Low Compression

[yater]

The low compression cylinder plug looks fine, new according to mechanic.

[MLA]

Originally Posted by jfw432

What about after cranking for a couple minutes over the course of testing all the cylinders? I'm sure some of that fuel will go out the exhaust valves but since it's an unburned liquid, it will start to build up over time. I don't have any idea...just throwing that into the mix.

The average cranking time per cylinder should be 5-6 seconds. So the total amount of time the engine will be spinning over during the entire test is less then 60 seconds on a V8. Any measurable amount of fuel entering the cylinder by a pulsing injector during the 5-6 crank time will exit the exhaust and open spark plug hole (on those cylinders not being tested) during the piston's exhaust stroke.

Originally Posted by JimN

But the standard procedure is to do the test, either by opening the throttle/crimping the fuel line on a carbureted engine or removing the fuel pump/injector fuse on an injected engine. Think about how many cycles are needed for the full test- at least two, with no combustion. Not much of the fuel delivered will leave through the exhaust port and that leads to an innacurate reading.

I am 100% confident that a non-functioning injector on one cylinder would not create a 40% drop in compression as compared to the other 7 cylinders with a pulsing injector. At this point, it is only speculation on whether or not the injectors were pulsing during the compression test. A leak-down test on that cylinder would take 60 seconds to determine if there is a leak and a few more minutes to narrow down where the leak is. Far less man-hours then whats wrapped up here in speculation.

[JimN]

Originally Posted by MLA

The average cranking time per cylinder should be 5-6 seconds. So the total amount of time the engine will be spinning over during the entire test is less then 60 seconds on a V8. Any measurable amount of fuel entering the cylinder by a pulsing injector during the 5-6 crank time will exit the exhaust and open spark plug hole (on those cylinders not being tested) during the piston's exhaust stroke.

I am 100% confident that a non-functioning injector on one cylinder would not create a 40% drop in compression as compared to the other 7 cylinders with a pulsing injector. At this point, it is only speculation on whether or not the injectors were pulsing during the compression test. A leak-down test on that cylinder would take 60 seconds to determine if there is a leak and a few more minutes to narrow down where the leak is. Far less man-hours then whats wrapped up here in speculation.

It might not account for 40% but it still skews the test and the only way to see how much it does or doesn't affect the results is to test the compression both ways. Since this is dry intake, the fuel delivered to the cylinders is only going to the cylinders, not wetting the intake AND going to the cylinders.

At this point, I would remove all plugs (I would hope this was already done for the compression test), attach an air hose with 50 psi to the bad cylinder and crank it over manually, to listen for air escaping- if it's coming out the oil filler hole, we'll know it's probably a problem with one or more rings and if it comes out the intake or exhaust, it's probably a valve/gasket problem.

[JimN]

Originally Posted by yater

The low compression cylinder plug looks fine, new according to mechanic.

Has this engine overheated recently, or did the problem start after anything specific occurred?

[yater]

No known overheating incidents. Unfortunately, I have not owned the boat for long and the only records I have are service records from the past 3 years that do not indicate any red flags or major service.

[Kyle]

Are you handy with tools?

Can you take something apart and put it back together again easily?



If so then you should be able to do the work yourself and save tons of cash.


I would be willing to bet that the engine does NOT have to be pulled. A simple cam swap or head issues can be done with the engine still in the boat.