Compression Test - Mercruiser 4.3

[BillK2632]

Anyone do a compression test on their engines just as a part of maintenance? My old girl still runs great after 17yrs and 550hrs, but I was thinking I would do a compression test when I change the plugs out next week. It's never been done on this engine and I am just curious and have always wanted to do this.

For any of you who have done it, I have a couple of questions.

1. Just went and bought an inexpensive compression test kit at Harbor Freight.
2. I know you need to disable ignition, removing the coil wire seems to be the straight forward way, but won't flipping the lanyard kill switch do the same thing?
3. It's a carburetor, do I need to worry about the fuel system? On EFI you can open the throttle to prevent fuel from being delivered. On mine should I disconnect the fuel pump? Or does the fact that there are no spark plugs in the engine pretty much prevent fuel from getting pulled into the cylinders?

 

[bbwhitejr]

I can answer #2 as I watched the Tech do the compression tests on my boat-yes-flipping the lanyard switch will do the same thing.

Bennett

 

[CliffA]

Bill,
you could always just pull the fuse/relay for the electric fuel pump to be safe.....

cliff

 

[Scott06]

I've never disabled the fuel pump, get it hot, pull all the plugs out, throttle wide open, pull the coil wire of the the distributor and ground to block. Get like three compression strokes on each cylinder before you take the reading and go on to the next. Minimum 135 psi and above is good. New engines will be 155-170 psi. I think my 25 yr old 3.0 was like 145-150

 

[BillK2632]

Thanks guys. Like I said, for some odd reason I have wanted to do this, hope I don't find out something I don't want to know.

 

[Lazy Daze]

You can either flip the kill switch to "off", or ground the coil wire (must be grounded, not just removed!). Throttle open to get a truer reading.

Don't get too concerned with the actual readings, more-so with any large discrepancies from one cylinder to another. Unless of course it's WAY off - but then, of course, you'd likely already know it from having running issues. HF tester should be good enough to give you a general idea, but don't count on it being perfectly accurate. Although, for what you're doing, it's not all that important for it to be dead on accurate. And it's probably not that far off, either. The main goal here would be to do it once or twice a year and look for variances from time to time. As long as you keep using the same tester, it will work perfectly for this case.

 

[BillK2632]

Thanks Dennis, going to do it this weekend.

 

[Thornton69]

Considering spring and all, if your boat has been sitting for any length of time, then your results could be misleading. In that event, I would give it an honest run before your testing to avoid unnecessary concerns and get a more accurate conclusion.

 

[BillK2632]

Ok, did the compression check over the weekend and replaced spark plugs. I never could find what the compression should be, Mercruiser just states it should be greater that 100lb and the lowest cylinder should be within 70% of the highest. My readings are well within that spec. Also, engine was not run before I did the test - Mercruiser states the engine should be at room temperature. Based on these numbers and the plug condition along with the engine is running pretty near perfect. I think the old girl is good for a few more hours.

1. 132 4. 132
2. 135 5. 137
3. 135 6. 135

Here are the plugs that came out - 3 seasons @150hrs

 

[Lazy Daze]

You're correct Bill, there is no "stated" comp number. It would create WAY too many problems and headaches to have a specific number.

30% range is too much for me, I prefer the 15% range (ideally 10%). Your numbers are a bit lower than I would expect, but since you did it cold that's understandable. The fact that they are so close together is more telling than anything else. Looks really good. Clean the plugs and pop them back in?

 

[BillK2632]

Dennis - Thanks for you guidance on this - it ended up being very easy. I put new plugs in, these were 3 seasons and I fog each winter, so I just replace them every 3. I might do the test again sometime with a warm engine, also even though I opened the throttle, I suspect the choke was set - now that I have a new tool in my chest, I might do it again at some point with the engine warm. Otherwise boat is running as good as it ever has.

 

[CliffA]

Based on these numbers and the plug condition along with the engine is running pretty near perfect. I think the old girl is good for a few more hours.

not to mention the boat looks like new......I'd say she has more than just a 'few more hours' left.....

cliff

 

[BillK2632]

So just an update. I was curious how much difference a cold vs warm engine makes on the compression test, so yesterday after the engine was warm, I pulled a plug from each side of the motor (the middle ones that are easy to get to). Checked the compression on those two cylinders - 155lb on both, in my case the difference between hot and cold was 20lb.