Marine: Boating, Sailing, PWC, GPS and Navigation - Exhaust Manifold problem on 4.3L Merc?

I have a 4.3L Merc inboard. I had a crack in my intake manifold and I have replaced it. When I start the engine, it seems to experience hydrolock (sounds like it's sporatically undergoing load). It runs rough and steam starts to come out of the valve cover vent on the starboard side of the boat. I have taken out all the plugs and turned over the motor. It turns over perfectly even with the earmuffs connected with the water turned on. When I put the plugs back in and turn it over with the coil wire disconnected, after it turns over 2-3 times, it seems to experience hydrolock. There is no water getting into the engine, the oil is still clear. The compression is 120 psi in each cylinder and no water in the cylinders. I'm wondering if it could be the gasket on the exhaust manifold on the one side? Any other suggestions?

Welcome to the forums.

It could be just a gasket on your exhaust riser but if your intake cracked I would bet your block is cracked as well.

since it's hard to say over the internet I guess I would just put riser gaskets in it and see what happens. They go between the exhaust manifold and the riser. If it still hydrolocks I would pull the intake off and look for cracks in the lifter vally and inspect the heads.

Thanks. I had a boat shop remove and install the intake manifold and they did an inspection and didn't find any cracks. Also, wouldn't the oil start to turn milky if the their was a crack in the block?

Are the gaskets fairly straightforward to replace? I haven't done those before. The boat is only three years old

If the block is cracked the oil will turn milky and will be at a higher level than when you started. I'm assuming you live in a area that freezes.
The older intake manifolds are self draining but the new ones have a pipe lpug that has to be removed to drain the water.

The riser gaskets are easy to change. You will see 4 bolts on the top of the riser just remove them and loosen the hose clamps where the exhaust bellows attatch to the riser.

Most times I have seen riser gaskets go is from overheating but I learn and see something new everyday.

I hope this helps and keep us posted on what you find.

Thanks for the advice. Yes, I live in an area where it freezes - Edmonton, Alberta. Yes, my manifold had a pipe plug. I made the mistake of thinking it was self draining.

The oil has not turned milky since the intake manifold was replaced.

Thanks for the advice on the riser gasket. I appreciate it.

I'm not really sure how a riser works. If the gasket is leaking, wouldn't water be getting into the cylinder and the oil?

Actually, I was thinking about this further and I know that there was no hydrolock prior to replacing the manifold. It's only after the manifold was replaced that the hydrolock occurred. Do you think it's possible that the shop installed the intake manifold incorrectly and the gasket is leaking causing the hydrolock?

It is possible for the intake gaskets to be leaking into the cyl. causing it to hyd. lock. If the riser gasket is leaking it will also cause it to lock, when the motor is shut off the water thats in the riser will leak past the gasket into the exhaust manifold and if the valve is open it can get into the cyl.

If it didn't lock before I would look at the intake gaskets.

Thanks. I think I will start with the intake manifold because the motor ran perfectly prior to stalling from the water getting into it from the cracked intake manifold and there was no locking before it was replaced. Secondly, I've checked the plugs 3 different times and there was no water in the cylinders. Thridly, the motor probably only has about 60 hours on it and i haven't had any exhaust/riser issues up to now.

Now I don't know what to think. The hydrolock is gone completely now. I noticed a spark by the battery when I was turning it over. One of the battery cables was slightly loose. I guess there was no hydrolock. The engine fires right up but there is still something wrong. There is still a little bit of steam rising from the valve cover vents and it's still not running quite right. There is still no water in the oil or the cylinders.

A good test is to put it in the water and check your oil level. Run the boat hard for about 10-20 min. Then let the motor cool completly and recheck your oil if the block is cracked the oil level will be higher. The 4.3 motor will crack in the lifter vally instead of out the side like a 5.7 or 3.0.

As for as the steam that is something I will have to think about might be a head gasket, but if compression is all at 120 it doesn't sound like it.

I put my boat in the water yesterday and had it in the water for about an hour. It ran fine at trawling speed but when I opened it up it seemed like it was being held back. There wasn't enough power to get it going. I checked the oil and it was fine. The level didn't change and no water in it. Could something electrical be causing the lack of power? I bought the boat in 2005 brand new and the condensor went on the boat after one year.

I'm glad it didn't make any oil.:thumbup:

Check your dist. cap and rotor, alos take your water sep filter and poor it into a clear,clean glass jar it there is water in your fuel it will seperate. Those are the most comon problems with lack of preformance in the io's that I see.

I took the boat back today and had the timing set properly. It still has a knock in it at higher rpms. The spark plugs are all still normal, compression is still 120 psi in each cylinder and there is still no water in the oil. It has steam coming out of the valve cover vent on starboard side only. Does anyone know what could cause that?

Is it steam or does it smell like fumes?

this steam could be exhaust fumes from blow by from the valves.

if your valves or cam are worn or not seating correctly it will cause power loss and a hydrolock type of compression feel during start up. you will also get lifter/rockerarm chatter.