Hi all,

Looking for some advice from anyone that is familiar with the Mercruiser 8.1 / 496 engines because I'm at a complete loss. I'll try to keep my story short:

Bought a boat last year with twin 370HP Mercruiser 8.1s Horizons. #OM319145. Cool Fuel 2, v-drive. Stock setup, no modifications. Survey and sea trail went well.

Shortly after purchase (still on the same gas that was in the tank when purchased), the port engine developed a knock. No loss of oil pressure, no alarms, no codes, no temperature changes. Didn't even notice any loss of power in the 3800 RPM range. Turned out to be a spun rod bearing and a melted piston in #5.
Had a marine technician install a new long block and got me back up and running. When pressed for a reason why something like this may have happened, the best explanation I got was "sometimes they just do this". Okay, not very satisfying, but he's the Mercruiser tech, what do I know?

Then, midway through this summer, one weekend I start the port engine and hear a knock. Once again, never noticed any codes, alarms, oil pressure, or temperature issues before. No loss of power. Same thing as before, except this time it didn't spin the bearing. I was no longer in the same area as I was the last time, and the technician had no interest in helping me, so I pulled and rebuilt the engine myself. No damage was found outside of the piston. It appears the top ring expanded beyond the room it had available, buckled, and blew a section of the top of the piston out in the vicinity of one of the valve reliefs. The piston itself had overheated to the point of binding on the wrist pin, causing the piston slap / knock. Installed a new piston, checked everything else out, and was good to go. This time, not wanting to engage in continued insanity, I had the injectors checked, cleaned, and calibrated for both engines. Some were stuck open, some where partially clogged. It made sense to me that a partially clogged injector resulted in a lean condition in one cylinder, eventually overheating the piston and ring, causing the damage. Everything back together, a good explanation of what caused the issue, happy rest of the summer.

But no, coming back from one last trip before being hauled out for the winter, this time I notice a loss of power on the starboard engine while running around 3600 RPM after maybe 10 minutes after pulling anchor and heading home. Come off plane, knocking in the engine, transmission rattling, here we go again. Except, yes that wasn't a typo, this time it's the starboard engine, not the port engine which was the one rebuilt twice before. I did have the injectors cleaned for both engines when I rebuilt the other previously. Didn't take anything apart yet, but the #5 cylinder spark plug is smashed, with a bore scope I see the ring land did the same thing as previously seen in the port engine, and this time there is considerable damage and molten looking metal all over the top of the piston. From the sound I suspect the bearing is spun as well.

So thus my predicament - how could I possibly be having the same issue, in the same cylinder, on both engines? Fuel filters were all changed less than a couple months ago. Separate fuel tanks. Fuel from different marinas. All injectors cleaned and tested less than 2 months ago.

I don't consider myself someone who abuses these engines at all, I always let things warm up before leaving the slip, climb up on plane quickly but not too quickly, and ever since the first blow-out I've been too scared to go beyond 4000 RPM so I always run either less than 1200 RPM at hull speed or around 3600 to 3800 on plane. I always use 87 octane gas and add Star Tron before each fill up. No performance issues have ever been noticed, nor any codes or alarms, temperatures always come up to around 170 degrees and stay there. No guardian mode. As they say, it runs great until it blow up.

Fuel rail pressure was measured last time the port engine blew, on both engines I measured 40PSI KOEO, dropping to 34 PSI after the pump stops running, 30 PSI at idle running and at 1000 RPM in neutral. It was the same on both engines so it seemed safe to consider this normal at the time...

If anyone has any idea what could be going on here, I would greatly appreciate any input. Until I can figure out a good explanation, I don't have any faith in the integrity of the port engine now either. I'm going to be too traumatized to ever leave the marina again!

Thank you in advance!

I'm thinking you should have low 40s like 43 psi for fuel pressure at idle. You've got 30 psi at 1000 rpm???


And typically when a piston fails on a 8.1/496 it fails at the top ring, it beaks a chunk off the top of the piston usually near the valve relief. There's not much material between the top ring and the piston face. Aftermarket forged 8.1/496 pistons usually move the top ring down to leave more material there before the first groove.

What year are the engines?

Certain 8.1s had an issue with the cool fuel system. The paint inside would come off and clog the injectors.

Thanks for the replies!



Yes, that's correct - on both engines. In hindsight maybe I'll be kicking myself, but it seemed OK at the time for two reasons:

1) it was the same on both engines

2) These are the only 8.1s I've ever owned, but they never seemed sluggish or underperforming. Would one not expect performance issues if the fuel pressure was 10PSI lower at idle and 1K RPM? That was my thought anyway. I'll retest when I get a chance.


I know both the boost and main pumps at least run. Any advice where I should look first? Perhaps trying a new regulator?



Yeah, that's exactly what happened on the port engine the 2nd time. I didn't see what happened the first time (tech never showed me). Also happened this time, with additional damage.

Not a great design, especially when it goes into a marine application. But it seems these are fairly popular engines used in a lot of boats of that 2003-2008 kind of era. I guess the design doesn't have much margin for when things go wrong...



It's a 2004 boat so probably 2003 or at latest 2004 engines. These have the Gen 2 Cool Fuel - the paint issue seems to be a Gen 3 Cool Fuel issue.

I do plan on taking the injectors to be tested again, just in case something clogged them up again. Previously it was varnish from ethanol fuel, so I doubt that happened again already.

You will have lower fp at idle/high vac than if the engine is under load.

Check the fp with the vac line removed from the regulator. If it tests good, you need to test fp at wot to see if the pumps are keeping up.

Do the engines reach recommended minimum RPM @ WOT? If not, it could be over-propped causing your issues. .

What does the spark plug look like from that cylinder vs the rest of them

I'll check with the vacuum line removed next time I get a chance. I should see the same pressure at idle with the vacuum line removed as I do with key on, engine off, correct?


The last time I ran it up to WOT was last year, not long after the first port engine rebuild. Both engines were at 4650 RPM. That seems to be in the expected range.

Haven't pulled more than the one in the bad cylinder yet. That one, besides being smashed up, looked like it has some carbon / black buildup nothing that seemed particularly "off". When I replaced all the plugs earlier this year, I checked to see if any looked different, and did not notice anything out of the ordinary.

Thanks!

I would think if you are melting the top off of the piston, you have to be running lean and detonating in that cylinder. A bad injector, small crack in the intake at that runner, head gasket between 5 and 7, could possibly be in the ECM tune. A little late now but pulling plugs at different RPM may show you a lot. Best of luck to you !

Could also be bad gas? Same gas for all the issues. Maybe it can be tested just to see how you need to pursue this.