Mercruiser 5.7L Carb Alpha/Bravo Overheating or Within Spec?

[silverseal99]

Folks, I have a perplexing manifold/riser temperature/water flow issue that has me stumped. I don’t know if I am chasing a valid overheating issue or am within normal operating conditions and chasing a ghost. Below is a bit long but I tried to address all the “Have you tried…” issues up front. I am new to forums so if I commit some egregious error in protocol or procedure, please forgive me.

Issue: Seeing 30 F temperature differential between port and starboard risers (port side hotter) at idle whether in driveway or at the dock. Cooling water to and from the port riser burps and gurgles with about 30-50% air or exhaust gas mixed in. Peak temperatures at idle are 151 F at the port manifold, 119 F at the port riser, and 121 F at the port elbow. Increasing idle speed to 1200 rpm results in temperatures balancing to within 10 F port to starboard.

Questions:
1) Do I have a temperature issue or am I within specification?
2) Could a persistent “no fire” in #2 result in a 30F temperature differential port to starboard?
3) Is exhaust gas in the cooling water at the riser normal, or indicative of a cracked manifold that only manifests itself at operating temperature?
4) Should I invest limited funds to replace the manifolds or install an aftermarket closed cooling system instead?

Thanks in advance for your interest and informed recommendations.

GORY DETAIL

Background: During an initial sea trial in May 2019, the engine ran fine at idle and on plane but at WOT only pulled 3900 rpm. While at WOT, I smelled a hot engine, eased her back to idle and shut her down. Dash temperature gauge hit 200F. The set-up is a stock, raw water cooled, 2003 Mercruiser 5.7 Alpha/Bravo Carb (Ser. 0M634xxx) with 2 bbl Mercarb, dry joint manifolds and risers, less than 50 hours on the clock, and 100% saltwater use. Timing is set to 10 BTDC with purple/white wire disconnected. The engine is clean with no apparent exterior rust (either surface or at joints/gaskets) but was fresh water flushed (but not winterized) and stored on a trailer in SoCal for 15 years. In 2017, the boat was serviced (bellows, impeller, fluids, tune-up, etc), shop run, and assessed ready to operate by a local, respected marine repair shop specializing in Mercruiser products. Boat remained idle while hull/other maintenance was performed.

Assessment: Post-mortem revealed clogged jets in the carburetor (resolved), stuck thermostat (resolved), clogged water passages in manifolds (resolved), no fire in #2 cylinder (probably resolved although occasional backfire remains on startup), and persistent 30F temperature differential port to starboard. Measured raw water flow to thermostat housing exceeds threshold by 18% so “Bravoitis” has been ruled out. Suspect either cracked port manifold permitting exhaust in cooling water, or this is normal operation and I’m chasing a ghost.

Actions to date:

• Manifolds and Risers: Removed and inspected. Bottom ports were clogged on the manifolds and the risers had some internal rust flakes. All were stripped, rodded, pressure washed, flow checked (garden hose), inspected (visually and with an endoscope), and re-installed with new gaskets. (A local machine shop assessed them to be clean and serviceable and that chemical boiling could do no better).
• Raw water pump: Rebuilt with new impeller and housing. Suction hose back-flushed from pump through out-drive. Supply hose back-flushed from thermostat housing to pump. Operational flow check yielded 10 quarts/15 sec at 1000 rpm pierside (exceeded spec).
• Power steering heat exchanger: back-flushed with garden hose.
• Thermostat: Housing is clear/clean with no rust. A new, tested good 140 F thermostat has been installed.
• Circulation pump: A new marine (SS backing plate, bronze impeller) circ pump was installed in 2017 with zero hours. It was removed, inspected, and reinstalled with new gaskets.
• Block: Water passages were flushed with a garden hose while the circ pump was removed. They flowed clear with no obstruction.
• Hoses: All hoses were checked clear and clog free. One soft manifold supply hose was replaced.
• Y-pipe and exhaust: Flowed clear from the flapper through the transom. Flappers, joints and elbows are clear and in good condition.
• Elbows: clear and clean with connections in good shape.
• Tune: Verified timing. Replaced spark plugs with AC MR43LT8. Checked spark to #2 with known good spark plug wire.
• Compression: Checked compression good at all 8 cylinders.
• Leak-down: Checked good (6% or less) at all 8 cylinders.

 

[silverseal99]

Forgot to mention two things:
1) I swapped risers (port to starboard and vice versa) with no change--port side still running hot.
2) I pressure tested the water jackets on both manifolds. The port manifold (the hot side) held 60 psi overnight while the starboard manifold leaked down after 20 or so minutes (go figure).

 

[Bondo]

2003 Mercruiser 5.7 Alpha/Bravo

Ayuh,..... Welcome Aboard,...... Which is it,..?? Alpha, 'n Bravo are very, Very different,......

Issue: Seeing 30 F temperature differential between port and starboard risers (port side hotter) at idle whether in driveway or at the dock.
I don’t know if I am chasing a valid overheating issue or am within normal operating conditions and chasing a ghost.

Yer chasin' a ghost,..... Manifolds/ risers rarely ever run the exact same temps,.....

 

[tpenfield]

My comments . . .

Sea Trial was pre-purchase or post purchase :noidea: I would not have bought the boat if the engine reached only 3900 RPM.

