Absolutely frustrated with this Mercruiser engine

[Ericinmich]

Its a crossplane like i have written , never said its a flatplane.

I mentioned it because the crossplane V8 has its typical 'rhytm' where indeed you can judge the overall rpm,s from this rhytm frequency . But judging at 2000 rpm if all cylinders fire or not , especially when it finally goes thru one underwater exhaust is vodoo science for me.

My bad on the flatplane, eyes don't read well on phone.

Not sure if you have driving the 8 to 4 cyl AFM on GMs trucks, but you head that change in frequency when it switches. Not sure why the same wouldn't work on a boat engine.

 

[markrsimon]

My bad on the flatplane, eyes don't read well on phone.

Not sure if you have driving the 8 to 4 cyl AFM on GMs trucks, but you head that change in frequency when it switches. Not sure why the same wouldn't work on a boat engine.

The GM AFM also includes collapsable lifters, which close up those other 4 cylinders, in effect disabling the valve train for them, thereby taking the energy from the compression stroke and allowing it to push the piston back down. I’ve had to replace two of the lifters - not fun - but Ive gotten 260K miles outta the truck.

 

[rustybronco]

I haven't read the entire thread.

My question is, what controls the total timing? ECM or distributor? Just curiosity as I don't have the manual.

Have you checked for total engine timing when the engine does rev to red line unloaded? I don't know if the manual lists total timing in addition to the base timing but a cursory search for other 7.4L GM engines should give you a rough idea what it should be.

 

[techmitch]

I'm still thinking about a plane ride.

My bad on the flatplane, eyes don't read well on phone.

Not sure if you have driving the 8 to 4 cyl AFM on GMs trucks, but you head that change in frequency when it switches. Not sure why the same wouldn't work on a boat engine.

The GM AFM also includes collapsable lifters, which close up those other 4 cylinders, in effect disabling the valve train for them, thereby taking the energy from the compression stroke and allowing it to push the piston back down. I’ve had to replace two of the lifters - not fun - but Ive gotten 260K miles outta the truck.

That God awful system wrecks havoc on torque converters!

PS The 2 bailed out bitch's use it.

 

[Scott215]

My 2018 Silverado has it knock on wood so far so good only 34,000 miles though

 

[Scott215]

I don't think total timing would stop it from going over 2,000 RPM it wouldn't make any power but it should rev up past that advance just comes in to make the power curve at a higher RPM pulls it out for less detonation

 

[ttmott]

It appears with everyone's help the fuel, ignition, and electrical has been taken out of the equation.
Remember this engine had a significant issue with water intrusion. @Hoplite808 pulled the heads and had them rebuilt but ran into issues with the valve train. The valve train issues were then corrected. The engine has never recovered from the initial issue.
I'm back in the camp that there are rudimentary mechanical issues remaining. Several pages ago I and a couple of others recommended to do a cylinder leakdown and cranking compression test with the engine at operating temperature. Was this ever done?

 

[scoflaw]

Camshaft ID, condition and valve timing were never addressed.

 

[aerobat77]

Nobody can judge from an online forum if the engine is mechanically sound or not so we have to assume it since hoplite claims it.
If not and in fact the culprit sits mechanically it was a nice brain storming for us but a lost season for him .

How the hell can we say from here if the timing is a teeth off or not ?

 

[Stee6043]

Well I found your problem.

You're parked with all sailboats!!! No wonder you can't find anyone to help

 

[ttmott]

Camshaft ID, condition and valve timing were never addressed.

From his past thread the bottom end never came apart so skipped chain is remote but heck what hasn't been looked at on this engine besides these things? It could be a wiped cam but these are roller setups..... The water intrusion was pretty severe so rings and cam are about the only things not investigated.

 

[Soulshine]

Consider just cutting your loss and selling the boat buy something smaller that runs just to get out on the water maybe a few years down the line step back up to a bigger boat this thread is killing
,me I haven't even got out yet I'm hoping to get out this weekend.

Or put carbs on it.

Spend some to avoid losing many.

 

[ttmott]

One thing not discussed is to cut open the oil filter and investigate for metallic things. A wiped cam will show up there.

 

[PlayDate]

Complete, comprehensive and proper diagnosis is 85% of the job. Any body can throw parts at it costing ton's of money unnecessarily. I wouldn't even hook my scanner to it with those low compression numbers. Any fuel injection system is worthless without a good base engine.

