Nadams

I bought a 1987 vette with about 100,000 miles. Previous owner told me that the block was cracked but was not very sure. It blows some white smoke out the back and has some loss in power. Did a compression test and most cylinders are between 165 and 175 except cylinders 3 and 4 which are at 145. Wondering if its as simple as bad gasket between the two or if it could be worse such as a cracked block or cracked head.

96GS#007

Frankly, if it was a cracked block you'd have an oil pan full of water. Could also be a cracked cylinder head. Most likely just a head gasket although if it's cylinders 3 & 4, then it's both head gaskets since the driver's side is 1-3-5-7 and the passenger side is 2-4-6-8. I'm going to guess that you're actually referring to the back two cylinders on the passenger side which are numbers 6 & 8. Nonetheless, only one way to find out. Tear it down.

Nadams

Yes i am referring to the back two cylinders on the passenger side. I guess the cylinders are named different on the corvette than other engines i have worked on. Thanks for the advice and your quick response.

Purple92

The engine will probably have to some out, but before you go there, run a leakdown test. (This is where you push compressed air thru a small orifice into the motor thru the spark plug hole, and see what percentage of the air pressue the combustion chamber holds.

The advantage to the leakdown test is that you get to listen for where the air is escaping. You run the test at TDC for the cylinder being tested (when the valves are supposed to be closed). If you hear air escaping thru the intake manifold - it's probably a bad intake valve. Hear air escaping thru the exhaust - probably a bad exhaust valve. See bubbles in the coolant - cracked block or bad head gasket. Hear air in the crankcase (PVC or breather) - bad rings, cracked block, broken piston or bad head gasket.

Honestly - the odds are high that the motor is going ot need to come out (although if it's a head or head gasket - you can pull those without pulling the motor), but isn't it a good idea to have as much info as possible before starting the job.

GOOD LUCK

89onlyZ51

Get a Block Tester kit, and it will tell you if you have combustion gasses in your cooling system. You can also cut fuel/spark to individual cylinders to determine which one's leaking.


You get get one here:
http://www.blockchek.com

or, NAPA sells them, too, but you usually have to ask the guy at the counter as they're not on the floor.

Nadams

But the block test would not narrow down between a bad block or just the head gasket would it? Because either way there would still be exhaust gases in the cooling system.

89onlyZ51

Not for sure, but if you had a cracked block, as someone else said, your oil would be milked.

If you narrow it to two adjacent cylinders, 99% chance it's just the head gasket. That's the most likely case with the alum head motors anyway.

Compression test or coolant pressure test aren't that reliable in outing a blown head gasket as you could have good static compression while still having a leak that shows up during combustion where pressures are almost 100x higher.

oldalaskaman

thats not much of a leak, get some blue devil head gasket seal, follow the directions religiously , should seal it right up, there are other products that work also

Nadams

The previous owner put some head gasket sealer in it because he planned to run it into the ground and then swap engines. I would prefer to fix this one and fix it right. Since its most likely either the gasket or the heads i think I'm just gonna tear apart the top end, replace gaskets and get the head tested and resurfaced and maybe do a valve job.

oldalaskaman

thats best, I'd bet money its just the gasket though, good luck

TravisMartin

Ditto...I just this week had to do the same on my '90. Same symptoms. Mine caused by previous owner or his "mechanic" installing intake manifold gaskets backwards with restrictors up front. My money says gasket. You have to pull the heads off regardless, so look there first.

leesvet

Yep....

as everyone else has stated...just from the piles of past problems an experiences, all solved, odds are without eventearing it apart that its a head gasket or worst case a cracked head...maybe warped. I cannot recall seeing a cracked block in a vette...head gaskets however, should come with the spare tire.

BTW..
do BOTH.
if 6&8 are eating your lunch this week....next week will be #7. You are not driving a C4 until #7 area head gasket leaks and/or blows.

trick to long life on new gaskets....clean ALL mounting holes with thread chaser. Use water sealant on ALL bolts in head to block. Some are wet and IF water can seep up and attack the gaskets from within, this hastens the destruction. Torque everything and after running for a week, go back and re-torque everything. There will be a couple that got loose. Aluminum vs iron.
Nows a great time to spend $250 on fresh injectors.....no BS. you WILL need them tomorrow. The stock injectors cause half the problems with C4s that folks come here to complain about. Call Jon. This inj "kit" gets you all the fuel o-rings (you need, know it or not) all new gaskets and the latest fuel inj design..Bosch-III. Its not about miles....its about alcohol in old technology.
If budget permits, lifters, check push rods, rockers, lots of options there for another $300 and of course intake : head : runner port matching/opening. Quite a few ponies can pass thru that stock intake after you clean up the mass production flaws...

Get yourself a real FSM before you start and have fun. Nothing to fear with a FSM handy. I would not attempt to do a top end overhaul without the FSM...too many details that matter to these very sensitive engines.

383vett

Gasket sealer is not going to fix a head gasket leaking compression between 2 cylinders. Head(s) have to come off. Sometimes leakage between 2 cylinders will erode aluminum heads at that point so don't wait too long. If the gasket is leaking, your heads may need to be resurfaced to get a flat sealing surface. At this point, maybe you should have both heads done, guides checked/replaced. New seals, valve job. Depends how much you want to put into your car and how long you intend to keep it.