2003 5.7 350 MAG MPI - Antifreeze overfill overflowing

Hi all. My 2003 5.7 350HP Mag Horizon MPI has been trouble free ( lately ) and today it decided to say "hello". I turned the bilge on as I just started to run and saw what looked like red fluid coming out. Took it for a quick 5 minute run and inspected the engine area.

THE FACTS:
The antifreeze was always topped off and the overflow was empty. Never had antifreeze issue.
Original thermostat.
Only the starboard bilge near the coolant overflow had antifreeze in it - 1/4 gallon, not counting what I bilged out on the quick run.
Upon noticing this and before I restarted the engine, the overflow was full - I checked the coolant level in the intercooler ( for lack of the right terminology ) and it was to the top.

Started the engine on land with a freshwater bypass - at idle, all ran great and did not overheat. I saw no hose leaks, but the antifreeze overflow filled a bit more.

What is going on ? Hoping it's just thermostat ?

Could be as simple as a bad pressure cap not holding pressure and allowing hot antifreeze to flow out of the heat exchanger into the coolant reservoir. If the engine does not overheat the thermostat is probably fine.

Proper antifreeze levels... Top off heat exchanger. FILL OVERFLOW tank to the "low" mark. Most overflow tanks have a low and a high line. Run engine until it heats up fully. DO NOT REMOVE CAP ON HEAT EXCHANGER. Check antifreeze level in overflow tank. It should be between the low and high line. Make note of level. Let engine cool completely ( overnight). Check level in overflow tank. Add antifreeze mix if below "low" line. DO NOT REMOVE OR LOOSEN CAP ON HEAT EXCHANGER. Operate engine as usual. NEVER remove cap on heat exchanger. After several operate/cool cycles, antifreeze level in overflow tank should cycle and stabilize between low and full levels. Under normal circumstances, you should never have to remove cap on heat exchanger, nor should there ever be antifreeze in the bilge. Antifreeze in the bilge cause by several reasons: 1) overfilled/improperly filled system, 2) leak in overflow tank, 3) leak in or disconnected tubing between heat exchanger and overflow tank.

Empty over flow tank once filled properly and no antifreeze in bilge: 1) leak in heat exchanger, 2) leak in "fresh" water circuit of the engine.

In order for the system to fill correctly and maintain the level between low and high marks, it requires the proper functioning "radiator cap" on the heat exchanger and no crap on the seating surface of the cap to the heat exchanger. Make sure that the CORRECT pressure rating cap is used on the heat exchanger.

If you fill both the heat exchanger and the overflow tank, normal expansion when the engine heats up will cause antifreeze to flow out of the heat exchanger, then overfill the overflow tank... and then into the bilge.

Also sometimes an air lock will cause antifreeze to be forced out, it may be necessary to bleed any air out of the antifreeze side of the heat exchanger, Lisle makes a special funnel kit with adapters just for this purpose.

Hmm. I have to study how the coolant and raw water flow through this system. I got a bad feeling that it is not as easy as an airlock, thermostat or heat exchanger cap.

I tried SandKickers idea and the engine filled the coolant reservoir that was low back to high and overflow. Once cooled down, the reservior fluid decreased a bit, but not back to low. I did remove the heat exchanger cap and saw there was no room to add any more antifreeze. Pressure cap and heat exchange mating looks fine.

Seems like there is plenty of fluid - too much - which brings me to the idea that the heat exchanger could be leaking between the raw water and antifreeze, allowing raw water to enter into the fresh water system. With the amount of coolant lost over the past day or so, it makes no sense that a vapor lock could do this ( I could be wrong of course - I'm a novice ). So, it leads be to believe that add'l raw water os entering the system, causing too much fluid, then too much pressure which springs the heat exchange pressure cap allowing water to escape via the fluid res.

Making sense ? What else could it be ? Engine runs great otherwise. No water in oil, etc.

If so, how would I go about testing this hypothesis ?

Just pulled the ends off of the heat exchanger - no coolant at all.

Then, I started the engine, with cap off and the fluid, even cold, was pushing up out of the top. I would think there would be flow ? Thoughts ?

Bad leak in heat exchanger... or depending on exchanger design, bad gasket or improperly installed gasket(s) on ends.

abl1111 said:

Hi all. My 2003 5.7 350HP Mag Horizon MPI has been trouble free ( lately ) and today it decided to say "hello". I turned the bilge on as I just started to run and saw what looked like red fluid coming out. Took it for a quick 5 minute run and inspected the engine area.

THE FACTS:
The antifreeze was always topped off and the overflow was empty. Never had antifreeze issue.
Original thermostat.
Only the starboard bilge near the coolant overflow had antifreeze in it - 1/4 gallon, not counting what I bilged out on the quick run.
Upon noticing this and before I restarted the engine, the overflow was full - I checked the coolant level in the intercooler ( for lack of the right terminology ) and it was to the top.

Started the engine on land with a freshwater bypass - at idle, all ran great and did not overheat. I saw no hose leaks, but the antifreeze overflow filled a bit more.

What is going on ? Hoping it's just thermostat ?

Click to expand...

You have what is referred to as a Closed Cooling System. What we don't yet known, is if you have a Full System or a Half System.

E/G = ethylene glycol.
H/E = heat exchanger.

*** The E/G coolant surrounds the exterior of the tube bundle (outside of the tubes), of which is non-accessible by average Joe user.

The end caps on the H/E give you access to the seawater side of the tube bundle (the interior of the tubes) ONLY.
You should see seawater ONLY in this area.

