7.4 Mercruiser overheat

@94SD370
Although a proper compression test should be performed on a hot engine, WOT and the fuel disabled, I don't think it's gonna make much difference on those low cylinders.
A good cylinder leak down test on all cylinders will tell you where the leak is most prominent and help you make a better decision when you pull the heads and get a chance to visually inspect the cylinders.
 
Follow up to our problem - Overheating Starbooard engine at higher RPM - Replaced thermostat (stuck open) - Problem solved.

Still have high Riser Temps on inboard side Port Engine (VDrives). Temps run 145-150 while all three others are 125ish. Good flow to Manifold, couldn't pull muffler to see flow through. Pinched other side manifold hose and flow continued (may have a bypass).

Closed cooling, raw water through intercoolers, exchanger and out manifolds. Have cleansed strainers and changed impeller (whole body).

Should the higher temp alarm me - Getting lots of steam running at 3,000 RPM but not much when piddling at 1,000.
 
Steam don't sound good
 
techmitch said:
@94SD370
Although a proper compression test should be performed on a hot engine, WOT and the fuel disabled, I don't think it's gonna make much difference on those low cylinders.
A good cylinder leak down test on all cylinders will tell you where the leak is most prominent and help you make a better decision when you pull the heads and get a chance to visually inspect the cylinders.
Click to expand...
Well, tried to do a leak down and I was having a difficult time finding TDC. I am a one man band and trying to rotate the fly wheel while holding a dowl in the spark plug hole isn’t possible with the manifolds in place. I pulled the rocker cover, and alligned both rockers in the up position, using a remote starter and that’s the best I could do. There has to be a better way? On cylinder 8 using this method, I set the compressor to 50PSI and got 48/45 on the leak down gauge. On 6 it was 50/10. Almost complete blow by. I can hear the “hissing” most of the time in the exhaust manifold. I can’t hear anything around the rockers. Nothing in the oil dip stick or heat exchanger. Worth noting, I have never done a leak down before. I noticed nothing unusual to me visually with rocker movement. Trying to decide, if this is an exhaust valve or ring/piston issue. I suspect it’s a valve, but would hate to spend $500-$1000 on a head if I need to buy a new motor. Marine mechanics are very hard to find where I am (and expensive) Thanks!
 
With that much air getting by the valves you won't be able to tell how much is getting by the rings.

Pull the heads and inspect the bores.

If you want to do that leak down, mark your balancer in quarters 90 180 and 270 degrees from the tdc mark that's on it.
 
94SD370 said:
Well, tried to do a leak down and I was having a difficult time finding TDC. I am a one man band and trying to rotate the fly wheel while holding a dowl in the spark plug hole isn’t possible with the manifolds in place. I pulled the rocker cover, and alligned both rockers in the up position, using a remote starter and that’s the best I could do. There has to be a better way? On cylinder 8 using this method, I set the compressor to 50PSI and got 48/45 on the leak down gauge. On 6 it was 50/10. Almost complete blow by. I can hear the “hissing” most of the time in the exhaust manifold. I can’t hear anything around the rockers. Nothing in the oil dip stick or heat exchanger. Worth noting, I have never done a leak down before. I noticed nothing unusual to me visually with rocker movement. Trying to decide, if this is an exhaust valve or ring/piston issue. I suspect it’s a valve, but would hate to spend $500-$1000 on a head if I need to buy a new motor. Marine mechanics are very hard to find where I am (and expensive) Thanks!
Click to expand...
You could use a borescope to look into the cylinder while rotating the flywheel to see TDC.
 
scoflaw said:
With that much air getting by the valves you won't be able to tell how much is getting by the rings.

Pull the heads and inspect the bores.

If you want to do that leak down, mark your balancer in quarters 90 180 and 270 degrees from the tdc mark that's on it.
Click to expand...

I was able to do the leak down by dividing the balancer into quarters. I also removed the distributor cap, to aid in finding TDC on compression stroke.

While I had the plugs out, I also did another compression test.

The results surprised me, and leaves more questions.

I only did the right side of the engine, which is where I am having the problem. I used 50 PSI on the compressor, and the tester showed approximately 40 PSI going into the cylinder.

Cylinder 8: 120 PSI on compression, 45/42 leak down.

Cylinder 6: 68 PSI, 40/10

Cylinder 4: 115 PSI, 40/5

Cylinder 2: 108 PSI, 40/32

When pumping air into cylinders 6&4 I can feel it escaping from the other plug holes. So I put the plugs back in and I could hear the air escaping from the exhaust.

Adjoining cylinders suggest a leak in the head gasket between the two. I am surprised cylinder 4 has such bad blow buy, but still has 115 PSI compression. That doesn’t make sense to me.

I will be removing at least that head in the coming weeks, and like you said, that will probably tell the rest of the story.

I went from thinking this is an exhaust valve issue, to thinking it’s a head gasket. Maybe I have multiple issues?
 
There is no downside to pulling the heads to see what is going on if you are doing the work. If the valves are not sealing......the numbers for compression as well as leak down will be all over the place.

I wouldn't read anything into those numbers at this point.
 
Help!

Solved initial overheating problem on Starboard with Impeller and thermostat. Changed same on Port engine. Port engine riser remained hot until this past weekend when we got no water flow at idle and little at higher RPM. Coaxed it home.

