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TOPIC: Mercruiser 7.4 MPI Overheated!

Mercruiser 7.4 MPI Overheated! 28 Sep 2017 23:08 #1

  • optimist_prime
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Well, we just had our first ever "breakdown" and had to be towed in. Thankfully I opted for the BOAT/US "Unlimited" Tow option... best $149 I ever spent. The problem started while we were cruising at about 1800 RPM/8 MPH. We heard a "rumbling" or "thrumming" sound from the engine, so we came to slow idle in gear. Quieter, but definitely something there. And then I noticed the temp gauge, instead of it normal just-below-175 degrees, was climbing steadily up to 185-190. We shut her down and checked the engine for obvious signs of trouble, but nothing visual. I am sure the overheat was real though, the risers were the hottest I've ever felt them. I thought about restarting it, but in its elevated temp state, I thought we might get vapor lock. So we called for help. We were only 5 miles from our marina, and the Vessel Assist got to us in 35 min.

My mistake I think, was not letting her cool way down, and retrying. I say this because after they brought us into the marina, we started her up to park in our slip. I let her run for almost 20 min. after that, watching and feeling the temp while flushing with fresh water/Salt-Away. She stayed cool as a cucumber. I even advanced the throttle in neutral to see if higher RPMs would show signs of trouble. Nothing! BTW we just had the raw water pump replaced a few months ago when we pulled the motor to replace the steering pin/seal.

So my theory is this. We picked up a piece of kelp, which somehow covered the outdrive intake holes. The holes would have kept suction on the kelp leaf until we shut it off- even in neutral. We have hit kelp before at higher speed (the Bravo 3 has a "notch" at its leading edge which can hold on to a kelp rope), but never has it blocked water intake- just caused a funny-looking wake. Since we were at much slower speed, I think it's possible the kelp wrapped around the leg. We will sea-trial it carefully just outside our marina before calling it okay.

Anybody ever have something like this happen? Any ideas on what else it could have been?

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Mercruiser 7.4 MPI Overheated! 28 Sep 2017 23:12 #2

  • green650
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Are you in salt water?
How old are the exhaust manifolds and risers?

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Mercruiser 7.4 MPI Overheated! 28 Sep 2017 23:19 #3

  • Jeffw
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Same thing happened to us, no apparent reason, but the temp alarm sounded. We were in a shallow bay in San Pedro, CA, windier than hell, 20-30 mph. So the first thing I did was drop anchor, then opened the engine hatch. No signs of trouble, no steam, no seawater, no coolant. We let it cool for 30-40 minutes, then all was fine. We cruised slowly for about 30 minutes, then on plane for the last 30 minutes to our marina, no problems. We came to the conclusion of kelp or a plastic bag blocking the raw water inlets on the drive.
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Jeff & Tara
(And Ginger too)
Lake Havasu City, AZ
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2000 Bayliner 2858
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In memory of Shadow, the best boat dog ever. Rest in peace, girl. July 2, 2010

Mercruiser 7.4 MPI Overheated! 29 Sep 2017 00:12 #4

  • watchthis
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Up here we have areas of eel grass that will stuff up the intake. I was talking to a captain of a vessel assist boat and he said it happens to him as well, often behind (west side) Bainbridge island, and he usually throws it in reverse and raises/lowers the outdrive to clear it..
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Johnson Point, Olympia, WA
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Horizon 6.2 and Bravo II

Mercruiser 7.4 MPI Overheated! 29 Sep 2017 00:37 #5

  • Nauti Mike
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Yes grass or trash can block the inlet holes on the drive. The grass in my area can build up going through shallow areas that the boat will not plane. I have to stop it and gun her in reverse then all is good. This leaves a large amount of grass floating in front of the boat so I steer to the side to avoid it.
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1997 Maxum 2400 SCR 5.7LX Bravo II

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Mercruiser 7.4 MPI Overheated! 29 Sep 2017 04:16 #6

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Yes, we are in salt water. Exhaust manifolds probably original, and risers two years old. I do need to replace the down pipes (the ones going into the outdrive) as they showed some wear upon engine disassembly. I have them in a box ready but have yet to pull down the boots and put the new ones in.

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Mercruiser 7.4 MPI Overheated! 29 Sep 2017 12:25 #7

  • businessgetmoney
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This happens to me alot when i leave my river. The leg will pick up some grass. Put in reverse, look for floating grass. If u have to shut down, raise drive and check it.

Also as far as noise, that happened once too. I had a giant gob of seaweed from the detroit river. I couldnt even get on plane and powered down right away and checked it.

Very use to this now. I ALWAYS check the drive for blockages before hitting the throttle

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1993 formula pc 31 twin 454 bravo 2
1989 2655 cierra 5.7 omc cobra
2014 "searay" tandom trailer

Anchor bay clinton river
Michigan

Mercruiser 7.4 MPI Overheated! 29 Sep 2017 12:34 #8

  • 88fourwinns
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We get barnacles blocking the water intakes here on moored boats sometimes you have to split the drive to clean em out.

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88 Four Winns 200 Horizon 4.3 OMC
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