I was losing coolant and thought I had it fixed when I replaced the rusted freeze plug (of course it was the one behind the motor mount):



My first clue there was more work to be done was when I was looking for leaks after replacing the freeze plug. There was steam coming out of the junction between the exhaust manifold and the front Y-pipe. I was hoping it was just residual water...

It ran fine for a few days and then: "rrrrrUMP!". I know exactly what THAT means (been there, done that). Hydrostatic lock sucks.

This is what it looked like after I took the head off tonight (This is why water injection was such a big deal back in the '70s -- it got a bad reputation because most people did it wrong):



My fancy schmancy FelPro 1094 head gasket (The machine shop said, "We've never had one fail". They charged me $70 for a pair in February, 2009 -- 30,000 miles ago):



The large hole on the right is a head bolt hole.
The hole in the center is a water passage hole and it's about twice its normal size. This is the same hole that rusted out in my original from-the-factory head gasket.

When I did this job back in 2009 I left out the coolant flow restrictors for the rear coolant passage in the intake manifold. I think I'll put them in this time...
.

 

That's odd Cliff. Wondering if either the deck or head may need surfacing? Maybe coolant leaking in from head bolt?? Could be that 50/50 sulfuric acid coolant you've been running.

 

Cliff, I've always wondered the same thing..There really should be nothing different on #7 then on any of the other cylinders,but #7 is the one that always has problems..Maybe the restrictors were put on the 350s for reasons other then just cooling the rear of the head better.In your case the extra heat from not having the restrictors may be why your gaskets didn't last very long, heat is a killer.....Maybe there's a greater flow of coolant at the #7 cylinder somehow causing the gasket to eat away faster and the restrictors slow the flow down...This is a very good question, but knowing how you are about getting answers, I have no doubt you will have a definite answer for us very soon........WW

 

I have always had #7 or #8 fail.....always at the coolant passage. My thoughts have been that the very rear of the block does not have the large coolant cross overs that are between the other cylinders (as shown in the pictures). Because of this I have felt that the rear of the head experience extreme heat variations than the rest of the head. And with aluminum heads on a cast iron block there is move movement because the materials expand and contract at different rates.

The other thought that has crossed my mind is that the head gasket (like shown here) has a very thin metal ring around the cylinder. Coolant that has been in the engine too long will turn acidic and eat the gasket material (and aluminum). There is a very small gap at this point between the coolant passage and the cylinder. As the gasket gets eaten away by the bad coolant it starts to eat the metal in the gasket. The pressures in the cylinder find the weakest point.

I think similar things happen with the intake manifold gaskets (L98 and prior Chevy SB). My intake gasket have always failed on #7 or #8.

 

Cliff, I'm sure you are aware that you could read for a month about the pros and cons of coolant flow restrictors. Do you think boiling coolant had an adverse effect on the gasket?

 

Cliff,
Did you burp the rear crossover passage in the intake?

 

More common on the aluminum headed motors, but my guess has always been that the restrictors in the intake gasket simply keep coolant sitting at 7 and 8. Once the coolant has been compromised, it simply eats away the head gasket. GM addressed the issue with a Service Bulletin blaming galvanic corrosion on crappy coolant that became crappy because the intake gasket was mis-aligned during assembly which led to leaks and contact between dissimilar metals. Offering a fix using counterbores in the front and rear bolt intake holes for gasket retainers, they eventually abandoned sending any coolant at all through the intake. That led to other issues: excessive oil consumption (mostly on the 6's) when heat cycling destroyed the gasket and cracked intakes when they switched to plastics.

A lot of the head gasket issues were showing up right around the end of the original warranty and were eventually noted on the NHTSA site (when it became operational which was pretty long after the fact so I'd also guess some of the lawsuits that were filed following warranty denials were still active). GM's Service Bulletin has been edited with "galvanic corrosion" being one of the phrases removed. Current replacement 113 heads include the fix. Don't know of anything for the iron heads, but any machine shop could add the mod assuming the intake will accept '91 intake gaskets.

 

Will be interesting to see what you find. Regarding coolant restrictors, I don't think that would be a factor due to most aftermarket intake manifolds (carb) do not even have a water passage crossover between 7 & 8.