Is the propeller reasonably correct? i.e not of excessive pitch for the boat/engine

Since your leak test came in good, I would not suspect a head gasket or cracked cylinder head. I would like to see the actual #'s of compression and leak for each cylinder.

I assume that this is an open cooling system (right?) As such, the cooling water will boil at 212˚ F . . . so . . . the air/water mixture that you are seeing to the port side could be from water reaching that temperature.

Take an IR heat gun and get temperature reading at various parts of the engine/hoses, comparing port and starboard.

What is the history of the engine? Rebuild? Valve job? other ?

IIRC - the top of the exhaust elbows on my 7.4 Merc run at about 95˚ F at idle (closed cooling system though . . .) One side may be about 10˚ F hotter than the other.

A few thoughts . . .

I assume that the engine is still over heating (200˚ F)? Should be 175 ish.

Thermostat housing . . . check, clean/replace

Hose at the outdrive bell housing - cracked, pinched, internally clogged - If it is a Bravo outdrive . . . maybe Bravoitis. The best way to check for Bravoitis is to take the fitting off at the transom assembly and look. A basic flow test may not be enough to tell. Not sure if Alpha drives suffer from a similar fate :noidea:

If there was engine work, perhaps the wrong type of head gasket was used. Marine engines should have a head gasket that provides 'parallel' flow from the block to the head. Regular automotive head gaskets often provide 'serial' flow. I wouldn't go ripping apart the engine at this stage to check, but something to keep in mind.

I went down a similar road a few years ago with my Merc 7.4 Bravo . . . it turned out to be a shell had grown inside the water duct of the outdrive, then dislodged and got stuck at the bell housing, restricting the flow of water to the engine.

 

[silverseal99]

Bondo and Tpenfield, Thanks for your quick responses.

For Bondo: the engine is 5.7L Alpha/Bravo Carb with dry joint mated to a Bravo III outdrive. Regarding the temperature differential, the local dealer said 10F is normal, 20F the upper limit, 30F too high.

For Tpenfield:
o Boat was purchased in May 2003 after a normal sea trial with proper rpm. Wheels were 22P cupped.
o In December 2003, previous engine (ser. 0L619xxx) blew an oil pressure switch while on plane, dumped the oil into the bilge, and spun a crank journal before I figured out what was happening and could shut her down. I found a brand new dry joint bobtail (ser.0M634xxx) at a price comparable to a rebuild and had it installed by a local marine shop. The new Vortec V8 engine revved too high with the 22P wheels so I replaced them with 26P which tamed her into normal operating range.
o Compression: (Yes, they are high, but I checked with three different compression gauges and inspected the bores with an endoscope to rule out water intrusion or other problems).
#1 200 #2 200
#3 199 #4 200
#5 192 #6 200
#7 190 #8 200
o Leak-down: I don't have the numbers but all cylinders came in at or below 6%. Test was conducted by a marine mechanic with a Snap On leak down tester.
o Water temperatures: At idle through 1200 rpm, Starboard riser and elbow remain cool to the touch in the 89-99F range while port side ranges from 99F to 110F. At idle, hottest manifold temp is #7 exhaust at 151F. Thermostat housing is 137F immediately above the installed 140F thermostat. While starboard elbow runs cool, port elbow is hot at about 121F.
o Engine history: New bobtail in 2004 with less than 50 hours on it, but sat in driveway for 15 years.
o Still overheating? I haven't taken her out for a second sea trial yet because of the temperature differential issue. Old 160F thermostat was stuck closed through 212F in kitchen test--this would have caused the initial overheat issue. I replaced it with a new/tested good Sierra 140F. Also, the temperature gauge sensor in the thermostat housing was grossly inaccurate and replaced with a new OEM part. Temperature at the dash is now 140'ish and steady at idle pierside.
o IIRC: Roger, good input, thanks. Question: do temperatures on port riser cycle somewhat at idle, perhaps in synch with thermostat opening and closing?
o Thermostat housing: Clean and solid throughout.
o Transom cooling water supply hose: Damn. The interior fitting is in a difficult/nearly impossible location. Local Mercruiser mechanic said to dunk her, rev the engine, and observe the raw water supply hose. If it sucks down at all there is a likely restriction at the transom and to proceed to the next step (replace the hose). I forgot to do this the other day when I did the pierside flow check of the RW pump. I thought that, since the pump flowed 18% more than spec, I was probably okay on Bravoitis but need to observe the suction hose per the mechanic's recommendation.
o Head gasket: I don't think so. The bobtail was a brand new item from the former factory boys at Stillwater and hasn't been molested since.

Residual issues:
1) Gaseous material in port riser cooling water. My marine mechanic buddy says that manifolds can have small cracks that open when heated, this could be the cause of the gas infusion, and that he can verify with a thermal imaging camera during a sea trial. He has such a camera and I will attempt to cajole him.
2) No fire #2 cylinder: I'm unsure whether a non-firing #2 cylinder would contribute significantly to the starboard bank running cooler than the port bank. Although I have installed new spark plugs throughout, #2 is clean and doesn't appear to be firing. I replaced it again with another new plug, tested it with a known good plug wire, and got indications of a good spark on an in-line spark plug tester. However, the plug still comes out clean indicating a no fire. Next step will be to replace the distributor cap (I have a known good spare).

Thanks again to you both for your interest and insights. All said and done I need another sea trial with a handy assistant. I hope I don't have to get towed back to the dock (again)...