3.7 with a knock - Questions!

Greets all. I'm helping my sister and brother in law sound out a variety of boats with the intention of buying a cuddy/cabin cruiser in the 27-30 foot range for travel on the Trent Severn waterway system.

We looked at a 30 foot cruiser today, 1989. Quite impressive condition for the year, obviously well cared for all it's life, and we were impressed with almost everything. Unfortunately when I lifted the engine compartment the interesting part began.

I knew it had the infamous 3.7's. I'm well versed with engines, I've rebuilt more than a few from the ground up, my 6.5 turbo diesel being the last about a year ago. I've done a lot of reading on the 3.7 and know the issues with the charging system, cooling, etc. This is a last-year 3.7 so as I understand it the cooling system was the larger and more reliable one on this year/model.

The starboard engine starts and sounds good...absolutely nothing I'd be concerned about, however when we started the portside engine I immediately picked up on a knock. It was difficult to pickup exactly where the knock was coming from, that was the worst part, but with so little external on this engine, one must assume it was internal.

Seatrial was totally uneventful. Engines run up nice and smooth under load, synchronize nicely, power output seems even across both engines, and oil pressure was consistent at both idle and high RPM.

So, first off...for those who know these engines, a knock can't be a good thing, but any further insight?

Our idea is this - make an offer on the boat with the assumption that by next spring we will need to rebuild/replace the port engine, and price our offer accordingly. From what I can see, I think I can do a rebuild for between $750-$1500 in parts, so long as the crank is salvageable - the big question given it has a knock, and if it's a main bearing...what exactly is going on in there.

That said, crate engines can be had for $2500 or so....and that's a possibility that they are prepared for and will budget for with a contingency fund.

They are NOT racers who will ever likely run these engines wide open - looking to putter from place to place with the occasional mid-speed cruising. My thought is the engine will probably last until the end of the season (the knock doesn't sound like it's reached the "gonna throw a rod out the side of the block" stage), and then the engine could be replaced in the winter months.

So, thoughts?

Re: 3.7 with a knock - Questions!

There is a chance, just a chance that it may not be a knock. There are two small plugs in the head, one fore and one aft. They were for the thermactor smog pump on the auto engine (460 Ford head). They can rust out and the escaping exhaust gas can sound like a knock. You can see the one in front, but you have to feel around for the one in back. Anyway with the engine running you should be able to feel the exhaust gas blowing out. $10 fix. Happened to me, and there is a thread around here somewhere about it with a video.

Aha! Here it is: http://forums.iboats.com/mercruiser...ves/doesnt-sound-good-609422.html#post4247650 does it sound like this?

Re: 3.7 with a knock - Questions!

Hmmm....Now THAT would be awesome.

Initially I wondered if it was an exhaust issue and I did run my hands along the exhaust manifold and such to see if I could feel anything, but it was pretty cursory. I will definitely look and feel closer tomorrow.

Re: 3.7 with a knock - Questions!

Just watched the video....I can't say for sure if that's an exact match to the sound, but it certainly could be.

So feeling around by hand at a mid idle it would be immediately evident by feel alone if this was the issue?

Re: 3.7 with a knock - Questions!

Welcome to iboats! Given all that you know about the 470, steer away...

Re: 3.7 with a knock - Questions!

bonzoscott said:

Welcome to iboats! Given all that you know about the 470, steer away...

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I've read this here and there, but I'm not sure I buy it. Like any engine, those who have had bad experiences with them suggest others run away screaming, but aside from the issues that need addressing (alternator conversion and cooling system upgrades for the earlier models, not an issue on this one)...it sure seems that they're otherwise pretty reliable engines which get good fuel economy on top of it all.

If they were known for cracking blocks, blowing headgaskets, breaking cranks, or something other catastrophic I'd be more likely to heed those words, especially considering I'm the one doing the homework on behalf of someone elses purchase. But, I don't see those issues. I see a lot of little (characteristic) issues that are overcome by someone who is willing to either proactively (most importantly) maintain the engine, as well as do some simple relatively low cost upgrades - like the alternator conversion.

Yes, the port engine on our prospect COULD have bigger issues, so I realize I'm being a little hypocritical here, but if it's a swap out situation and the boat goes for another 1000 hours relatively trouble free...why steer clear?

...or is there some elephant in the room that I'm just missing?

