1990 70hp Evinrude-chronic overheating

[Four]

So far I have replaced thermostat-engine overheats alarm sounds. Replaced water pump w/housing./Engine overheats. Engine overheats after about 4 minutes of 3/4 throttle operation. I can remove thermo and proceed without any problems. I am aware of some TSB's concerning this issue. Flow indicator nipple has been relocated to top of ex. manifold and restrictor removed by previous owner. Waterpump kit contained red straight spring and conical yellow spring + new thermo cover; but, no instructions. I installed red spring and stock cover. I am aware of new temp sensor p/n. My tan wire overheat sensor must be working correctly because when motor overheats it is very hot. I have to pour water on thermo housing to cool it, so I can remove thermostat without burning my fingers. This is and has always been a fresh water motor. Waterflow out of motor with thermo cover removed is substantial. I also tested new thermostat in a pan of water with themometer, it opens at correct temp. Compression is even 110 psi, motor starts and runs fine with termostat removed. Cold engine is killing my fuel economy and no doubt cokeing up motor. Any suggestions? Thanks!!!

 

[Walker]

Re: 1990 70hp Evinrude-chronic overheating

You might try a de-carb treatment and a cooling system flush. We have a couple of power plant lakes down here that cause a scale buildup in the cooling passages and I get a couple of engines a year that need flushing. Also carbon buildup in the engine will act as insulation and retain heat.

 

[clanton]

Re: 1990 70hp Evinrude-chronic overheating

You need to know what spring was in the engine, before you replace the spring and cover. I should have a set of instructions, may take a day or two to find them. Is the thermostat seat in the block?

 

[seahorse5]

Re: 1990 70hp Evinrude-chronic overheating

Unless the powerhead has been changed out, you should have the black plastic thermostat housing with the element inside of it. If it is a white housing, get rid of it, that should have been done years ago. The red spring is correct, and the rubber seat for the t'stat housing should be replaced if it is not perfectly round on the inner diameter. The water pump kit should have a 5 blade impeller, not the 3 blade one.<br /><br />Make sure the pump body oring is well sealed to the stainless steel plate and that the water inlet screen is in good shape. Are there any speedo pickups or transducers within 14" of the motor centerline that could disrupt water flow to the gearcase?

 

[Four]

Re: 1990 70hp Evinrude-chronic overheating

Previous owner said motor had piston replaced. oozing gel seal looks fresh.Top of pistons are fairly clean.Spring that was original was straight spring about 1/4 " longer than spring i installed.Thermo seat is intact.older 2 piece copper ring + rubber.New thermo is black with small holes on end seems restrictive to me.Old thermo was white with 3 flutes.Doesn't higher rpm=more pressure-pushes thermo off of seat? I am wondering if rebuilder installed wrong ex or head gask.

 

[seahorse5]

Re: 1990 70hp Evinrude-chronic overheating

Water pressure pushes the thermostat housing off the rubber seat at midrange and higher. If I remember the old service bulletins, there was an improved exhaust cover plate that should be used if the motor was taken apart.<br /><br />If the temp switch wire is solid tan, replace it with the tan wire version with a blue stripe.<br /><br />Deposit buildup can cause hot spots in the cylinderhead near the temp sender pocket. In extreme cases, the inner passageway of the head should be soaked with toilet bowl cleaner for about an hour, then rinsed thoroughly.<br /><br />You may have to temporarily hook up a water pressure gauge and monitor it at different rpms for troubleshooting.<br /><br />If a waterpump blade or tip ever came off, it could be lodged underneath the powerhead, restricting a water passage.

 

[Four]

Re: 1990 70hp Evinrude-chronic overheating

Thank you all for your help! Newer temp sensor raises temp/alarm threshhold correct? The small area i can see inside of head with thermo cover off looks very clean no scale ect.Iknow that doesnt mean passageways elsewhere are. Can i tap for water press. at old lower nipple What would correct psi be. New water pump as well as old is 6 blade.Pump housing to plate sealis correct and intake screens clear.The impeller obstruction sounds plausible to me.It seems like a pressure/volume problem. Maybe what seems like a substantual flow from thermo cover is not.I did mount transdcr when i got boat 16 inches from centerline.Thermo seat has slight deformation where 3 flute thermo was. There appeared to be a fair volume of water exiting from ex. relief holes at speed. If there is an obsruction can i backflush or remove ex cover, head, go through water tube ect.I do not want to remove powerhead if i can help it.Fish are still biting well here!

 

[clanton]

Re: 1990 70hp Evinrude-chronic overheating

If you remove the thermostat housing and thermostat start engine in water 1500-2000 rpms, you should have a gusher of water from hole in head. If not water passage restricted, or pump sucking air.

 

[Four]

Re: 1990 70hp Evinrude-chronic overheating

Thanks will check water flow this wkend.

 

[seahorse5]

Re: 1990 70hp Evinrude-chronic overheating

Change out that distorted rubber thermostat seat. Clean out any sand or salt buildup underneath the seat, also. Seeing those 3 grooves from the old style thermostate flutes means the old t'stat was sticking inside the grommet, and that is why they changed the plastic t'stat housing design.