Loss of compression, rebuilt engine - Page 2

DonE · 09-27-2006, 03:14 PM

My guess is a burned piston on #4. Not blown, but just enough to create excessive leakdown - probably damaged the rings (don't ask how I know this). Where is it leaking, intake valve, exhaust valve or crankcase?

patkeefe · 09-27-2006, 03:31 PM

Quote:

Originally posted by DonE
don't ask how I know this

I always find that phrase to be associated with a direct painful experience. It is leaking out the crankcase; if I take off the valve cover, it blows pretty good out the return tube. It's definitely, from that test, either going past the rings, or through a hole in the piston. Won't know 'til I get it apart.

There will be prizes awarded for the best guess diagnosis!

quattrorunner · quattrorunner's Garage

Any news?

patkeefe · 10-27-2006, 02:18 AM

I have the engine down to the long block, on the stand in the basement. I'm going to check cam timing, then take it apart this weekend.

T77911S · 10-27-2006, 06:41 AM

just for your info, the cams on my 2.7s were advanced almost 15*. compression was excelent, it did ping under heavy load or above 4000rpm. no bent valves. ive reset the timing and it runs great.
i may have missed it, but did u retorque the heads?

patkeefe · 10-27-2006, 02:41 PM

Well, the tear down starts soon. The lawn mower sound I may have described is from every exhaust gasket leaking at the port/exhaust tube interface. At least I was consistent. Timing check is coming up in a few minutes. Then, we'll see what the problem is on #4.

patkeefe · 10-28-2006, 02:51 PM

Pulling the cylinder heads off was like opening an anticipated gift...you kind of know what it's going to be, but you're really not sure.

First off, why it sounds like a lawn mower (yes, the old air injectors are welded shut). Also, the cam timing was indeed 2 mm overlap as per DR Camshafts for the DC-15's.

I have to get my bore gauge out and take a real close look at the cylinder bores and ovality. I do not see any signs of detonation, nor are the rings broken. What I did find was that the compression rings had the ring gaps migrate to the top of the cylinder (operationally "up"). I KNOW that I put the gaps in approximtely 180 deg apart (I usually put them somewhat near the pin clips).

I'm not real sure these rings seated (great...I could have put some 20 weight in the oil system, and ran the crap out of it for an hour). However, the installation of the turbo does introduce some other variables into the engine, so it's just as well I pulled it apart.

Anyway, it looks like a 500 mile engine inside. Just need to figure out the ring seating issue, and remove as many of the "what-else-could-it-be?" variables from this.

DW SD · 10-28-2006, 08:34 PM

Interesting, sounds like good news, kind of. The bad news is the problem isn't as obvious, at least to me.
It does look like the exhaust ports were leaking at the gasket, not that this would effect compression. They could contribute to the "mower" sound.

Good luck,

Doug

mb911 · mb911's Garage

I second what doug wrote. Very possible as to your major issue

patkeefe · 10-29-2006, 05:41 AM

According to my micrometers and bore gauge. I have somewhere near 0.003" clearance piston to cylinder. The cylinder taper on the thrust (skirt) surfaces is less than 0.001", and there is no appreciable ovality. New Mahle's are supposed to have only 0.0017" clearance. Bentley says the wear limit is 0.005".

I doubt I'm the first guy to ever put in pistons and cylinders with this clearance value. Also, I would imaging the gas pressures in the combustion chambers with the turbo would help to seat the rings.

I'll keep thinking about it today. BTW...Ben, the flanges on the heat exchangers arent very flat, they were sort of concave. I have to file them across the mount bolt direction to get them flat again. That should fix that problem.

mb911 · mb911's Garage

Pat

when I spoke with henry a couple weeks ago about cylinders and P-C clearence he indicated that .0035 was worn out for most porsche applications.

