Slow Coolant Leak - Mercruiser 7.4L (454) FWC Engines after Top End Rebuild

[rnace]

This spring I purchased a 1997 Cruisers 35' boat with twin Mercruiser 7.4L (454) carbureted engines, FWC full closed cooling system with inboards and 625 hrs. I had the engines surveyed prior to the purchase and was told both engines had bad cylinder compression due to water intrusion and required top end engine rebuilds.

I bought the boat at a reduced price and immediately had all the work performed by a Certified Mercruiser Mechanic. When work was finished, I made my maiden voyage to my marina located in the Upper Chesapeake 2 hours away.

Work performed as described by mechanic
• Resurfaced heads
• Reconditioned values
• Installed new head gaskets
• Installed new Mercruiser exhaust manifolds, risers and elbows.

Problem: Even though the engines run good, I’ve been chasing a slow coolant leak in both engines for the entire season. This coolant leak requires me to add 4 - 8 oz coolant in both engines every time I take the boat out. The coolant leak does not appear to be going into the engine’s oil or the boat’s bilge and my thoughts are it's going out the exhaust.

The mechanic has not been responsive to my requests to look at this issue. I've now taken it upon myself find the source of the coolant leak. I've been performing a battery of tests along with removing and replacing parts but still can’t identify the root cause.

Here's everything I’ve done so far
• Installed new heat exchangers and pressure (radiator) caps on both engines

– No impact​

• Performed cooling system pressure test

– Both engines held at 16 psi​

• Performed cylinder compression test

– Starboard engine 130-135 psi all cylinders​

– Port engine 130-140 psi all cylinders​

• Performed cylinder leak-down test (75 psi input)

– Starboard engine 5%-10%, except cylinder 4 was 20%
– Port engine 4%-12% all cylinders​

• Put UV dye in coolant system in both engines

– No leaks detected​

• Checked for combustion gases released into coolant with a chemical block tester

– No gases detected on either engine​

• Engine oil was analyzed end of season (25 hrs logged on engines)

– Sodium level 29 ppm on Starboard engine
– Sodium level 56 ppm on Port engine​

• Performed pressure test on exhaust manifold and riser isolated from boat's cooling system

– Starboard engine's manifolds and risers held at 20 psi​

– Port engine’s manifolds have not been removed and tested yet​

Observations
• Both engines run good but produce white smoke.

• Mechanic put the block off gaskets between risers and elbows and re-plumbed the coolant hoses to the riser fittings.

– Mercruiser's service manual specifies the block off gaskets MUST go between the manifold and riser​

• When starting the engines cold with the heat exchanger’s pressure caps off....

– Coolant pushes out of the heat exchangers in 60 seconds​

• When checking for combustion gases in the coolant with heat exchanger's pressure caps off and funnel attached....

– I noticed small bubbles (non-violate) when revving the engines. Bubbles in port engine were a little more noticeable.​

• All 4x exhaust manifolds have water/coolant sitting in the bottom of the manifold port chambers.

– Starboard engine's manifolds were removed,inspected & tested. I couldn't find obvious signs of a gasket failure.
– Port engine's manifolds still installed on engine and I can see water sitting inside with my borescope camera​

• Starboard engine's exhaust port openings show signs of moisture and rust

– Excessive rust detected in the exhaust port openings at the exhaust crossover passages, Cylinders 4 and 5​

Things left to try
• Remove port engine's exhaust manifolds to look for signs of gasket failure, rust and moisture in ports.
• Remove heads and Intake manifolds to look for signs of leaks. ***LAST RESORT***

Question: Does anyone have advice or personal experiences they could offer in my quest to find my coolant leak?

 

[Scott Danforth]

acetone test your manifolds

you have water entering the exhaust somewhere, could be reversion due to not tall enough risers, or could be a cracked manifold or bad gasket surface. however since you have coolant sitting there, that is where I would focus.

you should have a nice film of dark grey carbon on your exhaust passages.

