Horse pucky on the "no forces...which can break a ring" nonsense. Preignition and detonation are NOT the same, agreed. However, preignition can cause or lead to detonation, and detonation (regardless of cause) definitely IS a force that can break rings, pistons, and more.




To pressurize the crankcase like that, there has to be a combustion pressure leakage in to the crankcase. If not a ring, then a piston, or a chunk of cylinder wall (highly unlikely).



Cool! Diagnostics by a democratic process, a poll. And to think I wasted money on an FSM with trouble shooting charts. Next time I see a check engine light, I'll just take a poll on the repair. That should be a lot quicker than common sense or trouble shooting.

RACE ON!!!

 

That is a really good way to show something. Not.
90% of people who will vote, have probably seen 1 ot 2 engines opened up, and probably only their own.

I didn't say that it's common, but it's possible and I've seen it with my own eyes in more than 1 occasion.

 

I hadn't thought of listening for the leaking air. Very good idea, thank you. I will do that as I tear down.

I found a local L98 refreshed short block for $450. Would my costs in repairing bad rings exceed that? I told the seller I would tear the engine down and make sure it's not head gaskets before buying.

I don't think I'll be able to hold back on putting a bigger cam in if I go that route!

 

I'm betting on the broken rings. Spark knock often goes unheard or ignored. 70# compression is pretty much a dead cylinder, it must be running pretty bad. Now that you have us wondering, hurry up and tear it down.

 

I said except. Which part of except did you not understand?


Well, now we're getting down to it

Where do you get a piston or cylinder wall? He said he drove it home.

Who said anything about poll diagnostics? The purpose was to see if others have seen broken rings in Corvette engines and so far a few have so we (or at least I am) are learning something. I don't work with OEM stock rings and I had second thoughts about maybe the OEM iron rings have been a problem so let's find out.

You must have stayed at a Holiday Inn Express last night but next time the light comes on you might try Autozone!!

RACE ON... indeed!

 

If they have had their engine apart they might know if it was because of a top ring and I'm I'm OK with that. That's what we want to know!! Shout it right out in the piston ring thread if you have seen one!

 

OK, and detonation is?

 

Depends on what is hurt. If there is no serious damage, a quick hone and a set of rings would run you $200 or less. If you chunked a piston, you could need a piston, sleeve, complete hone, and a ringset. That would likely get you near the $350 mark.

 

Assuming the actual fault turns out to be less serious than some have predicted here I would lean toward fixing your original engine only because you know what it is and you know the previous history of it. The replacement shortblock you found sounds good but do you know for sure that it is what the shop says it is?

If you are going to invest the time and effort and money to pull yours down it doesn't make sense to shortcut the job and put it back together again unless doing it correctly is just not in the budget or you plan on selling it anyway. In that case I would be OK with doing as little as possible just to get it running again.

Doing the job "correctly" means the engine should be removed from the car and completely disassembled. Begin with a visual inspection of the cylinder bores looking for excessive wear at the top of the bores (ridge) or deep vertical scratches or scoring. If you see scoring the block will have to go to a shop which has the equipment to determine what oversize it will take to clean your bores and what costs are involved. If no scoring is present in any of the bores you will need to measure the diameter at the top just below the ridge to determine how much wear is present. If the amount of existing wear exceeds factory limits the block will have to be honed to the next available oversize which again, requires new larger pistons. Keep in mind the rings not only must seal to the cylinder walls they must also seal to the ring lands in the pistons and an engine with over 100K miles will have pistons too worn out to reuse (again, unless you are selling the car or because of the budget).

Measure everything when you get it apart to determine which parts are within recommended service limits and can be reused and which are outside factory limits and either need to be remachined or replaced and make a note of which parts have physical damage. Look carefully at the head gaskets for the presence of a combustion leak path and inspect the intake manifold gaskets to see if it appears oil was getting by there indicating a possible mismatch in the mounting surface angles of your replacement intake manifold. Once you have accomplished this you will be in a good position to decide which course of action is right for you and your pocketbook.

 

We just had a great post over at the piston ring thread that presented something I had not considered... bore taper in blocks with high miles and lots of wear. This would produce a force that will break the rings. I stand corrected. I missed this one but another forum member knew it and posted it. Thanks to all!

 

Andrew,
Pull the heads off the engine and have a look to see what is wrong. I assume you were out at Road America when you had this problem? When I used to race out there I was running my modified 1988 LB9 firebird (heads, cam, intake and bolt on's approx 300 HP) and it started to blow oil out of the dipstick due to high crankcase pressure (engine seemed to run well on the street). Disassembly of the engine (high mileage 130,000) showed that it was completly worn out. The cylinder bores were all grooved up and there was no way the rings were going to be able to seal those bores.

That track is 4 miles long and most of it is WOT high RPM. That track is hard on engines, my guess is the engine is just worn out from repeated use at Road America.

 

Update, head gasket looks fine and what I could see of the rings looked OK. The cylinder wall is scored though. Picture of #7 that had the 70 PSI compression test.

 

Once you pull that piston and rod out, you will know for sure.

 

Personally, I think you jumped the gun without doing enough testing to know for sure why those cylinders where low before tearing it apart.

You should have pressurized the cooling system with a pressure tester before tearing anything apart to be 100% sure WHERE it's leaking. All you know right now is that it isn't your piston rings leaking compression.

There are 3 areas where you lose compression. Past the rings, past the head gasket, or past the valves. That's it. It's about eliminating two of the three and leaving only 1 possibility. If there is a combination of the 3, testing will determine that also. You need to do a compression test with and without oil (which you did), a cooling system pressure test (which you didn't), and a leak down test for the valves if needed (like when the two previous tests check out fine).

