Mercruiser 140 overheating at WOT

[jagilbert]

I just bought a 1973 sidewinder 16' w/Mercruiser 140 and took it to the lake. It very quickly overheated. I then replaced the impeller which was, of course, completely shot. I also replaced the missing thermostat.

Now it warms up to about 160-180 degrees and stays constant until I push it hard. Then the temp climbs toward 260 on my gauge and I'm sure it would keep going if it let it. As soon as I back off the throttle the temp comes back down.

All the while the oil pressure reads fine (30-40psi)
Compression is 120,120,115,118

Could any of these be the problem?

• When I replaced the impeller, I left out the plastic guide tube which had melted from the previous overheat. My understanding is that this is only used to ease reassembly(?)
• I also didn't pay attention to which direction i flexed the impeller fins when I put it in the housing. I was told that they would flip themselves around as soon as you start the engine.
• The oil level is a quarter inch above full on the dipstick.
• The engine maxes out at 4000 rpm WOT while surging and sounding generally ill. (I am going to troubleshoot this problem separately, but I mention it here in case it may be related)
• There is a slight drip of water running down the side of the block under the manifold

Any suggestions on the overheating? did I ruin the impeller when i replaced it? Could it be the circulating pump? Could a fuel starved lean situation overwhelm the cooling system? Maybe I'm frothing my over-full crank-case oil and collapsing my lifters?

Thanks in advance for any help!.
Jeff

 

[jagilbert]

Re: Mercruiser 140 overheating at WOT

Any insight would certainly be appreciated.
Thanks!
Jeff

 

[jagilbert]

Re: Mercruiser 140 overheating at WOT

I only replaced the impeller, maybe I should replace the whole pump?
(I can get the whole pump on this site for not much more than I paid for the impeller at my local store)

 

[porker99]

Re: Mercruiser 140 overheating at WOT

All so get the mercuriser thermostat b/c auto is about 180 plus and the marine is about 140

 

[jagilbert]

Re: Mercruiser 140 overheating at WOT

@Porker, I did use a mercruiser 140? thermostat.

 

[porker99]

Re: Mercruiser 140 overheating at WOT

maybe defective I would take it back and say give me another one for free I would not pay for another one.

 

[jagilbert]

Re: Mercruiser 140 overheating at WOT

maybe defective I would take it back and say give me another one for free I would not pay for another one.

Or maybe I'll test it by suspending it in a pot of water on the stove and use a cooking thermometer to see what temperature it opens.

Thanks for the response

-jeff

 

[Falco]

Re: Mercruiser 140 overheating at WOT

I would look at the pump. Make sure it spins, check belt tension. If in doubt, pull it off and have a look.

 

[DarrinT]

Re: Mercruiser 140 overheating at WOT

Cracked the head on a mercruiser 140 after changing the impeller and the engine still overheated. The bits of the old impeller were hiding in the bottom of the outdrive water intake where I didn't think to look.(bonehead) Those four small bits of rubber would intermitently clog the intake causing an occasional overheat condition until finally they jigsawed themselves together to totally clog the outlet pipe from the pump and being in the wilds of Georgian Bay with the kids at the time I badly overheated her and wrecked the head to get back to shore.

 

[jagilbert]

Re: Mercruiser 140 overheating at WOT

@Darrin: Good idea, I'll check that. I did notice some bits of old impeller down there, but I didn't pay too much attention to them. figured they'd flush themselves out. sigh. live and learn.

btw: I checked the thermostat by heating it in water on the stove. It fully opened well before boiling.

I've already ordered a complete impeller pump & housing kit. When I replace it I'll dig for old bits of rubber and make sure the intakes are completely clean.

Thanks for the insight Darrin

-Jeff

 

[jagilbert]

Re: Mercruiser 140 overheating at WOT

I dropped the lower unit to replace the water pump and found that the water tube coming out of the pump housing was jam packed with chucks of the old impeller. Doh! Why didn't I check that when I had it out last time. Anyway, that would explain why it overheated under load. Hopefully I got all the junk out of it this time.
I was much easier to remove and replace the lower unit the second time, now that I have experience and the trick someone mentioned about using a bungie cord to hold the prop in forward gear.

Jeff