(Long post) 99' 7.4 MPI Merc exhaust mani's/head issues

[collawash]

I'll try to post some pics later...

I have 750 hours on this original engine. It's mostly used in salt, but with a full closed cooling sytem. I do a lot of long 20-50nm offshore runs so I'm up on plane for the duration. I'm never WOT for very long, but heavy with gear and hopefully fish on the return. Much of the time is trolling or drifting at idle. Runs fantastic, no hiccups, issues, nada...just been a great engine to say the least. I run a 160* tstat. The only overheating issues ever was a cracked raw water housing that allowed air into the cooling system. That was when I bought the boat on '03, but nothing since.

The last few years, the exh mani's were getting flaky at the exhaust openings, underneath. I know these powerplants tend to run warm and I did have saltwater dripping on them at one point in time. With 750 hours, it was time for new ones. My ring gear lost a tooth last year so now was as good as time as any to pull the motor for the first time. Before I started, the compression check was 152-153psi on all 8. When I bought the boat in '03 with 93 hours, they were all 155psi. Woohoo...so I figured!!! I ordered up some GLM's mani/riser kits and y-pipe o-ring. Yes, I've heard the GLM horror stories...but I have several friends who have used them with great success. All For the price I paid, it still an awesome deal even if they need resurfacing out of the box. Figured now was the time to change out some other components to have spares for the future too.

Yesterday, I pull the engine. I knew the water shutters were toasted so no surprise there. I was shocked to see the condition of the metal on the exh mani bolt heads and cyl exh port openings. Rust and flaking was horrible. The metal was splitting and flaking just like underneath the mani's. They looked like an overheated brake disc. The exposed cylinder head bolts looked like the exh mani bolts. Both port and startboard sides are the same. After 3 hours of drilling and grinding, I got one mani off. The head mounting surface is horrible but intact even with flaking metal. At a minimum, I'll have to pull the heads and TRY to have them resurfaced, but I don't think they can take that much off. Even if they can be trued, I suspect the condition of the heads at the exhaust may put me on borrowed time.

I need some advice:

1) Should I even bother with the current heads or just replace?
2) What can I do to reduce the temp/heat out the exhaust? Lighter loads? Different prop?
3) I can buy a 330hp-425hp MAG reman long block for $2600-$3100, 7 year/500 hour warranty from a reputable local company.
4) I'm leary of changing heads because of past experiences. Seems like head jobs are shorter term fixes. New heads tend to hurt existing rings sooner than later.

ANY INSIGHTS ON THIS IS GRETALY APPRECIATED, GOOD OR BAD! Just want to make an informed decision. This site has ALWAYS directly and correctly guided me through many a repair, but now it's on my own boat!

Kevin

 

[collawash]

Re: (Long post) 99' 7.4 MPI Merc exhaust mani's/head issues

Digital camera problems, I'll take some other photos with another camera tomorrow.

A little more info...

The risers look great even though they've had saltwater ran through them for over 600 hours. Truth be told, I could easily reuse them with confidence.

I was looking at the top of the exhaust ports on the heads where it connects to the manifold. The flaking metal extends about 1/4-1/2 from the edge. I pulled off a piece that was about 1/16" thick. Just looking inside the exhaust passage, below the flaking metal, there does not appear to be any damage. I put a socket to the head bolts, but the metal just flaked off. If I pull the heads to have the exhaust manifold mating surfaces resurfaced (and heads if needed), how much can actually be removed? .25mm? Less? More? Obviously I don't want too much or I could bottom out the bolts. Even tho' my compressionwas good, I'm concerned about cracks in the head between the valves which seem to be the norm if it occurs.

I'm already leaning towards the reman'd motor, especially if I can get more HP. I'll be calling the factory tomorrow for guidance too. I'm no metallurgist, but it seems the system should never cause the heat damage it did. Unless I find out why these things happened in the first place, I can only assume the replacements will do the same over time? I know others with the same powerplant who have replaced manifolds, but none have seen this type of rusty flaky metal, bolts. Thinking out loud...what about running a 140* tstat instead to keep the coolant/manifold temps down?

Yes, a picture is worth a 1000 words, will try again tomorrow.

Kevin

 

[HT32BSX115]

Re: (Long post) 99' 7.4 MPI Merc exhaust mani's/head issues

Howdy,


If you decide to get a reman engine or a long block, you might take a ride over to Kent and check out the engines that USENGINES does. they do a pretty nice job...


