Loss of compression, rebuilt engine
 

I recently rebuilt my 3.0 SC engine. I also added a BAE turbo kit. I have the following problem:

The engine is losing compression on 4-5-6, where the compression pressures are 150-135-130 respectively. 1-2-3 are at 160. I'm going to do a leakdown this weekend. Car runs good cold, sounds like a lawn mower warm. Turbo at 5 PSI boost. Plugs reveal no sign of internal damage or detonation. The 4-5-6 cylinder bank also leaks oil.

Leakdown will probably reveal whether I put the cams in correctly. I may have somehow forgotten to torque the barrel nuts. I have 993 head studs, which were, IIRC, torqued, then another 90 degrees. Engine sounds, when warm, that the compression is blowing out of the cylinder-head interface. I do not believe I have overheated the engine in it's first 500 miles. This condition has become progressively worse. I am unsure whether these studs needed to be retorqued.

I have some possible theories on what and why, but I am open to hear any other possible scenarios. Best case repair is from me being stupid and not torquing the heads correctly. Worst case is I detonated the heads loose. I am baffled as to why it only happened on the 4-5-6 side, other than that side may run hotter due to the oil cooler, thermostat, etc, on that side.

TIA
Pat

To venture a guess based on your information I have two guesses

Did you install the CE rings between the head and the cylinder? (the ones that fit in the groove on the cylinder)

Or you've pulled the head studs on that side of the motor.

As a quick check, take off the lower valve covers and see if the head nuts are loose. If they are, torque them down. Drive the car for another week and check again.

I think it is unlikely that you forgot to torque the heads correctly. If you have the knowledge and skills to install the kit, I doubt you would make that kind of mistake.

AFJ

If it sounds like 1 1/2 old VW vans, the head gaskets are leaking.

I've had 8 Microbusses, and only one sounded like that, after I blew a plug out on #3 cylinder.

AF, I didn;t find the CE rings in the pile of leftover gaskets, so I must have put them in. In order to retorque the heads, should I loosen the nuts to finger tight, and torque them as if it were the first installation? I will check on stud conditions, etc, after work tonight.

Pat

Pat,

This is very odd. I would NOT lossen them but simply recheck tourque . If you can hear Then I would be concerned. Any oil on the plugs? Is it consuming oil?? It is possible to injest oil which intern will lower octane on the A/F ratio which can cause big problems if prolonged. I don't think that is your problem though. I wish I could be more help but no guesses at this point.

When you built the motor did you check your deck height before assembly?

Ben:
This turbo crap gets pretty complicated, doesn't it?

hm, no, I didn't check my deck height as I would if building a high comp engine cause I really didn't do much to disturb the stock SC settings. I eyeballed it and determined that if anything, my CR bumped incrementally.Mine is the low compression SC 3.0. I doubt there are valve clearance issues. I will definitely know when I do the leakdown check.
Pat

I am thinking cam timing is off on the on side causeing for lower Compression numbers on that bank. that also explains the running crappy when warm. I did this by mistake on my first engine rebuild and retimied and no issues from that point on. just a little off will make a world of difference. It could dare I say cause to bend a few valves just enough to make it run crappy when warm.

I feel like we are now brothers pat. Huge learning curve ;)

Thanks for the sympathy, Ben. I know you know how it is.
I did a cold leakdown on #4 and 5. 5% leakdown on #4, 25% on #5, air coming out the crankcase breather. Had to stop the test to fetch pizza.

