What the HOT is going on here?!-gctid387936

[Guest]

2000 Mercruiser 3.0, inline 4 cyl, no thermostat; Alpha outdrive.

Engine has always operated at 125 deg (by gauge) regardless of load.

Straight to it - on last outing, temp fluctuated around 150 - 175 on the run...falling back to 125 - 140 idling along; eventually crept up to 175 by days end....and then finally and suddenly slammed over to 250, triggering the temp alarm.

I have rebuilt the pickup pump in the lower unit with new impeller, wear plate, gaskets etc. ; AND installed a new circulating pump on engine.

Ran fine in the driveway, sucking from a tub OR on ears - 125 deg.

TODAY -first "river" test since repairs, BUT - a repeat. Started out fine, crept up, eventually OVERHEATED.

Anyone wanna' speculate?? Did I do something wrong, or have I overlooked another possible cause??

(Other observable factors: there are two discharge holes; one on either lower side of the gimbal housing - but only one spews hot water...and cool hose water tricles with exhaust gas from the prop cone. If I block that hole, the other side will discharge - but seemingly less volume AND a greater amount of WARM water from the prop cone. Pertinent??)

Any advice, suggestions, clues welcome.

[Guest]

How long since the risers/manifolds have been checked? Could well be rusted nearly shut.

And running an engine cold (125) is not particularly good for them, nor good for fuel economy. 175, running a thermostat, will be much better.

[Guest]

Exhaust flappers broken off and blocking exhaust ?

[Guest]

orca wrote:
How long since the risers/manifolds have been checked? Could well be rusted nearly shut.

And running an engine cold (125) is not particularly good for them, nor good for fuel economy. 175, running a thermostat, will be much better.

Thanks . The riser / manifold Has never been inspected. It is still as it was when originally assembled . I am not completely familiar with the inner workings. Would you mind elaborating?

[Guest]

crowder.dr wrote:
Exhaust flappers broken off and blocking exhaust ?

Where are those located? ( The confounding thing is That I have good Water flow and discharge .)

[Guest]

On the port side of your 3.0L, from the top down is the carb, intake manifold, then the exhaust manifold (where the exhaust exits the engine). At the rear of the exhaust manifold is the riser. The riser is like a 180 degree chunk of cast iron that sits on top of the rear of the manifold. The riser goes straight up, then turns back straight down into a rubber boot. The exhaust flapper is in the cast iron pipe just below the rubber boot.

Over time, the manifold and riser can completely plug with rust and restrict the cooling water flow at higher RPMS. Temp and flow may look fine at idle or lower RPMS, but is inadequate for cooling the engine under load.

[Guest]

orca wrote:
On the port side of your 3.0L, from the top down is the carb, intake manifold, then the exhaust manifold (where the exhaust exits the engine). At the rear of the exhaust manifold is the riser. The riser is like a 180 degree chunk of cast iron that sits on top of the rear of the manifold. The riser goes straight up, then turns back straight down into a rubber boot. The exhaust flapper is in the cast iron pipe just below the rubber boot.

Over time, the manifold and riser can completely plug with rust and restrict the cooling water flow at higher RPMS. Temp and flow may look fine at idle or lower RPMS, but is inadequate for cooling the engine under load.

Excellent sir ! I am very familiar with the Assembly you speak of . I will investigate that passageway posthaste . Thank you for taking the time to explain.

[2850Bounty]

HighTidesOnly wrote:
2000 Mercruiser 3.0, inline 4 cyl, no thermostat; Alpha outdrive.

Engine has always operated at 125 deg (by gauge) regardless of load.

(Other observable factors: there are two discharge holes; one on either lower side of the gimbal housing - but only one spews hot water... and cool hose water tricles with exhaust gas from the prop cone. If I block that hole, the other side will discharge - but seemingly less volume AND a greater amount of WARM water from the prop cone. Pertinent??)

The 4 cylinder transom unit, I believe, will have the two discharge ports as does the V-8 transom unit.

The Aluminum V-8 Y-pipe directs low RPM exhaust through these ports as to eliminate back pressure from the otherwise "thru prop" exhaust.

These are often referred to as "exhaust relief" ports.

As seen here, the Aluminum V-8 Y-pipe relief ports align with the transom unit ports. When running, water exits both ports.



With the inline 4 cylinder Aluminum tube, both relief ports align, but the Port side gets the brunt of the water flow.

When you plug off the one, the other takes over. This is normal.

orca wrote:
On the port side of your 3.0L, from the top down is the carb, intake manifold, then the exhaust manifold (where the exhaust exits the engine). At the rear of the exhaust manifold is the riser. The riser is like a 180 degree chunk of cast iron that sits on top of the rear of the manifold. The riser goes straight up, then turns back straight down into a rubber boot. The exhaust flapper is in the cast iron pipe just below the rubber boot.

Over time, the manifold and riser can completely plug with rust and restrict the cooling water flow at higher RPMS. Temp and flow may look fine at idle or lower RPMS, but is inadequate for cooling the engine under load.

Orca pretty much nails it.

If you have not yet pulled your exhaust system apart for an expection......, it's high time to do it.

BTW, IMO, you should NOT be operating without a thermostat in place.

The OEM designed these to use a stat.

.

[Guest]

My thanks to ALL for their expert assessments, direction, graphical references and opinions...especially Orca!

My buddy and I pulled the riser off and INDEED it was rusted shut. A horrific sight! The center exhaust gas funnel cone was completely eroded away! The lower tube - housing the flappers was actually (and surprisingly) pristine. I have ordered another riser. Again - THANKS!

[Guest]

Did you resurface the face of that Y-pipe? That is the CLEANEST I have ever seen!

[2850Bounty]

HighTidesOnly wrote:
My thanks to ALL for their expert assessments, direction, graphical references and opinions...especially Orca!

My buddy and I pulled the riser off [SIZE]4 wrote:
and INDEED it was rusted shut.[/SIZE] A horrific sight! The center exhaust gas funnel cone was completely eroded away! The lower tube - housing the flappers was actually (and surprisingly) pristine. I have ordered another riser. Again - THANKS!

Good for you! Yeah! :coo-

This should be a testimonial for all to see. Often we tend to over-look this and chase our tails in all other directions.

Speakrdude wrote:
Did you resurface the face of that Y-pipe? That is the CLEANEST I have ever seen!

No, Jim.... Google Image did that for me!

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