5.7 mercruiser over heats at trolling speed.

[Rich L]

My 5.7 mercruiser over heats at 650 rpms while trolling. Changed the impeller, thermostat ,exhaust boot and back flushed the cooling system and the problem still exists. Any solutions will be welcomed.

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[Kuba]

Timing???

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[slow rollin]

Rich most engines will overheat at 650 rpms. Your impeller is not spinning fast enough to supply the water your engine needs to keep cool. Also you will have dangerously low oil pressure which will eventually cause major engine damage. Go buy a set of trolling bags the resistance will make you run a higher rpm ( should be 1000 to 1500 when trolling) this will allow your engine to pump the water it needs to keep cool and keep your oil pressure at a safe level. Hope this helps? Good luck

[vetting]

I have an 89 mercruiser 5.7

 

I idle mine at 600 and never have had a problem.  Oil pressure is usually 30+ at idle on 15w-40.

 

Does it only overheat at idle?  Are you sure the fins are going the correct way and all going the same direction when the impeller was installed?  Has this always been a problem since you've owned the boat or is it something more recent?

 

Was the old impeller shredded at all?  A common issue at idle is that the old bits of the old impeller are stuck in the system and blocking low pressure water.

 

If it overheats at idle, but is better at a higher speed, it almost always points to a water flow/pressure problem. 

 

Another common problem is the check valves/balls in the hoses - thats if thats the system you have.

Pull the the two upper rubber hoses off thermostats housing to check balls.  If they are too loose or not seated correctly, they will let water through without it cooling the block.  You can tighten the nut on the shaft and check to see if that tightens them up.  If they dont tighten up, I think you can stretch the spring and put it back together.

[Xxx]

I own a 1982 350 mrec. And 650 rpm at 30 plus with sae 30 oil and barely move the temp gauge, I had every issue you could imagine with over heating, first I would check your risers, after the checking the ball thing, mine doesn't have that, but a lot of trouble can occure in the riser, I was told to take them off and inspect the area where the water dumps out the bottom, if you flip them over there is I slit make sure that is open along with the other ports, use a coat hanger to loosen the rust and slag that gets hung up in the ports, then blow them out with air compressure. Another problem I had was the exhaust shutters, mine rusted off and lodged in the y pipe but I doubt that's your issue, I'd check out your risers first.

[holy mackerel]

I would mount a 10 horse, 4 stroke kicker on it. Less fuel, stealth trolling, less hours on main, and security.

[Rich L]

I should have mentioned that the motor is new with only 100 hrs on it.

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[lost a lure]

Warranty!

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[slow rollin]

Try the bags Rich I'm sure this will solve all of your problems. Let us know how it works out for you.

[Chas0218]

I should have mentioned that the motor is new with only 100 hrs on it.

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Why even screw around do what lost a lure said!

 

 

Warranty!

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Yup exactly what I was thinking!

Edited June 4, 2014 by Chas0218

[vetting]

Ditto the warranty.  The worst case scenario is overheating to do blow head gasket/low compression on a cylinder. 

 

Can you provide more details on this "new" motor.  Was it just a block or was it a full long block with all of the bolt ons that are all new?

[slow rollin]

Any mechanic will tell you that your rpm's are way to low. This is a very common problem people have with 5.7 motors. It is much better to run the motor at a higher rpm to ensure proper oil pressure and water flow. But many other point the guys on here mentioned are worth looking into. I'd run bags that's just my 2¢

[G-Daddy]

Just put a new 350 in the Sportcraft last season and it idles at around 800 rpm.  It is a factory new model.  Depending on current we may run her at up to 1200 rpm just to maintain lure speed based on the readings we get from our Subtroll - and that's without bags or buckets.

Edited June 4, 2014 by G-Daddy

[Rich L]

Thanks Mackerel. I know where there is one on a red boat next to me at the dock.

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[Rich L]

Thanks guys. I will try the bags.

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[Tim Bromund]

I have 2 28" Amish Outfitter Beefy Bags for the 24' Thompson.  I usually only have one in if needed, but when I have both out, they will drag me to almost a dead stop at idle.  I like running 1 bag to get the RPM's up a bit, plus you have better throttle control if you have the throttle engaged a bit.  There always seems to be that point at the bottom of the throttle travel between "idle" and "not at idle", where it is really difficult to fine tune speed.  Getting the rpms up a bit alleviates that condition.  I only put both out if we're fishing in big waves or trolling into a severe subsurface current and just cant get slow enough downspeed without them. 

 

Tim

[Bad Habit]

I had the same problem with my 1987 5.7 merc. Tried everything I could think of. It turned out to be exactly what Vetting described with the nylon check balls by the thermostat. I pulled the hoses, tightened the nut which put more pressure on the nylon balls so less water was being pushed out the exhaust manifold. I'm not sure how to correctly set them, mine was trial and error but worked like a charm. At least it's an easy try. Good luck

[vetting]

Check your balls!

[Rich L]

The problem was in the gauge/wiring. Mechanic checked engine with infrared heat sensor.

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[Xxx]

Yea, that's the best tool to have in the boat!! I had so much over heating troubles I bought a infer red gun, and I still don't believe my gauge, and its a new one. I still pull the motor cover back and shoot my exhaust sleeve, rubber that goes between the riser and the exhaust pipe, which reads 137 degrees and my exhaust manifolds read 125, along with the heads, yea I'm paranoid, but its a piece of mind, before they got so hot you could smell the rubber, temps around 190* good grief,but all is well now, but I still shoot my engine after a long haul JUST to make sure. I haven't used the boat enough to build my confidence yet. I'm glad your was a easy fix.

[TyeeTanic]

I have a 5.7 and troll with it at 600 to 800 RPM.

No temp or oil pressure problems.

[hooknup]

Water pump impeller?

Thanks

Al

HOOKNUP 2

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[MWOLF451]

Having the same problem. It is currently in the shop. So far they found that the water pump housing is warped and three "O" rings are missing. This is after I had another shop change the impeller and water pump. Haven't got it back yet to see how it is running, but I am sure that when things are fixed right, it will run great. I was told that I should not try to troll under 1000 rpm. It is not good for the engine as the oil pressure is too low and the impeller is not able to pump the water fast enough, so I bought two trolling bags.