OMC Cobra 5.0 overheating

[Reinell-BRXL-191]

Re: OMC Cobra 5.0 overheating

First off, you have to understand how the thermostat housing moves water... once you understand that, it is very easily possible for one riser to be colder than the other... once the stat opens, it flows hot water from the engine, and cold water from the drive, directly in to the manifolds, the starboard riser is closer to the diverter in the housing, it will see more of that hot water... so those temps don't mean much at all, and it also depends on the temp of the water coming in to the drive, the thickness of the steel in the casting.. yada yada yada - I can run across my lake at WOT, and then put my hands on my manifolds... cool to the touch... the intake, that's a whole different story!

I am betting it's a bad sender... I would actually try a new sender first... it is what sends the signal to the gauge, the gauge only interprets the resistance the sender is telling it...

 

[bruceb58]

Re: OMC Cobra 5.0 overheating

You're just not getting what I'm explaining. Basically I am just wanted to verify the readings are accurate

If he hadn't even posted his manifold readings and just posted the sender readings, we wouldn't be having this conversation would we? So what makes you think he doesn't know how to measure temps with an IR gun...its pretty darn easy and its pretty accurate!

 

[Idlespeedonly]

Re: OMC Cobra 5.0 overheating

You would argue with a fencepost. Explain to me how can the starboard manifold can be 82 degrees but the riser is only 68. Where do you think the water from the riser comes from? One with any common sense would question those two measurments. I guess the further and longer water travels in a hot manifold the colder it gets? The reason I was questioning the supply water temp is because if it was 80 degrees than something is wrong water wont get colder in the engine. I dont know if he knows how to use a temp gun, neither do you. I have seen people that have a hard time using a wrench. No fault of theirs, that is what this forum is for.

"If he hadn't even posted his manifold readings and just posted the sender readings, we wouldn't be having this conversation would we?" I wish now he wouldnt have.

 

[bruceb58]

Re: OMC Cobra 5.0 overheating

He probably didn't run any longer than it took to get his gauge temp up to 195. My manifolds don't hardly warm up at all until I run the boat on muffs for at least 20 minutes and that is with Los Angeles tempertaure water.

We will see what readings he gets with his gauge when he replaces the sender with the correct one.

 

[Idlespeedonly]

Re: OMC Cobra 5.0 overheating

Thats it? I figured you would have come up with something better than that. I think he ran it long enough. Im not questioning his ability to do anything. Just stating the readings dont add up.

 

[Reinell-BRXL-191]

Re: OMC Cobra 5.0 overheating

Explain to me how can the starboard manifold can be 82 degrees but the riser is only 68. .

The manifold is subjected to direct fire from the exhaust ports, the cooling system is injecting water in to these manifold that flows towards the exhaust exits...the riser is getting the first exposure to this cold water being sucked in from the lake or hose.. they should be colder than the manifolds... the risers connect to rubber tubing that needs to be cooled substantially in order not to burn. In a water cooled exhaust, the further away from the manifolds you get, the lower the pipe temperature should be. Average WOT in a engine produces about 1200 degrees of temp as verified by anyone with an EGT probe, yet anyone can go and put their hand in the exit water from the drive, and see it is less than 100 degrees. Your logic that the risers should be hotter is flawed.

 

[Ck111484]

Re: OMC Cobra 5.0 overheating

I replaced the sender with the appropriate Quicksilver brand marine sender yesterday, but have not run the boat yet. Plan on taking it to the lake today to make sure things are sorted.

However, I took temperature readings before swapping senders when the engine was still warm from having ran earlier.
Old sender - infrared gun showed 108 at the sender base, boat gauge showed 145-150 degrees.
New sender - infrared gun showed 106 degrees at the sender base, boat gauge showed slightly over 100 degrees.
So I think we found the problem

As for the manifolds and risers, I don't know what to tell you... I took those readings multiple times I can verify that the risers were absolutely cool/cold to the touch. I'm very confident the readings are accurate. The thermostat in my house is set to 67 degrees; point the gun at surfaces in the house, and it shows 67-69 degrees.
I don't know how you could "not know how" to use an infrared gun... you point the laser at a surface and pull the trigger for a few seconds.

 

[Idlespeedonly]

Re: OMC Cobra 5.0 overheating

The manifold is subjected to direct fire from the exhaust ports, the cooling system is injecting water in to these manifold that flows towards the exhaust exits...the riser is getting the first exposure to this cold water being sucked in from the lake or hose.. they should be colder than the manifolds... the risers connect to rubber tubing that needs to be cooled substantially in order not to burn. In a water cooled exhaust, the further away from the manifolds you get, the lower the pipe temperature should be. Average WOT in a engine produces about 1200 degrees of temp as verified by anyone with an EGT probe, yet anyone can go and put their hand in the exit water from the drive, and see it is less than 100 degrees. Your logic that the risers should be hotter is flawed.

The riser gets the water first? Thats new. The water goes to the manifold first. Then on to the risers.

 

[Ck111484]

Re: OMC Cobra 5.0 overheating

The water goes to the manifold first. Then on to the risers.

