Yamaha 250 SX overheating

[vangrady]

Hi Guys, I am back for help. My boat has twin '2000 Yami 250 SX's with 180 hours. Last summer my port engine was overheating at various rpm's and after several helpful comments, regarding, t-stats, prv and waterpumps, it was narrowed down to a shreaded impeller. I replaced both waterpumps last fall and shut down for the winter. This weekend on start-up, both engines overheated within about 1 minute of each other at low rpm after warm-up of 10 minutes at (600 - 700 rpm). To summarize, I have replaced T-stats, PRV's and Waterpumps and now both overheat! My mechanic says that some of the machines when run in the test tank are on the verge of overheat at low idle because the pumps are barely able to pump the water to the top of the head. He has used an infarred heatgun to monitor the temps and some enigines are within a few degrees of triggering the temp sensor. This is good information but before all of these problems started, both engines could idle for long periods without overheating.
Has anyone else experienced these problems and what would be the next step?
Many thanks.

 

[Tuna Man]

I have read many posts on various websites with this overheating issue as well. Many people experience overheating on 'ear muffs' or in a test tank of some sort, then put the boat in the water and the problem goes away.

This past weekend I tested my own twin 2006 F250 engines after replacing the water pumps and the thermostats. Mine did not overheat after running on the 'ear muffs' for twenty minutes each so I assume I am ok.

However when thinking about Saturday, I did notice a few potential problems that others may have witnessed:
A. Water pressure/volume may not be adequate from the source. Most people I know have to run as much as a few hundred feet of garden hose which certainly restricts flow substantially. Most hose spigots mounted to a building produce about 5 pallons per minute (exact amount depends on many factors). When a 5/8" x 100' garden hose is connected to that spigot and attached to a engine flush device I guess the flow is about 4.5 gallons per minute (gpm). As the hose gets longer (or smaller like 1/2")the gpm decreases. If your water source is suspect, you could time how long it takes to fill a five gallon pail, divide th time by learn the gpm delivered to the flush attacdhment. I am guessing that I need at least 3.5 or 4 gpm as a minimum, although I have never actually measured it.
B. Most 'ear muffs' (engine flush devices) do not fit all that well to the pick ups on lower units. I have wrapped them with cable ties on occasion and had pretty good results. I have seen witnessed as much as 1/3 of the water leaking from the muffs instead of going into the water inlet.
C. I know this has been mentioned before, but I suspect that if the impellor was installed with the vanes going the wrong way they would straighten out and go the correct way when spinning at least 600 rpm.
D. I have seen sand and seaweed and other debris on the black plastic inlet strainers. I guess it is possible to have thm restrict flow if loaded.

Just some ideas for you to consider when running on ear muffs.

 

[Strikezone]

I had this problem a few years ago where the engine would alarm below
1000 RPMs. Anything over that and the alarm stopped. I changed
pressure release valve and this solved my problem. There was a good
bit of sand and gunk that didn't allow it to properly seal. I also checked
and changed the thermostats after solving the problem.

You might check the poppet again to make sure there isn't debris before
moving forward.

 

[Grog]

The T-stats are most likely OK. The poppet may have something holding it open. Take out 2 bolts and you can verify that, but the odds of both having the same issue are slim. It's only 2 bolts so not a biggie to check. I have a feeling it's the water pumps. When I fire up the motors in the Spring it's only enough to verify they run because I don't trust there's enough water getting to the pumps. If the pumps run dry or without enough flow they burn up. I'd drop one of the lower units and check the water pump if it looks OK put her in the water and try the engines.


It's not hard to drop the lower unit (1 bolt is under the zinc) and there's 4 screws holding the water pump cover on. If you pull the cover up a little you can see how the impeller looks. To put the lower unit back on be sure the shifter aligns with the lower unit and bolt'er back up.