Overhauling my 1988 MerCruiser 4.3 ltr. engine

[Scott06]

What is the purpose of the metal cover underneath the intake manifold? As I was getting the manifold ready to install I noticed a few little rust holes in that cover (see circle). Is that cover important or shall I ignore the little rust holes? I have a good spare one that I could replace this one with but I noticed that the corners are riveted (see arrows) to the bottom of the manifold. What a bummer. What shall I do?

Its to keep hot oil off the underside of the intake runners. I wouldnt put something that can drop rust or pieces into the engine. Either take it off or replace it. I suspect you can grind the rivets off and run without it just fine

 

[Rick Stephens]

If not mistaken, those pry off pretty easily.

 

[Lou C]

The cover is to shield the oil in the cam valley from the heat in the exhaust crossover above it. The main issue with that one is you don’t want bits of rust falling onto the cam and lifters. I agree with Rick I’d try to find a better one.

 

[Rivergator]

So removing the whole thing versus rust pieces falling in to the engine is the lesser of the 2 negatives. I can see that. If there are no adverse effects removing the cover I will do that. I will try to pry the studs off and if that works that will be great. I do have a good spare cover to install and all is well. If I have to grind the studs off and won't be able to install the good cover you all say that it won't do much or any harm. Btw the thermostat is 140 degree which runs the engine pretty much on the cool side if that means anything.

 

[Lou C]

Only thing I can say is the oil may get very hot when it hits that crossover under the intake. This was used extensively on both auto and marine engines to open the auto choke and also to warm the carb base so you don't get carb icing. Not relevant with a marine application obviously. The newer Vortec intakes do not have the tin cover because there is no exhaust crossover. They use electric chokes.
Take a good look at the inside of the intake where the thermostat housing is. If the cast iron in there is not solid they can rust through right under the stat and put water right in the oil. If you find that start shopping for a better intake (ebay often has them), I've picked up a couple of spares for mine that way because I'm in salt water and sooner or later you will have to replace it due to corrosion in the stat crossover passage.

 

[Rivergator]

Scott, Rick and Lou. Thanks for all the good info, that really helped. Yes Rick, I was able to pry all 4 studs out with relative ease and now the new shield is installed and all is good. Lou I took a good look inside the manifold at the thermostat area and it too looks good and solid. I appreciate all that detailed info. Knowledge is everything! Oh, one more thing. I painted the outside of the shield with Rustoleum paint since it was originally painted black. I hope that is ok. I will give it plenty of time to cure before I put oil in the crankcase.

 

[Lou C]

Sounds like you got it handled. I am still running my original 1988 intake, its had close to 20 years in salt water. Going to check it again this spring before re-commissioning the boat I have 2 spares just for this reason.

 

[Rivergator]

For the intake, yes the "china wall" front and rear vertical ends of the block get a bead of sealant, like Permatex Ultra Grey and continue it up the cyl head by about 1/2" or so. When I did my intake gaskets I used a bit of OMC gasket sealer on the water passages front and rear, to keep the gaskets from sliding around when you drop the intake into place.
Above is a quote from Lou's post #26. I am getting ready to install the intake manifold. I don't have OMC gasket sealer. I have Permatex Aviation Sealer #80019 and Permatex Ultra Grey. Can I use Aviation sealer instead of OMC gasket sealer. Shall I put Aviation Sealer on the entire intake gasket or just around the water passages? I know the Permatex Ultra Grey goes on the china walls. Any other good advise you can give me putting the top back on is appreciated as always.

 

[Lou C]

I used the Aviation just on the water passage area at the front and rear ends of the gasket to keep it in place when dropping the intake down so the gaskets don't move around. Use the Ultra Grey for the china walls, go 1/2" up the gasket. Before so doing make sure the thread holes in the cyl heads and the bolt threads are clean, so you get accurate torque readings.

BTW this may be obvious but: some gaskets have the front water passage area open and the rear one closed, this is because the cyl heads are the same casting and the gasket is the same too, just reversed from one side to the other. This is because there is flow only at the front of the intake, not at the rear, the hot water flows from the block, into the heads, then forward to the head to intake passage. Make sure that if you have this type of gasket that the closed ends go toward the rear of the cyl heads and the open ends go toward the front or else the engine will overheat quickly because the hot water can't escape. It sounds ridiculous but don't laugh it happened to someone on here whos engine was rebuilt by some 'professional'. LOL!

