Overheating Merc 5.7/260

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We recently bought a 1986 Cobalt 21 with Merc 5.7/260. We have put only ten hours on it, but everything has been normal. The other day we took it out and it overheated. Hate to admit it, but it went over 200F before I noticed it. Temp had always stayed at 130F before. Kitchen stove test on thermostat was inconclusive because I had to use some force to remove it and may have damaged it. There was no debris in water jacket that I could see. Normal amount of rust (I’m comparing to a car. Some surface rust, but no flakes.) Hoses look good inside and out. Removed stat housing and cover, cleaned thoroughly, replaced with new Merc.gaskets and new Merc stat. New stat tests ok. Now it still runs hot, but only about 160F at 2500 rpm. At idle, temp just keeps going up. If I waste water to bilge from top of block by removing plug in stat housing, water flows freely, not like fire hose, but steady stream and temp drops.<br /><br />Sounds like worn impeller to me, but I’d like comments before I tear into it. I’m a fairly good driveway mechanic, but new to marine engines. I see frequent postings re bad impellers, and also note that marine stores have them on the racks, so I guess this is not rare. But, …do they wear out or do they fail catastrophically? Mine seems to be the latter, but it still works sort of. Is there a typical life span for these things?<br /><br />Should there be a measurable pressure at the stat housing?<br /><br />Oh yes, boat has always been in fresh water (Wisconsin) and I’m pretty sure was winterized by dealer and stored indoors.

 

[Trent]

Re: Overheating Merc 5.7/260

I like to recommend changing impellers every two or three years. Your describing the classic case of a worn impeller. Replace the housing and wear plates..Not just the impeller.

 

[Boatin Bob]

Re: Overheating Merc 5.7/260

Had a similar problem but with OMC Cobra, manual had a water test where you would take the raw water hose that came from the outdrive water pump and disconnect from T-stat and hold it vertical. Start engine (only for a 10 secs or so) and check for a good stream of water that went about 4 or 5 inches out of the hose, if OK then your impeller should be OK. My problem never came back (temp guage over 200) Concensus was that it was probably a plastic bag stuck/sucked to the water intake and when engine was shut off it floated away.

 

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Re: Overheating Merc 5.7/260

First, thanks (late) to Trent and Boatin Bob for your prompt good advice. Re Trent's reply, I will definitely change out the water pump, have kit, but haven't done the job yet. I must comment as an engineer that I find it disappointing that they would design a machine with an internal component which requires replacement every two years. I sure would not have lasted long in any of my jobs as an engineer if I did that. Oh well, I'll just accept it.<br /><br />Re BB's reply, that also was helpful. I didn't mention in my first message that I had taken the hose off. I did get water, but not the 4 or 5 inch stream you mentioned-maybe only an inch or two.<br /><br />I have had very little time to tinker with this. Actually spent a lot of time replacing the hydraulic trim lines, because they melted/chafed where they rub against the exhust when the thing overheated.<br /><br />Anyway, here's where I am. I accept that the water pump should be changed, but it does pump some water.<br /><br />The hose coming from the water pump is in very poor condition near the stern drive housing. I couldn't test it for leaks, but I will certainly replace it. Looks like a knuckle-buster to me. Any advice?<br /><br />I have removed the thermostat.<br /><br />I have put a 3/8 pipe and ball valve into a plug in the stat housing and can bleed water through a 3/8 plastic tube.<br /><br />If I drive the boat at moderate speed, say 2500-3000, it quickly runs up to 160 deg. More power and it gets hotter, so I backed off. If I open the valve and run water into the bilge, it helps, but it's still pretty hot. Yes, I know I have a defective pump, but I am getting about 1 gal/min of fresh water in there. The water is hot coming out-burns my hand, so we are removing heat from the engine, and the water is not short-circuiting.<br /><br />Based on experience with engines other than marine, the two basic causes of overheating are late timing and lean mixture, neither of which are happening here.<br /><br />So, gentlemen, am I simply faced with the pump problem, or is something else going on here? Pardon my stubbornness, but it's just my nature to question the obvious.<br /><br />Thanks!!

 

[Wellcraft 89]

Re: Overheating Merc 5.7/260

Sonofbill,<br />as Trent has said you need to replace the waterpump in the outdrive. I would suggest getting a manual for your engine and out drive first. If ,when you take your drive apart and the impeller has disintegrated you need to try and back flush and pieces that could find their way to block <br />the water tube going toward that that knuckle buster hose you mention.<br />I once had a piece of an impeller block the flow of water to th engine kept overheating even after changing the pump.<br /><br />Also trent has posted lots of useful info in other posts regarding this problem.<br /><br />Yes at one time or another we have all wondered why <br />mercrusier designed the drive with a pump internal that has to be replaced every couple yrs.<br />Yet alot of us know people who go 5- 10 yrs without changing it.

 

[bbracken]

Re: Overheating Merc 5.7/260

To wellcraft89<br /><br />Thanks for the advice. I'll just take the time and do it tomorrow. I have the Clymer manual. Do you recommend another?<br /><br />Thanks again!

 

[Walter]

Re: Overheating Merc 5.7/260

I recommend the Mercury factory repair manual. I bought one when I purchased my boat new in 1980. The manual has paid for itself a hundred times over. It will answer nearly every question you've posted here and on another thread as well. It'll provide you with a cooling schematic for your particular model, which as an engineer, I'm sure you'll find very easy to follow.<br /><br />Walt