Engine Rebuild -- What's Actually Involved
 

Hey Everyone,

I've got a 2003 Mercruiser 6.2, which has apparently lost compression in #2 cylinder, and is in need of a rebuild.

Here's the story:

About a month ago, launched the boat for the first time this year -- my ramp to open lake is about 10 minutes of idling / no wake zone. Boat was idling a bit rough, but figured this was just because it hadn't been run in a while. Get to open lake, try to get on plane, I have no power, and now my engine alarm is going. Idle it back to the ramp with the alarm going, turns out it was overheated -- my water pump was toast. Replaced, no big deal.

Next time out, boat is still running a little rough, and I'm getting a "banging" sound over 4000 rpm -- told it's probably bad gas -- and to take it easy until I can get it down real low, and replace with premium. Couple weeks later, that's done, boat running a little better, but still the banging over 3/4 throttle.

Back to the marina we go, and apparently I have no compression in #2 cylinder, which means engine out rebuild.

The quotes I've generally gotten (without anyone having taken it apart yet) tend to be somewhere between $4000-$5000 Canadian, plus removal & re-installation. My question is -- am I going to get taken for a ride here? I've watched my fair share of DIY car shows, to know all of an engine's moving parts and what they do. Let's say, I've got a bad valve and piston in #2 cylinder as result of some metal on metal contact. When we say rebuild, is the expectation that we're replacing all 8 pistons, rings, 16 valves, springs, rockers, etc? or is it sufficient to simply replace whatever is broken?

Also, if I'm having the engine out and apart, is there much that can be done to these motors to bump up performance, without impacting reliability?

How many hours are on the motor? If your up in the 600-800 area it might be wise just to replace everything. If you are less than 400 probably can get away with only fixing what's broken. Those 6.2's will run a long time. Also depends how long you plan on keeping the boat.

Less than 200.

Probably intend to keep the boat another 3-5 years, but don't want it to be rendered worthless after that.

until the engine is out&apart you don,t really know how much damage you really have.the prices you have gotten are most likley for worst case .if someone quoted you a price for a minor repair and it turns out the engine needs major repair most people would say,BUT YOU SAID 2000 BUCKS,WHY IS IT NOW 5000 BUCKS.my guess is that after you ran it as long as you did while it was overheated a complete rebuild is going to be needed.

Plan for the worst case, hope for the best outcome. Until the engine is out and apart you just do not know.

I see if you could get a bore scope in the # 2 hole to see if there is anything obvious, then pull the valve cover and take a look, maybe the intake, it might be able to be fixed without pulling the engine.

Makes sense.

Perhaps it's as simple as clearly communicating with the engine builder that the $5000 estimate I'll have to agree to assume that pistons/valves/springs/rockers all need to be replaced, when in fact that may not (and hopefully isn't) the case.

The amount quoted could be spot on, a little high, or low...bottom line is you won't really know until it's torn down. With an overheat and a cylinder down there is a fair shot a good rebuild is in order. You likely had an injector lean out and take #2 with it.

Get a good quality Pressure Tester and do a "Leak Down Test". This will tell you why you have no compression, then evaluate/reevaluate the situation. Just MHO.

Yeah -- the marina is doing that this afternoon... why they didn't do that as soon as they found out the compression was gone -- I'm not sure.

Doing some reading, seems like I'm hoping for something in the top end of the motor.

While I'm certainly in no position to debate this assesment (and fear it may be accurate), it does seem a bit strange to me. Even after the overheat, the boat was running absolutely fine, except with more than 3/4 throttle.

How do can you do a leak down if there's no compression in the cylinder, or did I miss something.

The leak down will isolate where the problem is resulting in the loss of compression. The air will escape either thru the intake, exhaust, or crankcase.

I understand how it works, but if there's NO compression you cant tell anything, could be a hole in the top of the piston, the OP said no compression not low

If there is a hole in the piston you will hear the air escaping from the crankcase (breather), instead of out the exhaust or through the carb if there were a stuck/broken valve. It might not tell you for certain that it is a hole in the piston versus a broken ring, but it would help to isolate the problem a little more and maybe tell you if the engine has to come out versus possibly pulling a head and fixing a valve.

I had a chunk come off of the ringland of #6 and it busted off a 1/8" piece from the intake valve. Boat ran fine from idle up until about 3800 RPM. If you tried to lean on it, it would backfire through the carb. I originally thought I had a weak fuel pump causing a lean pop, or an ignition issue. Only after a compression test showed about 80 PSI in #6 did I realize I had a major issue. When I pulled it apart I found a chunk our of piston #5 as well. Compression was fine in that cyl.

