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Losing coolant on engine cool-down - 1989, 5.7L Mercruiser

D

Dave Larson

New member
Hello, I've read the forums and will do a pressure test next. The problem: I have an 1989 28' Carver Voyager that has rebuilt 1989 5.7L Mercruisers (gas). Unfortunately, the port rebuild has had lots of trouble so I have kind of lost track of what was fixed/replaced when. We recently had a broken head bolt which then blew a head gasket and my mechanic repaired it by taking off the head and replacing all the head bolts, gasket etc. Right before I blew the head gasket (it was immediate and I shut the engine down when it happened), I was losing a slight amount of coolant so I suspect that was because of the broken bolt and the eventual head gasket failure. So, fast forward, I get the boat back and I can run 2-5 hours at 1750 RPM, then as soon as I back down the RPM to around 750 (coming into a dock), the engine burns through coolant and overheats within 2-10 minutes. This happened 4 times in a row so it doesn't appear to be coincidence. I had to add about a gallon of coolant each time. It runs fine for 2-5 hours at 1,750 RPM and overheads and drinks coolant as soon as I drop down to 750 RPM. I thought it might be a seawater pump issue, so I had that replaced...it didn't work. There is NO coolant in the bilge. There isn't any evidence of coolant in the oil or crank case. The water pump (circulation pump) doesn't have any leaks or squeaks so that appears to be working fine. My assumption is the only place the coolant can be going is out with the exhaust with the seawater. I don't see any steam or smoke in the exhaust. Has anybody seen these symptoms before? My next step will be a pressure test on the coolant system, and I saw some sort of dye test that appears to look for exhaust in the coolant. I also saw a note about the 'elbows or riser' needing to be torqued to 20-25 pounds and if they aren't, then coolant could be going out with the seawater. If anybody can chime in, I'd appreciate it. I'm not a mechanic...just trying to learn what the problem is so I can talk coherently with .a repair shop.
 
We recently had a broken head bolt which then blew a head gasket and my mechanic repaired it by taking off the head and replacing all the head bolts, gasket etc. Right before I blew the head gasket (it was immediate and I shut the engine down when it happened), I was losing a slight amount of coolant so I suspect that was because of the broken bolt and the eventual head gasket failure
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Not knowing any more then what you just typed

Appears the motor overheated, head warped and Mech replaced head gasket and it blew again

Was the head and block checked for flatness to be within 0.003 ??
 
AllDodge said:
Not knowing any more then what you just typed

Appears the motor overheated, head warped and Mech replaced head gasket and it blew again

Was the head and block checked for flatness to be within 0.003 ??
Click to expand...
Thanks so much for your reply! The head and block were not checked for flatness after the failed head bolt and replacement gasket. I hadn’t really thought of that. We probably should have done that when we had the head off since the head is portable. Can the block be tested for a warp while it is still in the boat…ie, without pulling the engine?
 
Dave Larson said:
Thanks so much for your reply! The head and block were not checked for flatness after the failed head bolt and replacement gasket. I hadn’t really thought of that. We probably should have done that when we had the head off since the head is portable. Can the block be tested for a warp while it is still in the boat…ie, without pulling the engine?
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sandkicker said:
Check heat exchanger
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Thanks very much, I appreciate it. I assume a pressure test would help diagnose a heat exchanger leak. I’ll have to read about how to properly perform a pressure test. Do you know…when you pump up the pressure (I assume to about 15 PSI), if it leaks out, how do you know where it is leaking from? Do you just listen for hissing and/or look for ‘bubbles’? Kind of the same way you find a hole in a tire? I haven’t read about how to do them so just looking for tips/tricks. Thanks!!
 
when you do the pressure test, read the instructions that come with the tester...you pump it up to the specified pressure for the rating of the system...and its usual to find the value on the cap to the heat exchanger..you DONT want too much pressure in the cooling system and you can make things worse.

typically, the head will warp before the block...both can be checked with a straight edge and a set of feeler gauges...If the mechanic just changed the gasket without checking for warping, I'd suggest you find another mechanic...

as far as tips, I'd suggest pulling the spark plugs and then do the pressure test...if any coolant is leaking into the cylinders, pulling the plugs will let you see/hear it...
 
Yes you can measure the block deck surface for flatness with the engine in the boat, the main thing is cleaning all of the old gasket material off and clean out the cyl head bolt holes with a thread chaser. Measure flatness with a mechanics straightedge and feeler gauges. That’s how I did it on my 4.3 when I installed remanufactured heads 7 years ago. No leaks no failed head gaskets no water in the oil. Not a hard job but you must not take short cuts….
 
Agree with the others pressure check cooling system and do compression test.

Possible if just one side was replaced that the other is leaking.

How did the head bolt break? would think that was from poor assembly
 
that, or over torquing the bolt because the cyl head bolt holes in the block weren't cleaned out 100%
cleaning the block deck and those holes (26 in total on the V6) is what took the longest when I replaced the heads on my 4.3.
on a closed cooled motor they shouldn't be that crudded up though...what can happen is if you don't clean them out all the way down to the bottom of the hole the bolt won't thread in all the way.
always make sure all the cyl head bolts will thread in all the way before installing the heads.
 
