Continuos problems with 4cyl mercruiser i/o

[Sascwatch]

I'll try and keep this post short but we just blew a head gasket on the boats fourth #1 engine.

The boat is a 26' Searay Sundancer that was repo'd and had dual 4cyl mercruiser i/o in it. Newly rebuilt engines, all new outdrives and gimbal housing/bearings.

We have had zero problems with engine #2 which was rebuilt by the same mechanic at the same time.

Original motor had a knocking noise which turned out to be a loose rod end cap, engine rebuilt for the second time.

Got the motor back, everything tested fine on the test bench so they installed it in the boat. Went for a test drive and the motor still had a noise while it was in gear, ran fine otherwise for about 5 minutes then the head gasket blew.

Pulled the boat out, sent the motor out to have the head and block resurfaced. Motor came back but checkout out fine, new head gasket, gimbal bearing and rebuilt out drive installed.

This is now the 3rd time this motor has been rebuilt. Took boat out for the 3rd test drive and the noise was still there but not loud at all. Not even 2 minutes into this test drive the head gasket blew in the same spot!

Shop decided not to rebuild the motor for a 4th time and got us another rebuilt engine, this one rebuilt from a different shop. Had to transfer the alternator, power steering pump, distributor and wires over to this motor.

Got this new engine in (the 4th #1 motor) and took it for a test drive. No more noise but there was a slight vibration at all rpms. Mechanics were onboard for this ride to make sure it wasn't something we were doing wrong. Motors ran good at about 4500 rpm cruising along at about 32mph. A few minutes into the ride the head gasket blew again! On the new motor.

At this point they are out of ideas and cannot figure out what is causing this problem.

I'm leaning towards a broken timing advance in the distributor which was causing detonation which in turn was the source of all our problems.

Any ideas? Thanks.

 

[Bondo]

Ayuh,..... Merc has used several 4 cylinder motors, so which one are We talkin' 'bout,..??

2.5l,..??

3.0l,..??

or the 470, 3.7l,..??

 

[stonyloam]

Are they retorquing the head after bringing the engine up to operating temperature the first time? Bring up the operating temperature at idle, then retorque the head. Mercruiser says this is a must. There is some question as to wether you do it hot, or after it cools. I did mine hot, worked fine.

 

[Sascwatch]

Yes the engine is brought up to temp on the test bench and heads are retorqued before installed in the boat.

I forgot to add that the engine in question will loose power and drop rpms at times, increasing the throttle does nothing. You have to bring the engine down to an idle and you can reapply the throttle again. If you don't decrease the throttle then the engine will continue to drop in rpm by itself while the carb remains open. The throttle itself isn't falling back either, you can hold it in place and the rpms continue to drop.

I'm not sure about the engine size but I will grab that tomorrow, we are going to pull the distributor and check the timing advance out.

 

[Benny67]

Did you do a compression test or do a leakdown test or check the cooling systerm and see if it held pressure? the replacement motor that is.

could be just real bad luck

You might want to think about shopping around for another mechanic as well

 

[Bt Doctur]

Care to inform us on what engine it is

 

[Sascwatch]

I will get the motor size tomorrow morning when we check out the distributor, weird thing is the motor was built the first time by a shop that does racing engines, the second and third time it was rebuilt by their in shop mechanic who only does marine engine rebuilds (30+ years). This second motor came from a completely different shop.

The fuel pump was also transferred over to the new motor as well, it's gotta be either a problem with timing advance or fuel. When the rpms drop the motor won't recover by itself, you have to manually bring the rpm down to idle and rev it back up. Either running out of fuel or the timing isn't advancing is my guess.

 

[Sascwatch]

Engines are mercruiser 140's. The distributor had a well worn spot inside where the timing weights have been rubbing, hopefully this is the cause of all our problems. Mechanics think there could be an exhaust pressure problem too but I doubt it, two different outdrives and manifolds so far.

Oh and this has all been covered by warranty, but it sure has but a damper on fishing.

 

[Benny67]

The distributor had a well worn spot inside where the timing weights have been rubbing, hopefully this is the cause of all our problems. Mechanics think there could be an exhaust pressure problem too but I doubt it, two different outdrives and manifolds so far.

I don't see how these issues could blow a head gasket....

I also think that these guys are feeding you a s**t sandwich about what they are actually doing and what they are telling you they are doing.

If this was my boat I would wait till they were done with the next motor and have them do a leak down test and pressurize the cooling system IN FRONT OF ME....and then see what happens after that.

 

[Sascwatch]

We've been at this marina for almost 20 years now and have never had problems like this. Every time they've done work on our boat it's been perfect. They have one guy dedicated to rebuilding motors and several mechanics to do everything else.

My theory is the timing advance is getting thrown off due to the distributor problems. You can see the weights have been scraping about 1/4-1/3 the way around.

The distributor, fuel pump and carb are the only items that have been transferred over to the new motor. Maybe the 5th rebuild will solve our problems.

Edit: the head gasket always blows out the side of the head just under the exhaust manifold on cylinder #2. Shoots exhaust out into the engine compartment and sounds like an old farm tractor.

 

[stonyloam]

140's? Sounded like 470's to me blowing head gaskets. Yeah, this is a pretty strange one, 140's are pretty bulletproof, let us know how it works out. Good luck.

 

[bbook83]

I don't claim the expertise of others posting here, but I do recall blowing a head gasket on my 140 years ago and learning that a head bolt hole in the block had not been tapped deep enough into the block to allow that head bolt to be properly torqued, causing the head gasket failure. That wouldn't explain the failure on engine number 4' unless you had the incredibly bad luck to have the same defect in 2 different blocks.

 

[Benny67]

I don't claim the expertise of others posting here, but I do recall blowing a head gasket on my 140 years ago and learning that a head bolt hole in the block had not been tapped deep enough into the block to allow that head bolt to be properly torqued, causing the head gasket failure. That wouldn't explain the failure on engine number 4' unless you had the incredibly bad luck to have the same defect in 2 different blocks.

I don't claim to be an expert either but this sounds like something worth investigating on that first bad block that blew the head gasket over and over

 

[JerryIrons]

I'd be real tempted next time you go out to hook up a timing gun and watch it while a buddy drives the boat.

What is the "noise while in gear" ?