Overheating Mercruiser 7.4L MPI

[tpenfield]

Aren't MPI engines fun

 

[FuelishBehavior]

Just reading this thread has been interesting and baffling. I had a similar overheat issue on my '93 Crownline 223 CCR that had a 7.4L/Bravo but was carbed and not EFI. It seemed to idle a bit warmer than usual but when the revs would come up it would start to overheat. Changed everything...all pumps and thermostat. Turned out there was a small, less than a dime sized hole in the intake hose from the transom to the sea water pump and it was sucking air and losing more flow as the revs came up. It would idle ok...warmer than usual, but get hot under way. It was really difficult to see anything behind the engine due to the design of my boat...I only found it once I removed the distributor so I could see back there. No water was even coming in the boat...just sucking enough air to mess up the vacation. Hope you find your problem.

 

[DracoDan]

Just reading this thread has been interesting and baffling. I had a similar overheat issue on my '93 Crownline 223 CCR that had a 7.4L/Bravo but was carbed and not EFI. It seemed to idle a bit warmer than usual but when the revs would come up it would start to overheat. Changed everything...all pumps and thermostat. Turned out there was a small, less than a dime sized hole in the intake hose from the transom to the sea water pump and it was sucking air and losing more flow as the revs came up. It would idle ok...warmer than usual, but get hot under way. It was really difficult to see anything behind the engine due to the design of my boat...I only found it once I removed the distributor so I could see back there. No water was even coming in the boat...just sucking enough air to mess up the vacation. Hope you find your problem.

Thanks, I'll definitely check for any leaks. I have been working off the assumption that my bypass hookup for winterizing would provide the extra water volume if there was an intake restriction (it T's in just before the seawater pump), but if there's a hole/crack then it could still be getting air in the line. One thing though, was your engine above the water line? mine sits very low and therefore the intake line always has a bit of pressure on it, if I disconnect it from the seawater pump water pours in (since the intake and line are below the water line). I have to elevate the host to keep water from coming in.

 

[DracoDan]

So next up (other thanNautiNights' suggestion) on my game of "replace parts until it works" is to replace the ignition coil. My thought process is that I may have caused damage to it by using it for 5 years with a problematic tachometer. Many of the symptoms I have read about with a failing coil would fit with what I have been seeing, plus it was only $50... I'll be installing it tomorrow and then (if things don't improve) also checking my spark plugs for fouling and possibly doing a compression test. Can anyone tell me what an acceptable compression would be for a nearly 20 year old 454 with ~450 hours on it?

 

[DracoDan]

So... the boat is 100% fixed, had it out Saturday and Sunday and both engines purred like a cat!

I have to apologize, I left something out of the details because I didn't think it was relevant... I WAS WRONG!!!

At the end of last season I had to replace the ignition solenoid after my ignition switch got stuck in the start position I also replaced the ignition switches!). While replacing the solenoid I forgot to disconnect the battery and I accidentally caused a short that blew the 90A engine fuse (yes, I'm an idiot). I had trouble with this fuse in the past and I had jumped the port ignition switch off the starboard ignition wire to supply power to the solenoid/computer to start the engine (I was only connecting the wires during startup). That worked just fine for months last year, and after I blew the fuse I just went back to using this trick to start the engine until I could get someone to replace the fuse (I have bad shoulders that prevent me from being able to replace it because of the location). On Friday I was going over everything and I wondered if the engine would do the same weird rev thing if I held the wires together for the jump wire... Low and behold it suddenly started acting perfectly fine!!! Sure enough, once my brother came up and was able to replace the fuse, everything worked perfectly!

I'm still quite confused about how this could cause overheating, my best theory is that the limited power was somehow causing a backfire that temporarily slowed coolant flow out of the exhaust. I've always learned that power is supplied by the alternator when the engine was running, so I didn't expect the lack of a battery connection to cause any problems. I may still have ignition coil problems (I was sent the wrong coil so I couldn't swap it out) that are causing it to need more power than normal to generate a spark, both coils looked like they were in poor shape so I will be replacing them just to be safe. The engines now run about 183 for the port and about 177 for the starboard, even during spirited driving.

TLDR: The problem was with my 90A (lawyer) engine fuse and me taking a shortcut around it.

 

[tpenfield]

Sounds amazing, but glad it is fixed. :thumb:

 

[kenny nunez]

There is a metered hole in the thermostat housing that is the bypass until the thermostat is open. Yours is probably plugged with corrosion and you cannot see it.