tell tale problem

Orlando · #1 01-25-2012, 08:25 PM

ok, i got the engine running the right direction wich was the previous post on this motor,, it is a nissan 15hp ns15b2, but i dont think this problem is model relevent. it had been sitting for a few years when i got it, i rebuilt carb, and replaced the entire ignition system. when i was working on it i knew the impeller was shot because it had been sitting, so i only ran it a couple minutes at a time. so i havnt let it overheat badly. i replaced the impeller the other day, and run it in a 50 gallon barrel, pumps water fine at idle, good steady stream from tell tale, however if i increase rpms say from 700 to say 1500 (about halfway from the slowest setting to the start position on the tiller handle)the tell tale stops almost immediatly and it looks like exhaust coming out, water also stops coming out the exhaust ports on the back, then the motor starts to warm up, but if i lower the rpms again the water starts flowing again and the engine cools right back down. there was no thermostat installed when i bought the motor, and i have a new one that is currently back out of the motor. no difference. this is the first time i've really messed with an outboards cooling system, so any suggestions are greatly appreciated.

Blue_Runner · #2 01-26-2012, 10:17 AM

Not familiar with the cooling on the Nissans but assume it is like others.

On the Johnny rude 9.9 - 15hp when you re-install the lower unit you have to be careful when lining up the copper pickup tube with the rubber grommet on the impeller housing. Sometimes the tube can pinch the grommet causing leakage there which could possibly be enough to divert water thru the leak as RPM's increase? If I remember correctly there were 2 different grommets in the OMC impeller kit - slight improvements made in newer models so it would be important to use the right one for your year model.

Does it have the wedge shaped key that locks the impeller onto the shaft? Although forgetting to install that key would probably prevent any water from reaching the tell tell and/or water jacket at any RPM? The shaft would just spin inside the impeller causing it to not pump water.

One other thing, has it been run in saltwater? If so there could be salt deposits/corrosion blocking the water passages. This would especially be a concern if run in saltwater and the p.o. was not diligent about flushing after use.

Destroyer · #3 01-26-2012, 10:39 AM

Understand, I am NOT an outboard mechanic nor do I pretend to know everything about them. But another thing to consider is the rpms of the engine. I know that it used to be that when engine rpms went above a certain speed the impeller was designed to stop working because the speed of the boat thru the water forced water into the engine water pick up and through the engine without the use of a pump. I do NOT know if that's still the case, or if it's the case for your particular engine, but it might be worth considering.
Another thing to look at is are you sure that your impeller is facing in the correct direction? It's very easy to install them backwards, and since you say your engine was running backwards perhaps it got flipped?

Orlando · #4 01-26-2012, 03:55 PM

I will take the lower back off today or tomarrow and have a look. im familiar with car engines, not outboards, so this is a learning curve. i figure if im gonna learn i'd rather learn on the cheap little motor then the big motor. if it all checks out i will try bringing it down to a lake on a little john boat i have and try it moving through the water. the cooling passages looked clean from what i could see under the thermostat housing, the motor used to be on a sailboat and was only used a handful of times by previous owner.

Blue_Runner · #5 01-26-2012, 04:14 PM

PM Sent

steplift20 · #6 01-26-2012, 04:40 PM

I agree with the last post check to see ifit was installed right the fins of the impellor should only face one was and make sure the key is in place on the shaft of the impellor. im sure your using a new impellor cause if your not the rubber could have sepperated fron the housing

reelapeelin · #7 01-26-2012, 05:46 PM

Your description of what it does sounds like the Sierra pump we 1st used on the Johnson tried on my '84...wound up swappin' it out for a real factory replacement which solved the problem instantly...no more off brand pumps for me...

phatdaddy · #8 01-26-2012, 08:15 PM

ditto reel, had 2 sierra's go bad within 2 weeks, went with merc's and all was good. also learned to change out the housing along with the impellor.