 

Simple, the -5-7- firing order lends itself to fuel charge robbing so #7 tends to run leaner/hotter. Changing the Firing order with a cam moves the sharing to -2-4- or in the LS motors, to -3-1- where the coolant can offset the effect as it come right off the waterpump at its coolest.

 

This makes sense if you're running a single plane carbureted engine, but how can this be true in an EFI long intake tube TPI engine?

 

The heads were surfaced back in 2009. I did a complete valve job back then since it was apart.

One thing I thought was very interesting is that the machine shop owner (Joe Jill -- they've been in business a VERY LONG time) told me he had never seen valve rotators on anything but truck engines. He gave me all my parts back in a box, except that one of the rotators was missing. Maybe he kept it as a souvenir? He also told me all my valve springs were weak and he replaced them. I always thought that maybe he didn't take into account the reduced installed height caused by the valve rotators. The machine shop is Superior Automotive Engineering -- formerly Speedwin Automotive Engineering when they were in Long Island, NY. I ate lunch today at the restaurant across the street from them. I was thinking of stopping over there but didn't.

 

For the record, I have an early '86 with the cast iron heads. I got it in December, 1985, so obviously I know the complete history of this car. I have done all the maintenance and repair myself except for the valve job and having the 700r4 rebuilt (I did rebuild it myself but just didn't want to go through the agony of taking it out and putting it back in the last time).

 

Did the #7 spark plug look real clean, or was it just the cylinder?

 

But it does hold true with batch fire injector systems and sub-sonic airflow. While it's more pronounced in short runner efi, it still happens. Carbs are actually less prone to it since the plenum is wet.

 

^Still doesn't make sense. If the paired cylinders (5/7) are drawing air at similar/same times, then those would be starved for air...not fuel. That's rich...not lean.

 

The fuel is in the air down in the runner, and the dynamics are not easily visualized at sub-sonic airflow dynamics. You don't have to believe anything, but as the OP asked, why is it always #7. Put a 4-7swap cam in and you'll see that the rear cyl 7 doesn't go as lean as it does under a normal firing order. I have 3 rebuilds and numerous intake tests under my belt and know you have to deal with 5-7 sharing if you're trying to run a lean tune.

 

L98 Vette uses Restrictors to keep coolant at the rear of the heads longer for additional cooling. A Bypass from the Heater Core is used to provide a sufficient amount of coolant. Neither the F-Body TPI or the ZZ4 Long Block (which uses the same late model 113 heads) has this feature. It was unique to the Vette L98 which seems to suffer an inordinate amount of head gasket failures at 7 and 8. Generally, it was such a small weepage you didn't even know it's going on and a lot didn't completely let go until about 50,000 miles. By then, it may have worn a groove into the deck of the block meaning it needed more than heads. Some have repeated failures because the Deck was never squared or checked after it failed. A groove is fairly obvious and easy to find once you see where the gasket let go, but it's still good insurance to place a straight edge at a couple of points around the cylinder.

 

I guess I'll remain "skeptical" on this. The same amount of fuel is injected into all 8 ports. Where does the fuel in ports 5 & 7 "disappear" to? It doesn't disappear, so it has to end up in the combustion chamber, ultimately. You talk voodoo...but don't back it up ("not easily visualized/sub-sonic...."), which seems a little patronizing. But maybe you've run L98's with EGT's on all 8 cylinders. I haven't so I don't know for sure, but I've read that #5&7 end up running rich from the 5/7 "suck", and that makes sense to me.

Either way, I don't think the cylinder being very slightly rich, or very slightly lean, is going to blow head gaskets.

 

I never said the fuel in 5 disappears, in fact #5 is robbing the charge from 7 because it is in full intake cycle while #7 is in overlap/reversion cycle. (Firing order) The back cyls are farthest away from the TB and 5 pulls it's charge from wherever it easily can. This is a batch system, the fuel has been supplied and is sitting on the valve and vaporizing. As #5 is at its peak vacuum draw, the charge in #7 is being pushed back into the plenum and is drawn into #5. Add airflow dynamics at high rpms, and anyone that's pulled a plenum knows the reversion is happening, so there's no voodoo.

I won't argue the case, you either know (by measuring exh temps as I have) or you keep guessing.

 

So you have measured individual cylinder temps. I have not (on a V8). If you have done that and the result was a higher EGT on 7, then that is pretty strong evidence. I can believe that.

Just curious; what EGT's have you seen? And what was the delta between 5&7?