Mercruiser 470 heat exchanger

[NOLA-boy]

Hello everyone,

I have a 79' 3.7 470. Serial# 5388729. I recently installed a 4' heat exchanger and I've been troubleshooting my overheating problem using advice from this great forum.

I pulled the raw water hose off the front of the heat exchanger to check the flow. I have the boat in the water. I brought the RPM to 2,000. You can see in the video that the flow isn't great. I suspect a bad impeller.

My actual question is about the back pressure "spitting" off the front of the exchanger. Notice in the video that I have a valve installed.

Head gasket possibly? I don't think it's the heat exchanger because I don't see coolant. No milky oil or oil in coolant. I haven't done a compression test yet. Thats my next step. Pressure test the closed system after that.

Thanks for any advice.

 

[Bondo]

Re: Mercruiser 470 heat exchanger

Hello everyone,

I have a 79' 3.7 470. Serial# 5388729. I recently installed a 4' heat exchanger and I've been troubleshooting my overheating problem using advice from this great forum.

I pulled the raw water hose off the front of the heat exchanger to check the flow. I have the boat in the water. I brought the RPM to 2,000. You can see in the video that the flow isn't great. I suspect a bad impeller.

My actual question is about the back pressure "spitting" off the front of the exchanger. Notice in the video that I have a valve installed.

Head gasket possibly? I don't think it's the heat exchanger because I don't see coolant. No milky oil or oil in coolant. I haven't done a compression test yet. Thats my next step. Pressure test the closed system after that.

Thanks for any advice.

Ayuh,...Welcome Aboard,... The pressure is probably backin' up from the exhaust at the riser...

The water pump will have a higher pressure than the exhaust, Normally...

 

[stonyloam]

Re: Mercruiser 470 heat exchanger

Yeah that is probably just hot exhaust blowback as it burns up your shutters and engine exhaust bellows. It is probably not wise to run the engine at that speed with no water cooling the exhaust. If you are going to do that remove the aft hose from the exchanger (that goes to the elbow) and feed water from a garden hose to the exhaust.

 

[NOLA-boy]

Re: Mercruiser 470 heat exchanger

I only ran the engine to check the impeller for maybe 15 seconds.

Would y'all agree that the flow from the impeller looks weak for 2,000 RPM?

Thanks for the responses.

 

[Bt Doctur]

Re: Mercruiser 470 heat exchanger

Why the valve in the hose? that right there could be your problem

 

[NOLA-boy]

Re: Mercruiser 470 heat exchanger

The boat came that way. I assumed the PO used it to flush the raw side while in the water.

I just use the muffs. It appears to be a 1/2" valve.

Do y'all think this could restrict the
flow enough to cause it to run hot?
Does the flow from the impeller look sufficient?

Some background?
I've checked the riser, shutters, coolant cap, thermostat and flushed the closed side.

Thanks again

 

[Bt Doctur]

Re: Mercruiser 470 heat exchanger

there is no real reason to flush the raw water side of the heat exchanger. get rid of all the extra plumbing at retest it.
flow appears to be a bit weak but its really hard to tell. so easy to replace an impeller anyway.

 

[Bondo]

Re: Mercruiser 470 heat exchanger

The boat came that way. I assumed the PO used it to flush the raw side while in the water.

I just use the muffs. It appears to be a 1/2" valve.

Do y'all think this could restrict the
flow enough to cause it to run hot?
Does the flow from the impeller look sufficient?

Some background…
I've checked the riser, shutters, coolant cap, thermostat and flushed the closed side.

Thanks again

Ayuh,.... It looks like a full flow gate valve,... is it,..??

The flow don't look too bad,... usually the hose is held Up-right....

What's the History on the Impeller, 'n pump,..??

 

[NOLA-boy]

Re: Mercruiser 470 heat exchanger

I run in salt/brackish water. Wouldn't the flush valve work if the boat was in the water? Like I said earlier, I usually flush on muffs.
Thanks!!

 

[NOLA-boy]

Re: Mercruiser 470 heat exchanger

Ayuh,.... It looks like a full flow gate valve,. is it,..??
I think it is. It is 1/2" all the way through.

The flow don't look too bad,... usually the hose is held Up-right....
I'll try that.
What's the History on the Impeller, 'n pump,..??

PO claimed to have replaced them.
It's cheap and easy. I'll do that next week.

Thanks!!!

 

[southwest395]

Re: Mercruiser 470 heat exchanger

PO claimed to have replaced them.
It's cheap and easy. I'll do that next week.

Thanks!!!

Try runing it at idle speed. See how your flow is then. If the flow is there ur impeller fin but if you have no flow at idle your impeller is shot just like mine is. I had ok flow in the higher rpm but normal 1000 rpm no flow at all.

 

[southwest395]

Re: Mercruiser 470 heat exchanger

You will have some exhaust pressure threw heat exchager not alot. But some if you have alot. Replace the water flap in the lower y trap in the exhaust boot. Took care of my issue.

 

[NOLA-boy]

Re: Mercruiser 470 heat exchanger

Ok. Good idea.
Thx

 

[NOLA-boy]

Re: Mercruiser 470 heat exchanger

Sorry I tried to use the quote feature and failed.
I'll try the test at idle.
Thanks

 

[southwest395]

Re: Mercruiser 470 heat exchanger

I had a bad water pocket gasket. But i had replaced impeller once before and then i had a faulty heat exchanger . And my impeller burned out. But at higher rpm took hose off the voltage regulator and water flow. but at idle speed took same hose off and no water at all had 3 bad fins.

 

[NOLA-boy]

Re: Mercruiser 470 heat exchanger

I'm gonna change the impeller.

It heats up over 2,000 RPM.

And the flow at idle is low.
I think that will solve my problem.
The video is at idle 750 ish.
Thanks again!!

 

[stonyloam]

Re: Mercruiser 470 heat exchanger

It would be best to change the impeller and housing this time, and the impeller every couple of years in the future.

 

[NOLA-boy]

Re: Mercruiser 470 heat exchanger

I'll do the housing as well.
Thanks!

 

[stonyloam]

Re: Mercruiser 470 heat exchanger

Hay! You any relation to Nola mike, another 470 guy here?

 

[NOLA-boy]

Re: Mercruiser 470 heat exchanger

Nola mike? I'll have to look him up. Maybe my long lost cuz.
LoL!!