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502 flow issues please help

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paulysess

New member
i have a 1996 carbed 502 in my 1999 baja 232. at idle boat has great water flow. once i throttle up to get under way i lose total flow to my risers. This was confirmed by running clear hoses to risers from tstat housing. New impellar,circulation pump as well as thermostat. Riser are good and clear as well... Its pulling water feom a throughhull pickup right to the raw water pump... THANKS YALL!!!!
 
paulysess said:
i have a 1996 carbed 502 in my 1999 baja 232. at idle boat has great water flow. once i throttle up to get under way i lose total flow to my risers. This was confirmed by running clear hoses to risers from tstat housing. New impellar,circulation pump as well as thermostat. Riser are good and clear as well... Its pulling water feom a throughhull pickup right to the raw water pump... THANKS YALL!!!!
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Try backflushing your oil coolers, and seeing if you have any impeller pieces stuck in them.

Have you ever pulled your risers to check for corrosion/rust? Freshwater boat?
 
390Express said:
Try backflushing your oil coolers, and seeing if you have any impeller pieces stuck in them.

Have you ever pulled your risers to check for corrosion/rust? Freshwater boat?
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riser are a year old and clear as can be. I have good flow through the cooler no pieces or blockages. its the dual oil and steering cooler. its fresh and some salt.. mostly brakish water .. It has super strong flow at idle and slow speed. up to say 1500 rpm, once i start to get into it flow stops totally .. Super confusing..
 
paulysess said:
riser are a year old and clear as can be. I have good flow through the cooler no pieces or blockages. its the dual oil and steering cooler. its fresh and some salt.. mostly brakish water .. It has super strong flow at idle and slow speed. up to say 1500 rpm, once i start to get into it flow stops totally .. Super confusing..
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Do you have a worn out hose that is collapsing under pressure/suction?
 
the hose from transom to seawater pump is new and soild type hose. That should be only suction correct? the other side out to coolers and motor is all pressure push.. thank you for getting back to me
 
paulysess said:
the hose from transom to seawater pump is new and soild type hose. That should be only suction correct? the other side out to coolers and motor is all pressure push.. thank you for getting back to me
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If the hoses are soft or weak, they can collapse. I’ve seen the long, bendy manifold hoses collapse completely when they’re old and worn out.

Due to thermal expansion, the stat opening and closing, and different sized openings at each end of the hoses, there can be odd pressure/suction cycles throughout the system.

It’s not as straightforward as the raw water pump pushing water through the system. If you have a go pro, or if you can run the engine with the cover off, check the hoses that supply the manifolds.
 
390Express said:
If the hoses are soft or weak, they can collapse. I’ve seen the long, bendy manifold hoses collapse completely when they’re old and worn out.

Due to thermal expansion, the stat opening and closing, and different sized openings at each end of the hoses, there can be odd pressure/suction cycles throughout the system.

It’s not as straightforward as the raw water pump pushing water through the system. If you have a go pro, or if you can run the engine with the cover off, check the hoses that supply the manifolds.
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we put clear hoses on from the tstat to the riser. once i throttle up the pressure goes away and the hoses have zero flow. if i pull in neutral and rev the motor flow comes back up strong
 
paulysess said:
the hose from transom to seawater pump is new and soild type hose. That should be only suction correct? the other side out to coolers and motor is all pressure push.. thank you for getting back to me
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Look up bravo it is, where cooling water line comes through transom housing it can collapse restricting flow
 
make sure the belt driving the pump is adjusted correctly....

Should be a strainer on the inlet side of the raw water pump - make sure the lid it secure and the gasket not leaking.

When was the last time the raw water pump was serviced? Changing the impeller isn't the only maintenance they need...
 
paulysess said:
It doesn’t pick up from the drive for that to be an issue . It’s a threw hull pickup right to raw pump
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Dieter said:
Ok missed that
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No worries man thanks for tryin to help
makomark said:
make sure the belt driving the pump is adjusted correctly....

Should be a strainer on the inlet side of the raw water pump - make sure the lid it secure and the gasket not leaking.

When was the last time the raw water pump was serviced? Changing the impeller isn't the only maintenance they need...
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no strainer, bent is tight . Not sure on it being serviced it’s a new to me motor this season . It’s the pump with the mechanical fuel pump mounted on it . What service needs to be done ? Should the whole pump be replaced ? Thank you !
 
paulysess said:
No worries man thanks for tryin to help

no strainer, bent is tight . Not sure on it being serviced it’s a new to me motor this season . It’s the pump with the mechanical fuel pump mounted on it . What service needs to be done ? Should the whole pump be replaced ? Thank you !
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Belt is tight . Sorry
 
paulysess said:
....no strainer, bent is tight ....
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That's not optimal....that said, assuming it has run ok for years, may not be an issue. Tight bends increase friction...
paulysess said:
Not sure on it being serviced it’s a new to me motor this season . It’s the pump with the mechanical fuel pump mounted on it . What service needs to be done ? Should the whole pump be replaced ? Thank you !
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Replacement will make you cry when you see the bill....what needs to get changed is a function of what is worn...wear plates and gaskets....sometimes, the seals and bearings, and a body depending upon the operating environment....any air leak will impact the 'pumping ability' of the pump.

