I have a 99 volvo penta 5.0GL with a fair amount of time on it. (~1500 hrs). All salt water. Was away this weekend and boat started hard at sandbar, I hoped was low battery from stereo. Checked oil when running back (was about 2 hrs north of condo) oil looked clear. Boat sat overnight in slip and this morning was hydro locked. Cylinder number one had fair amount of water in it. Sprayed wd40 in it and got towed to ramp. Compression test shows 175psi in that cylinder and about 150 in others. (Last time I checked was about 150 in all.) Maybe #1 is higher cause I hosed it down with wd40. Manifolds and risers are about year old GLM, I had both risers off about 2 months ago to do steering ram replacement but has been fine for about 50-60 hrs since it was apart. Boat didn't overheat. You think there is chance it is just riser gasket? Seems like center cylinders would be first to get water but maybe those exhaust valves were closed? I guess tomorrow rip manifold and riser off and go from there. Do people make block off plates to pressurize them? If it's head gasket it's probably long block time with that many hours on her.

At 1500 hours...all salt water, I would not be surprised at anything letting go. Start at the manifolds...then take a close look at the heads and block. I have seen Small Block Chevys and Fords loose water jackets and cylinder heads down here with half that time. Good luck.

water port and gasket seal of the intake manifold, I take it this a NON closed app ? I would have put closed cooling on this along time ago for saltwater use.

Correct, it is raw water cooled. I bought it used and I have always heard if it starts life as raw water cooled you can't convert it .

Worked on it this week. Removed the exhaust with riser intact and filed it wait water and let it sit and it stayed dry. Then rigged up pressure gauge to block drain and pumped water to riser water feed hose and plugged other side. . With about 10 psi saw the leak. Water leaking into exhausted valve area around the guide . So it seems for sure I have cracked head. Halfway thinking about doing top end but with 1600 hrs of salt water maybe a new long block is proper route.

At 1600hrs in salt I'd say that you should be three long blocks into it by now, lol. I'd scrap pretty much everything that you have so that you don't sabotage the new motor, and find a drop in unit already rigged with closed cooling.

I leaked manifold and riser and they checked out ok. Then plugged the manifold riser exit hoses and rigged up water and pressure guage to the block. Let sit with about 10psi and water leaking thru head, hard to tell but looks like it's by the exhust guide boss on the number one cylinder. I halfway thought about two reman heads but with the time on the motor probably not wise. No way to put it in and out and never shaped complete one before but found local shop that said if I disconnect everything he will pull it out and sit in in truck and back for like an hour each way.

Brand new GM marine longblocks are between 2500 and 3000 bucks. I wouldn't mess around with the old engine with all the salt water hours. You could also get a 350 for the same price or a bit cheaper although you would have to change up the fuel induction to match. Add a closed cooling system, and good to go.

Of course you could also find a good running take out. I would do one of these options before buying a reman from some online site, they never seem to work out.

ditto!!!!!

I would replace the manifolds and risers while you are at it. Even though they pressure tested as OK, you never know how many hours they spent in salt water, and they might be paper thin from corrosion. Would be a shame to trash a brand new motor over something like this. A set of GLM's would work just fine on a stock motor. Or you might be able to find a clean set of freshwater takeoffs for a good price.