At my wits end with this damn motor. Its a 1997 HP500 Carb. Had the heads redone with new valves, springs, ect this winter. Changed cam over to a Teaque 525 cam hoping it would change the reversion issues the 500s were known for and I had.

Got it all together a few weeks ago and got it on a pallet running and fixing any little issues it may have had before I put back in the boat. Well Ive been chasing 2 things. First one is milking down the oil with water at a decent clip.....over a qt in 15 minutes or so judging by dipstick. Changed oil and tested again today to no avail. Same thing on dipstick. I have done compression test on all holes and all are within 5% or so of each other with highest being around 150psi. Have done a leak down test on even numbers and all showing great. Within a few psi of each other at TDC. I have pressure tested the block coolant passages by going in top hose with a garden hose with every other hose blocked off. Scoped bores while this was going on and not one drop of moisture to be found. I pressure tested it with air as well at 25 psi and other than a small hose block off fitting leaking it seemed ok. I have pressure tested the odd side exhaust manifold and no bubbles in the dip tank. Have not tested other side. I just now pressure tested the oil cooler at 90 psi in dip tank and again no bubbles. Im lost at this point. Everything was fine last year as far as I know. Just tried to stay on top of the maintenance since I just bought the boat and it had 330hrs on it and heads hadnt been touched. I do know the head gaskets on there when I took off seemed to be sharing compression into the other bores. Didnt notice driving it though. Ran fine. I didnt have the block decked as I figured it was ok since it ran fine and I wanted to keep the bottom end together if it didnt need it. Didnt notice anything unusual on deck. Used Hylomar (per local guy who does these heads) and a Cometic MLS HG with ARP bolts. Really starting to irritate me now as I dont know what else to check besides tearing it all down again and Im hoping I just missed something.

Second issue is Im having water drip just below the odd bank exhaust manifold above head gasket so doubt its that. Seems like its coming from the exhaust manifold and very well be since I just discovered the ARP head bolts are hitting bottom of the exhaust manifold prolly making it not seal all the way. BUT with it being dry there only thing I can think of is reversion which I thought being a 109 CL and 119CL intake and exhaust cam it would have helped that. I also see water seepage outside at the riser joint on even side. I can get my hands on a Gil dry exhaust for $1k from a local guy so may just do that if thats the fix.

Any suggestions or insight would be great as Im lost at this point.

Thanks,
Chris

Fyi: exhaust leaks can/will/do cause reversion.

So, you had no issues with motor reverting until rebuilding motor ?

What exhaust is on it now ?

What did the block deck surface look like? Cometic gaskets need a fine finish to work. If there were groves or scratches on deck, can be an issue.

Some say put cometic gaskets on dry, others say spray with copper spray. I prefer cool copper spray.

Stock Mercruiser Manifolds witj Gil risers. I can see what ur saying with the leak creating pulses. Main issue is the milky oil at this point though. Wouldnt it hydrolock motor if leaking that much into exhaust ports?

No grooves or deep scratches. Looked good imo. Ive done several on automotive applications and never had any issues with lots of boost. Just weird how all the tests show good.

Sorry didnt answer first part. Yes I had a little reversion with it before but not nearly as bad now. It would milk oil a little but not a qt or two in 15 to 20 mins.

Rule #1 with any issues :Fix known problems before chasing unknowns.

You’re exhaust leak is sucking water in.

Speaking of known issues, stock exhaust will revert with larger than black motor cams. Gill exhaust with non hp500 tall risers will revert with blue motor cams, thus why Merc changed design of risers for them.

Common water leaks in bbc’s is intake manifold gaskets. Not all intake gaskets line up correctly with all the different heads/intake manifolds. Always check how the head and intake manifold water ports line up with the gaskets.

You said heads redone. OE cast iron ? Were the guides replaced / redone ? Seats ? This can cause issues.

502 blocks have blind head bolt holes. People have run into bolts a tad too long and not clamping the heads down properly. Sure, torque maybe fine but that doesn’t mean it’s just from headbolt not being able to thread further.

So…….fix what is sucking water now (exhaust leak) and decide what is next to ck.

btw: hydrolock doesn’t always lock up a engine. I’ve had several engines apart that seemingly ran well have a bent connecting rod or two, which was caused by water getting in.

Will do. Will fix exhaust leak today and report back. Stock 088 heads. Guides and seats were good from what I was told.Only had 1 broken inner spring on factory springs.