Low Compression and Ping Under Load Mercruiser 5.7

[Begester]

I had my boat out this weekend and noticed that when putting the engine under a significant load, there is a high pitched pinging on the front starboard side of the engine for a few seconds. At low speeds and in neutral, the condition is not present. Boat is a 1989 SeaRay 220DA with a Mercruiser 5.7 that has about 1350 hours.

Here?s what I've done so far:

1. Got the boat home and ran a compression test. Cylinder #2 was 90 psi, Cylinder 4 was 135 psi. In running the boat in the water and under load with the engine compartment up, this seems to be the area that the pinging is coming from.
2. Cylinder #3 (other side of the engine) is 115 psi. All other cylinders are 150 psi.
3. Changed all spark plugs and wires. All plugs are fairly consistent in terms of wear and have about 75 hours on them. They were used for the past two seasons.
4. There does not appear to be water in the oil, there are no abnormal smells, and I don?t believe I?m burning any oil.
5. Boat is not overheating. I?ve owned the boat 10 years (~700 hours) and sometimes the oil pressure reads a little high when I?m getting out of the hole but always comes down to normal range. I had my mechanic look at this a few years ago and he said it was normal.

What does this point to? In doing a little research I could see this being a head gasket, but wouldn?t a blown head gasket show symptoms at all levels of engine load?

Any insight is greatly appreciated. I don?t want to run the boat until I can find the root cause and fix it.

 

[Begester]

Also note that the pinging only lasts 5-10 seconds and then goes away.

 

[Bondo]

Ayuh,.... The most common cause of pinging or spark knock is a lean condition because of water in the fuel,...
Yer compression numbers indicate it's been goin' on, 'n has caused significant damage,...
Could be burnt/ detonated valves, or pistons,...

 

[Rick Stephens]

Low compression all by itself will cause pinging. Sometimes you can get away with upping the octane on your fuel for a little, but a rebuild may be in order.

Bondo is correct that a little bit of water will also introduce pinging. Getting water in E10 fuel will combine with the ethanol and separate. Being heavier the mixture of water and ethanol sinks to the bottom. Ethanol by itself is about 15-18 percent of the octane in your fuel, so separating it out leaves your fuel down to under 70 octane rating, which is cause enough for pinging. You can try draining and replacing your fuel to see if that gets rid of the ping.

Also check your timing.

Rick

 

[jimmbo]

And yet water injection has been used for years to stop detonation. Of course this gasohol garbage being sold causes lots of problems, plus lowers hp and reduces mileage.

 

[Begester]

I've used ethanol free gas religiously for at least the past 5 years. I've also used fuel stabilizer when I winterize. I re-did the timing 4 years ago...does this still point to bad fuel being the cause?

Thanks again for the insight

 

[Bondo]

does this still point to bad fuel being the cause?

Ayuh,.... Check the Contents of the fuel filter, 'n you'll get yer answer,....

Water displaces the gas, so the motor can't get enough gas flow,...

Could also be crud in the carb, but I start at the fuel filter 1st,...

 

[flipbro]

I'd say 90 psi is the problem! Time to do a leak down and find were that pressure is escaping. And as Bondo said check the contents of filter to see if that's caused the damage.

 

[Scott Danforth]

Time to do a leak down test. I would use a bore scope camera for your smartphone (about $10) and look in the spark plug hole. My guess is you will be doing a rebuild.

You need to find the cause of the lean condition and correct it.

 

[Lou C]

Agree on the leak down test. You may have an exhaust valve stsrting to burn or a slightly leaky exhaust riser gasket on that side causing water to get into a cyl and rust exhaust valve stems. So in addition to the leak down test Id remove the exhaust and inspect.

 

[Begester]

So I had my mechanic take a look at my boat yesterday and found the following:

1. My compression test gauge is crap. His readings on 7 cylinders were all in the ~175 neighborhood. One cylinder was 145. These readings were taken when the engine was cold.
2. The leakdown test yielded a leak in the intake valve on the #5 cylinder, which was the one that read 145 psi.
3. There are no issues with cooling, water in the oil, etc.
4. He did not rule out poor fuel for the pinging under significant engine load. I've not run the tank "bone dry" in awhile but I always use ethanol free gas and fuel stabilizer in the winter.

The mechanic and I couldn't get the pinging problem to duplicate on the muffs as the boat was on the trailer. This being said, the mechanic said that from his standpoint and from what he saw and checked out, the engine was running fine, sounded good, had generally good compression, etc.

He also said it would be ~$2k to remove and re-machine the valve and change the gaskets, with the vast majority of that cost being labor related. He said that while 145 is slightly out of the "normal" range, if it was his boat he would think twice about dropping this much money for a cylinder that's still making 145 psi compression on a 27 year old boat.

What do you guys think? I trust the mechanic but I'd like to get a second opinion to better understand if I'm driving around with a ticking time bomb or if this is something to live with in an older engine.

thanks!

 

[Bondo]

4. He did not rule out poor fuel for the pinging under significant engine load. I've not run the tank "bone dry" in awhile but I always use ethanol free gas and fuel stabilizer in the winter.

Ayuh,.... So was the Contents of the fuel filter inspected,..??

Crap in the fuel will show up there 1st,....

 

[thumpar]

$2k to work one head is insane unless the way the boat is constructed is very unusual.

 

[ghunttwo]

If your mechanics gages are correct, 175* is darn good for a 1300 hr engine. Have you checked the timing? I dont know the factory specs are for your eng , it might be only a couple of degrees high