I experienced much the same when I bought my GM 6.5 diesel. LOTS of people have had bad experiences with them over the years as a result of mostly small stuff that creates bigger problems, but the engine itself is remarkably simple and durable if it's maintained, and some simple and cheap upgrades are done. The result was that, to this day, there are people who scream "RUN AWAY, Run away screaming!" when someone mentions buying a 6.5 in certain circles...but in all honestly, it's not justified.

Re: 3.7 with a knock - Questions!

They are very prone to blowing head gaskets. The open deck design does not lend itself well to an overheat. Overheats of course could cause the engine to mix some coolant with the oil and create a milkshake that can cause a rod to knock and blow if it's left to knock... Nobody pays much for broken 470's.. Also another consideration is if a full repower was in order, many boats with twin inlines are spaced so that only inline engines can go back in without more work.. Only other inline is the 3.0L.. It's not a really bad design just not great. As I understand it, they were only discontinued because Ford folded on the parts supply for the crank, head and pistons and the 4.3L Chevy was a comparable better option anyway

Re: 3.7 with a knock - Questions!

There's plenty of room in the engine bay for a potential repower down the road, that was something I thought of.

Headgaskets, yes, I read about them, but again, it seems to only be an issue during overheats, correct? This boat is unlikely to be spending much of it's future much beyond 1/2 throttles with few exceptions, and of course when they are run beyond I will stress the importance of watching gauges.

I've spent quite a bit of time looking at the aftermarket parts supply for these engines and it seems relatively plentiful. The local Mercruiser dealer that I have ties to and spoke to a tech was actually surprised at the number of parts that were still available by Mercury themselves, and what's not (there is some glaring absences, admittedly) seems readily available online.

With reman crate engines in the $2500 range, that's what's got me thinking that even if it comes to that...it's an economical possibility to just swap the potentially problematic engine out and be done with it.

Most of the bigger boats I've seen with this engine are of course the late 90's, but more importantly, seem to have in the 1000 hour range on them - again, if the engine was a total turd that was prone to blowing up every weekend I don't think there would be many 1000 hour boats out there with them in the engine bays. That's the thinking I'm going along.

Secondly, I have a GREAT friend with a well equipped shop, and rebuilding an engine ourselves isn't out of the question - heck, I think three people could lift the thing out of the engine bay with just a few ropes, much less a hoist.

With a parts-only rebuild probably being in the $1K range (like I mentioned earlier), my fears are somewhat lessened. Heck, even oversized mains are available for these things in the aftermarket, so even if it is a main that's wearing on the crank, get her ground and polished .030 over and throw her back in. If the crank comes back bad, crate engine.

I can TOTALLY understand that someone who doesn't know a spark plug from a distributor would potentially soon gain a "run away!" feeling for these engines, but having spent some time investigating and reading about them, they seem like they're a decent little powerplant that just can NOT be abused and expected to survive it unharmed.

Re: 3.7 with a knock - Questions!

Sounds like you are determined and as long as you are comfortable with all the input, go for it. I myself currently own a dreaded OMC and think that if you keep an eye on things you have success. You could possibly have many more sister visits in the future.

Re: 3.7 with a knock - Questions!

You said oil pressure is consistant! What are the numbers and how much drop when it gets to temp? A bearing going bad you would notice a good drop in pressure when it got to temp!

Re: 3.7 with a knock - Questions!

Oshawapilot said:

Headgaskets, yes, I read about them, but again, it seems to only be an issue during overheats, correct?

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My 470 is from 1978 and the head gasket has never been changed... Correct, don't go above the max operating temp and you'll be fine. If only it had an aluminum head I think it would be more tolerant with overheating. Good luck.

Re: 3.7 with a knock - Questions!

Mine is an 87, replaced head gasket once. Never overheated that I know about, so it can happen for no apparent reason. The head gaskets will blow if overheated. The engine will overheat if neglected! A well maintained 470 will never overheat if the cooling system (even with the 3" exchanger) is kept in good shape (unless you pick up a plastic bag or something on the intake and that is just bad luck). Keep the system full of 50/50 antifreeze, keep the exchanger clean, make sure that you have good raw water flow out through the exhaust elbow and change the impeller every other year and you should be good to go for a long time. Keep an eye on the temperature gauge and if it starts to creep up, find out why. Yes I am a 470 fan:facepalm:

Re: 3.7 with a knock - Questions!

Thanks guys. Glad to hear from both sides of the story, and from actual owners.

