Donman

1992 Mercruiser 502 carb ( 390 hp) ----

On the first day out this season my 502 started missing. After evaluation it had a blown head gasket between #1 and #3. I could not figure out why.

I pulled the heads and had them fully reworked, including milling. I installed the heads using Fel-Pro Marine gaskets.
I installed a new sea pump impeller. The engine has a cross-over.
The engine started up great and I tuned it on the hose. Initial timing at 14 degrees with Thunderbolt IV ignition, 34 total.

During lake test, it could barely plane and will only pull 4200 rpm max. The engine temp rises pretty quickly and gets hot. The water coming into the sea pump is cool and the hoses are cool going through the oil cooler and into the engine. The water coming out of thermostat is scalding hot (I removed the t-stat). There is sufficient water flow out of the exhaust and it is steamy as it comes out. It is getting plenty hot. There is no water in oil.

I am puzzled and looking for advice.

Could the head gaskets be wrong ? This is my first guess. I do not know the number ( I have the box at home ).

Cracked block ?

I still do not know what caused the blown head gasket to start with. I am thinking this is all related, obviously.

Looking for advice. Thanks in advance.

stevesxm

you didn't get the hoses on the sea water pump backwards, did you ? or stuff a rag in them when you had them off and forgot it ? ( assuming all else like firing order and all the easy stuff is correct)

Donman

No, it seems to have good water flow. Water is definitely coming out of the exhaust in what seems like a normal flow rate.

stevesxm

correct me if i'm wrong but didn't that tstat housing have a little l shaped by pass hose ? and if so then the tstat didn't have a bleeder but on the ones where that bypass is blocked the tstat has a little tiny bleeder hole/bead looking thing... you maybe get the wrong tstat ?

oh... and does that ignition need the jumper to set the ignition base timing ? and are you certain you got that right ? like maybe you used one of those lites with the dial it in advances and somehow doubled what you put in ? it sound like you are way down on power as well...

blue thunder

I would pull the hoses off the risers, start the engine and see if you have compression coming out the water jacket circuit (hose from engine). That will tell if you have a crack somewhere or another blown head gasket. This assumes your engine is sea water cooled. Also I presume you checked the deck for a low spot?

Donman

I pulled the t-stat out.


I thought TB IV does not need to be jumped. ?

Donman

I am about to go back to the dock and I will check that. I did check deck best I could with straight-edge. Everything looked fine.

blue thunder

No jumper needed on tb IV. But I've made that timing light mistake before that stevexm mentions with the advance setting.

Donman

I have Gen V block and the Fel-Pro box said head set was for Gen V 502, so ...

I will get part number tonight.


I keep thinking head gaskets could be wrong .............. ?

stevesxm

this is a long shot but i got my boat originally because the prev owner blew up every 454 that he could buy and finally there wern't any more. . the starboard ones all blew up because he had the oil lines backward and the port ones blew up because the seawater inlet hose got soft ( no wire reinforcement ) and would suck itself closed and roast the motors. you don't have something crazy like that going on, do you ?