No compression…

[Mac-Porsche]

Been a while since I last posted! I conducted a full rebuild on my ‘81 911sc back in 2019/2020. Rebuild went great, car was much more alive following the build, life was good. Had done compression and leak down tests over the years, kept up with valve adjustments, no problems.

I drove the car a few times this spring so far (After snow melted), car would fire right up after days of sitting, maybe a single puff of smoke - nothing to cause concern. However, the other day I went to drive it after sitting overnight and it took 15-20 seconds to start. Then gurgled and slowly got up to rpm. This was then followed by a lot of white smoke.

Decided it would be a good time to do a leak-down/compression test. All cylinders were 97-99% on leak down and compression was 180-190psi (I increased the CR with new pistons during the rebuild)… EXCEPT… cylinder 1…

It would not hold pressure at all, as if the valves were wide open. Compression test showed 150psi but it took twice as long as the other cylinders to get there. I removed and reconnected the test valve multiple times, even turned the crank while holding pressure on it (To see if maybe the timing was way off), no change.

Ran a a little borescope camera in there, looked relatively fine. Maybe a little oily. Pulled the valve covers (Both injection and exhaust sides) to see if anything was off, the gap was right where it should be.

My best guess is piston rings failed. Before I drop the engine and take it all apart - are there any other tests I should run??

Thank you
-Mac

[Tad240]

Check the valve clearances on#1.

[Mac-Porsche]

Are you referring to the .004” gap between the rocker arm end and valve end? If so, that’s what I was checking when I pulled the valve covers and the gap was good.

[Tad240]

Yes. Just to confirm you have adequate clearance / valve not lifted when on the base circle of the cam.

[yelcab1]

You have done all the tests you can. Drop the engine and remove the cam ... that will immediately tell you what is wrong. If not, then take the cam tower off and it will be obvious.

[Dpmulvan]

Did you use JE pistons and rings?

[Jim2]

With your leak down tester on cyl 1 listen to the tail pipe, throttle, and breather to determine where the leakage is. Remove the valve covers and while watching the tester, tap on the rockers with a plastic hammer to unseat the valves and blow any carbon or debris off the valve seats.

[stownsen914]

Sounds like you may have a couple things going on. 1. Slowing starting and smoke, 2. Low compression on one cylinder.

For #1, you may have experienced oil migrating from the tank to the engine while the car sat, and seeping into the cylinders. Can make for a smoke bomb on startup. Might also explain the oily cylinder.

For #2, to confirm - you are getting full compression on a compression test but 0% on a leakdown test? Are you sure you were at TDC when checking that cylinder?

One question - how did it run once started after the sluggish start? If it were not running on one cylinder, it would be a bit rough.

[Mac-Porsche]

All very good comments/suggestions. I’m gonna mess with the valve clearances a bit more today, I’ll try the light tapping to see if it’s maybe debris holding the valves open.

They are JE pistons, can’t remember the ring brand, I’m pretty sure they were Goetze (it was a set I purchased from a fellow Pelican member, they use 97mm turbo rings). Although it pressured to 150psi, it took a while to get there (10-15 turns). I have close to a 10:1 CR, so the cylinders have always shown ~190psi and usually only take 4-6 turns to get there.

Definitely at TDC, that was my concern too at first, but double/triple checked with rockers and distributor. I even kept pressure on it and slowly made two full turns with crank just to see if it was off for some reason.

I pulled the fuel injector on the cylinder to see if I felt air coming through, nothing. I did feel a slight amount of air coming out the tailpipe. Haven’t tried yet with the valve covers off, will do that today.

The car didn’t seem rough, however, it did feel like it had less power. I was actually shocked when it didn’t hold pressure. Thought ‘maybe’ I’d see like 90-95% if there was an issue, but not 0%… still feels like something is not adding up. Hence posting… I’ll experiment a bit more today (tapping a repressuring without valve covers) and post what I find.

[porschedude996]

Check the difference in the valve adjustment screws. Compare number of threads. Mine are always the same. With zero leak rate, there are three was it goes. Intake, exhaust, and crankcase. Use a smaller diameter hose and listen for air noises. That will tell you where it is leaking.

Intake: Hose down the filter box past the throttle plate. Not just the Mass Air Sensor Plate. Throttle needs to be open.

Exhaust: Just up the muffler.

Crankcase: Should hear noise all the way to the tank filler if it is quiet environment around you.

[Mac-Porsche]

So… the rocker was holding the valve slightly open. It had play, so I thought it was good at first, but I did see it move after readjusting the gaps. It’s now holding pressure, but still bad. Showing ~75%.

Based on that last comment, the thread count is visibly fewer on cylinder 1 (exhaust side), almost as if the end of the valve is sticking out further.

Air is definitely coming out of the muffler. You can hear it best on underside in heat exchangers. So my theory is that the exhaust valve is not sealing/properly sitting for some reason.

I think it somehow protruded into the seat, and is sitting deeper in the head, letting air go by. Seat failure? I didn’t see any metal chunks/seat dislodged with the borescope.

I think no matter what, the heads are coming off…

[Alan L]

If you do have to go that far, you only need to pull #1 head. That will save a lot of work. And the 4-6 side will be untouched. So not quite as bad as it may feel.
BTDT a yr ago - twice. In a week.
Alan

[porschedude996]

Valve seat loose is something to look at. You mat see that with your bore scope.

[ant7]

Exhaust Valve sticking in the guide, or something stuck under the valve to valve seat, or worse scenario, bent valve, either way, IMHO its at least head off on that side to investigate.
As for the white smoke, either its condensation, or is it posible that you have some diesel in the fuel tank ?
Ant.

[Mac-Porsche]

Thanks for all the replies. I realized after the last post that I was backwards in my thinking, the exhaust valve is not returning to the seat, so it's getting caught on something. Either damage or debris.

I will admit that there was some soot on the plugs. Which was actually to be expected because I've always run my car rich. We don't have emissions testing, I only drive it as a "Sunday-driver" not daily, and I wanted to get the most power out of the car (especially with my new P&C's and higher CR). So was okay with the poor fuel efficiency. I unplugged the O2 sensor and targeted ~13.8 AFR (2% CO) - I confirmed this through both an AFR sensor and a CO meter years ago.

That being said, I wonder if it's carbon build-up. Although all the other cylinders test just fine, so maybe not...

The white smoke and hard start has baffled me over the last couple years... Sometimes it will sit for weeks, fire up immediately and no smoke. Other times it will only sit a day or two and hard start, with lots of white smoke - and the smoke always follows a hard start. I actually have considered the chance of it being excessive oil leakage, but it's odd that it's intermittent.

When I rebuilt the engine years ago, there were two areas that never left my mind:
1) I had to increase the piston ring gap on the new rings, it was my first time - although I did everything by the book, I was always fearful that I gapped them "too much". The car had very good leak-down and compression tests, so that fear faded over time.
2) When I was 'breaking in' in the engine. I followed Wayne Dempsey's engine rebuilding book and ran the engine stationary at 2000rpm for 20 minutes. My uncle thought I was crazy, but that's what the book says and I was inexperienced. I've since seen a lot of people question this... Nonetheless, the engine got VERY HOT... So I always feared that I damaged the valve guides. Not doing that step next time.