The H/E pressure cap must be the correct style for a recovery system.
As E/G heats and expands, the expanding E/G over-rides the pressure cap valve causing the E/G to be pushed into the reservoir.
The cap is designed so that when the E/G cools, the vacuum created pulls E/G back into the H/E, returning it to FULL.

NOTE: the full line on the reservoir (when cold) is not necessarily a direct correlation to that of the actual FULL in the H/E.
If/when all is working correctly, it will be fairly accurate when we begin with all topped off with a cold engine.

The recovery system (when working correctly) should create a no-gain/no-loss scenario.

Make sure that these are all working correctly.

abl1111 said:

Hmm. I have to study how the coolant and raw water flow through this system. I got a bad feeling that it is not as easy as an airlock, thermostat or heat exchanger cap.
The thermostat should be equipped with a small air bleed hole. If so, this virtually eliminates any air lock.

I tried SandKickers idea and the engine filled the coolant reservoir that was low back to high and overflow. Once cooled down, the reservior fluid decreased a bit, but not back to low. I did remove the heat exchanger cap and saw there was no room to add any more antifreeze. Pressure cap and heat exchange mating looks fine.
DO NOT remove the pressure cap until the engine has cooled down.
Otherwise, you will cause the system to loose it's ability to pull coolant from the reservoir back to the H/E.

Seems like there is plenty of fluid - too much - which brings me to the idea that the heat exchanger could be leaking between the raw water and antifreeze, allowing raw water to enter into the fresh water system. With the amount of coolant lost over the past day or so, it makes no sense that a vapor lock could do this ( I could be wrong of course - I'm a novice ). So, it leads be to believe that add'l raw water os entering the system, causing too much fluid, then too much pressure which springs the heat exchange pressure cap allowing water to escape via the fluid res.

Gasoline combustion creates hydrocarbons.
You can purchase hydrocarbon test strips that will indicate the presence of hydrocarbons in the E/G, of which may indicate a failing head gasket.

Making sense ? What else could it be ? Engine runs great otherwise. No water in oil, etc.
If so, how would I go about testing this hypothesis ?
See above.

Click to expand...

abl1111 said:

Just pulled the ends off of the heat exchanger - no coolant at all.
See above comments *** .

Then, I started the engine, with cap off and the fluid, even cold, was pushing up out of the top. I would think there would be flow ? Thoughts ?
A thermostat that is stuck open will cause excessive E/G flow that may be visible from the H/E pressure cap port.
In other words...... we need the restriction created by the thermostat to prevent excessive coolant flow, AND to hold back coolant releasing it ONLY on an "as required" basis.

Click to expand...

.

UPDATE:

I spoke to my mechanic today. He helps me when I need it and does the heavy lifting when I need that too. He was focused on the H/E pressure cap and the thermostat.

To cut out any BS ( they're both original at 1000 hours ) - I bought both today. I installed them and it seems that all is running properly, no resevoir overfill, temp seems tight, while on the hard at idle. Previously, in the same scenario, the overflow was filling and temp seemed to be creeping past where I remember her to . I am waiting for the engine to cool, and will add antifreeze if needed and put her in the water for a 'bay test'.

The thermostat seemed to me like the 'blocker' that rides the brass shaft had play and it could get jammed at a miniscule angle. Not sure which is bad, the thermostat or the H/E pressure cap. Another thing my mech said is that I might be able to smell antifreeze in the exhaust if it were a head gasket - I did not smell that...

I will let all know what the Bay test shows -

Another thing my mech said is that I might be able to smell antifreeze in the exhaust if it were a head gasket - I did not smell that...

Click to expand...

Rather than relying on your nose..... pick up some hydrocarbon test strips.
Dip one into your E/G mixture and take a reading.


.

Bay test complete. I believe I'm 'good to go'. Not sure if it was the HE pressure cap or the thermostat - but all seems well again. I am running her a bit, cooling, then filling HE with antifreeze until its; full.

Until the next time....

You may have needed to "burp" the system a few times to get all of the air out.

Jeff

PS: Any Pontiac Fiero owners out there? With their engines in the back, and their radiators in the front, you have to burb the HELL out of them or they boil over like crazy.

UPDATE:

Issue still occurring. Had mechanic look. Says its the Heat Exchanger - replaced it $$$$ and hopefully that'll do it. The price for Mercruiser parts is really ridiculous. This lists for over $2K. A really reputable, local guy manufactures them as good or better than OEM, so I bought it from him for a lot less - but still $$.

Before you dump money into a part you do not need.

Let the engine cool, put a pressure tester on the HE and pump it up with about 15lbs of pressure. If it does not lose pressure you do not have a leak.

Now after you determine if you have a leak or not, with the engine cold, leave the pressure tester on the HE, make sure there is no pressure in the system, Start the engine and raise the RPM to around 1500.

Does the pressure gauge start to move immediately? IE like to 5psi, If yes you have a compression leak into the cooling system. Pressure should only build with heat.

This is a simple way to test.

Chris Thx for your input - too late on the part. Installed already. Mech said he tasted antifreeze in the HE fresh side. I hope this was it and that I didn't waste $$.

For academic purposes - when you say put a pressure tester on the HE, when cold, and pump to 15 psi - where does that pressure tester attach ? The HE pressure cap location ?

I do understand the 2nd part - it all make sense and I wish I had known this initially as I could've/ would've done this test myself...

The tester replaces the cap during testing.

OK Got it - I hope all is OK at this point. I should ask mech if he pressure tested the system.