5.7 running warmer at speed

[Larson5.7]

I have a 1998 5.7 GS - this season I noticed that the temperature was a little high and erratic at speed so I put in a new 160 degree thermostat - today running at idle/ slow the temperature stays dead at 160; running at 2500-3k it is going to 180-185 - and staying right there - I rant it WOT and it went to 190 - as soon as I slow/ stop it cools fine - the boat has a new raw water pump and impeller in the last 10 hours as well. From the rear at idle it appears plenty of water is coming out the back. the recirculation pump does not appear to be dripping (no water is leaking in the engine compartment). My mechanic is telling me this is not unusual and I should not worry about it and that if the recirculation pump was going the temp would just keep rising - I am wondering if it is going out / missing a blade.

Is this truly normal? the boat has never done this before - but last season I did put in a new (used short block) - new manifolds and risers so I do not have many hours experience with the motor..

thanks in advance for any Thoughts/ ideas...?

 

[skydiveD30571]

Re: 5.7 running warmer at speed

Are you doing this on the driveway on muffs? You shouldn't be getting anywhere near WOT while out of the water. Keep it down under 2000rpm.

If you were out the water, I believe that there simply isn't enough water volume from a hose to run at higher rpm's. Drop it in the water and try it out.

If you were in the water, you may have a leak in the water inlet line in the drive. There's a seal that can fail...and while at slow speeds it is under water so the impeller draws water like normal. At faster speeds on plane, the seal is out of the water and the impeller is sucking in air also.

 

[Don S]

Re: 5.7 running warmer at speed

No, it is not normal for them to go to 190?. It may run 160 at idle, but it should never go over 170-175? on a raw water cooled engine.

Do you run in fresh or salt water? What condition are the exhaust manifolds and risers in? Are they getting plugged up?

Also, what is your engines full model number? Year is only for the boat, not the engine and drive.

 

[Larson5.7]

Re: 5.7 running warmer at speed

It is a 5.7 GS; with a cobra sx
the risers and manifolds were replaced late last season - so I am assuming they are still in good condition - I ran the last set for a number of years.
I am running in salt water in Maine - the water is still pretty cold.

thanks in advance.

 

[Don S]

Re: 5.7 running warmer at speed

It is a 5.7 GS

That is the first part of the model number, what are all the letters AFTER the GS?

You may have a new thermostat, and new impeller, but you still have problems. They do not run that hot.

You could have improper timing, over propped, worn out pump that just won't pump as much as when it was new. Could be a slight air leak in the water inlet side allowing air into the system.

Is the timing right, what's your WOT rpm?

 

[Larson5.7]

Re: 5.7 running warmer at speed

Don - I'll have to check out the letters in the morning - it's at the dock and its dark now.

I don't think it is the prop; its basically the same pitch/ diameter I have been running on it since she was new. I thought after I put the motor in last summer it was running about 5k wot; but when I got on it today I only got up to maby - 4400 - but note I only had it wot for maybe a half mile and it was the first time I've had it more than half throttle this year - I don't typically have need to push it.

We set the timing when we put the engine in late last summer and it ran fine the rest of the season - I've put about 2 1/2 hours on it this season and this is just starting.

Note the first time I noticed it was after I towed my very beached dock off the beach - I had it running 2k rpm basically sitting still for several minutes (2-4 mins probably) before the dock came loose - would this stress the recirculating pump or perhaps the seal that you mention may be sucking air? I was not overheating when I was pulling it off the beach - but the next time I got on plane I noticed it was getting close to 180...

The recirculating pump has almost 500 hours on it, put it in 8 years ago; never replaced the any seals related to the water inlet/ tube in the outdrive however I did replace the oil seals in the out drive when I put the engine in. I don't know the part numbers for the seals/ air leaks you reference (do you need the addl engine model numbers for this - I thought a cobra sx was pretty much a cobra sx - I am assuming that is where this leak would have to be if we are talking about it occurring due to being on plane.. )but I am not opposed to switching out the pump for good measure - it's not expensive but it is a little bit of a pita to do for no reason.

 

[dypcdiver]

Re: 5.7 running warmer at speed

How deep was the water when you were pulling "beached dock off the beach " you might just have churned up some of the sand/mud and it might be reducing the flow of raw water. The oil cooler for the power steering might be a good place to look. I have also heard of sand collecting in the manifolds and reducing the outflow, after running in shallow water. If you remove the outlet hose from the raw water pump and put a temporary hose on it into a bucket, then run the engine briefly to get an idea of the flow it might give you a clue.

 

[Lou C]

Re: 5.7 running warmer at speed

If the boat sucked up a lot of sand that could have worn the impeller or wear plate in pump so that it does not pump enough water.
The rubber impeller is much more likely to experience damage than the circulating pump (those typically last a LONG time).

 

[Larson5.7]

Re: 5.7 running warmer at speed

No sand - the "beached" dock was on a mud flat (we have to move our docks to shallow water here so the snow/ ice flow dies not sink them in winter) and the boat was in ~10 feet of water. I ordered a new recirculation pump today - should be here Wed or Thursday.

If it is still heating up, I guess I will check the raw water pump next to ensure nothing is going on there and then I guess pull the outdrive off and replace the o-ring water seal to see if it is sucking air - people have talked about the water tube guide/ seal but that seems like a pretty significant seal - struggle to see that leaking and my dealer (been in the Volvo biz all his life) told me he had never ordered that part before...???

thanks again for your help.

 

[Lou C]

Re: 5.7 running warmer at speed

I'd be surprised if the circ pump had anything to do with the hot running problem. The OE one on my engine lasted 22 years before it got noisy. Impellers are much more often the cause of running hot issues. That and clogged exhaust systems.

 

[Larson5.7]

Re: 5.7 running warmer at speed

Ok - I will check it - but the impeller, raw water pump itself, manifolds, risers, & thermostat were all replaced last August / less than 10 engine hours and I do not run in shallow water often - and in maine we don't have sand. I usually replace my impeller every few years and it typically has no wear - just beginnings of dry rot-stress cracks and a few marks from the gate.