View Full Version : Still Overheating - impellar and therm changed

07-10-2006, 01:51 PM
I need some help in diagnosing a problem of overheating. I checked the site very carefully and couldn't find anything to get me past simply changing the impellar and thermostat.

Last week, I was using the boat for the first time this year. The engine started overheating and burned out the insides of the mufflers. I checked the impellar and it was fried.

I replaced the impellar and it still started overheating and mroe smoky mufflers. I then changed the thermostat.

I have a PCM engine in a 1997 Moomba Outback. Engine ID - PLD 5.8 L

When at the auto dealer, we looked up the parts for a PCM engine and it called for a 142 deg therm. The one I took out was stuck shut and was labeled as 160 deg and was stock to the engine as I had to break the paint seal on the parts.

I changed the therm and took it for a ride, no more smoke and I could feel heat on the pipes coming from the engine into the headers. However, the engine temp gage still went over halfway, ran about 185 deg.

I took all the freshwater hoses off and blew them out and ran water through them to check flow. I also did the same on the transmission cooler and muffler and headers. All flowed well, in fact I could see the insides of my mufflers flow out into the lake as ash from the flareup.

I dont have a freshwater screen on the system.

The dealer said they are 3 weeks out on repairs and not taking appts. I want to get skiing for the summer before I have to take it in for storage. Any comments would help!

Anyone got any clues on what to check next?

Thank you


07-10-2006, 02:58 PM
when you say the impellar fried, how many fins where left? if any? There is a good chance if the fins are missing that they are clogging the engine manifold, or water passage ways in the block. thus not letting water flow through the engine? As well check to see if your boat has a primer or pre-impellar pump that might have gone bad.

07-10-2006, 03:15 PM
The impellar was missing pieces and you could tell it might have melted, then shredded from not being able to pump water once the system was full and the therm didnt open. I should have taken a pic of it.

I tried gravity flushing the manifolds with clean water under gravity head and got a bunch of rubber material from the impellar out of the system. I also gravity flushed everything else and then blew through all the lines and was able to clear them, I thought. I am starting to wonder if there is a strainer in the engine water pump that may be plugged.

I can be reached directly at a_grosser@msn.com with comments to my original post.

07-10-2006, 07:39 PM
Where are you located? What is the air temp and water temp. Running 185 should not be a problem. I am in the South in the middle of Alabama. Our Mobius runs 180 all day long. If it is 70 degrees or 90 degrees it runs 180. Seems to be the "comfortable" range for the enigine. I would only worry if it goes 200 plus. Anything under 195 is acceptable. I have almost the same engine in my Tahoe and it came with a 195 which I replaced with a 160.

07-10-2006, 09:00 PM
I am in Northern WI. Water temp was 76 deg. It was 90 deg outside. We were pushing 200 on the trial after the new therm went in and I shut it down and got a tow back to the dock after roasting in the sun for a half hour waiting for help. I believe the gage max is 225.

I guess I am pretty concerned about overheating the engine and any damage I may have done already (burned up exhaust already). I have tried to find contact info on the PCM page and talk to them but they dont look too helpful. A word from someone at Moomba would be nice too.

I only have about 175 hrs on this 97 and hope to run it many more years.

07-11-2006, 01:17 AM
I don't know how likely it it is, but the engine water pump may have been effected as well. Maybe it's no longer running as efficiently as it should be.

07-11-2006, 08:38 AM
Maybe I missed it, but the strainer on the bottom of the hull is the first place I look. A friend burned up his impeller due to a plastic trash bag getting drafted up into it. He had to take it off the boat and pull the remaining pieces out to clear it completely. If this had been checked then, like you, I'm stumped.


07-11-2006, 06:03 PM
I'll pull the strainer and check. Maybe a mouse or something got into it this winter. I am able to blow water back through it from the tranny cooler though.


07-12-2006, 09:42 AM
FYI. For those of you with possible flow problems from the raw water pump. I talked to PCM and they recommended a flow test from the pump. At 3000 rpm, the pump should fill a 5 gallon bucket of water in 15 seconds for my engine.


07-13-2006, 12:09 AM
i would be careful with that.Not many garden hoses will fill a 5 gallon bucket of water in 15 seconds.If I use the fake a lake i never come off idle.Just my .02.

07-13-2006, 08:16 AM
I left off the most important part, Put Your Boat In The Water.

You would probably need a 3" line in order to get enough water from a hose and even then, I would worry that the line would fall off the intake unless it was plumbed right into the intake hose inside the boat.