You can rent the pressure tester at Autozone with a refundable deposit. That would have told you right away if you're holding pressure in the cooling system or not. If the pressure would have held steady, then you know your head gasket was fine. Top that with the fact that adding oil changed the compression little (meaning the rings aren't leaking), that would have meant your #7 valves must be leaking and you would want to do a leak down test for final confirmation before even taking anything apart. But since you didn't do a pressure test of the cooling system to begin with, you don't know if you head gasket was leaking. Just because it "looks" fine doesn't mean it is. I've replaced many head gaskets that "look" fine but leaked. You can't always tell by eye because they don't always totally blow out in an obvious way, and sometimes it's a warped head surface causing the leak not the gasket itself.

If the pressure would not hold during the test, then with the cooling system partly pressurized you would start the car. If the needle on the pressure gage went up any (it shouldn't since cooling system must be totally sealed off from cylinders), it's a GUARANTEE you have a leaking head gasket. Because if you're loosing coolant/pressure into a cylinder, then cylinder pressure can also enter the cooling system in the opposite direction from the same leak point in the head gasket when the car is running, raising the needle on the pressure gage. So the pressure tester can test in both directions for a reliable result.

Your oil test indicates that your piston rings aren't leaking in those low cylinders. If they where, the reading would have gone up significantly (which 3 psi isn't). Only when you see a significant increase in pressure with adding oil does that indicate leakage past the rings. Otherwise, it's up top (head gasket or valves) which a quick cooling system pressure test would tell apart.

But since you already have the head off now, go have the head vacuum tested on a bench for leakage. Pay attention to what they find on your low cylinders' valves and see if it matches up. Basically, the head is placed upside down now that all valves are in their closed position, and a flat suction plate is placed over both cylinder valves by hand. This tests weather any vacuum can leak past the valves while they rest in their closed position, or if they hold. Have the head deck surface milled flat if it needs it before you put it back on the car.

If you're keeping this engine, do a straightness test on the block's deck surface with a feeler guage and straightedge while the head is in the shop, after scraping off the old gasket, and before putting the motor back together. You can get long aluminum straightedge at sears for $5.

 

If you end up honing and re-ringing the motor, here are some pointers you should know to properly seat those new rings for less wear, less scuffing, and to prevent excessive oil consumption.

Triple check your ring end gaps after honing. I like staggering my gaps like this:


Get a good cross hatch pattern with the hone. Not too flat and not too hight an angle. Finish off by spinning in the opposite direction for a good cross hatch. Medium up and down speed and always keep the hone lubed and moving when the drill is turning. Use a hand drill on LOW setting. Go all the way up, and all the way down. Clean the bores till a white towel comes out clean. I use brake cleaner since it leaves no residue and dries fast. This cross hatch is what is going to act as a file to seat your new rings so you better do it right!

This is about right:


Get the motor back together as best you could, check everything for potential leaks, and set the timing. You want to try and not have any leaks or other problems so that when you first fire up the car, you can let it fully come to operating temps till the fans kick on, then go for the break in drive. DO NOT use synthetic oil to seat new piston rings! It's too slippery. Use 10W-40 for the first 1,500 miles. Then you can switch to whatever you want.

DO NOT pussyfoot the car after you re-ring it. You need to do hard acceleration/deceleration under load in short quick burst after the car has reached full operating temps (coolant and oil) to seat those new rings right. You have a very small window of opportunity here to get this right, so the first 25 miles are critical. Otherwise your rings may never fully seat right for an even seal all the way around after that. The only solution then is to tear it apart, re-hone, and start over. Simply reving the car in the drive way won't cut it. You have to put it under real load with acceleration. I suggest you don't even start up the engine until you'll be able to take the car out on it's break in drive.

For the first 25 miles find an isolated road with little traffic and do at least a dozen short random WOT blasts from 40-60, 30-50, etc, then slow down almost to a stop (when nobody is behind you) and do it again, keep repeating at random speeds and times within the first 25 miles. You want short burts of hard acceleration and deceleration. Run it through each gear. If you must get on a freeway, vary your speeds. Do not coast at a constant speed. For the first 200 miles, run it through the gears at different speeds.

What this does is put the piston rings under heavy load. When you hammer it under load, the compression gets behind the piston rings and forces them harder against the cylinder walls, where your fresh cross hatch patter acts like a file to wear the new rings into the specific bores for a proper seating all the way around. New rings won't seal all the way around till you seat them in. And it's load and the fresh cross hatch in the cylinder walls that you use to seat them. If you pussyfoot the car during this short window of opportunity with an easy break-in, your cross hatch pattern will wear before your new rings are able to fully seat. By that time it's too late no matter how you drive it after that! You need to force those rings against the walls under load while the cross hatch is fresh.


Change the oil and filter after the first 25 miles. You'll see tiny metal shavings in the oil that came from your new rings being filed by the cross-hatch pattern which is what you should see. Don't wait any longer to change your oil for this reason. Change the oil and filter again after the first 100 miles but stick with 10W-40 for the first 1,500 street miles or two days if you race hard. After that, switch to whatever you want.

 

That was my guess, you will have to bore and hone the engine. Give me a call and let me know what your plans are.

 

The scratches go right up to the ridge in the cylinder. That is not typical. Scratches suggest a ring, or ring land is busted.

 

The top of that piston looks way too clean to me ???

 

That can't be, Pete. We have it from the expert:




If you aren't familiar, you are playing the "expert" in the wrong thread.

Maybe if you take a poll, you will get an answer you can live with. It is pretty obvious that if I explain it, it will go in one ear and out the other. No thanks.

RACE ON!!!

 

good catch.. i would think there should be way more carbon on that.