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tjwieseler 05-17-2010 04:12 PM

Engine overheating problem
I purchased my 1997 2400 SCR in January, so no water test. Purchased from a reliable seller, at least he seemed to be. The boat looked like it was very well taken care of, and only 500 hrs on her. I have taken her out for probably 30 hrs or so without a problem. Then Friday I had it at full throttle and it started to overheat, so I backed down on the throttle and it cooled off. So I was going to look into why it was overheating, but had friends coming out on Saturday to go boating. We took off and got 10 miles from our dock and decided to anchor for a while. On the way back to the dock I brought the boat up on plane at full throttle and noticed the temp getting up there (200) so I backed it down to 3400 rpms and the temp was showing 150 I kept it at this rpm for a while and noticed that when I tilted the trim up the temp would climb again, so I left it down and the temp stayed right at 150. Then all of a sudden I noticed a loss in power, and I tried to give it a little more gas and it wouldn't pick up rpm's so I shut her down. When it reached 1000 rpms I noticed the temp guage was pegged at 250 and the engine was knocking. I immediately shut her off and she wouldn't stop running, it dieseled for a couple of seconds and then quit. We were towed back to the marina, where I started her back up to get into my slip. She ran just fine, oil pressure came up and everything seems fine for now. I checked the dipstick and looked in the valve covers for any sign of water in the oil and didn't find anything.

Any ideas what the problem could be? The previous owner just had the impeller changed when I picked it up, and made me pay the bill from the shop. So it shouldn't be the impeller. I was told to check the thermostat, and the oil cooler for possible impeller parts plugging it up.

I really don't want to taker her out of the water, but I can do whatever I need to do.

ss3964spd 05-17-2010 04:57 PM

Bummer. Has the engine always run at 150? If yes, that's too cool and suggests that either the wrong t-stat is installed or the t-stat is getting stuck too far open. Could be that the gauge isn't correctly calibrated so verify by getting the engine to operating temp then shoot the t-stat housing with a hand held IR thermometer.

However, having the temp go up when tilting the drive up suggests the impeller - especially if you've never seen that symptom before. If your drive has one it could also be the raw water hose from the drive to the engine is loose or has a crack in it and tilting the drive up creates more of a leak in that hose.

Could be clogged exhaust risers - which would be a buzz kill, but you can help diag that with the IR gun.

Some good reading here:


tjwieseler 05-17-2010 05:08 PM

The boat normally runs at 175-180. Someone told me the 150 could have been because there was NO water in the thermostat housing.

The raw water hose was replaced when I replaced the bellows before putting on the water, but I am also wondering if maybe it came loose and tilting the engine was allowing it to come off, and then it finally came all the way off. I will have to go swimming in 62 deg water or take it out to check.

ss3964spd 05-17-2010 05:40 PM

Someone told you correctly. If the temp sensor isn't in water then it can't read the water temp. Does not fit though - since you mentioned that you saw the temp gauge pegged at 250.

When you did the drive tilt test, was the symptom repeatable? Meaning, were you able to tilt the drive up several times with the same results? If yes I'd be focusing on the raw water pump or hose.

I'd be pulling the boat, because I don't like cold water. Also, if you are able to actually see the problem underwater are you going to be able to fix it underwater? Under cold water?

shrew 05-18-2010 03:35 PM

1) Manifolds. It's Possible enough scale has clogged passages, but only enough to restrict water flow. At lower RPM's waterflow is enough to cool.

2) Impellor. Similar. It may still be pulling water, but not enough at higher RPM's to maintain lower temps.

3) How long has the boat been in the water? I had a similar issue late last year where barnacles were blocking the pickups in the outdrive. The restiction was enough so that the engine temp. only rose at higher RPM's. This was because I stupidly covered the pickup holes with masking tape before I primed and painted the drive with anti-fouling paint. The holes were the only thing on the drive without anti-fouling and the only place the barnacles grew. (I didn't try to run on plane with the drive completely in, so I don't know if I had that specific symptom).

Just a few thoughts. Engine overheating is a common issue, and this is where I would start looking.

By the way. If the engine dieseled, you may want to do an oil change and see what the oil looks like. If it's anything but black (particularly if it looks like coffee with cream) run about 5 or more oil changes through the thing. It's my understanding that a it's possible for a 'dieseling' engine to actually run in reverse and suck water up the exhaust. That would be les than desirable.

edrey1996 05-19-2010 02:16 AM

I had this same problem with my boat . the problem was 2 things. first I had my impeller change. just out the shop . and at slow idle the boat ran fine, then when i raise the rpm the motor will get hot, 220 hot ! ! so i taken the out drive off, and found the hose that went to transom housing there is a water hose, it was kink . just enough to let some water throw, but not enough . so i replaced that hose. and after i cut that hose off. i found some of the old impeller in that hose and in the metal hose . with some long nose pliers. i pulled out some more impeller. Then went inside the boat. taken off that hose in back of the motor and blow some air with my compressor . a little more impeller came out . put it back together . That was the problem.I hope this story help you . it sure help me . PS. i think if you get under your boat and look up by the bellows yous should be able to see that hose. transom housing to the out drive it's a 3/4 or 5/8 hose

tjwieseler 05-19-2010 01:32 PM

That is one of the things I will be checking. We just changed the bellows and the raw water hose before putting it in the water. I am thinking that it could be twisted or maybe came off???
Is your outdrive an Alpha or Bravo? I was told by a Merc mechanic that the Alphas usually do not break pieces off of the impeller.

seapuppy 05-19-2010 06:35 PM

sounds like an impeller the temp sensor is on the stbd side of the engine...down in the block..I don't believe it's in the thermostat housing...


Bella Sera 3300SCR 05-19-2010 06:50 PM

one other thing to consider, is the past boat use and geographic location; boats that don't get winterized as a result don't usually get the block bung drain plugs removed letting the accumulated sand and tubidity out. Sand/dirt gathers in the waterjackets especially if the boat is beached. Which is why I do not over idle my 350mags while on shore. I know that a lot a crap comes out of my mills when I drain the blocks during the sad lay up process. That junk will otherwise settle and displace vital cooling area which can cause the aformentioned over heating while under way but not while idling.

Bella Sera

edrey1996 05-19-2010 11:04 PM

it was a alpha 1 out drive. and if you get the impeller hot enought it will come apart . my boat was sterted with no water ! yes started dry !@ well i bought it like that. it was a used boat . you just don't know what the person did befor you bought it .and when you check the raw water hose, turn the sterring wheel both ways, all the way . i think when i turned left, that was the worst on the hose.

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