Mercruser temp fluxuating HELP

"Hi,

I have an 96 Sundancer

"Hi,

I have an 96 Sundancer with a 5.7LX Mercruser engine (Alpha 1). I started getting an alarm last weekend and noticed that the temperature gauge was pegged at 200. I stopped the boat to take a look at the engine compartment for something obvious but found nothing. On my way back to my dock, I noticed that the temperature gauge was fluxuating from all the way left to 200 say every couple of mins. Note the alarm would go off ONLY when the temp was at 200. Starting the troubleshooting process, I changed the thermostat. No resolution. I checked the pressure from the impeller and it seems ok and steady.I back flushed the oil cooler. Still no resolution. Checked the intake on the sterndrive. All clean and no objects blocking. My mechanic thought it was the sender to the gauge (WRONG!). Now he thinks its exhaust gases going to the cooling system but has not proven it. Suggestions ???"

"Pull the Spark plugs, and ins

"Pull the Spark plugs, and inspect them (for some reason #3, and/or #5 usually seem to be at fault). See if any look like they're REAL clean, like steam cleaned, or show water droplets. Typically a temperature fluctuation that bad will indicate a blown head gasket, or cracked head. Other than inducing air into the water flow at some point in the system (such as, bad inlet hose, or bad grommets between the lower and upper, or an improperly seated slinger on top of the water pump) Head gasket (or cracked head)is usually the cause of a severe fluctuation like that.

I'm not saying it cant be something else(and it may well be), but in my experience thats the most likely cause.

Mr. B"

"when was the impeller changed

"when was the impeller changed last?Risers changed when?Can you hold your hand on the manifolds or risers when the alarm goes off?
might be clogged risers,burned flappers clogging the exaust ports, connector hose at transom may be kinked or collapsed internally.pump could be picking up the exaust instead of water"

"I'm with Bt Doctur; if th

"I'm with Bt Doctur; if the impeller has never been changed I'd say it's fried. It's easy and cheap to inspect and change, and I'd look at that before doing anything else.
If fragments of impeller blade are missing, they have gotten into your cooling system and could be blocking a water inlet passage somewhere. You could backflush from the T'stat housing while the lower unit is off to see if they dislodge and come out.
Rod"

"All the plugs seem fine. I ch

"All the plugs seem fine. I checked the oil to see if it has changed color. Looks ok
The pressure from the impeller seems fine. I disconnected it from the engine and ran it for a moment. constant pressure even when I try to cover ( block ) the hose from pushing the water out.

I think I am getting air in the cooling system. When I hold the hoses from the top of the thermostat housing, I feel a pulse like air is going through. Much like that of a wet vac picking up water then air. I ran waterr through both exhaust manifolds. does not appear to be any. blockages Suggestions?"

Riser surfaces are shot. Build

Riser surfaces are shot. Build up with epoxy file smooth and be done with it....Terry

"I have a 1987 5.0 LX Mercruis

"I have a 1987 5.0 LX Mercruiser/Alpha 1 that is in excellent running condition. After warm-up and running at higher RPM's, the temp gauge climbs toward 200 in neuteral or idle but can be brought down by running the engine at around 1500 RPM or higher. Normally, the highest temp reading, even after a full day of water sports is <150 degrees. Where should I start looking? Thanks. Big Joe."

"It sounds like your impeller

"It sounds like your impeller may have taken a set. I would start by replacing that. When they're starting to go, a lot of times it will only run hot at idle because the impeller isn't pumping efficiently at low speed. The blades are not making good contact with the walls of the pump body. Another thing I've seen cause overheating at idle is, (if your model has it) there are two hoses coming off the top of the thermostat housing, and going to the exhaust risers. Pull those off, and there will be a couple of plastic check balls. Sometimes those will become loose, or get debris in them causing them to hang open which can also cause it.

Mr.B"