4-8 ounces is not much. factory HX system is marginal (OK, its about half what it needs) on expansion volume (hence the puke tank) use the UV light and check for leakage around the puke tank as well, however I am thinking the wet-joint manifolds need to go. when you pressure tested, you most likely had the engine cold. most manifold leaks show up when the motor is warm.

I would find a set of dry-joint manifolds off the 8.1

 

[tpenfield]

I have the MPI version of the 454's (Gen 6) w/ Full closed cooling systems (San Juan Engineering after market cooling systems). Here are my observations and suggestions . . .

1) Your mechanic may not be very good. Was it a regular mechanic or an engine rebuild shop? Marine engine expertise?

2) Re-visit the cooling system pressure test. Make sure to fully drain the cooling systems and then go for 15 PSI with a leak-down test. There should be absolutely no leak. Use soapy water around hose fittings and joints to look for any small leaks.

3) When the cylinder head work was done, did the mechanic/shop change the camshaft? If so, then they may have used an automotive or a performance cam with a fair amount of valve overlap. The cam on the 454's should have almost no overlap. A cam with overlap will cause reversion of exhaust at idles speeds and pull water from the tip of the elbows, back into the exhaust manifold and on into the cylinders.

4) Since the engine was given a top-end rebuild for essentially the same problem that you have now, I am wondering if the wrong cam was pre-existing in the engines from prior engine work?

5) Have the block-off gaskets at the manifold/riser joint.

6) Your engine is essentially back where it started before the cylinder head work, so I would imagine that the mechanic may not want to make good on their mess-up.

 

[scoflaw]

Were the heads pressure tested ? The exhaust guides go into the water jacket and are prone to leak there.

Your compression and leak down numbers aren't good. If the valves aren't leaking then the rings are.
Does not matter where the block off gasket goes if it's plumbed correctly.

 

[Lou C]

Did this engine come with full closed cooling from the factory? If so then the cyl heads should not have rusted to the point where you have leaks from the valve guides although it is possible. You found antifreeze in an intake port, correct? You will need to learn how antifreeze flows through the intake manifold on the big block to determine if leaky manifold gasket can cause this. On the small block engines the crossover is in the front only, not in the center area. So if there is a leak there it can leak into an intake port or into the cam valley. Any sign of antifreeze or water in the motor oil? The antifreeze in the bottom of the exhaust manifold gas passage suggests a leak in the sealing gasket or the surface is not flat enough for it to seal well. I would remove all 4 exhaust manifolds and test with acetone. Careful because it is very flammable. It will show leaks that water might not get through because it has less surface tension than water.
Pay close attention to the flatness and quality of the sealing surfaces. Are the clean flat cast iron or pitted?

 

[tpenfield]

+1 on checking for rust-through on the valve guides. My Carb'ed 7.4L (Mark IV) had that problem shortly after a valve job. I would think a cooling system pressure test would indicate such, but it is worth mentioning.

 

[Scott Danforth]

Ted, the OP didnt state how he did the cooling system pressure test. as you indicated, it needs to be done with the cooling system empty.

 

[tpenfield]

Ted, the OP didnt state how he did the cooling system pressure test. as you indicated, it needs to be done with the cooling system empty.

Yes, I think if the cooling system was full, then a leak would not be very apparent during a leak test.

I'm thinking either the valve guide thing or the camshaft.

 

[aimlow]

So the barf bottle keeps going down? Of course an engine pushes coolant out of a full heat exchanger when the engine is cold...that's why you have an "overflow" bottle. As the engine cools it should suck the water back in. Is that happening?

 

[tpenfield]

So the barf bottle keeps going down? Of course an engine pushes coolant out of a full heat exchanger when the engine is cold...that's why you have an "overflow" bottle. As the engine cools it should suck the water back in. Is that happening?

It sounds like the cooling system is sucking more back in than it pushed out, as the engine cools. It seems like some of it is ending up in the exhaust.

 

[HT32BSX115]

Coolant leaks in a closed systems are frequently riser gaskets leaking if there's no coolant in the oil, bilge, or cyls
(or even if there is!)

