Overheat - How hot is too hot?
 

Had an impeller issue yesterday, overheated the motor (5.7 efi merc) shut it down, let it cool and limped back in. Temp hit ~200 degrees when the alarm went off. Smoke was in the engine compartment. mostly from a blower hose that was on one of the exhaust downpipes. When limping in the temp hovered around the 185 mark, no alarm sounding. How hot is too hot with these engines and what damage should I look for. I replaced the impeller and water is flowing good now. I'm going to change the oil and get that fresh, but other than that, what other damage was likely?

It seemed stumbly at idle today, temp gauge is all over the place, you can see the thermostat open and close, water is cold still in the lake. Boat use to stay at 175 degrees all day long, but that may have been the impeller going last year. Is it normal to see the temp go from 100 degrees to 150-175 and back down again so quickly? Again it is may and the water is only 59 degrees. How hot would I have had to get the motor to cause piston/ring etc damage? Thanks again, Jeff


also had a hard time starting after sitting for 2 hours after a good run. Engine was stumbling at idle I'm on last years gas still, so I'm not ruling that out, but my boat has always started within 5 sec of turning the key. again it was stumbly then

those motors can get really damn hot and not have a problem.

if it only hit 200 degrees you're fine. they run close to 200 degrees during normal operation in cars.

There was Zero water running through it, only the antifreeze that was left over from winterization. Water had not made it as far as the thermostat housing.

What about the erratic temp readings?

We overheated my friends 350 last year. Maxed out the temp gauge, burned the boots on the exhaust, melted the water pump housing. The only thing keeping it from running right (after fixing the water pump) was a melted (sticking) shifter cable that had been sitting against the lower y-pipe when it overheated. When he shifted into gear the ignition would cut out (alpha drives have an ignition cutout that momentarily cuts ignition when you shift... if the cable sticks, it's not momentary anymore and the motor dies).

I would say just change the oil before you run it again, if it got really hot. Other than that, just fix the water pump and fire it up.

If it started to burn the boots between the exhaust risers and the y-pipe, replace them. If they don't leak now, they'll crack and leak. Later in the season my friend had one go out, because it was burnt and cracked from the overheating.

As for the erratic temp readings, that is probably something wrong. Maybe a thermostat acting up, maybe a water flow issue. Swap the thermostat and see if that makes a difference.

Thanks for the help. Can the boots be done in the water, or are they too low and allow water in?

What kind of temps do piston/ring or head gasket damage?

I would check tht the thermostat is in the right spot and the spacer is init....that may make the temps fluctuate like that

Depends on the boat, for the boots. My friend was able to do those boots in the water. On my boat, water would come rushing in if i tried to replace them in the water.

As for temps to do head gasket / piston damage, hard to say. It's going to depend on a lot of things. Aluminum heads would be a lot more susceptible to damage due to overheating. If you overheat when you're running real hard and get detonation as a result, you could get piston / ring damage. If you're idling, it would have to get a lot hotter to do damage. Too many things come into play.

It's a real crapshoot with those motors, if the exhaust rubber didnt burn you may be ok, best bet would be a compression check.

Unfortunately the temp guage does not indicate temp very well when no water is flowing. I saw 3 smallblocks that blew head gaskets last year with very mild seeming overheats.

That makes 4 with the addition of mine. Stupid brand new mercruiser circ pump. :mad:

yeah, there was no water in the thermostat housing. So it could have been hotter than read

Ok, the bad news is here. Checked the first bank of cylinders, 1 good, 2 full of water, 1 light grey plug. However, compression looks good. 180lbs all cylinders. What is likely the damage? Exhaust manifold gasket? Please help inform and direct me what to do!

Could be a cracked exhaust manifold if you have water in one of the cylinders.

I wrecked an engine with a bad header. Filled one cylinder with water, then at started up it would hydraulic and it blew the head gasket out btween two cylinders. It still ran, but sounded like crap. I was to young to care. I eventualy had to repair the head and block.

It was running ok, not 100%, but 95%. Hard start though after a good run, assuming water leaking into cylinders from exhaust gasket. Otherwise boat got on plane and ran well. I hope I can get this fixed without pulling the boat out of the water or the motor out of the boat.
Ti confirm, compression looks good, no water in oil. Just water in cylinders, both banks. Intake gasket? Exhaust manifold gasket?

I say cracked Exhaust manifold filling the cylinders when you shut it off. Sorry

I vote for this as well. Water in cylinders is never a good thing.

no, never a good thing, but is a cracked exhaust manifold really difficult to fix. Correct me if I'm wrong, but a couple new manifolds and gaskets and I should be off to the races again!

As long as that water didn't do any damage like hydro lock or break a valve/valve seat. But since you've ran it already, I'm guessing you'll be OK.

both banks have water in them, would that not indicate a intake gasket failure as opposed to a exhaust manifolds? Would it be likely that both cracked. Sorry for all the questions, trying to learn and understand, try to fix it myself.

If you want the "easy" way out, get some Revolution exhaust. They are the least expensive "performance" exhaust. But the real reason (at least for me) to get them is the manifold/riser joint is dry, so there is no chance for the joint to leak into your motor.

I bought them. I got them for about $1K, which is about twice what new stock style manifolds/risers will run you. cheap peace of mind for me.

Either way, this is not something I would mess around with. Replace everything on both sides, because the next time water gets into your cylinders it could destroy your motor.