The boat was purchased today. Ironically, in the craziness that was the first day (between purchase, hand over, and "hey, it's our boat, woohoo!" festivities, I never actually lifted the cowl when the engine was running and checked to see if it was (or was not) the rotted plug issue causing the noise.

A few hours after handover was complete and we had oriented ourselves, we put $400 of fuel in it (which appears to be about half tanks, ouch) and went out for a few hours. I observed temperatures and oil pressures like a hawk today and noticed absolutely nothing out of the ordinary - actually, both were exceptionally good with both engines maintaining about 170-180 degrees and not a touch above, and oil pressures cold in the 60 range, and even at idle hot well over 45 on both engines, even the one with the mystery noise.

So, here's hoping that it is just a rotted plug, and not a main/rod bearing on it's way out - I would have expected to see more oil pressure symptoms of such if it was such, though, and it was rock solid all day long despite a few hours of running.

Fingers crossed...but even worst case scenario we went into this knowing and accepting the possibility that they'll be into a crate engine or rebuild before next season.

Re: 3.7 with a knock - Questions!

Welcome to the "470 Club":facepalm:

Re: 3.7 with a knock - Questions!

stonyloam said:

Welcome to the "470 Club":facepalm:

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Thanks, welcome by brother and sister and law though, it's theirs, not mine. I'm just the boat guy helping them along this journey. No, they're not members here, although I think one of them is reading this thread as I post on their behalf.

Re: 3.7 with a knock - Questions!

stonyloam said:

Mine is an 87, replaced head gasket once. Never overheated that I know about, so it can happen for no apparent reason. The head gaskets will blow if overheated. The engine will overheat if neglected! A well maintained 470 will never overheat if the cooling system (even with the 3" exchanger) is kept in good shape (unless you pick up a plastic bag or something on the intake and that is just bad luck). Keep the system full of 50/50 antifreeze, keep the exchanger clean, make sure that you have good raw water flow out through the exhaust elbow and change the impeller every other year and you should be good to go for a long time. Keep an eye on the temperature gauge and if it starts to creep up, find out why. Yes I am a 470 fan:facepalm:

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In my dads case, the water pump seals went and it wept out all the A/F into the bilge and it wasn't noticed until the gauge was pegged. Limped to the dock after keeping water in it. Fixed the WP and it never got hot again, but not long after it started knocking. $2,500 for a new engine was unreasonable to my dad and he sold the boat non running for much less than he paid. Now he has a 120 Mercruiser powered Starcraft Super Sport

Re: 3.7 with a knock - Questions!

I'm thinking that installing a warning indicator (preferably a big loud buzzer) of some sort is prudent on these engines - it doesn't seem that they were equipped with anything beyond a gauge...which is great, if you happen to look at them regularly, or of course, happen to look at them at the right moment when something is happening.

A big loud buzzer that goes off if an engine surpasses 195-200 or so would seem like a very worthwhile investment. Heck, our jetskis both half overheat warnings that are impossible to miss, so one would think that a huge cabin cruiser should have the same. I will look into the logistics of such.

Re: 3.7 with a knock - Questions!

Oshawapilot said:

Thanks, welcome by brother and sister and law though, it's theirs, not mine.

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Yeah but guess who's in the uhhh... hot water if something goes wrong.

Re: 3.7 with a knock - Questions!

Oshawapilot said:

I'm thinking that installing a warning indicator (preferably a big loud buzzer) of some sort is prudent on these engines - it doesn't seem that they were equipped with anything beyond a gauge...which is great, if you happen to look at them regularly, or of course, happen to look at them at the right moment when something is happening.

A big loud buzzer that goes off if an engine surpasses 195-200 or so would seem like a very worthwhile investment. Heck, our jetskis both half overheat warnings that are impossible to miss, so one would think that a huge cabin cruiser should have the same. I will look into the logistics of such.

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Achris posted a while back on adding a pressure sensor/gauge for the seawater flow as it enters the boat from the drive. With a faulty/worn drive impeller or obstruction over the drive's inlet, that alarm would go off before any damage to the engine would happen.

Re: 3.7 with a knock - Questions!

Here is a thread on the subject, but not the one I'm thinking about:
http://forums.iboats.com/mercruiser-i-o-inboard-engines-outdrives/cooling-system-alarm-559511.html


edit: here it is:
http://forums.iboats.com/mercruiser...ressure-alarm-stern-drive-engines-390801.html