Not all of the water goes through the block...
Look at the first diagram on this page: http://www.boatinghowto.com/content/how-boat-cooling-systems-work-240/

The risers do get raw, cool water.

 

[Idlespeedonly]

Re: OMC Cobra 5.0 overheating

All water exits through the exhaust. You have two hoses that go from the stat housing to the manifolds then to the risers. I meant exhaust not intake. The raw water pump supplies water to thetstat housing. Some of this water goes to the circulation pump the rest of it goest to the exhaust. The water that goes to the exhaust it a mix of water fom the engine and raw water. Believe me, i know how it works. I have worked on one or two.

 

[bruceb58]

Re: OMC Cobra 5.0 overheating

Depends on the engine but for OMC the manifolds do get the water first followed by the risers. And just for clarification sake, the manifolds/risers do get raw cooling water before the thermostat opens up. There is a bypass channel in the thermostat housing for this. If you didn't have this, the exhaust, especially the exhaust hoses, would overheat within a minute.

As you can tell, i am not really concerned about the temps of the manifolds/risers for this test since they are meaningless to the discussion unless you believe the OP doesn't know how to use the tool, which idlespeed is implying. All we wnat to know is if the sender gauge temp matches what the IR tool indicates.

 

[Idlespeedonly]

Re: OMC Cobra 5.0 overheating

The raw water just keeps the cooling water "topped off". Im smarter than the average bear.

 

[Idlespeedonly]

Re: OMC Cobra 5.0 overheating

Where did i imply he doesnt know what he is doing? Quit putting words in my mouth!

 

[Reinell-BRXL-191]

Re: OMC Cobra 5.0 overheating

If you read what I wrote, the water gets injected in to the manifolds, and the first thing it cools is the risers, water doesn't sit around in the manifold, the waters purpose is not to cool the manifolds at all.. they do not have to be cooled, they would be quite happy at "burn your hand temps"... what cooling the manifolds get is from the water being injected in to them.. the main purpose of the water is to cool the rubber exhaust tracts, and they start right after the risers... the risers are the first part cooled by the water... they will always be colder than the manifolds, the front part of the manifold will also be hotter than the back part, as more water is introduced in to them from each of the 4 injection points. and as I stated before, when the t-stat opens and dumps the hot water in to the mix, the shortest path of resistance is always the starboard side, which will cause a slight temp increase on the starboard manifold vs. the port side.

I have no idea what the argument is... it makes no difference.. the OP at this point has resolved the issue by replacing a inaccurate sender...

and idlespeedonly, I too, from reading your posts, got that you were insinuating the OP didn't know how to take temps or was working with inaccurate equipment. Maybe you didn't mean to come off that way, but I though you were insulting him.

/thread

 

[bruceb58]

Re: OMC Cobra 5.0 overheating

Where did i imply he doesnt know what he is doing? Quit putting words in my mouth!

Thought you said that here!

I dont know if he knows how to use a temp gun

 

[Ck111484]

Re: OMC Cobra 5.0 overheating

Didn't mean to start an argument here! Haha.

Like I said, I'm taking the boat to the lake today to confirm everything is sorted, and I'll re-check the temps. I believe I've felt the risers significantly warmer than they were yesterday, so perhaps they do get warm under load.

 

[Idlespeedonly]

Re: OMC Cobra 5.0 overheating

the waters purpose is not to cool the manifolds at all.. they do not have to be cooled
/thread

The main purpose of the water IS to cool the exhaust so the boat doesnt burn up.
There are not 4 injection points like you describe. The manifolds are basically flooded with water first, then on to the risers. The risers do not get the water first like you are saying. I have no idea why you think this.
The front part of the manifold will be cooler. This is where the cold water is introduced.
I never said he didnt know how to use the temp gun either. I basically said he might or might not.

 

[bruceb58]

Re: OMC Cobra 5.0 overheating

The main purpose of the water IS to cool the exhaust so the boat doesnt burn up.

I do agree with you regarding this.

 

[Idlespeedonly]

Re: OMC Cobra 5.0 overheating

Woohoo I win!!
just kidding.
I do hope everything works out for the op. Nothing is worse than loading up everything to have to turn around and go back home.
Ask my kids!
I hate to see anybody get the wrong information, it can be dangerous!
I dont claim to know everything. But I usually wont speak up and give advice on somthing I know nothing about. Thats why you dont see me on the Volvo and Mercruiser threads, unless it is engine related. Reinall jumped in on this with inaccurate information. Is it his fault? I dont know. But like I said wrong info, especially on boats can and will get you hurt. And most know this.

 

[bruceb58]

Re: OMC Cobra 5.0 overheating

I measure my riser temps occasionally throughout the year since I boat in salt water and typically replace every 4 years or so because of it. It takes my engine a good 20 minutes running at around 2000RPM on muffs for all the exhaust components to get to temperature so I take my measurements then. I bet that is what happened here with the OP. There is a lot of mass to get up to temp.

I am very conservative with manifolds and risers as I don't want a leaky one to take out my engine. Amazes me still that people don't put new ones on rebuilt or new engines.