I used the Fel/Pro marine gaskets for this engine (Pre-Vortec like yours) and they sealed fine.

 

[Lou C]

guess you're up to this point.
what intake gaskets are you using?
I picked up a pair of reman 4.3 cyl heads (same casting #s as my originals) for about $450 for the pair. I was able to reuse the lifters, pushrods, rocker arms etc.

 

[Rivergator]

Nice heads, Lou. I am using the Sierra 18-1275 gasket set. I got it all straight in my head now and I am ready to put the top back on. Just out of curiosity, why not put a thin film of aircraft sealant on the whole gasket instead just the water outlets? Wouldn't that eliminate the possibility of intake air or vacuum leaks?

 

[Lou C]

You can, I don't see any harm it could do but Fel/Pro in their instructions says to put on dry. I used the sealer because there is often a little pitting around the water outlets on the manifolds, even on old heads that have not been redone.
I got the heads from a local machine shop here in Long Island. Remember to use a thread chaser on the threads of the bolt holes in the cyl head. They can get clogged up with crud due to raw water cooling. Before installing the heads make sure each bolt will thread all the way in. I used ARP head bolts because my originals were too rusty to clean up and re-use.

Felpro Marine Intake Manifold Gasket Set 17310: Advance Auto Parts
I used these and the Felpro Marine head gaskets.

 

[Rivergator]

Thanks Lou. Yes, I cleaned up everything thoroughly, all holes, all threads male and female with my tab and die set, then vacuumed it out and finished it off with a blow of compressed air.

 

[Lou C]

don't forget to use sealer on the head bolt threads. I used Permatex Aviation.

 

[Rivergator]

don't forget to use sealer on the head bolt threads. I used Permatex Aviation.

The heads I did some time ago but I used Permatex 2B sealer on the bolts. I hope that was ok. If not, I can do the head bolts over again without having to take anything apart, thank God!

 

[kenny nunez]

The sealer you used will work fine. I have used it many times. The main thing is you used sealer.

 

[Lou C]

Agreed that’s the main thing it’s not a super critical spec esp with raw water cooling there is not much pressure in the system.
I’ve see Merc Perfect Seal, OMC gasket sealer, Permatex thread sealer and Permatex Aviation all recommended for this application. All are non hardening sealers as far as I know.

 

[Rivergator]

Thanks Kenny and Lou. Looks like all is good! Next thing, the distributor and the thermostat housing go back on. Both have been serviced and are good to go. Btw. those of you who said I should take a closer look inside the carburetor were absolutely correct. What a mess! I am overhauling both carburetors right now (I happen to have 2 Mercarb 3304-9565). Unfortunately both have bad power pistons, one is totally stuck and the other one has a broken spring. If anybody has a spare Mercarb power piston (not the same as Rochester) let me know. Either way I will try to free the stuck one carefully. I couldn't find one on the internet. Btw. these Mercarb carburetors are easy and fun to work on. Great, simple design IMO.

 

[Scott06]

Thanks Kenny and Lou. Looks like all is good! Next thing, the distributor and the thermostat housing go back on. Both have been serviced and are good to go. Btw. those of you who said I should take a closer look inside the carburetor were absolutely correct. What a mess! I am overhauling both carburetors right now (I happen to have 2 Mercarb 3304-9565). Unfortunately both have bad power pistons, one is totally stuck and the other one has a broken spring. If anybody has a spare Mercarb power piston (not the same as Rochester) let me know. Either way I will try to free the stuck one carefully. I couldn't find one on the internet. Btw. these Mercarb carburetors are easy and fun to work on. Great, simple design IMO.

i think you should be able to take the spring off the stuck one and put it on the unstuck one they are just staked into the airhorn so you should be able to pull the stuck one out may free it up to use in the other one .

i like the mercarb too , if you consider its lineage back to the rochester 2 jet that came out in 1955, to the TKS that was used until 2015it pretty amazing. They work great if taken care of, cheap and plentiful.

 

[Rivergator]

The stuck one is the good one actually, nice, shiny and clean but stuck. But no longer a problem. I unstuck it already. So if all fails I will be able to make one good carb out of the 2. The one with the broken spring will be my next experiment or challenge so to speak. I will try to remove the stem from the piston, make a new stem (finding the right spring will do some research) and put it all back together and hopefully make both carbs whole again. Btw. if I had a lathe I would manufacture a small run of 200 or so new Mercarb power pistons and sell them, because according to Mike at Mike's Carburetors that part is nowhere to be had.