Now I could be wrong but you seem border line. but still that does not mean it would not work fine for you if you are not looking for a 100,000 mile engine.

patkeefe · 10-29-2006, 07:47 AM

Yeah, Ben
I think it's borderline. However, that doesn't explain to me why I have almost perfect compression and leakdown in five cylinders, and #4 having such excessive leakage.
I'm going to check it again after I finish the pre-winter estate maintenance...

mb911 · mb911's Garage

maybe a bent valve?? Improper seating?? Not enoguh spring pressure

patkeefe · 10-29-2006, 02:47 PM

The leakage in the #4 cylinder is past the rings, without a doubt, which I established when I did the leakdown. The valves are fine, cam timing, etc was all right on. I'm going to pull the rest of the pistons and cylinders apart for comparison, and check all the sizing really well.

I should clarify that the 0.003" clearance was in #6, which had no leakdown. I have to pull the rest of the pistons from that side. Not tonight, however, as it was a hard day at estate maintenance. Trees sure do grow an awfully lot of leaves each year.

Ben, you did your turbo on a running engine?

mb911 · mb911's Garage

Quote:

Originally posted by patkeefe
The leakage in the #4 cylinder is past the rings, without a doubt, which I established when I did the leakdown. The valves are fine, cam timing, etc was all right on. I'm going to pull the rest of the pistons and cylinders apart for comparison, and check all the sizing really well.

I should clarify that the 0.003" clearance was in #6, which had no leakdown. I have to pull the rest of the pistons from that side. Not tonight, however, as it was a hard day at estate maintenance. Trees sure do grow an awfully lot of leaves each year.

Ben, you did your turbo on a running engine?

shoot Trees do grow allot of leaves but My house is only 4 years old so it has all these tiny trees on my acre lot so not much of that clean up for me.

But as to your leakdown and my experience let me list what I had and what I am switching to

running 81 sc engine with rerung aluisil 9.3-1 with stock cams and EFI with .4 bar of boost no IC

I am switching to 98 mm JE 8-1 with QSC Cylinders with CTR cams(maybe stick with SC cams as this is Cam grinders suggestion) And a full BAy IC .5 bar

I do have that brand P&C if you want it you can have it for 125 bucks plus shipping. Any interest.

also I should note I still have not pulled my engine down as I have been busy with my new storage building/work shop so I am finishing the insulation and hooking the furnace up so as soon as that is done game on with pics. Oh and for the record I hope its nothing major pat..

patkeefe · 11-01-2006, 03:27 PM

The problem is solved! Unfortunately, the problem was created by my own carelessness (nice way to call myself stupid).

I evidently put in a set of Goetze rings made for a later SC or Carrera, which has the top two rings the same, but a thinner (3.5 vs 4 mm) oil ring. So, I put in the thinner oil rings, and paid absolutely no freaking attention to it. Thus, I let oil blast past the oil rings, overlubricate the cylinder walls, and never get the rings to seat properly. I checked the top compression rings for end and side gap before assembly, but not the oil rings.

The good news is it didn't damage anything but my fragile psyche. One of the reasons I make checklists for stuff like this.

DW SD · 11-01-2006, 03:34 PM

Pat,
Good news! Interesting findings.

Were those rings specific to the bad cylinder, or just more acute in the one? If they were the same in all cylinders, why the compression in only one?

Doug

patkeefe · 11-01-2006, 03:46 PM

Doug:
These rings are in the three cylinders I have apart. I have no answer as to why #4 was a bad leaker. I have to examine the cylinder bores in greater detail, maybe it has more ovality or clearance than the others.
Pat

DW SD · 11-01-2006, 04:00 PM

Gotcha. Are you still confident the problem is solved? Seems like the explanation is very plausible.

Doug

patkeefe · 11-01-2006, 05:03 PM

Well, the #4 cylinder was pretty round, all well within 0.001", with less than 0.003" clearance, and less than 0.001" taper. I'll pick up some new rings and a gasket set and regroup.

Maybe it's just as well I had to take it apart. I did get to see what it all looks like after run-in, and how the turbo effected the internals, etc. Hopefully, this is the answer to the problem.