If you are truly getting zero compression it almost has to be a really big hole in a piston or valve.

I would guess warped head and blown gasket.

I think my heads warped for posting on here

Let us know please, the final verdict

What's actually involved? Money. Lots of money....

i am not sure how an engine can run fine when it has a dead cylinder.

Well if the motor was knocking and it was the marina that said 'just go easy on it', if you take it to that marina for a rebuild, it will most likely involve you dropping your shorts and grabbing your ankles.

Interesting -- sounds exactly like my symptoms -- which felt almost like a small backfire versus metal on metal contact. I also did notice a little bit of smoke coming out of the intake.

Obviously not what I'm hoping for given that means full rebuild.

That being said, I wasn't given the compression # over the phone, so perhaps the "no compression" I was told was very low compression.

It seemed to -- just not above a certain RPM. It also had a hissing sound at idle, once the motor was warmed up.

Update: Leakdown test apparently did not reveal anything through the crankcase, so they're pulling the head.

since you overheated it, you may have cracked a head and tuliped an intake valve. Just saying this to make sure they magnuflux the head because a crack can be very hard to see.

Marginal injectors may not manifest their problems until at higher throttle settings when the motor needs the fuel but the injector can't keep up. Cylinder goes lean, detonation ensues, timing is retarded which delays the combustion event, elevates exhaust temps. Intake valves begin to tulip, valves and valve seats take a royal beating as do pistons etc. You may or may not hear, or more likely feel, a slight miss at cruise speed.

Motor will also tend to lay down at the upper end.

What do you mean by lay down at top end?

Injectors are often overlooked. I just roached a piston in a 502mpi due to a #6 injector that flowed 24% less and had a chitty spray pattern. Luckily I caught it quick and could do a garage repair without full tear down. Still sucks though. Worst part is I blew it up at an easy sustained cruise not WFO. Just bought boat this spring and was already planning on full rebuilds this winter. Had notion to pull them when I first bought it and have them tested after learning how inconsistent Merc injectors are. Boat ran great every time until it didn't. Piston lost some ring land material, chunked pieces of top ring and embedded in top of piston. Heat cracked the exhaust valve. Got lucky with hardly any cylinder or head damage.

Injector testing WILL become a standard preventative maintenance item from now on.

betting there is a valve tulped bad or piece missing..

What does a "tuliped" valve mean?

cylinder overheats causing exhaust valve to become malleable and extrude.

Not pull the max RPM it did in the past, or well short of rated WOT rpm with reasonable load and prop.

Usually the intake valves tulip because of the larger diameter.

I had the exhaust seat come clean out of the head for the #1 cylinder to consume. After that it spread the cheer to 2 other cylinders on the opposite bank in about .3 seconds. Outcome was complete failure of exhaust seat, crunched #1 piston, broke all three rings, bent all three ring lands, bent wrist pin, breached hole in head near spark plug, drained water from breach into cylinder, spit pieces of broken valve seat upstream through the intake valve (when it opened) up through the runner into the plenum and #2 and #4 cylinder then ate pieces of #1's valve seat. In addition the pieces embedded themselves into the piston top.

The carnage was righteous.

So a little more information after speaking to mechanic in person.

Compression test on all 8 cylinders -- good between 180 and 190, on all, except #2 which had zero compression.

Leakdown test revealed air rushing out the intake, and nothing out of the crankcase, so they are pulling the head next week to see what's going on.

instead of looking like a "T" looks like a "Y" and sometimes big pieces leave !!!

So, an update:

Intake valve in #2 cylinder was bent, causing the loss of compression. Upon pulling the head, they discovered that many of the valves are sticky, so have recommended that both head be rebuilt, which is the route that I'm going with.

I guess the question now is, do i have them just replace whatever parts are broken? or does it make sense to do upgrades at this point in time?

Budget wise, if you upgrade the entire heads you'd buy bare castings and have to have a machine shop put in all brand new parts anyways. The piston that bent the valve is fine? If this is an OEM motor why would the valves be sticky? I'm no expert and curious myself.

Yes -- the piston was apparently fine... saw it yesterday and didn't look problematic to me. They claim some bad gas and detonation has caused all of the sticky valves.

What is the typical head upgrades for these motors?

i would sure like to know how bad gas caused sticking valves.i would ask them to explain that theory.

Overheating the valves. More common the center exhaust ports of SBC's as they are right next to each other.

http://paceperformance.com/images/F40300551.jpg

i have seen tuliped valves from to little timing and seen my fair share of detonated junk but never seen one stick a valve from it but i guess if the stem to guide clearance was to tight it could happen.