250 Pro XS Heating up

[Bryan O]

I'll try to keep this short. My '08 250 Pro XS (1B55074) has started idling at less than 5 lbs. water pressure, and if left idling long (5 mins) will heat up to ~ 150 degrees. If just a little throttle (900 rpms), the temp falls to ~ 118. Up on plane, it runs around 125 degrees. I changed the water pump twice. After the first time didn't cool it off, I found that it had pushed the gasket out under the water pump. I replaced it, and replaced the pump and gasket, but the temps are running the same. Wide open used to be 25 lbs of pressure and above, now it's down to 15 lbs or less. At WOT the engine is running at 128 degrees. I just pulled the lower unit and this time it didn't push the gasket out and the impeller looks fine. I'm going to replace the gaskets again, another new impeller and a new pump housing this time. I'm hoping it was the housing, but I've got my doubts. Should I be looking at the thermostats? Something else? It hasn't sounded any alarms, or thrown any codes (that I know of..), and if it wasn't for the temperature mode of the smart gauge I wouldn't even know of the condition. In these water temperatures (Rayburn is at 64) the motor has typically idled at 90 degrees, and heated up to 105 at WOT. The motor is running fine also. What the heck is going on?

[KCLOST]

Could be a t-stat or poppet valve problem.. The easiest to start with and check... If the poppet valve is stuck open or faulty by some debri for instance it will dump water out of the block at lower rpms before it should. And that explains your low water pressure and higher temps at idle... When you raise the rpms like you mention, enough water will fill up the block more and will cool it down some.

[Bryan O]

Update - I talked to friends, the mechanic where i buy parts, and KC's post above, and decided to jump in and repair it. KC you were spot on. I thought it may be the t-stats, but I really didn't know what the poppet valve was. It is a valve that opens at higher RPMs to dump water out of the block so that it's not getting too much and allows the engine to keep operating temperature. It is located on the starboard side of the motor down where the powerhead meets the leg. The most intimidating part was figuring out how to get off the lower cowl. It's really pretty simple. There is a grommet that covers access to a bolt in the back of the lower cowl, and there's also 2 more in the front of the cowl, and one on the bottom of the outside of the front of the lower cowl. It has to slide out of the rubber panel for the tell-tail and fresh water flush, and unplug the trim switch also.

The poppet valve is held in place with 2 bolts and easy to get to with the lower cowl removed. It is a 3 part (layer) piece, with 2 gaskets, and if you are careful you can save the gaskets. I'd hoped to find a small red-ear turtle, or the bill of a 10DX in it, but no such luck. However, it was filthy. A lot of scum and silt and just overall very dirty. It is a diaphragm with a spring push rod with what looks like an impeller on the end (that goes in the water jacket). I guess at speed there is a enough water flow to turn the impeller and push the diaphragm open and dump water out of the water jacket. I cleaned and lubed it up had reinstalled it. As dirty as it was i was pretty certain it wasn't seating or closing. I buttoned it up and tested it on the hose, and the idle pressure went back to where it should be, and the temperature settled in at 125 and stayed there. Took it to the lake, and the top-end pressure went back up to in excess of 25 lbs. when wide open. And the temperatures stayed at 125. FIXED!!

I love the nose-cone water pick ups on today's motors, but if you fish and scout shallow and muddy trash filled lakes this likely will happen. They vacuum stuff off the bottom in shallow water when tilted. I was fishing way north on Livingston last summer, and hopping in and out of creeks on the river that had silted in. The temperatures and pressure went down while I was practicing for a tournament there later that month. Probably shouldn't do that.

Sorry for the long post. Hope it helps someone down the road

[SilverFox579]

Good explanation in the FAQ at top of forum - "Water Pressure 101"- Poppet Valves and Variables with Water Pressure

[Babe Winkleworm]

Would the poppet valve stuck cause water to not get to the compressor?where did you get the parts?

[EuropeanAM]

Possibly... as can a number of other cooling-system related problems.

See Rhonda's post (pinned at the top of the forum)- she can set you up with most any part your Mercury engine desires or needs.

Bryan: Two items to keep in mind would be:

- Engine should ALWAYS reach 120 degrees or higher at idle. If not... DON'T accelerate.

- Make sure you're tilting your engine up enough to avoid dragging into stumps or debris when on the trolling motor. That's where a lions-share of debris comes from...