Mercruiser 5.0 overheating

[Oldone88]

I have a 94 Sea ray 200 Signature, and for the last 17 years the boat has been predominately in salt water. The engine block was changed approx 5-6 years ago when it was inadvertently ran out of oil, due to a loose oil filter(ugh). I've had overheating issues in the past (pre engine change), that have been remedied with impeller changes, thermostat, or thermostat housing replacements. After the engine change, the issues persisted on and off till the present.

In the last three years ive changed the impeller, exhaust manifolds, risers, thermostat, and temp gauge sensor. In 2018 i had the boat in the shop at the end of the season due to leaking drive oil.. They ended up changing some seals, and the complete impeller setup. Never had it back out that year, and then Hurricane Michael hit. Did some cosmetic damage to the boat, so it was late 2019 before i was back on the water. Noticed if i idled into the dock (about 800 rpm) for a long ways, my temp gauge would go up around 200-220 and i would get the overheat buzzer.

Went ahead this year and changed the Impeller again. If just sitting at idle it will still overheat, near 220. If i take it out of gear and bump idle to about 1100 rpm the temp will come down to about 180. I had a laser pyrometer with me last time, and took some temps when the boat gauge was reading over 200. The Thermostat housing was reading about 175, shooting the sensors in the housing showed about 169-175. The elbow risers were about 85, the manifolds were about 145. The riser elbows actually felt cool to touch, and a quick touch of the manifolds weren't too hot. running on the muffs it seems to be pumping water pretty well. Its making me wonder if im really having an overheat issue, or just a gauge issue? Im almost positive that the sensor is not OEM. Every year that goes by the salt water takes its toll on wiring and grounds, so i try to stay on top of it.. Just wondering if there were any ideas or suggestions? I've done a lot of researching thru forums but haven't found the answer yet.

 

[alldodge]

What is the motor serial number?
Which outdrive is installed?

 

[Oldone88]

Not sure if the engine S/N is the same as whats on the stern drive.. But the number on the stern drive is 0F198776. Last number may be an 8, very hard to read. Ill look again tomorrow to get a better read on the number. Its an Alpha One, Gen 2 drive..

 

[alldodge]

Your year motor could have one of 3 different thermostat housings. Here's 2 of them, and last one is a V6 but just for reference

 

[Scott Danforth]

you have changed a bunch of bits, however have not mentioned back-flushing the power steering cooler. that is the first place where the system plugs as the ID of the tubes is 0.2" and the ID of the inlet strainers in the alpha casting are 0.3"

overheating is a sign of lack of flow.

 

[QBhoy]

Hi
There is a bit of a chance that after the engine overheated the first time, you melted a flapper exhaust shutter enough to have it drop down the cow horns behind the engine and partially block your cooling water flow. I’d check each side for temperatures with this in mind.

 

[Oldone88]

My system is more like the second diagram..

I did disconnect both inlet, and outlet hose of the power steering cooler, and it seemed to be clear. I may need to go back and give it another look, as to me it appeared to be just a large tube with zero restriction.. Just for clarification, you're referring to the cylindrical object by the oil pan of diagram #2?

If i wanted to flush that system, where would i disconnect?

What would i have to remove to check the cow horns for a melted exhaust flapper?

thanks

 

[Oldone88]

Just more thoughts from off the top of my head..

The gauge in the boat is the original Sea-ray.. The sender is not original, its just a basic P/N 806490T. The Sea-ray gauge reads to 220 degrees. Any new gauge you buy goes to 240. While doing research, i seemed to determine that the gauge and sending unit should be matched, due to OHMS values?

Also my overheat buzzer sensor is not the original. I changed it a while back when i was getting a buzzer, but the engine wasn't showing an overheat condition.. Anyways the buzzer sensor is supposed to trigger at 212, and then back off at 195. All these times I've been out lately, and I've seen the engine temp approaching 220, the buzzer has never sounded..

I pulled the sensor last night and heated it in a pot of water while connected to an ohms meter to try and determine the opening temp. I discovered that its really hard to get water past 210 without it just wanting to vaporize.. using a laser pyrometer i was seeing about 235 on the bottom of the pan, but i could not get the sensor to close.. Don't know if the sensor is bad, or i just cant get it hot enough.