 

[aimlow]

It sounds like the cooling system is sucking more back in than it pushed out, as the engine cools. It seems like some of it is ending up in the exhaust.

I'm not sure from his post if indeed he has a barf bottle. If he's just looking in the heat exchanger itself with no overflow, of course the level will drop several ounces after a heat/cool cycle. If he does, then you are indeed correct.

 

[Scott Danforth]

all Mercruiser motors with factory HX's have over-flow bottles from the factory because the expansion volume required by any of the V motors is about 2-4 times that of the little tank on the HX itself.

 

[HT32BSX115]

After reading your initial post again, It appears that you did indeed already pressure tested both systems.

You might install 0-30 PSI gages on both systems to observe how high the pressure actually gets during normal operation.

• Performed cooling system pressure test
– Both engines held at 16 psi

If this is true, how long did it hold?....... and have you tried another pressure check of the system? It's sort of unlikely BOTH systems would leak the same (although not impossible)

How hot are both engines getting (assuming 160° stat)........... don't rely on the gages.

Verify using known gages or an IR "gun"

With the pressure gages installed and known temp, and if system is holding pressure, you might find that the caps are venting slightly into the overflow tanks (if installed) or overboard....

 

[scoflaw]

Doesn't really explain the green coolant in the exhaust crossover and manifold though

 

[HT32BSX115]

Doesn't really explain the green coolant in the exhaust crossover and manifold though

True...That or a cracked riser/manifold should point back to the riser blocking gaskets.. If it holds pressure, maybe it doesn't hold it for long....

 

[Scott Danforth]

OP never stated if he pressure tested the system dry or full

 

[HT32BSX115]

OP never stated if he pressure tested the system dry or full

It should hold 15 psi indefinitely dry or full. Although it might not leak cold if there was a hairline crack that opened up when hot.

 

[Scott Danforth]

If the system is full, and there is a minute crack, it may not leak until hot, however would leak when cold if the system is empty. Hence the prefered method of testing empty

 

[rnace]

Status update, I finally got a chance to remove the exhaust manifolds on the port engine and found water laying in the bottom of all the ports. It wasn't a lot of water and I'm thinking it could be from condensation. Most things in the engine compartment were wet from condensation from the cold winter temperatures. I pressure tested the exhaust manifolds/risers and they held at 30 psi. When I disassembled the elbows, risers and manifolds, I didn't see any obvious signs of gasket failures or leaks. Still no smoking gun.

So to answer some of your questions and address some of the comments, see below.

⦁ Did I pressure test the engine's cooling system full or empty? Yes, I did both. All tests held at 15 psi. I tested for 15+ mins and the psi did not change.

⦁ Did the boat come from factory with FWC closed cooling system? Yes

⦁ Were the manifolds, risers and elbows mating surfaces flat and free of pitting? Yes

⦁ Was the mechanic a regular mechanic or an engine rebuild shop? He was a certified Mercruiser mechanic that owned his own business. He was experienced but I don't know much about his reputation. He doesn't do head work, he sends them to a machine shop.

⦁ Were the camshafts changed during the recent work or were they ever changed? Not to my knowledge.

⦁ Do I have an expansion (puke) tank connected to the heat exchanger? Yes, what's strange with my system, I get a lot of coolant that pushes out of my engines (approx 30 oz) and it will overflow my expansion tanks. I stopped filling my expansion tanks and only fill the heat exchangers. Could the amount of coolant that pushes out of my engines have anything to do with the way my cooling system is plumbed?

⦁ What temperatures do the engines run at? 170-175 deg F.

⦁ As far a performing an acetone test to the the exhaust manifolds, I would think a pressure test would be more reliable than an acetone test. I pressure tested the water jacket side of the manifolds/risers which held at 20+ psi.

⦁ As far as the comment that it doesn't matter where the block off gasket is located as long as it's plumbed correctly. Hum, if this is correct why does Mercruiser's service manual and bulletin tell you the block off gasket must go between the exhaust manifold and riser. Does someone know the reason Mercruiser wants it this way?