You can always check the pumps capacity my measuring its product volume of a short period of time. If that is satisfactory, you need to look downstream of the pump.

With it being 'new to you' I'm guessing you don't have a detailed service history... Its possible you inherited the issue. I would also take the clear hose (may have to get different sizes) and verify the flow thru the oil cooler and into the T-stat housing...old impeller remnants can cause "interesting cooling water flow features"....
 
I meant the belt is tight . I will try to run a clear hose from the cooler up tonight . I priced the pump at 500 which isn’t horrible if it’s what’s needed . Just have never gets if one going bad . And it doesn’t make any noise or leak in any way . If it’s driving the fuel pump as well and I have fuel pressure isn’t it working properly ?
Thank you again !
 
Marko
paulysess said:
I meant the belt is tight . I will try to run a clear hose from the cooler up tonight . I priced the pump at 500 which isn’t horrible if it’s what’s needed . Just have never gets if one going bad . And it doesn’t make any noise or leak in any way . If it’s driving the fuel pump as well and I have fuel pressure isn’t it working properly ?
Thank you again !
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just want to make sure it is not sucking air or bypassing at worn end plates. Some where you are not getting enough flow, obviously a lot of possibilities to work through. Has a compression check been done sometimes a bad head gasket can cause bubbles and over heat

While this has no bearing on the issue- there is a reservoir on fuel/water pump that has gear lube and needs periodic changing.
 
Compression was close across all cylinders. But all good numbers .
I’m just confused how the flow totally stops but dropping into neutral and revving the the throttle brings it right back . I would think a little sir would cause bubbles not complete stoppage . It’s super weird
 
paulysess said:
Compression was close across all cylinders. But all good numbers .
I’m just confused how the flow totally stops but dropping into neutral and revving the the throttle brings it right back . I would think a little sir would cause bubbles not complete stoppage . It’s super weird
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not sure either wonder if you are leaking air on suction side and reving under no load gets rpms up on pump to get it moving again?
 
If the fuel pump is working, it means the shaft is spinning...the pump may or may not be delivering its rated volume depending on its condition. You can check the pump's condition by doing the flow rate test.

If the boat is new to you, are you sure the problem didn't already exist?

What triggered putting on the clear hoses to check for water flow?

I'd be inclined to check the t-stat housing to make sure you don't have a piece of crap in it...

FWIW, there were lots of changes to the cooling designs in that time frame...and lots of variations depending on the application (MCM vs MIE). If you have the housing with the check balls in the outlets to the elbows, they are spring loaded and will open as the pressure builds when the system is cool. so the water flow may appear intermittent.
 
makomark said:
If the fuel pump is working, it means the shaft is spinning...the pump may or may not be delivering its rated volume depending on its condition. You can check the pump's condition by doing the flow rate test.

If the boat is new to you, are you sure the problem didn't already exist?

What triggered putting on the clear hoses to check for water flow?

I'd be inclined to check the t-stat housing to make sure you don't have a piece of crap in it...

FWIW, there were lots of changes to the cooling designs in that time frame...and lots of variations depending on the application (MCM vs MIE). If you have the housing with the check balls in the outlets to the elbows, they are spring loaded and will open as the pressure builds when the system is cool. so the water flow may appear intermittent.
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The clear hoses were an idea from a shop . The issue was first the boat wound hsve good temp at idle then running drop to no temp then climb to hot fast . They said try the clear to see if you have an aeration issue . Which let us see the flow stop totally . It’s a new impeller , circ pump and Tstat . No check balls in housing it’s clear through out to risers . To threw hull exhaust .
 
If you want to test the output of your pump, the pump should be able to fill a 5 gallon bucket in around 20-25 seconds at 1500 rpm.

If you have two people on the boat, you can remove the output hose and run the boat briefly for 30 seconds to perform the test. If possible, try to use a garden hose to supply the engine while you're testing the pump.

Personally, I think you either have a clog somewhere, or a collapsed hose. You may want to try running no stat, and seeing if that helps your problem. Stats can be faulty, some "marine" stats have two 1/8" holes drilled in them, to provide some constant water flow through the engine.

You may want to try running a radiator flush and/or barnacle buster through the block/manifolds. to see what comes out the other end.
 
Sound more like you loose all raw water flow vs just loosing flow to the elbows....

with the new data, I'd measure the raw water pump's output...the spec is at least 7.5 qts in 15 seconds at 1000 RPM...

Is the boat moored or trailered?
 
makomark said:
Sound more like you loose all raw water flow vs just loosing flow to the elbows....

with the new data, I'd measure the raw water pump's output...the spec is at least 7.5 qts in 15 seconds at 1000 RPM...

Is the boat moored or trailered?
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That’s why it sounds like a collapsing hose to me.
 
390Express said:
That’s why it sounds like a collapsing hose to me.
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its it a slip at the marina currently. i can watch if flow great up to 1000 rpms steady. not till its under load with more throttle does it stop. I was gonna bypass the cooler go right from the raw pump to the tstat housing with a clear hose for a test to see if it keeps flow.
 
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