This is not looking good. My worst fear on every engine I've ever built is breaking the rings. I'm going out to pull the valve covers off and see how everything looks, while I try to think of any other way for air to escape out the breather if not by the rings. Maybe I'll try it again warm, but a 20% difference in cold from cylinder to cylinder is ugly, no matter how it's cut

I turboed a honda accord about 25 years ago . It doesn,t take much of a run with a lean condition to scorch the rings and cylinder bores.I think you said you were running A/F of 14:1? (Way too lean) I would guess you might even have broken rings. The telltale blowby in your previous post reminds me of a similar experience with the Honda. I couldn,t hear audable detonation either but a sustained high speed run of about 5 miles caused the engine to start blowing out volumes of crankcase fumes. We found the top inch of the bores burnt blue. We reassembled with a hone and new rings and ran richer mixtures and less advance. You might notice I also have a Corvair turbo (owned since 1972) They were engineered without a wastegate and used pressure retard and rich mixture under boost but were also prone to fits of detonation and often suffered broken rings and cracked. pistons. I hope the damage is minimal but you need to dismantle to find exact cause. Don,t run without boost enrichment and less advance as a minimum.

Johnsjmc
Thanks. I have seen similar from the VW's we used to turbo in the 70's. We'd put them on the Stuska dyno, and run em without any boost control. They could make 230 HP, fail, but they were just little junk 1600 engines we tested with. Unfortunately, I don't recall details of 25-30 years ago as well as I should (but I'm sure I was having fun!)

So, I went back out to do a bit more testing. I drained the oil, no magnetic bits in the sump. I pulled the exhaust valve cover off 4-5-6. The head studs seem tight. Valve clearances on the exhaust were good. I leak tested #5 again, still cold, but I turned the engine by hand a few times prior. Now, I have like 1% leakage. I tested #6, negligible leakage again. I think next week I'll have to run the engine and get it warmed up, and leak check it again, unless I do it tomorrow.

I'm starting to get a bit baffled by this, but the warm test should be definitive. I initially am thinking broken rings also, but have trouble figuring how the little bit of boost did this, unless I broke them during assembly. I did check the end gaps on a few of the rings, but not all. The ones I did check were good, so I didn't feel it necessary to do them all.

However, detonation is a different story. Again, I have a water injection system set at 3 PSI which should limit the detonation (that's why I installed it). Plugs do not indicate any detonation at all.

Thanks to all of you guys for the help and ideas!
Pat

I am not to be listened to but my guess is cam timing? Just wanted to subscribe. I read on the engine rebuilding board about similar symtoms from experienced (somewhat) people.

Can you hear where the air is rushing to during the leakdown? There are only 4 places for the air to go; intake valve, exhaust valve, headgaskets,or past the piston. You may be able to listen in the exhaust, open the butterfly and listen to the intake, and take off a breather hose or the oil cap and hear where the air is going.
Good luck!

Warm leakdown test results:
1-2-3-6 all less than a couple of percent
5 has about 5% past the rings
4 has in excess of 25%, past the rings (90psi/68 psi)

Either the ring(s) broke, or they didn't seat. I hate when this happens.

I've had problems with using synthetic oil too soon and having the rings not seat. I'm sure that others here have used synthetic from the beginning and had no problems, but maybe you could satisfy my curiosity, what kind of oil were you using?

20w 50 dino, Cam 2 to be precise. Oil may be a bit heavy for break in WRT rings, but it's better for the cams.

pat I have 90 warm on one of my cylinders and my engine is smoked I am afraid you may have done the same

I would guess broken rings, time will tell. I think with poorly seated rings you would see 15% at the worst.

Cheers

Ben
90 was the leakdown test pressure, of which it retained 68, or a 22 PSI loss. The warm compression pressure was 160 PSI.

I'm going to have to take the top end apart and see what's going on in there. Maybe next week when I get back from the Glen.

The engine still runs ok, but it needs to be fixed before I do smoke it.

oh got you.. Man You know how it is missory loves company:D

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?

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!

Any news?

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.

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?

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.

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.

http://forums.pelicanparts.com/uploa...1162074849.jpg


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.

http://forums.pelicanparts.com/uploa...1162075285.jpg

http://forums.pelicanparts.com/uploa...1162075338.jpg

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.

http://forums.pelicanparts.com/uploa...1162075559.jpg
http://forums.pelicanparts.com/uploa...1162075582.jpg

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

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

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.

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.

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...



http://forums.pelicanparts.com/uploa...1162140401.jpg

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

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..

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.

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

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

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

Doug

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.