steps:

1) Drop engine.
2) Mount engine on stand;
3) Remove sheetmetal;
4) Remove intake and exhaust;
5) Remove valve covers;
6) Remove chain cover;
7) Remove cam nut and slip chain down;
8) Remove rockers;
9) Remove cam box;
10) Remove #6 head and inspect;
11) Remove #6 cylinder;

At that point, any severe damage would become obvious, like a broken ring or a pie-slice out of a valve. I would then have to mic everything (piston, cylinder, valve margin, stem, etc) to see whether anything was out of spec.

Then, as the British car repair manuals so hollowly admonish, "Installation is the reverse of removal." Oh, and throw in timing the cams at that point, too.

A big job, certainly one I would be willing to attempt, but probably 20 hours (at LEAST)-- basically half of an engine rebuild. The shop doing the PPI said expect $2000-3000 just to find out what's wrong.

If Colasante and a few of the NYC Pelicans and I did it, we could spend $2000 on beers alone, I reckon.
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'66 911 #304065 Irischgruen
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Old 02-24-2005, 01:19 PM
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Well, I had another discussion with the shop and the seller last night.

Turns out the compressions were on the order of

125
160
145
150
125
95

Or pretty close to that, i left my notes at home.

Now, following the rule that no jug should be more than 15%, that would mean that 136 psi should be the minimum. Which puts #1 and #5 in the suspect territory as well.

The shop quoted a minimum of $2-3k to simply diagnose the problem, and that's assuming working on cylinder #6 only. Which I would never do, but I wanted to get a sense of the impact on pricing.

The owner is going to check the compression and leakdown himself this weekend to form his own conclusions.
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'66 911 #304065 Irischgruen
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Old 02-25-2005, 05:55 AM
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I don't disagree with that. The only thing I can think of is that it's been sitting for so long that the rings are stuck and could use some Marvel Mystery oil in the sump and in the fuel, and a good high-speed drive (such as the recovery drive back to NY) to loosen things up. This engine was rebuilt like 20 years ago, and has only about 12K since, so that would be about 600 miles or 10 hours of highway run-time per year. . . it wouldn't surprise me at all if that cylinder were carboned up, particularly if the car has been periodically started and NOT run up to operating temperature.

BUT the results are the results, and I'm not about to rationalize away the PPI's findings because I think there's an outside chance things could improve with time. In any case, it's a question of who should bear the RISK, and in this case it's certainly not me. (Do I sound like I'm trying to convince myself? No accident there.)

So what could be causing it? Here's my short-list:

Burned intake or exhaust valve;
Broken ring
Broken piston
Scored Cylinder
Leaking head to cylinder junction
Carbon deposit on valve(s)

That's just for jug #6. The compressions being off on two other cylinders are symptomatic of greater problems.
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'66 911 #304065 Irischgruen
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Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Old 02-25-2005, 08:39 AM
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John, I suggest an Italian tune-up. If it has been sitting for a while, give it a good thrashing, then check the compression again. As you probably know from your aircraft experience, listening for where the leak is coming from will probably tell you where it is leaking (at least on the lowest one). Although I ended up rebuilding my '79 SC shortly after purchase, I was able to bring up my low cylinder from 50 to 90 just by driving it hard. This car had been sitting for about a year with only an occasional start up. The offending cylinder appeared to have partially fouled a plug and a bunch of crap had built up under the valves. If you really want the car, the thrashing/retest idea might be worth a try.

Good luck!
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Old 02-25-2005, 09:37 AM
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Ahh, Noah, you and I are thinking the same way.

I actually grilled him pretty good on the procedure followed, also on the leakdown procedure. Plugs out, throttle WOT.

I have already discussed the matter with the owner, who was surprised, but not unpleasantly so.

The owner and a friend of his are going to "verify" this compession test this weekend. I can understand that they may not get exactly the same values, but I would think it highly unlikely that they would NOT see the same pattern, and particularly #6, where the number is about 60% lower than where it should be. Also, the leakdown test on that cylinder showed 20%, so I think it highly unlikely that my mechanic was completley wrong.

In any case, the burden's on the owner now to establish otherwise.
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'66 911 #304065 Irischgruen
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Old 02-25-2005, 09:37 AM
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I'd rather it be in excellent condition: the adjustment to the price I'm contemplating in no way would cover the cost of a rebuild, as it's nearly equal to the price of the car. But I do want to buy myself some insurance in the event the engine has some sort of catastrophic issue.
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'66 911 #304065 Irischgruen
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Old 02-25-2005, 11:45 AM
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