Nostradamus on Page 1.

I hoped it was something simple…..but wishful thinking doesn’t always work.

No one ever said about a Sea Ray: “it is overpowered”. In fact getting 8,500 on plane using a single 7.4 engine is a big ask.

I have two 7.4s that came from SR rated at 310 hp each that took a while to get 15,000 pounds of boat on plane. The only thing that changed that was bumping the horsepower up to near 400 hp and adjusting the prop pitch accordingly.

Without healthy internals…..the OP’s boat can’t get over the hump of planing the boat. As I said before…..he needs every pony to get on plane.

 

[Scott215]

@ playdate. I agree with you , that's a lot of weight. Internal issues could be affecting this but only one real way to find out, complete tear down

 

[Overvolted]

Nostradamus on Page 1.

I hoped it was something simple…..but wishful thinking doesn’t always work.

No one ever said about a Sea Ray: “it is overpowered”. In fact getting 8,500 on plane using a single 7.4 engine is a big ask.

I have two 7.4s that came from SR rated at 310 hp each that took a while to get 15,000 pounds of boat on plane. The only thing that changed that was bumping the horsepower up to near 400 hp and adjusting the prop pitch accordingly.

Without healthy internals…..the OP’s boat can’t get over the hump of planing the boat. As I said before…..he needs every pony to get on plane.

Is he using his trim tabs to get on plane/over 2000 rpm? My boat will not get on plane without them. I initially did not know this and asked an old salty grey beard about it and he said trim tabs everytime to get on plane. A couple of taps on the switches and im on plane in 30 seconds or so.

GM did not always put roller cams in every block after 1986. I have an L98 I tore down and there was no roller cam. It is drilled and tapped for one though. Something about GM emptying parts bins during assembly. Could Mercruiser have done this as well?

 

[aerobat77]

That all reminds me that my happiest day in boating was when i sold my 06 240da for a premium price in the times of the pandemic .

The worst hull i ever had and the merc 5.0mpi/b3 was maybe not as bad as the boat but also not troublefree.

 

[Hoplite808]

@ttmott you keep saying there was severe water intrusion into the engine. I have found no evidence to suggest such a thing. I assume it's because of the one photo I had previously shared where I had spilled a small amount of sea water on the heads when I was removing the exhaust manifold and had the valve cover off already. The rust in that photo was from me failing to wipe it all up right away and was just surface rust. That was all cleaned off during the head job. I did not pull the cam out but I have a lifter tool and while the top end was apart I pulled each lifter out and inspected everything. I found no evidence of damage or excessive wear to any of the cam lobes or the lifters. I did have two bent push rods from the seized valves that I replaced. When the heads were off I inspected all eight cylinders and found no obvious damage to the pistons or cylinder walls. There are also no signs of any excessive blow by getting into the block either to support the idea that the cylinders are a problem. The crank turns free and easy and there did not appear to be any play or knocking to suggest that a con rod bearing or crank bearing are going bad. The oil is clean and free of metal shavings I had even cut the old oil filter apart to double check for shavings in the filter, and there were none. The oil also has never shown any signs of water contamination at all. If it were the timing chain I would expect the engine to run horribly throughout its RPM range. Instead it idles smoothly at the appropriate RPM and accelerated normally and feels normal until it gets to 2000 RPM's in gear then it just simply won't accelerate further. Which gives the boat a top speed of about 10 knots on smooth water. This project has beaten me and I'm just going to let it go for a song and the next owner can at worst case scenario repower it and still have a good deal. I've already found another boat that works and is powered by a pair of good reliable Perkins diesels to replace this with now anyways. As soon as I can sell this I can get back on the water.

 

[JayhawkCurtis]

@ttmott This project has beaten me and I'm just going to let it go for a song and the next owner can at worst case scenario repower it As soon as I can sell this I can get back on the water.

Make sure you give a link to this thread to who ever you sell her to so they keep us updated and we can see how the saga ends.

 

[Hoplite808]

Per J. D. Power the low retail blue book is $22,040 with the options I have installed and the suggested list price is $58,462. With the new bimini, outdrive, charger/inverter, and other electronics and a few other things do you all think it's unreasonable to ask for $22,000?