318 - Time for a new head gasket... what else?

[Fish Stix]

I used to have a Luhrs with a single 360, now I have a Trojan with two 318's - all was well until...

I took the Trojan out after setting idle(s), installing new FWC riser gaskets, and a few other minor things (rust removal and paint). Boat ran fine on the way out, but at 2500+ RPM, the port engine went to 200 degrees. I dropped back to idle and watched the temp go back to 160 degrees. Throttled up again and same thing happened - within a few minutes. Limped back to port at 1000 RPM. I then noticed a little steam coming from port exhaust at idle and above - not much at all, but just enough to be noticed.

I checked oil for water (none), antifreeze levels (no change), temps of the headers at idle (OK). Tonight I did a compression test with all plugs out (all were tan / brown and a few black). All were 120 psi to 140 psi EXCEPT #5 at 60 psi and #3 at 80 psi.

Going forward - I know I need a head gasket - but perhaps someone can enlighten me...

Which happens first - head gaskets don't fail for the heck of it, right? Did I over heat the engine and cause the failure? Or did the head gasket fail and cause the over heat and steaming issue?

Why would the head gasket fail and cause the two mid block port side cylinders to drop pressure?

These are fresh water cooled center risers (not the log type) - is there a potential for trapped air causing the over heat? Do I need to bleed the air out somehow?

While I have the one head off - is it best to change the other head gasket too?

Should I change the freeze plugs too? The motor is from 1975!

What's the standard "while you are in there" preventive maintenance that can be done?

Many thanks for reading all this and any input offered. I replaced a head on my old 360, and while not fun, it was pretty straight forward - any insight offered will be appreciated!

Brooks

Sorry for the barrage of questions!

 

[RicardoMarine]

Brooks................................

I checked oil for water (none), antifreeze levels (no change), temps of the headers at idle (OK). Tonight I did a compression test with all plugs out (all were tan / brown and a few black). All were 120 psi to 140 psi EXCEPT #5 at 60 psi and #3 at 80 psi.

Going forward - I know I need a head gasket - but perhaps someone can enlighten me...

Which happens first - head gaskets don't fail for the heck of it, right? Did I over heat the engine and cause the failure? Or did the head gasket fail and cause the over heat and steaming issue?
Cast iron will react and move with excessive heat that is out of it's intended heat range. When it does, it warps.... even if temporarily.
As it moves (warps) the clearances change, and the head gasket's seal becomes compromised and the gasket becomes susceptible to failure.

You may see tell tale signs of failure anywhere from mild, to extreme.
This one shows tell tales signs of an extreme failure.
(example only)

Why would the head gasket fail and cause the two mid block port side cylinders to drop pressure?
The fire ring area between the two cylinders became compromised. One cylinder can now bleed off pressure into the other.

These are fresh water cooled center risers (not the log type) - is there a potential for trapped air causing the over heat? Do I need to bleed the air out somehow?
When you say Fresh Water Cooled, are you talking about river/lake water..... or as in a Closed Cooling system?
The term FWC (in a misnomer fashion) can mean two different things to people.


While I have the one head off - is it best to change the other head gasket too?
I sure would.
You could also take both in and have them looked at for straightness, and for seat/valve condition..... straightness being the more important.

Should I change the freeze plugs too? The motor is from 1975!
Again.... what does FWC mean to you.
Is this engine equipped with a closed cooling system... or is it Raw Water cooled?

BTW, these are casting core plugs.... (aka welch plugs). If they are brass/bronze, they should be OK... in particular if closed cooling.

What's the standard "while you are in there" preventive maintenance that can be done?
Your cylinder pressures would appear to be a bit low over-all. (All were 120 psi to 140 psi EXCEPT ........)
Perhaps the Chrysler guys will chime in on this.

If these numbers are low, you may want to do cylinder leak-down test prior to removing the heads.

BTW, if the Chrysler engines are similar to the GM SBC's, and if your engine is equipped with Closed Cooling , you can use a high grade automotive head gasket set.
The gaskets won't know the difference, unless you tell them!
This will save you a few $$.


.

 

[fastjeff]

Damn, been through that on my old 360. And I didn't ever find out WHY it blew the head gasket.

One thing you MUST check, and carefully, after the head is off: See if the block is 'fire slotted" from the exhaust roaring through the break in the gasket. If so, the block will have to come out for milling (and a rebuild).

Hoping that's not the case. It was in mine, but I ran that way for at least a half hour (doh!)

Jeff

 

[brookster]

Update: I figured out why the engine over heated and caused the HG to blow. Evidently the previous owner didn't know what a zinc was and never changed them out in the heat exchanger. When the heat exchanger was tested - they guys reported that it was simply "no good - do not put it back on the boat".

I now have newly surfaced heads (took .001 off), new seals, new head gasket and of course a new lenco heat exchanger - and an overall cleaning that further evidenced the previous owner's inability to maintain a boat. I also took the opportunity to put a new moeller 50g fuel tank in! Lots less to worry about.

My only fear is that I have a whole 'nother set of problems on the starboard side now.

Thanks for the information guys - I certainly appreciate this site and those who hang out at it.
Brooks

 

[fastjeff]

I hope you checked the block surface for "fire etching" as I suggested or you'll soon be ripping it down again.

Jeff