Low compression on one side

[Tomco]

My wife and I are just in the process of buying our first cruiser and have an offer on a 2001 Sundancer 340 with the 7.4MPI MerCruisers. We have had a survey completed with only one finding based on the sea trial - at WOT the starboard engine runs at about 4150 RPM, which is about 250RPM less than the port engine (4400RPM). When the throttles are pinned the port engine spools up quickly while the starboard engines takes a bit longer. We have left this with the owner to fix before completing the sale.

They had a mechanic put a computer on it and found the sensor on the exhaust manifold (CO2 I think) was defective and had it replaced - it did not solve the discrepancy. After a compression check they found that one side of the starboard engine was 10 - 15 psi lower on all four cylinders than the other side. Common problem is usually the head gasket or the timing chain skipped a tooth ... they are in the process of pulling the head to further diagnois.

Has anyone else had a similar problem and what was the fix. I've informed them that I would like verification that the head is not warped - anything else I should be tracking down to have confidence it has been fixed correctly?

I'll post an update on what they uncover when they call next - probably tomorrow.

Thanks,
Tom

 

[gerryb]

How many hours on the engines? I believe a 10psi variation on one cylinder on the 7.4 is acceptable especially if the engines have hours on them. One side consistently lower than the other...? probably a gasket issue would be my guess..

 

[Larry]

Without any other background info I would be suspicious of an exhaust failure were water has gotten into the cylinders and rusted the exhaust valve seats a little. This could account for the drop in compression. I don't think it is a head gasket as it has affected all cylinders on one side, usually when a head gasket goes you could get lower compression on just one or two cylinders.

 

[dicor]

You will find countless posts on this and other boating forums concerning exhaust manifolds failing internally due to salt water corrosion allowing saltwater to find its way to the heads and destroy the valves. If left unchecked it will eventually hydro-lock the motor then you will be staring at a $12,000 + bill. You stated you had a mechanic inspect the motor. Is he a boat mechanic? If not you, would be wise to pay for a complete survey and make sure it includes a compression test. Although it is basically the same motor there are issues you would never ever see on a car as you would on a boat. Good Luck

 

[Larry]

You will find countless posts on this and other boating forums concerning exhaust manifolds failing internally due to salt water corrosion allowing saltwater to find its way to the heads and destroy the valves. If left unchecked it will eventually hydro-lock the motor then you will be staring at a $12,000 + bill. You stated you had a mechanic inspect the motor. Is he a boat mechanic? If not you, would be wise to pay for a complete survey and make sure it includes a compression test. Although it is basically the same motor there are issues you would never ever see on a car as you would on a boat. Good Luck

+1!

 

[gerryb]

The funny thing is... wouldn't the mechanic see water coming out of the cylinders when performing the compression test if there was a breakdown on the exhaust side...?

 

[jackson]

Tomco...has the exhaust ever been replaced? The manifolds/elbows should have been replaced by now. If not, don't trust it. Also, make sure the boat has the newer style exhaust with waterlift mufflers. The old log style is known for causing water ingestion. The set-up was changed half way through 2001 on the 340.

 

[Tomco]

Thanks for the feedback ... a little more info. I have had a complete survey done and it was the surveyor that found the RPM problem when he did the sea trial. The boat is out of Montreal so it has only been used in fresh water - it is my understanding that the manifolds should not . The engines have 380 hrs on them. I'm not leading the repair, it is the current owner and they have been keeping me appraised of what they have been doing. They have a marine mechanic doing the work.

Larry - the mechanic should be able to determine if there was any water infiltration when he pulls the head off. I'll ask next time they call.
Jackson - I don't think the manifolds/elbows have been changed ... since they need to pull them to get to the heads I assume they will inspect before putting them back on. What is water-lift mufflers vs. log style and how would I tell?

Thanks,
Tom

 

[Tomco]

Well, here is the update for today! From a magnaflux inspection they discovered some small crack(s) in the head ... they have ordered a new head from MerCruiser and expect installation in the next couple of days. I've asked for confirmation after the repair that the valves were also changed, a compression test on all 8 cylinders (they are likely to do this anyway), and RPM verification at WOT using the diagnostic computer (in case the gauges are off), for confidence before accepting the repair. Anything else I should be considering?

I received the survey report yesterday, according to the gauges, the RPM difference was 500 RPM at WOT (4400/3900) vs. the 250 I thought ...

/Tom

 

[Steve500dancer]

Cracked head in Montreal. If I remember correctly, and I do... Damn cold there. I would check every system for signs of things being frozen at one time or another. Sounds to me like they didn't do a good job winterizing that motor. Check everything, everywhere for signs of poor winterizing causing water tanks, toilets and all other water holding systems to freeze and crack. Fill the water tank to the top and look for any leaks.


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[Tomco]

Just thought I'd provide an update on this thread. The new head was installed and made no difference to the compression or the operating performance. They then thought it was a faulty sensor ... after replacing the exhaust sensors with no luck, they then discovered that the timing was not advancing past 12 degrees. Nope, it wasn't the computer ....

In the end we decided that this was not the boat for us and withdrew our offer - last I heard, the owner had't found the problem.

As fortune has it, my wife found a 2003 340 Sundancer that was not on our shopping list and we ended up purchasing it in September - its fully loaded (bow thruster, 496 engines, electronics). Couldn't be happier how it turned out!

If I do find out what the root cause was I will post in case others have to track down a similar problem.