03-09-10, 05:37 PM
My friend's mechanic told him that his 98 Deville's overheating problem was definitely either blown head gaskets, or a cracked cylinder head. I've read countless posts about head gaskets, but I've never read of cracked cylinder heads. Is this common? Personally I think that it is the head gaskets, but could it be both? He doesn't want to go through with the HG repair only to find a cracked cylinder head. Thanks.
03-09-10, 06:18 PM
The heads have to come off to replace the gaskets, so a cracked head (unlikely) will be apparent. More likely he'll need some cam and cam follower (valve lifter) work. Any of that will be apparent if the mechanic knows Northstars.
Cracked heads are almost unheard of.
03-09-10, 09:23 PM
I just wanted to make sure. He had two different shops mention cracked cylinder heads to him (they obviously were not familiar with the Northstar though).
Could they have been talking about a cracked block near the cylinder?
03-10-10, 10:39 AM
what leads you to believe there is cam/follower wear? I did not check my cams when i had my heads off. i was looking in the combustion chamber side of the head, not the top of the head.
03-10-10, 11:43 AM
Flat tappet cam and lifter wear is a fact of life whether the engine is OHV or OHC. Both cam and lifter sliding faces are hardened by heat treatment - the "hardening" is extremely thin in depth - less than .004" - so as wear progresses softer and softer metals are sliding past one another and the result is worn cam lobes and dished lifters (or cam followers). ANY time you go into an engine you should check the cam(s) and lifters.
The Northstar uses fairly high valve spring pressures coupled with high valve opening rates (cam ramp angle) and, as Jake will testify, pre-2000 units have a pretty significant valvetrain wear factor which is one of the reasons GM went to roller cam followers in 2000. Also one of the reasons why early Northstar gearheads were running a few ounces of GM EOS (old style Engine Oil Supplement: assembly lube) at each oil/filter change. That product has now been changed due to EPA pressure so it no longer supplies the same levels of scuff protection.
Why use higher valve spring pressure ? - that's one step toward high rpm operation, which is where a small engine like a Northstar gets its power way up in the rpm range. Remember that at 60 mph - 2,000 rpm - that L37 engine is only putting out about 50 - 75 hp at cruise loads. The engine never gets "on the pipe" until 4,500 rpm+.
Find a recent Northstar thread about something like "valve keepers missing" or "valve stuck in cylinder" - maybe one or two pages back in these threads. Couple of pics will curl your hair.
Here's the new 2000+ valve gear with the roller followers. The hydraulic lash adjuster on the left never moves - it preloads the "top hat" follower/roller assembly. The whole thing should last forever because it's all pressure lubricated, not by splash only as in the flat tappet system.
03-10-10, 09:17 PM
Sub just said it all about lifters/lash adjusters. He hit the nail right on the head. The 93-99 lifters are heat treated to a hardness of 60 R.C. Everything is nicely hardened, but there's not quite enough oil flowing over the exhaust cams/lifters. The biggest causes of failure, I believe, is either cheap oil, incorrect change intervals and/or viscosity, and overheating. Overheating causes oil to get thin. So it sure doesn't help when the head gaskets blow. I can pull apart some 93-99 engines and the cams/lifters are in perfect condition. Eyekandyboats's 94 Eldo - perfect set of cams and lifters. No wear. Clean. Good job Taylor, you and your aunt took care of that car.
Ranger is 100% correct - cracked heads are unheard of. I have not seen one yet. Cracks from freezing would probably be the only cause of head cracks. I haven't seen this yet (surprisingly, in Canada and the northern U.S. we experience a few months of sub-freezing temperatures).
Less than 5% of the blocks Ive seen have cracks, and the ones that do, are mostly cracked because of hydro-locking, either from fuel (injector stuck open) or coolant (bad head gasket and filled the cylinder).
Speaking of Eyekandyboats's old engine, it was run on green coolant. Original HG's and engine. Cause of HG failure? Head bolts. The gaskets themselves were fine- no rust/flaking. Again. Dex wasn't used and the gaskets are fine. No pitting in the aluminum anywhere. I am 100% convinced that green is the way to go and Dex is NOT. I've seen way too much proof- way too many cars and gaskets to not be 100% positive. Anyone buying a new Cadillac or GM product - if they are still using Dex, and if it won't void your warranty, get rid of that stuff. If nothing else, change your Dex coolant every year until your warranty is up - then switch to green & distilled water.