Temperature gauge parallels RPM.

N

norman

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1992 Mercruiser 5.0 litre V8 with alpha one drive.

Something's going on with my raw water cooling system.

As I throttle up and rpm increases temperature gauge increases to above normal.
As I throttle down and rpm decreases temperature also decreases to below normal.

Normal operating temp is 175 degrees, at 3500rpm but rises to 225 at WOT 4400rpm.

When I throttle down to idle about 800rpm temp falls to about 150 degrees.

My first thought is thermostat.
Don't think it is the impeller. If it was, wouldn't temp stay high at idle because of lack of coolant?

Any ideas?

Thanks,
~Norm
 
Impeller might just be warn out, I'd replace it.
 
Raw water cooled T-stat should be a 140-142 degrees stat. Depending on the raw water temp...you operating/cruise temp may/should be in the 175 degree temp you say is is/was. At idle/slow speed the 150 degree temp would be normal. Check your strainers and oil coolers for any blockages. How old are your elbows-if in salt water and more than 3-5 yrs old, it may be time to change them out.
 
Is the overtemp alarm going off to help confirm both the gauge and its temp sensor on the block are accurate? An IR temp gun aimed near the t-stat housing is also very helpful for that.
 
You describe classic symptoms of insufficient raw water flow.
Engine producesmore waste heat at higher load than at idle
 
Well maybe it is the impeller then.
Rick, I boat in fresh water so elbows and manifolds should be okay.
Sandy...overheat alarm is not sounding and I do have a temp gun but checked temp with it yet.

Thanks guys.
~Norm
 
If the givens are correct, then there are at least two things going on.

1.) Something is limiting the cooling capacity of the engine. Which is just a fancy way of saying that you notice that you overheat.

2.) You also run too cool. The only item in the cooling system that controls the lower limit of cooling is the thermostat. The failure mode of a too cool engine is a themostat that is either stuck open or miscalibrated to be open too far. But a stuck open thermostat by itself would not cause too high of temps, it would in fact work against that.

So....unless as others have noted that you have a raw water cooled engine with a 140/160 degree thermostat you have at least two problems, also in which case your "normal" 175 temps as a given would be incorrect.

I have traditionally found that cooling systems are best troubleshoot/maintained as a system. In which case if I was in a situation where I had cooling issues, I would be overhauling/zero timing every component in the system. That would include the water pump(s), hoses, intakes, manifolds, risers, exchangers, and impellers. The only exception is where I had very recently zero'd out the collection of those items.

The problem of course is that just about every individual item casts its symptoms on all the other items in the collection. While it might take me a very full day or even two to replace/overhaul all of those items, it could take me most of the week to troubleshoot them individually, often chasing my tail somewhere along the way. So I simply choose to work as a system and make life more predictable. Especially since if you find one failure, others are usually soon to come.

In the case of being forced to troubleshoot, start at the beginning and work your way component by component. Just because you find a smoking gun along the way does not mean you found the issue. The collection has a very bad habit of getting old/constricted at an even pace.
 
Go to the oil cooler before doing anything. Most times debris gets stuck in the oil cooler limiting water flow to the rest of the system. Coincidently I had the same issue this week. I removed the pencil zinc fron the oil cooler and put a plug back in. Then removed the intake hose to the entering Dixie of the cooler from the raw water pump. Mixed a 50/50 mix of muriatic acid and water and carefully filled the oil cooler and replaced hose and tightened. Leave it sit for 15 minutes and restart the engine. If you had a blockage there before it should be clean now. My temperature is now running a steady 170 from 190 and very little steam. Just be careful with the acid. If you want something easier to use try barnacle buster but that you have to circulate with a pump. You can watch their video for more info
 
Good points Ghost. Boat is being hauled tomorrow (Tue). I'm going to keep that in mind while we try to pinpoint the problem(s.
 
Sounds like an impeller. Since the boat is getting hauled anyway, it doesn't hurt to drop the lower half of the drive and change it. They are relatively inexpensive, and fairly easy to change.
 
Sounds like an impeller. Since the boat is getting hauled anyway, it doesn't hurt to drop the lower half of the drive and change it. They are relatively inexpensive, and fairly easy to change.
If the impeller looks good, you can always take the rest of the drive off, and rig a hose to push water through the intake and hopefully dislodge anything that might be clogging up the flow.
But it really does sound more like an impeller.
The other thing I've come to realize is that you're better off using OEM parts and oil. The Mercruiser stuff costs a couple of bucks more, but I've had an aftermarket impeller fail prematurely on me in the past.
The sleeve that goes on the shaft seperated from the rubber part of the impeller.
 
UPDATE: Impeller was beginning to wear so of course replaced... and replaced t-stat for good measure.
Back on the water and all is good.

Thanks for all the help.

~Norm
 
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