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Temperature rise at cruise

M

major_tom

Regular Contributor
Twin 1984 Crusader 454’s. Both engines idle and run below 1800 RPMS at 160F rock solid. However at a 3500 rpm cruise the port motor goes to 180F; starboard increases to about 170F. Throttle back down to 1000 RPM’s and the starboard temp drops back to 160F within 30 seconds; the port motor takes several minutes to return to 160F.


Other observations. There is a suggestion of rusting at the elbows/risers of the port motor, and I noticed some significant flakes of corroded cast iron when I replaced all the coolant hoses on the port motor last year.

The heat exchanger on the port side was removed and cleaned and the impellor on the port side is new. The starboard impellor is older and the heat exchanger has never been cleaned, yet this motor only has about ½ the temp. rise and returns to baseline quickly.

Both gauges and senders are the same and new last year.

I think the exhaust system may be partially restricted by rusting elbows and risers and foresee a new set of elbows and risers on the starboard side. I have a infrared temp gun and want to shoot some riser and exhaust temps on both sides at cruise but its tough to get the wife to turn up the engine beyond about 1200 rpms.

What do the experts think?

What is an acceptable temperature rise?

What is the “Do Not Exceed” engine operating temperature?

I'm aware 180F is now the recommended operating temperature but I don’t like the 20 degree rise and I am worried I have lost some cooling capacity and some day soon it might go right past 190F.
 
I found that I had some air leaks at my strainer and the gasket was the culprit. Replaced gaskets and used some grease around gasket to help with the seal and it makes both run at 160. watch the flow through the strainer as you rev to see the difference, air getting in will not fill strainer all the way up.
 
Tom,

I have had similiar problems over the seasons as they pass and have discovered there are essentially 3 important zones for cooling. Have you checked your U-coolers. These have been a persistant problem for us and temperature rise. Once I see that 160 breach I know it is time to start looking at water flow thru the system and it is either u-coolers, heat exchangers or impellers. We rule out manifold riser combos since we have replaced them a few seasons back. I have seen the situation you describe on our boat and know that as we are getting to the 180 mark, it will unexpectedly begine to cross that line as well.

When we had had a major system failure, I was removing cupsful of "iron ore" from the mufflers that had begun to deteroirate from the elbows..That infrared temp gun will provide some great information if you can get the engines going a bit as you describe. I do not have the logs here but I recall VAGUELY about 120 at the elbow?
 
If the risers/elbows have seen at least 4 years of salt water use, replace them now. If the oil coolers are original (or over 10 years old) replace them now.
If the HEx is not flushed after salt water use daily, and is over 10 years old, get it boiled out. Mid to high 160's is normal at cruise power at the thermo housing.
 
I've seen as high as 190 on the gauge but that was going south in 82 deg gulf stream water. Also seen it a couple times in August, when running in shallow (hot) water. 180 is preferred as the upper end.

if your risers have the drain plugs, you can also check for restrictions with a pressure gauge. You can use the starboard side to establish a baseline.

If you find the port risers/elbows needing replacement, I'd consider doing the starboard, too - they won't be too far behind.
 
Thanks. All good suggestions. I haven't investigated the condition of the oil coolers even though I know they are a ticking bomb --human nature I guess.

I'll start by eliminating air leaks at the strainers and work forward from there. I suppose I could remove and flush the oil coolers but even pressure testing them won't prove they are good. May have to write a check for new ones, along with risers and elbows. I'm down to no cooling reserve capacity to speak of on the port motor. This weekend it rose up from 160 F at displacment speed to 178 F at 3100 RPMS in 54F sea water up north here. The starboard side hardly moved from 160F.

I'm going to change one thing at a time until its fixed.

I do get the benefit of at least a partial fresh water flush after every run because my marina is 5 miles up the Merrimack River from the sea. I'm not sure what the frash/salt mix is at the slip but the color and appearence would indicate mostly fresh water.

Thanks for the help.
 
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