How warm do you run?

SCORPIO

SCORPIO

Well-Known Member
Apr 7, 2008
2,742
Delaware
Boat Info
1989 300 DA
Engines
Twin 5.7 Merc Alpha I
For those with raw water cooled small block GM engines (350s 5.7s) how hot does your temp gage read at idle and under cruise conditions? Especially those of you with 86-89 300 DA's.
Both my engines had been running 145-150 degrees at idle and under load until just recently. Now, my port engine wants to run at 165 at idle and 150 under cruise at about 3000rpm. The starboard engine is still rock steady at 145-150. I replaced my thermostat(140 degree) and thermostat housing. I pulled and cleaned my inside riser, it was already clean, no scale. Both drives had new water pumps installed before the start of the season, and I haven't used the boat that hard this year. I scrubed the water inlets on the lower units and backflushed the power steering cooler.
I even swapped the gages from port to stbd to see if it was an instrument problem, no luck, the port engine is definatly running warmer than the stbd. The mechanic that rebuilt the drives said that unless the temp climbs to around 200+ and stays there, not to worry about it. I'm concerned that I have something restricted that may get worse.
Does anybody have any ideas? I'd also like to hear what your operating temps are.
BTW I'm in salt water and the temps are in the mid to upper 70s right now.
Thanks
 
Our 350 mercs had 140 stats and we would indicate about that on the gauges running in cool Lake Michigan water. Sounds like something is going on with the port side.
 
Have you taken a Raytek infrared temp reading off the thermostat housing to see if the sending unit is on the fritz? I changed my stats just prior to our trip, on the 350 Mags they are rated at 160 degrees. Setting in my slip warming up the engines at idle, they warm up to 154 to 160 degrees. I cross checked the Smartview gauge with the Raytek and they were within a degree.

Under load at any rpm they will climb up to 162.
 
Forgot to mention that both engines had new risers and manifolds at the start of last season. I'm thinking of replacing the circulating pump to see if that helps. If nothing helps, I may replace my intake maniflod this winter as I believe that is an original part.
 
Stray Cat said:
Have you taken a Raytek infrared temp reading off the thermostat housing to see if the sending unit is on the fritz? I changed my stats just prior to our trip, on the 350 Mags they are rated at 160 degrees. Setting in my slip warming up the engines at idle, they warm up to 154 to 160 degrees. I cross checked the Smartview gauge with the Raytek and they were within a degree.

Under load at any rpm they will climb up to 162.
Click to expand...

Not yet but I want to do that soon as I get hold of an IR gun. I did replace the sender with a new one and even cross connected the port and stbd hages to see if the head unit was off, no luck.
 
I replaced my raw water pump in the drive. For some reason it ate its self after about 16hrs. All I can figure is I sucked a bag around the drive. When I pulled it the impeller was intact, but soft, the metal part the impeller sits in had rotated about 90 degrees and the plastic housing has somewhat melted inside and cracked on the outside. I replaced it before the last trip and it was fine. I don't know why it failed.:huh:
 
Curious as to the 140* thermostat... both of mine have 160* and both run at the 165-170 range. Is the 140* original or was it lowered for a reason? I second the IR gun - you can also clean the connector to the water temp sending unit and on the gauge itself.
 
140 is what Merc calls for in a raw water setup if run in salt water, prevents salt buildups in the water passages.
 
SCORPIO said:
140 is what Merc calls for in a raw water setup if run in salt water, prevents salt buildups in the water passages.
Click to expand...

hmmm - hadn't heard that. I boat in salt and brackish water so sounds like I should be switching - boat sure runs good with the 140* installed. Sorry didn't mean to hijack... glad I asked my question though!
 
tdappleman said:
Curious as to the 140* thermostat... both of mine have 160* and both run at the 165-170 range. Is the 140* original or was it lowered for a reason? I second the IR gun - you can also clean the connector to the water temp sending unit and on the gauge itself.
Click to expand...

Late model and fuel injected motors run 160, older motors run 140 t-stats.
 
I have the 5.7's in my 87 30' Sundancer with 160 Thermostats and both motors run at about 170 all the time. The boat is raw water cooled and I boat in salt water.
145 sounds a little low. I could be wrong, but I would think the motors have to run a little hotter than that to run efficiently.
 
I have 7.4's and I have been trying to figure out why these engines wont go above 135. I have changed the stats twice in 18 months and if im not mistaken I thought they were 160 stats, im pretty sure they are..

A dock neighbor had a gun and both read within a couple of degrees of the gauges. My concern is that Im not getting the most efficient fuel burn. The motors are made to run at 160 and i just cant get the temps that high.

I never thought about salt build up and temperature, I didnt know one had anything to do with another, I wonder what running so cool is doing to the water passages. The engines are raw water cooled and the manifolds and risers are 3 years old.

Any suggestions would be appreciated, Im out of ideas.
 
My engines run a consistent 145. I installed new impellars at the beginning of the season. We run about 10 days a year in salt water, the rest is all freshwater.

As far as the temps and salt water go, my understanding was that by keeping the salt water temperature lower, less would come out of solution and stick in the block somewhere. That's why most use the 140* thermostats.
 
The W.A.V.E. said:
I have 7.4's and I have been trying to figure out why these engines wont go above 135. I have changed the stats twice in 18 months and if im not mistaken I thought they were 160 stats, im pretty sure they are..

A dock neighbor had a gun and both read within a couple of degrees of the gauges. My concern is that Im not getting the most efficient fuel burn. The motors are made to run at 160 and i just cant get the temps that high.

I never thought about salt build up and temperature, I didnt know one had anything to do with another, I wonder what running so cool is doing to the water passages. The engines are raw water cooled and the manifolds and risers are 3 years old.

Any suggestions would be appreciated, Im out of ideas.
Click to expand...

It should be running warmer. If a fuel injected motor runs too cool - you are dumping too much fuel - which builds up carbon or worse case could get into the oil blowing past the cylinders. Not to scare you. But, been helping a friend deal with this. His motor started running bad. He had tons of carbon in his engine, to the point that it was running bad.
 
From what I've learned by talking to a few Merc mechanics; the older carbed engines with raw water cooling in salt should run 140 stats to minimize salt buildup, EFI engines need to run warmer as do closed cooled engines. As some have said, warmer is better than too cool for efficency, but not for salt buildup, its a trade off. Like Sundancer said, my engines used to both run steady at 145, so I think I have a problem lurking somewhere, just have to find it. I thought the higher water temps in late summer had something to do with the prob. but the stbd engine still runs at 145, so that theory went out the window. Still suspecting the impeller or maybe a kink/restriction in the hose from the OD to the transom. I'll keep you posted.
 
One thing I've noticed while working on replacing my exhaust system on both engines is mineral buildup inside the hoses. I pulled them all off of both engines and they would crackle and pop when squeezed or moved. I flushed them all with fresh water and flexed the hoses in all positions to loosen things up and you'd be amazed at the crap which came out.

These hoses are relatively new and I flush very regularly so it was a bit surprising to see that much build up in them. Might be something to check on yours just to make sure you have good water flow through all of them. Mine has the main input hose from the drive to the thermostat housing, one large hose down to the water pump and 2 hoses feeding the exhaust manifolds and elbows. If you haven't used the boat much it may have built up even more. Easy enough to check and gives you a chance to double check all of the clamps, etc
 
All hoses replaced this spring, even the big exhaust hoses from last year look like new.
 
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