How would I diagnose where the problem is? Or do i just replace intake and exhaust gaskets? I dunno where to go from here

A leaky exhaust can be hard to diagnose. I would replace your manifolds/risers. Depending on the age of them, they may be due anyway.

Last year I had a similar problem, I took mine off, tested them with a hose, and they showed no leakage. They still leaked when I put them back on the boat. Probably something to do with the heat from the exhaust opening up a crack.

I wouldn't worry about your intake gasket. If your intake gasket was leaking, you would have milky oil, and most likely no water in cylinders.

Okay, so no water in oil, only cylinders indicates exhaust manifold/risers? Pull them off, put new ones and and new gaskets on? The boat is a 1999. I think this can be done with the boat in the water

Update: pulled the header off and it had water in the exhaust runners. So...Guess I'm closer to my problem. gasket was pretty tourched, particularly on the manifold to elbow area
Probably my leak?

Please keep the help coming, I'm working live on it. Everybody's help is getting me through this one!

replaced manifold and riser gaskets, problem still exists.
runs great once started, water must be running in there when the motor is off

you could have another problem also. i would first check your thermostat. it should be a 160 degree stat. the next thing is if the water temp is erratic it could be the circ pump and what could be happening is it is not circulating and just relying on the raw water pump to move the water. also if the temp is going up and down the ecm see's a hot cold situation and constantly adjust's the fuel and timing curve and idle air control valve to kepp the engine running. you might also have fouled some spark plugs due to this condition all causing you rough running condition. you are also going to wanna check the exhaust flappers in the y-pipe chances are you burned them up. run through all of you coolers to check for impeller debris if you broke off any veins. they can make it up to the t-stat and get stuck in it and hold open the stat enough to allow water to flow to a certain degree but not enought to get the engine hot enough to open the stat fully open.

If your flaps are good, Replace manifolds and risers. They can crack, or have other problems, especially at their age.

You are seriously playing with fire here, I would almost guarantee that your valve sealing has been compromised at the minimum at this point. The water in the cylinders is not exactly clean, and the valves are closing on it.

I learned this the hard way.

B.O.A.T. stands for "Break Out Another Thousand"

I dunno if you said, but what is compression test numbers? Rough running soundslike you hurt a valve , but a compression test would show that. Do all 8 cylinders and look for good correlation between them. I over heated a SMC (Volvo Penta but same deal) about 2yrs ago by hitting a plastic bag and shut water off while cruising about 3500 RPMs. Got WAY hot, motor shut off, water boiling everywhere in engine manifolds, melted all the exhaust bellows (lower "Y" pipe is under waterline, boat almost sank, ...sweet design there huh). Anyhow. Got it running and immediately filed cylinders with water. Did a compression test and it showed OK, weird. Turned out torched both riser gaskets. Was due for them anyhow so I replaced both manifolds and risers. Did a total tune up and lots of oil changes and I have put about 300 more hrs on motor. Moral is sometimes you get lucky, don't give up yet.

I'm ruling out exhaust system as the problem, when I had the manifolds and risers off there was still water coming in the cylinder after cranking it over. Must be the head or intake gasket, guess the top end is coming off.

On the compression test, I was getting good numbers, 180lbs unit I got to the cylinders with water in them, and the water was throwing off my compression numbers, so I stopped there.

It is most likely going to be the head gasket, you may be able to check the intake gasket on disassembly, but it is not likely the cause. With the heads off, have them crack tested, complete valve job may be a good idea depending on the hours, relatively cheap once the heads are off anyway, the head will need to be surfaced normally.

thanks wobble. What's a marina usually charge for that kind of work?

I had the exact same problem 2 weeks ago as you have now.Starboard exhaust riser leaking into the cylinders and port head gasket blew which allowed water into the cylinders as well.
Had the entire top end removed and found valves not seating properly.Good valve job,good intake and head gaskets and new Imco exhaust installed and all is good now.
If you do have water in the oil you need to do a couple oil changes asap.That water is tough on bearings.

worked on a 5.7 efi this week that ran hot 2 cracked heads one broken exhaust valve two blown head gaskets!! ran hot enough to burn exhaust hoses.

Thanks,
Was it exhibiting similar problems as mine?
estimated costs to fix something like that?

Here is the deal on the cost. If you know of a mechanic that can be trusted, then you have to do the work he wants done for the price he wants, in return if it's not right he takes care of it.

You could be looking at 1500-3000 per engine + any major parts that need replacement.

Now if you can remove and replace the heads yourself, the rest of the work without a new head could be less than $500 parts and machine work.

If you are not comfortable with this job consider pulling the motor yourself, which is fairly easy with a buddies help. This will save on the mechanics time, however he may not warranty his work if he doesn't do the whole job.

Good luck

Do you have to change the oil after overheating?

Why not change the oil ? You might be changing cranks rods and bearings. At least check the oil for signs of bearing debris and metal dust.
Water boils at 212 and expands into steam. This is not good and can lead to head gasket failure as well as actually burn a path between 2 cylinders as well as the head if severely overheated. For some reason usually the heat riser passage , this usually happens in the center cylinders. This steam also pushes hard and because of higher cylinder pressures it temporarily can stretch head bolts and may not seem to be the culprit and head and gasket may seem fine sometimes. Small cracks in the upper head bolt holes in the block indicate a severely overheated motor , and motor may have other issues as well.
When motor gets mad hot it actually seeks cool water and wants to suck it up the exhaust with vacuum. This could 'Splain rust in exhaust. Post some pics and some experts will tell you whats going on. BBB