Overheating while we idle/move at a slow rate of speed

[RoJo]

Hello. I have a 1990 Sport Nautique with 469 hours running a 351 PCM. When we idle or are moving forward at a very slow rate, we are over heating. If I put the engine in neutral rev up or simple speed up to about 10 - 15mph, the temp will drop back down.

We checked the hoses for grass or foreign objects, didn't see anything and the strainer was clear minus a couple pieces of small grass or wood. We changed the impeller even though the other one was in great shape and are still having the same problem.

I was thinking maybe the thermostat but wanted to ask if anyone may have some suggestions.

Thank you for your time.

[Andrew]

RE: Overheating while we idle/move at a slow rate of speed

the thermostat would be my next step.......

[darrel409]

RE: Overheating while we idle/move at a slow rate of speed

Things to check
snug up all your hose clamps
check o-ring in strainer, you may be sucking a little air
check your belt for slipping
good luck

[RoJo]

Awesome. Thank you guys. I will check that O-ring first, that totally slipped my mind.

[MiracleDriver]

I have a 143 degree thermostat if you need one.

triple check the hoses, try and run the boat on land with a hose on the water pump to determine if the problem is before or after the intake. you can then attach the water directly to the hose that leads from the RWP to the thermostat housing. this will determine if the problem is with the RWP or not.

I had similar problems this year and after much head scratching because everything seemed perfect I bought an infrared thermometer. Needless to say the boat never fluctuates more than 1-2 degrees of 140... the gauge will read over 190 at times.

these older boats have crappy grounds and the senders are junk.

[MiracleDriver]

also..

to check the RWP put a hose on the intake and a bucket, it should suck the water out very fast. 5 gallons @ 3000rpm in 15seconds.

you can also just pull the outlet side and get a bit wet... it should gush out substantially.

then check the circ pump. they rarely go bad from what I'm told, but real easy to check.

pull the engine block drain plug and rev the motor. it should shoot out.. and I mean SHOOT! not drip...

I paid $50 for an infrared thermometer... after doing all this and finding it work.

I even tested the guages and they seemed to act normal. they just read high... so does my oil pressure gauge.

[RoJo]

Thank you for the information MiracleDriver.

Just to make sure I am correct, the RWP is Raw Water Pump which is my impeller correct? We did buy a new one and I thought it strange that is only came with the one rubber O-ring gasket and no paper gaskets. The one I purchased last year did. I am very curious if maybe the gaskets were going bad and I was taking in some air here.

Regarding the Circulating Pump. I was also curious about that. I have my engine manual, it mentions there are two heavy duty marine type pumps. A flexible impeller and a high volume circulation pump that maintains full water circulation to prevent steam pockets and hot spots, yet the diagram does not show this pump. I will have to figure out where this pump is hiding.

[MiracleDriver]

the Raw Water Pump houses the impeller. if the impeller looks good, it gushes water out and the pump doesn't leak, doubt there's a problem there.

do the tests like I said though!

also check the FAQ on skidim.com

circ pump is housed on the front of the motor as well. it is driven by the main pulley in the center (2 belts). you don't have to do anything but pull the main drain plug on the RWP side of the motor. unscrew it with the motor off and then fire it up. won't take more than a few seconds to determine if you have pressure... its will shoot out!!!! otherwise your circ pump may be bad. they rarely do though...

if you can run it on the trailer with a garden hose you can determine if you have a problem, before the RWP or after.

again, I bought an infrared thermometer from Princess Auto for $50... I have an electrical issue, not a cooling issue...

[RoJo]

Nice, so the circ pump is where the auto water pump used to be then. I feel dumb now. Thank you very much for the information. I will be heading back over to where I keep the boat this weekend to tinker with it.

[MiracleDriver]

trust me on the electrical factor. these old boats are notorious for having bad grounds and strange electrical issue. water and electricity don't mix... nothing looks corroded on my boat, but the gauges are off.

[Andrew]

yeah, its got nothing to do with corrosion and everything to do with the idiotic way the entire electrical systems are designed

[RoJo]

Very cool. Thank you for all your information. It's crazy to think all this might simply be piss poor design. I will perform the checks you all gave me and see what I come up with. Sounds like I should buy one of those infrared thermometers as well.

[MiracleDriver]

one other thing.

touch the risers and the thermostat housing when its running and showing an overheat on the gauge. if its overheating you should not be able to touch them... so be careful!

you should also feel the flow through the rwp and into the thermostat housing.

[RoJo]

Thank you all for your help.

I was over in Wenatchee this weekend tinkering with the boat. To make a long story short, the hose coming from the C-Strainer to the intake of the impeller was leaking. Odd thing, the hose was fine. I bought a replacement hose but this one was a lot thicker, even using two of those ring clamps the leak was worse. I took the original hose, cut a couple inches off each side and put it back on. The overheating issue is now fixed, however I am still getting a very minor leak coming from this hose when the engine is off. My guess the impeller housing may need to be replaced after 17 years of use.

Thank you MiracleDriver for all the tips on checking different portions of the engine, I am sure that will come in handy over the years.

[MiracleDriver]

we had a leak and were told to trash the pump. pulled it apart, new bolts, gasket and tightened it up properly and now no leak. saved $300 for a new pump...