Hi All, I wanted to thank you for the input and report an update. I 'think' the engine is repaired. Neither the mechanic nor I have a perfect explanation however, here's what happened. We hooked up a garden to hose the seawater intake and ran to temp at various RPM. It has a closed cooling system so there is a heat exhanger system. The mechanic noticed, at some points in time, the coolant water circulating through the engine was foaming in the heat exchanger. Not all the time, but sometimes. He temporarily spliced in a clear plastic hose (about 1.75" diameter) between the circulation pump and the heat exchanger intake. We could watch the foam circulating through the engine. The fact and theory combined: foam won't cool the engine correctly AND, when exiting the engine block and running over the temp sensor, if it were steaming, that would make the temp gauge spike quickly to over 210 degrees (which it was doing). We decided to replace the circ pump as a precaution in case it was part of the foam problem ($125). He then disconnected the 'outflow' end of the clear hose and hooked up a high velocity garden hose to the heat exchanger intake. We ran the engine for about 5 min which flushed all the coolant out of the engine and heat exchanger. We watched the fluid in the clear hose turn from foamy, to less foamy, to clear water and ran it about another 3-4 minutes. The foam entirely disappeared so we reassembled everything. I ran the boat for 4 hours up and down RPM and the overheat problem didn't reoccur. It appears to be fixed. Neither of us could explain the specific reason for the foaming. There was no apparent problem with the old circ pump but we didn't want to leave anything to chance for $125. Fingers crossed that the problem is solved. Side note, we learned the boat had a name change without the proper traditional name change ritual which..... according to long legend, can bring unexplained bad luck to a ship. Just in case, we are going to reverse that and go through a proper name change ritual. Good excuse to get people together onboard and pop some bubbly.... PS: we Googled "foam in the coolant/radiator" and none of the reasons sufficiently explained why it was foaming. And, we never were able to explain where roughly 7 gallons of coolant water disappeared to over the last 3 weeks. One final note, when we drained the coolant, I did drain it into containers and dispose of correctly via a spent antifreeze collection point. We'll see if this is fixed for the long term....
 
Wonder if prior owner put something in the heat exchanger other then AF (gas, alcohol, something else) which has a lower boiling point?

Don't know but sure glad it's working and thanks for coming back with update
 
Glad things are back to normal.

Though there are some 'holes' in your explanation, I'd venture to say the circulating pump's seal was leaking allowing some air in when it was spinning...the residual 'foam' that was seen was likely due to the air pockets created when the pump was changed and they purged out thru the cap when the engine was run...

FWIW, the 'fat' hose from the bottom of the heat exchanger is its outlet and that feeds the suction side of the circulating pump.
 
AllDodge said:
Wonder if prior owner put something in the heat exchanger other then AF (gas, alcohol, something else) which has a lower boiling point?

Don't know but sure glad it's working and thanks for coming back with update
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I've owed the boat for a while and had the engine rebuilt so if there was some 'foreign' substance in the coolant, it would have been from me or the engine installer....it's just a weird scenario and I hope it's resolved. Thanks for the comments.
 
makomark said:
Glad things are back to normal.

Though there are some 'holes' in your explanation, I'd venture to say the circulating pump's seal was leaking allowing some air in when it was spinning...the residual 'foam' that was seen was likely due to the air pockets created when the pump was changed and they purged out thru the cap when the engine was run...

FWIW, the 'fat' hose from the bottom of the heat exchanger is its outlet and that feeds the suction side of the circulating pump.
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The air leak would make sense but we never saw any dripping water from the circ pump area or coolant in the bilge. I'd think when the engine was hot and shut down, we would either see a coolant leak at the circ pump, hear hissing air, or smell coolant steam.? We did run a heat exchanger pressure test for about 30 min at hot and cold temps. It was holding pressure during that 30 min. When we spliced in the 'clear hose' it was from the outlet of the circ pump directly to the intake of the heat exchanger so we were seeing the coolant as it pushed out of the circ pump and into the heat exchanger. Note...I'm not a mechanic so could have some of my terminology mixed up :). The weird thing is I added about 6-7 gallons of water/coolant in the last 16 running hours and it ONLY overheated and ran short on water when the engine was at low RPM (1,200 or less). Losing 6-7 gallons in 16 running hours is a LOT of fluid or steam so it seems like we would have seen it in the bilge, block, or engine exhaust. We could never find where it was exiting. I didn't see or smell any 'steam' release from the heat exchanger cap area or overflow when I checked in the engine compartment every hour or so in the last 16 running hours. It's just really weird as I'd think with that much missing coolant/water, it would be evident where it was going. We also checked for exhaust gas in the heat exchanger...none, and, when there was green coolant in the cooling system, we captured the exhaust 'seawater' at the 'exhaust pipe'...whatever you call it...and that was running perfectly clear...no evidence of green coolant in the exhaust/seawater going out the back. It's just really strange....there is an explanation, we just couldn't find it. I hope it works...thanks for your comments!
 
The volume of fresh water vs the loss of coolant over time you would NOT see this in a sample of exhuast. It would be to deluted.

So if hou are still adding fluid to your closed cooling system,

1. Bad or Wrong gaskets at ex manifolds to elbows.
2. Exhaust manifold small crack leaking antifreeze into exhaust.
3. Possible cracked head/bad head gasket allowing water to mix exhaust.
4. Possible still, heat exchanger issue when it cools down, a crack that may cause this issue opens and allows antifreeze and water to mix.
5. If issue is resolved, and all my prior spew is unwarrented, then you used bad antifeeze.....it can go bad...
 
FWC systems run about 40 GPH at "cruise" RPMs so 6 gallons over 16 hours not likely to be noticed in the exhaust. Pressure check the cooling system with an auto parts store cooling system checker. Have heat exchanger looked at by a good radiator shop. On some design heat exchangers with end plate gaskets, removing the end plates and reinstalling with old good gaskets can cause a leak between the FW and RW paths and since FW at higher pressure..... water loss.
 
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