Just added a harden marine water pump housing and impeller,thermostat was replaced last summer. Engine is heating up tp 170 @ 4000 rpms, port manifold is 146 and starboard is 134 degrees. On the way back to the dock the engine started to miss. Original 2004 mercruiser exhaust manifolds. The boat was used in saltwater by the previous owner. Fresher water use now. Any thoughts....manifolds leaking?

Was it running hot before the pump change? Check the water hose where it bolts to the transom, its two 11mm bolts.

It was not over heating, but it was running warmer (165) and took a little longer to cool off after a good run.

I'm not sure how long the water circulating pumps last. Since it started missing I'm leaning toward faulty exhaust manifolds ( 10 years old- salt water use). I plan to check the plugs and pull the manifolds - replace with cmi etop headers. The missing has me concerned.

I have a Rinda scan tool,erased all faults and ran a system check. No faults other than the normal ones that I get.

I see on smartcraft gauges these motors run 164 to 172 all the time even new. Anyways always one side exhaust manifold is warmer than the other side because of uneven water flow. IMO if you stayed steady eddie at 170 degrees in all running conditions you are fine but of course running closer to the 160 to 165 is always better. If this motor sets a warning horn and or goes into guardian mode then most likely you are overheating.

Ok, I guess I should have seen if it would have gotten hot enough to go into guardian mode. I also got concerned when the engine started missing.it will be interesting to examine the plugs tomorrow and test the coils with the Rinda.

How old is the antifreeze? Is it still pink or is it starting to get grey/metallic? 6-7 seasons tops on antifreeze in any of these closed cooling motors. It's starting to silt up when it gets dark, and will not flow well or cool the motor.

Is this a new temperature you are hitting? That doesn't seem too high to me. Does your Rinda say you have plenty of water pressure?

Yes, definitely hotter than normal. Last year I had similar issues and you assisted me. Antifreeze, thermostat and flushed heat exchanger within the past year. All new sensors. New Hardin marine pump and impeller. I should have noticed the water pressure but was to focused on the engine and exhaust manifold temps.

Agree with Brian here as well. The anit freeze is a Dex-cool extended life mix 50 / 50 from Mercruiser. You can buy Dex-cool use distilled water50 / 50 mix if you change your anti freeze . Tap water has minerals in it and can over time leave deposits in the cooling system. Anyways OEM Merc spec on the anti freeze is 5 years or 500 hours which ever comes first. We change it at 5 years or 400 hours whichever comes first. If you are not using extended life anti freeze then Merc spec is 200 hours or 2 years which ever come first.

Water circ pump and or t-stat can play a role here for higher temps especially t-stat with very old anti freeze still used in the cooling system. Old anti freeze loses it corrosion protection properties over time thru heating & cooling cycles along with oxidation. Also old anti freeze persay not to get into a story book write up here adds surface tension in its flow patterns thru the cooling system. .

If you have smartcraft gauges you can watch your water pressure running. I leave water pressure displayed on my gauge all the time to monitor any changes with it. .

Is there a way to determine if the exhaust manifolds are bad or leaking? Any good advice to breaking the bolts loose on the manifolds? Since they've been on for 10 years I would imagine this could be the biggest challenge.

CRC freeze off or the new Loctite formula for frozen fasteners but before you apply the CRC might want to heat the bolts with a hair dryer on high setting. Also old machinist trick is transmission fluid & acetone mix to free up nuts / bolts / fasteners