Troubleshooting - Pulled hoses
Impeller to Transmission cooler - Good flow
Transmission Cooler to Heat Exchanger - Good flow
Y fitting to inside riser drain - Closest to raw water inlet of heat exchanger - Good flow
Y fitting to outside riser drain - other side of heat exchanger - not so good flow

I think my issue is in the heat exchanger, steadily got worse over the past weeks until its completely clogged now (I think).

What is the best way to clean/troubleshoot?
Can I remove the two ends and blow it out?
Would the raw inlet hose to impeller be collapsing under pressure?
Freshwater boat.

Thanks in advance!
 
D Bell said:
Help!

Solved initial overheating problem on Starboard with Impeller and thermostat. Changed same on Port engine. Port engine riser remained hot until this past weekend when we got no water flow at idle and little at higher RPM. Coaxed it home.

Troubleshooting - Pulled hoses
Impeller to Transmission cooler - Good flow
Transmission Cooler to Heat Exchanger - Good flow
Y fitting to inside riser drain - Closest to raw water inlet of heat exchanger - Good flow
Y fitting to outside riser drain - other side of heat exchanger - not so good flow

I think my issue is in the heat exchanger, steadily got worse over the past weeks until its completely clogged now (I think).

What is the best way to clean/troubleshoot?
Can I remove the two ends and blow it out?
Would the raw inlet hose to impeller be collapsing under pressure?
Freshwater boat.

Thanks in advance!
Click to expand...
I removed both end caps on mine and found impeller bits. I then used a long drill bit (a dowl will work)to clean out the tubes. I then flushed it with water. It didn’t fix my problem, but the heat exchanger flows well now.
 
Thanks. Cleaned heat exchanger(s) yesterday.

Port engine is fine. Starboard needs RPM’s to get water flow. No water flow at idle, lots of steam.

Engine runs and water flows fine at 1,000, better at 2,000 keeping engine at 160. Water flow stops when I reduce down to idle and engine begins to overheat. Inside riser still hotter than outside by 20 degrees (140-150).

Replaced impeller two weeks ago but wondering if it burnt since then with the heat issues.
 
OK - Two more hours today. Backflushed everything. I even backflushed the backflush. Still no water flow at idle. Fine at 1,000 RPM and keeps the the engine temp at 160. Begins to overheat at idle. What could it be?

So far -
Impellers
Cleaned out trans cooler
Cleaned out Heat Exchanger

Backflushed:
Heat Exchanger
Trans Cooler
Hoses from Heat Exchanger to other side of Trans cooler

Front flushed from Heat Exchanger out Risers

All clear.

HELP!
 
I would be looking at a re-do on that pump if you still have now water flow. Or, since it is not an outdrive, have you checked the water pickups for blockage / growth, or maybe a wonky shut off valve or strainer? Possibly a collapsed / deteriorated raw water hose before the pump.

Not too many things can cause it on a closed cooling engine with sea water pickups / strainers.
 
You may be getting air in the system before the pump. That would explain the no water at idle and water flow at 1,000 rpm.

As Bill mentioned……it is something simple. The pump could even have a crack in it.
 
What about end plates
 
PlayDate said:
You may be getting air in the system before the pump. That would explain the no water at idle and water flow at 1,000 rpm.

As Bill mentioned……it is something simple. The pump could even have a crack in it.
Click to expand...

Thanks. I'm going to hook up a clear hose between the impeller and Trans Cooler and check for air bubbles. I assume that's a good test? If no air, then I assume I have a blockage.
 
Bill Curtis said:
I would be looking at a re-do on that pump if you still have now water flow. Or, since it is not an outdrive, have you checked the water pickups for blockage / growth, or maybe a wonky shut off valve or strainer? Possibly a collapsed / deteriorated raw water hose before the pump.

Not too many things can cause it on a closed cooling engine with sea water pickups / strainers.
Click to expand...

Thanks Bill. Will check for air bubbles (hopefully today).
 
D Bell said:
Thanks. I'm going to hook up a clear hose between the impeller and Trans Cooler and check for air bubbles. I assume that's a good test? If no air, then I assume I have a blockage.
Click to expand...
Yes....that will work. It doesn't take a lot of air for the pump impeller to cavitate and not move water.

That said....I replaced both impellers and pump housings on my boat 3 weeks ago. For the first time, I didn't use Quicksilver impellers because the new pump housings came with impellers.

The Starboard impeller fell apart in ten minutes. The wife had 8 friends coming to the boat for a cruise up the Potomac and dinner. It started off pumping water fine then no water at idle. I sent her and her friends to a local restaurant for 45 minutes while I put the old pump housing and impeller back on the Starboard engine.

I knew the Port engine impeller would probably end up in the same place but I only had one spare pump housing/impeller and a bunch of people that wanted to go boating.

45 minutes into the trip (8-10 knot cruise) the port engine's temperature starts to increase. So I shut it down and did the rest of the trip on the starboard engine (keeping a very close eye on the Starboard engine's temperature).

This is what the impellers looked like:

The top impeller lasted 10 minutes and the bottom one lasted 45. A lot of quality parts come from China......these weren't those. There is a reason that Quicksilver impellers are engineered with wear guards on both impeller ends and high quality rubber.

1694526559956.png
 
Damn thats brutal. I use quick silver or factory Merc, whatever the marina has on stock
 
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