87 Mercruiser 4.3L overheating at at idle

Looking for help. Anyone have any suggestions?

I have an 87 mercrusier 4.3L w/ Alpha One that recently started to overheat at idle and no wake throttle. When cruising 1/2 throttle and more, the temp remains 165 - 170. However, during no wake zone and idle, temp climbed to 200-210.

I've removed and checked all hoses for blockages (clean and no debris), replaced t-stat (160 deg) and double checked for water leaks (Motor on Muffs) and kinked hoses with nothing found (temp around 165 on Muffs at about 1,000 RPMs)

Put her back in the water and temp seemed OK (170 after about 10 min idle). Cruised around the no wake zone for a about 5 min and temp climbed to 200 again.

Any suggestions will be greatly appreciated.

Re: 87 Mercruiser 4.3L overheating at at idle

Sounds like it needs a new impeller.

Re: 87 Mercruiser 4.3L overheating at at idle

Thank you for your input... I have also thought of the impeller to be a possible source, but from all the postings I have read, it appeared that in most symptoms, the temp increased with engine speed and would cool back down at idle when the impeller was suspected of being damaged or defective. My overheating issue is just the opposite, which made me guess that it may be something other than the impeller. Maybe I was wrong with my "guessing".

I still haven't ruled out the impeller, I just needed additional input and suggestions as the next place to look at based on my overheating symptoms.

Re: 87 Mercruiser 4.3L overheating at at idle

It's your impeller. It's not supplying enough water at idle to cool the engine. Once the out drive starts moving through the water at any speed above an idle the water is forced into the cooling system and the impeller becomes a non issue.

Re: 87 Mercruiser 4.3L overheating at at idle

Limited-Time,
Thank you very much, it's makes plenty sense to me... guess I'm off to buy an impeller kit.

Re: 87 Mercruiser 4.3L overheating at at idle

You might also want to have a look "down stream" of the water outlet. The debris for an impeller usually gets into the oil cooler and/or the power steering cooler and plugs it up.


73/Rick

Re: 87 Mercruiser 4.3L overheating at at idle

Update -
I bought the impeller / water pump kit and on the advice of the Service department I purchased the base also. Glad I did. The impeller did have some damage to a couple of the fins. The entire fin was not gone, just a couple of small pieces off each one. The remaining fins were also on their last legs (lots of cracks).

The plastic tube was deformed from heat I'm guessing as well as the base had some deformation (glad I purchased the base).

Replacing the impeller was not as bad as I feared thanks to the amount of information I was able to find from searching threads.

I was still a little nervous as a lot of threads suggested a second person to help when re-installing the drive and I would be doing this project by myself.

Long story short.... From the time I started draining the lower unit, 2.5 hours later, it was back together and running on Muffs. I will lake test tomorrow which will be the true test, but all is looking good right now.

Again, thanks to all those who post questions and a special thanks to those who make those replys with excellent suggestions and advice.

Re: 87 Mercruiser 4.3L overheating at at idle

Final update on overheating problem..

Before going to the lake, I went ahead and swapped the new 160 deg thermostat with a 140.

Lake test for overheating went excellent. Temp never got above above 145. Let it idle about 15 min, then cruised around the docks and slips for about 30 min since this is when the overheating issue was occurring. I Then cruised around for about another 30 min between 1/2 and 3/4 throttle while keeping one eye on the temp gauge. Although temp did great and changing the impeller was the fix, I would like to say all is well that ends well.... but a new problem has shown up but that's for a new thread. Thanks again for all the help with this issue.

Re: 87 Mercruiser 4.3L overheating at at idle

May I pipe in here?

I have the same engine. A few weeks ago I was trying toi get it running and I did. WHile I was idling on muffs the heat was going up. I increased the H20 pressure to the muffs and it cooled down to the middle of the temp gauge.
However I realized my carb was shot and just today replaced it. THe boat started on muffs again and this time the temp went up and stayed there even with increased H20 pressure. THe impeller was replaced before I started doing anything to the boat. Could I have damaged the impeller berfore I increased the H2O pressure and it is just showing the problems now?

Could it be the carb, maybe too lean?

Or should I just replace the thermostat and hope that God loves me and all wil be right with my boat and the world?

Thank YOu foir any help

Re: 87 Mercruiser 4.3L overheating at at idle

Could I have damaged the impeller berfore I increased the H2O pressure and it is just showing the problems now?

Click to expand...

Yes...