water wont pump continuously tell tale cuts out at high rpms

[sparky84]

i have outboard 83' mercury 135 v6 2 stroke and i am completely out of ideas. it started at the begining of this season first trip to the lake after i replaced the impellor which seemed to be working fine in my driveway. on the lake it would only pump water at a low idle, if i pushed the throttle any farther the check stream would stop and only steam would blast out untill returned to idle.i went home and replaced the impeller again and double checked everything and replaced the thermastats which i found one was missing and the other was seized. the new thermastats seemed to help a little could now idle slightly higher before water would cut out and even when stream was present it was weak. so i checked the impeller again and rebulit the pressure release valve replaced all hoses and checked for blockages using compressed air which seemed to have no problem blowing thru. i put everything back together and this time left both the thermastates out. now it had a strong stream and would increase when throttled up. took the boat to the lake today all seemed well till i went full throttle and again stream stopped and steamed untill i pulled throttle back a little then strong stream came back. SOMEONE PLEASE HELP I DONT KNOW WHERE TO GO FROM HERE!!!!! anyone with any input would be greatly appreciated cause i dont think there is anything left to fix haha at least nothing i can find in the manual.

 

[Texasmark]

First off after having worked on my son's 150, without the stats in place the tops of block don't fill with water leaving part of the cylinders exposed to air. I sugggest you put them back in. Look at your plumbing to your pee port. If you have a hose from the rear of each stat housing teeing and going below with a tee off to the pee port, your stat has to open before you get any noticeable water out of the port. So on cold days it will be delayed and it will be intermittent as the engine heats and cools from the stat opening and closing.

At the top, rear, center of the block was a hose that connected down below the powerhead to the pop off valve which is pressure activated and used for increased flow at the higher rpms where more cooling is required. You said you "rebuilt the pressure relief valve", what was that? The pop-off? How did you rebuild it?

If you disconnect this pop off hose and run the engine you should get a constant stream of water out of it (once the block fills) with the pressure and volume dependent upon the engine rpms assuming that if on muffs, you have adequate water supplied to the LU.

On the boy's engine, I replumbed the pee off the pop off hose where I had a reliable water supply to tell me the impeller was pumping. On worrying about the stats working or not, that's the job for the OT sensor.

For the record, the stat in his was stamped 143F for the opening temp. There is no steam at that temp; hot water yes. 140F is the temperature of a US domestic hot water heater when the tstat is in the N position for a reference what 143 feels like.

HTH,
Mark