overheating on plane

[jackd1023]

Hello, I have a 1995 mercruiser 5.0, Boat runs great, but once I hit plane speeds, the temparature skyrockets, as soon as I come off plane it cools instantly. If I sit at idle and rev the engine it also stays cool. I am confused please help.

 

[Reel Poor]

Re: overheating on plane

Is this engine raw water cooled or is it a closed cooling system (uses antifreeze) ?

 

[QC]

Re: overheating on plane

Welcome to iboats jack!!

Could use some more info like boat size, maintenance, drive, but here is some starter stuff. At idle, reved up or not, there is no load . . . load means fuel, fuel means heat. Here are the troubleshooting sections from a Merc Service manual. I assume you have an Alpha drive, so disregard the comments about a belt driven seawater pump. Yours is most likely in the drive.

Engine Overheats (Mechanical)Cause Special Information
1. Engine RPM below specifications at wideopen-
throttle (engine laboring)
1. Damaged or wrong propeller; growth on boat bottom;
false bottom full of water
2. Wrong ignition timing 2. Timing too far advanced or retarded
3. Sticking distributor advance weights
4. Spark plug wires crossed (wrong firing order)
5. Lean fuel mixture 5. Refer to “Carburetor Malfunctions” in this section
6. Wrong heat range spark plugs
7. Exhaust restriction
8. Valve timing off 8. Jumped timing chain, or improperly installed
9. Blown head gasket(s) 9. A blown head gasket(s) normally cannot be detected
by a compression check. Normally the engine
will run at normal temperature at low RPM,
but will overheat at speeds above 3000 RPM.
Engines that are seawater cooled: Using a
clear plastic hose, look for air bubbles between
seawater pump and engine. If there are no
bubbles present, install clear plastic hose be
tween thermostat housing and manifold(s).If air
bubbles are present at a higher RPM, it is a good
indication there is a blown head gasket.
10. Insufficient lubrication to moving parts of engine 10. Defective oil pump, plugged oil passage, low oil
level


Engine Overheats (Cooling System)
Cause Special Information
IMPORTANT: The first step is to verify if the engine is
actually overheating or the temperature gauge or
sender is faulty
1. Loose or broken drive belt
IMPORTANT: Best way to test gauge or sender is
to replace them.
2. Seawater shutoff valve partially or fully closed
(if equipped)
3. Clogged or improperly installed sea strainer
4. Loose hose connections between seawater
pickup and seawater pump inlet (models with
belt driven seawater pump only)
4. Pump will suck air. Pump may fail to prime or will
force air bubbles into cooling system.
5. Seawater inlet hose kinked or collapsed
6. Seawater pickup clogged
7. Obstruction on boat bottom causing water
turbulence
7. Obstruction will be in front of seawater pickup,
causing air bubbles to be forced into cooling system
8. Defective thermostat
9. Exhaust elbow water outlet holes plugged
10. Insufficient seawater pump operation 10. Worn pump impeller
11. Obstruction in cooling system such as casting
flash, sand, rust, salt, etc.
11. Refer to water flow diagram for engine type being
serviced
12. Engine circulating pump defective
13. Also refer to “Engine Overheats (Mechanical)”

 

[jackd1023]

Re: overheating on plane

Is this engine raw water cooled or is it a closed cooling system (uses antifreeze) ?

it is a raw water cooled system. I did pull off the top hose off of the water pump and it did shoot water out like mad. does the impeller feed the water pump when on plane??

 

[jackd1023]

Re: overheating on plane

The boat is a 1994 Chapparral 1930 SST with the 5.0(240hp) mercruiser. . Alpha 1 Gen II drive. The boat has had very good maintenance and probably has less than 200 hours. I know the water pump (attached to the block) is working , We poped the hatch and disconnected the top hose and it was pumping and the water increased when I reved the motor. That means the pump is working. Does the impeller in the drive feed the system once on plane?? Is there a way to backflush the system(in case of obstuction). Is there a way to check the impeller without dropping the lower unit?