Regards,


Rick

 

[Bondo]

Re: (Long post) 99' 7.4 MPI Merc exhaust mani's/head issues

and I did have saltwater dripping on them at one point in time. I know others with the same powerplant who have replaced manifolds, but none have seen this type of rusty flaky metal, bolts.

Ayuh,... You're dealing with Salt rusted metal,...
Motor Heat had little to Nothing to do with the current condition of the motor...

 

[Philster]

Re: (Long post) 99' 7.4 MPI Merc exhaust mani's/head issues

Thinking out loud...what about running a 140* tstat instead to keep the coolant/manifold temps down?


Kevin[/QUOTE]


Argh, a 140* t-stat opens at 140... and 160 opens at 160. They affect how the engine warms up. Once the t-stat opens, everything else is the same. A 160 doesn't make an engine run hotter than a 140. Some engines just won't warm up right with a 140, so they should have 160, which holds off the cooling fluid a little longer. In some engines, if the coolant flows at 140, the engine temp might drop or struggle to climb, and this is not best for the engine.

So... if you have an engine that just won't warm up and get to optimum operating temps, you might need to check to see if there is a thermostat (some dummies remove them) or if the thermostat is too 'cold'. If the rating is 140, and the engine won't warm up.... you go up to a 160.

 

[collawash]

Re: (Long post) 99' 7.4 MPI Merc exhaust mani's/head issues

No photos yet, at work today...

Thanks to everyone for their responses. I?m a bit frantic having a boat on the trailer and the engine on a stand in the garage. You might say I find this situation a tad uncomfortable. So I spent hours browsing the iboats forums last night looking for similar issues. Several other 7.4 owners obviously have the same rusty mani?s bolts issues. I?m concerned about the rust damage on the top of the head?s exhaust ports, but I know it would not affect performance. Keeping saltwater off of them in the future is a major priority. Maybe a little less aggressive spraying with the washdown hose wouldn?t hurt either, but tuna blood sticks to everything.

It?s painfully obvious that my engine hatches need modification to prevent saltwater from dripping down on the engine. Should of have done this years ago in hindsight, duh! On my boat, it?s not the hatch design, but rather the water absorbing sound deadening open cell foam liner the factory put in. Looking from within the engine compartment, the foam drips water right between the riser and head. Hmm, do you see a problem? I?m looking at some real closed cell neoprene weather stripping to solve this issue. I?ll be ripping out the current foam very first thing. I?m thinking Silent Running coating would be a better option. I?ll do a separate post on this later. Someone else must be having the same problem like mine. Hopefully I can share the wealth like everyone else does.

Pulling the heads and resurfacing is the much cheaper option, but it will take more time. With work and overtime, time is not one of my more plentiful commodities. Someone suggested I try US Engines which I?ve used before for cars, great success-highly recommend. They warranty their marine engines for 7 years/500 hours. Better than OEM reman?d motors. For $2895, they?ll remanufacture my engine. When completed, it?ll be a higher compression ?torky? 415hp motor instead of the low-compression OEM 330hp. I believe I?m only getting about 305-310hp at the prop right now so any increase would be good. For the same price, I can buy one ready to install long block with a $1000 core charge. Any higher that 415hp with the MPI will require reflashing the computer and maybe new injectors too. I?ll take more torque over speed. Realistically, running WOT at 34 knots doesn?t happen all that often on the Pacific.

Here?s my choices:
1) Remove, repair/resurface current heads & reinstall. More time involved, may find something else to repair, but keep OEM motor. Simply the CHEAPEST option, but may not be the best in the long run. No warranty whatsoever.
2) Remanufacture OEM motor, more hp when completed, MORE time involved with a 5-7 day turn around. Still have the rust damage on the current head, but will have a warranty.
3) Buy reman?d off the shelf without rust damaged exhaust port heads. Back on the water quicker, maybe by this wknd! Leaning heavily towards this option.

Yes, I write a lot but I type fast. Information (and pics) are key!

Let me know what you think.

 

[Bondo]

Re: (Long post) 99' 7.4 MPI Merc exhaust mani's/head issues

Ayuh,... With your story, #3 seems to be the Only Option....

For #1,... I'd just replace the heads with rebuilt used,... Just match the casting #s...
Ditto on option #2,... Used replacement heads...