7.4 overheating

[rooferjoe]

Recently purchase 95 crownline 250 cr. Had been hydra locked due to no exhaust baffles
Shut off was due to overheating relating to raw waterpump failure
Engine was cleaned of water installed new plugs and new raw water pump after test run all ckecked out well
On next trip out after 30 minutes alarm sounded and sounded like water boiling after short cool off ran good ten minutes and did the same again kept motor ideling alarm went off and idoled to dock any thoughts

 

[V-Tech]

Re: 7.4 overheating

How did you know it hydro locked? Was it because you tried to crank it and it would not crank? If so, you may have a blown headgasket or worse. Also, simply overheating an engine can ruin a headgasket or crack a cylinder head. Do a compression test and it might reveal the cylinder with a faulty headgasket or crack. Any of those can cause overheating. How is the thermostat?

I know, you did not want to hear those thoughts. Good luck!

 

[Bondo]

Re: 7.4 overheating

Recently purchase 95 crownline 250 cr. Had been hydra locked due to no exhaust baffles
Shut off was due to overheating relating to raw waterpump failure
Engine was cleaned of water installed new plugs and new raw water pump after test run all ckecked out well
On next trip out after 30 minutes alarm sounded and sounded like water boiling after short cool off ran good ten minutes and did the same again kept motor ideling alarm went off and idoled to dock any thoughts

Ayuh,... Did ya find ALL the missin' pieces of the old impeller,..??

 

[rooferjoe]

Re: 7.4 overheating

Compression test was done and was told all ckecks well also a new thermostate 160 deg installed
Only running hot under load
Also no water in oil showing up

 

[rooferjoe]

Re: 7.4 overheating

I had that thought aswell went to repair shop and they did not seam to think they checked as I also first found them not to have changed the oil twice after hydralocking

 

[rooferjoe]

raw waterpump

raw waterpump

Recently purchased 7.4 95 model 250cr
Had a new raw water pump installed prior to purchase after learning of prior overheating issue earlier this season
Test drive went well , only out about 15 minutes this past trip out 25 minuites into running alarm sounded and heard sounds of overheating
Also apeared to have little water if any in starboard side hoses, after short cooldown boat ran fine at idol speed but again after running full throttle 10 minutes alarm sounded again temp about 200
Reading. Slowed and shortly after alarm went off and idoled back to boat ramp. Any ideas

 

[Don S]

Re: raw waterpump

Re: raw waterpump

Check the impeller first, one plastic bag and it can take a toll on the impeller.
Could be the exhaust risers and manifolds are plugging up with rust and corrosion.

 

[jimh1626]

Re: 7.4 overheating

Are you sure that is the right thermostat? You might need a 140 or 143 degree stat. If you are using a 160 that is the temp it starts to open and opens fully at 185 from what I have read.

 

[Fishermark]

Re: raw waterpump

Re: raw waterpump

Any ideas

Run at idle for the rest of the time!

Either that or what Don says.

In all seriousness... who replaced the raw water pump? What it missing any pieces? If so, were they found? A lot of questions to be answered.

 

[Fishermark]

Re: raw waterpump

Re: raw waterpump

I see you have two posts going with the same problem.... best to keep it at one post. Maybe the moderator will combine them.

 

[Don S]

Re: raw waterpump

Re: raw waterpump

Merged both posts into one. Please don't start different thread on the same subject.

 

[rooferjoe]

Re: 7.4 overheating

Does a bravo one outdrive have a pick up waterpump in the lower unit and if so would that if bad akso contribiute to high rev overheating

 

[Bondo]

Re: 7.4 overheating

Does a bravo one outdrive have a pick up waterpump in the lower unit and if so would that if bad akso contribiute to high rev overheating

Nope, it's motor mounted, rather than in the drive...

 

[tpenfield]

Re: 7.4 overheating

I feel your pain as my Mercruiser 7.4L started experiencing the same overheating problem this summer.

At cruising speeds the temp goes up to about 170 - 180 and then when you go back to idling speeds, the temp drops back to the normal operating range.

I did all the 'normal' stuff regarding the cooling system to no avail. Then I performed a compression test.

My compression test came back good with 134 psi +/- 7 psi on all cylinders . . .

Sooooo, I performed a cylinder leak test . . . different story. 4 of the 8 cylinders showed excessive leak 30% - 50% leakage. The others were less than 10% leakage.

You may want to do a leak test as that can be much more revealing than just a compression test.

Your situation sounds like the early stages of a blown head gasket, but let the leak test tell the story.

I am currently tearing down my engine for a head gasket and valve job. Good luck with yours, let us know how the further diagnostics go.

 

[rooferjoe]

Re: 7.4 overheating

Thanks I will try that latter this week

 

[rooferjoe]

Re: 7.4 overheating

Rechecked everything again this weekend
Any suggestions on how to leak check each cylinder some are open and some are closed
Also ran n hose and all water and exhuast running out lower bell in drive is this correct

 

[tpenfield]

Re: 7.4 overheating

You can Google procedures for a cylinder leak test. You will need a 'leak test gauge'. Essentially, you have to turn the engine manually to TDC on each cylinder to perform the leak test.

A blown head gasket will reveal itself by air escaping into an adjacent cylinder and/or air going into the cooling system. Also, it would be good to check for water in the oil (what we call the 'milk shake').

 

[rooferjoe]

Re: 7.4 overheating

Oil is clean and compression is good