 

[QC]

Re: overheating on plane

The top hose you removed is probably # 11 in the diagram shown at this link: Alpha Cooling System

If so, flow there proves that the seawater pump impeller in the drive is working, not your belt driven circulation pump. The impeller in the drive should get waterflow always, on plane or not. It is fed by the intake slats below your anti-ventilation plate (flat thing just above your propeller) which should never run dry. If you get good flow out of your propeller hub (assumes thru prop exhaust) with the engine running on muffs, then you probably do not have a restriction. Back flushing of all of those hoses is a good thing, and especially the oil cooler, but if you already have flow in (#11) and out (prop hub), then restriction is probably not your issue.

 

[jackd1023]

Re: overheating on plane

I had this problem at the beginning of the season and I went to pull the drive off and do some checking. I found a large piece of hard plastic in the exhaust outlet the problem the cleared up for a while it just now started again. almost like it is a restriction on high rpms and something is closing off the system

 

[QC]

Re: overheating on plane

You may have a restriction as pressure (RPM) rises. No question. If you can run her on muffs you should be able to see a change in the flow through the prop at higher RPM or it slowing etc. Do you have muffs? They're cheap and allow you to run in the driveway . . .

 

[jackd1023]

Re: overheating on plane

I do have muffs, but the funny thing is is that it only does it under load on plane. I can rev it and it still pumps great. I can run it all day long at 2900 rpm and the boat will not plane just push alot of water and it runs cool as a cucumber which is alot of stress right? but the min I get over 3000 rmp the temp rises almost instantly. within a matter of 30 sec it is over 200 degrees. I then can take it off of plane and rev the motor to 4200 rpm and it cools right away. That is why I am wondering if the impeller is bad and maybe the water pump is pulling in the water when the lower unit is sitting still but once it gets to plane........

 

[QC]

Re: overheating on plane

Does seem strange, and the previous and similar issue would clog my judgement as well. Actually the propeller (connection to engine, right?) doesn't really know too much of a difference between on and off plane. A lot of guys will argue with me, but propeller load curves go up with speed almost linearly. There is probably a little more slip as she fights the bow wave, but not much. Either way it doesn't explain why she overheats.

The pump should know nothing, but RPM, so the quick cool down at 4200 makes no sense either except there is no load (heat). I guess I'd still want to see what the output looked like from the prop on the muffs at that speed. It could still be progressively clogging from 3000 up and the on plane off plane thing is just a coincidence of the RPM. I'd also pull that thermostat and check it. It may open just enough to cool the bhp you are developing at 2900 and not enough at 3000 . . .

Hopefully someone else will chime in as I am just reviewing basic stuff and have no experience with your specific issue.

 

[Reel Poor]

Re: overheating on plane

Check the supply hose from the drive bell housing to the transom plate for cracks. If it's cracked, on plane it will pump the water out of the crack instead of into the engine. Off plane and submerged in the water it may be able to overcome the faulty hose. If the water pump impeller (in the drive) is over two years old, replace the complete pump assembly, not just the rubber impeller.

The only way to back flush the supply hose is to remove the drive foot. The water pump impeller will not allow back flow.

It could also be the exhaust riser are cloged and not letting the water out of the engine fast enough. You would have to remove the risers and inspect the water passages.

 

[jackd1023]

Re: overheating on plane

The exhaust would make sense since I did find some plastic in the prop exhaust. Am I supposed to pull the for bolts on the top of the exhaust?? I have never gotten too involved in the exhaust on these boats . I will try to run this on muffs but this cannot be done until next week.

 

[Reel Poor]

Re: overheating on plane

The exhaust would make sense since I did find some plastic in the prop exhaust.

I dont know of any plastic, but it could be a part of the exhaust flapper that has broken off.

Am I supposed to pull the for bolts on the top of the exhaust??

Yes,,,,Also remove the exhaust elbows from the Y-pipe and look in the top of the Y-pipe for the exhaust flappers and their condition.