I.m about to pull the trigger on a new 240 degree Auto-meter gauge, with matching sensor. But holding off, hoping that someone can see something ive missed. All help is definitely appreciated..

Thanks

 

[QBhoy]

My system is more like the second diagram..

I did disconnect both inlet, and outlet hose of the power steering cooler, and it seemed to be clear. I may need to go back and give it another look, as to me it appeared to be just a large tube with zero restriction.. Just for clarification, you're referring to the cylindrical object by the oil pan of diagram #2?

If i wanted to flush that system, where would i disconnect?

What would i have to remove to check the cow horns for a melted exhaust flapper?

thanks

Take the two rubber exhaust sleeves off. If you are lucky you might get them off without taking the risers off, but I’ve never managed without. Wouldn’t take long. Then have a look down into the Down pipes at each side with a torch.

 

[alldodge]

Gauge sender should be 240 to 33 ohms (cold to hot).

Temperature / Ohms Reading
140° F (60° C) / 121-147
194° F (90° C) / 47-55
212° F (100° C) / 36-41

Temp switch
Temp switch 87-86080
Black 220 degrees F
Red 196 degrees F
Sierra are 240 and 200

Temp switch 87-866089 or 87-86080
175 to 195 degrees and lower it should be open
215 to 225 degrees and above it should be closed

Temp switch 48952
150 to 170 degrees and lower it should open
190 to 200 degrees and above it should close

 

[alldodge]

Many issues come from the Thermostat housing. This is the 3 types

 

[Oldone88]

I have design 1.

 

[Oldone88]

The thermostat housing has been replaced.. But its probably been about 10 years. I had the housing off in 2018, replaced gaskets, thermostat and spacer and verified it was clear. What kind of issues should i look for in the housing?

 

[Oldone88]

My Sending unit sensor OHMs values, are nowhere near the parameters you listed on the cold end.. At 20k it reads .77. At 2000 it reads 771. Am i running this test incorrectly, or is my sensor showing too much resistance?

 

[alldodge]

Don't know if your doing it right and your readings are way out there. If your meter is not auto ranging, it should be set on lowest scale for 300 ohms. One lead on the stud and the other on the case

Look for the check balls in the Tee and see if they are not stuck.
Next get a piece of clear tubing from the hard ware store and place inline with hose going to T stat housing. When the motor is running in the water look for air bubbles

 

[Oldone88]

My multimeter is not auto-ranging. The lowest setting is 200. At that setting there is NO reading. I have been connecting as you stated. My thermostat housing does not have the check balls in it. I’ve questioned that before but was told that my version didn’t use them. Looking on marine engines.com housing p/n 87290A 4, appears to be the one I have. And it incorporates the check balls. Could that be my issue?

 

[alldodge]

Yes, without check balls the most the water doesn't flow thru the block, it goes out the exhaust. This is way it will cool down only when revving motor.

Could replace thermostat housing with new style and changes or replace the check balls

 

[Oldone88]

Can I just order the check ball assy, and install it on this “T”?

 

[Oldone88]

Found this while doing some more research. I'm pretty sure i have an Alpha Gen II drive. I believe i had seen this before somewhere, and that's why i didn't think i was supposed to have the check balls. But looking at pictures of my style thermostat housing, they all show the check balls.

My question about purchasing the check ball's separate from the housing, or T fitting, is that it would be much cheaper. But I'm wondering if the "T fitting" is different from ones with the check balls, versus those without.

Thanks

Bondo
Moderator

• Join Date: Apr 2002
• Posts: 66282

#2
July 18th, 2009, 08:11 AM
Re: overheating mercruiser 5.0

Ayuh,... You must still have a blockage Somewhere,....
The Alpha 1, Gen.IIs don't use the check balls in the T-Stat housing...

 

[alldodge]

And Bondo does find some thing

The check balls are part of the Tee assembly 22-98477A2

https://www.amazon.com/GLM-Boating-13236-Assembly-22-98477A2/dp/B004AQS7U0