4.3 low compression

[Boater31]

I know there is a lot of this out there and was hoping not to join the club but here I am.

During summerizing today I ran a compression test per Merc bulletin 97-25 revision June 1999. My engine is 0L323711, a 1999 4.3. I did the test WOT, engine room temp as it said ( I thought it should be warm but bulletin says room temp). Anyways here are the results. The brackets are after 3 pumps of oil
1. 70(80) 2. 148
3.141 4. 142
5. 128 6. 80(100)

I have another thread going about a few oz of water that was in the block before I started anything but I think they are unrelated. Here

When I got this boat 4 years ago it came to me with batwings that leaked water into the exhaust. It was discovered cause I was making milkshakes and when I removed manifolds there was water sitting on the valves. Specifically the back 4, this makes number 1 even more strange. I tested back then and was around 120's pretty even. guessing this has to do with that, although 6 looks like it might be rings? I plan on renting a gauge tomorrow and verify the low numbers.

My questions are,

If it starts can I run it to temp on the muffs to confirm my other thread and that the block isn't cracked?
I am assuming if it starts I should't go boating with it in fear of making it worse?
Assuming block isn't cracked what are the next steps.

As always thanks

 

[Bondo]

If it starts can I run it to temp on the muffs to confirm my other thread and that the block isn't cracked?

Ayuh,..... Sure,...... why not,..??

Assuming block isn't cracked what are the next steps.

Verify the compression numbers,.....

 

[tank1949]

I know there is a lot of this out there and was hoping not to join the club but here I am.

During summerizing today I ran a compression test per Merc bulletin 97-25 revision June 1999. My engine is 0L323711, a 1999 4.3. I did the test WOT, engine room temp as it said ( I thought it should be warm but bulletin says room temp). Anyways here are the results. The brackets are after 3 pumps of oil
1. 70(80) 2. 148
3.141 4. 142
5. 128 6. 80(100)

I have another thread going about a few oz of water that was in the block before I started anything but I think they are unrelated. Here

When I got this boat 4 years ago it came to me with batwings that leaked water into the exhaust. It was discovered cause I was making milkshakes and when I removed manifolds there was water sitting on the valves. Specifically the back 4, this makes number 1 even more strange. I tested back then and was around 120's pretty even. guessing this has to do with that, although 6 looks like it might be rings? I plan on renting a gauge tomorrow and verify the low numbers.

My questions are,

If it starts can I run it to temp on the muffs to confirm my other thread and that the block isn't cracked?
I am assuming if it starts I should't go boating with it in fear of making it worse?
Assuming block isn't cracked what are the next steps.

As always thanks

Make sure pressure testing tool is accurate. One of mine had a leaking hose valve that cause me incorrect readings. Luckily, I found and repaired it before pulling a motor. That being said and your pressure tool is accurate, I fear it is time to pull and rebuild motor. 4.3s are common. If block is cracked, you should be able to buy a rebuildable vehicle engine and retrofit it to a marine motor. Or, if money is burning holes in your pocket, buy a complete marine engine from MC. Good luck!

 

[Boater31]

So I rented another gauge and tested the week cylinders again. I also tested a good cylinder for a base, the new gauge red about 10 psi higher. So I tested 1 and got 82 so 2 psi gain. Then tested 6 and got 125 so +25psi I decided to test 1 again and it jumped to 102 and repeated that twice. Again room temp open throttle 1 and 6 probably still have oil from last night. So the final numbers are

1 70 (80) new gauge ((82)) (((100))) 2 148
3 141 4 142
5 128 6 80 (100) new gauge ((125))


So I ran it on muffs, first start of the year, about 10 seconds rough idle then instantly turned super smoothed. Kept it at a fast idle of 1200 and couldn't feel any vibes, no noises or a puff of smoke out back. Ran silky smooth the entire time. So I know it's not a perfectly healthy engine but I'm thinking it's not a dead one either now. What's my next steps. Do a cylinder drop test on 1. Run it another season or more. Or my least favorite option rebuild/replace. I'm assuming since the pressure came up with the oil it's probably the rings and not valves so a doing the heads would fix it.

I will say we are looking to upgrade boats in the hopefully near future so if we need to replace the motor it becomes a real question cause that money could be going to a new purchase and know I won't get the cost back out.

 

[tank1949]

So I rented another gauge and tested the week cylinders again. I also tested a good cylinder for a base, the new gauge red about 10 psi higher. So I tested 1 and got 82 so 2 psi gain. Then tested 6 and got 125 so +25psi I decided to test 1 again and it jumped to 102 and repeated that twice. Again room temp open throttle 1 and 6 probably still have oil from last night. So the final numbers are

1 70 (80) new gauge ((82)) (((100))) 2 148
3 141 4 142
5 128 6 80 (100) new gauge ((125))


So I ran it on muffs, first start of the year, about 10 seconds rough idle then instantly turned super smoothed. Kept it at a fast idle of 1200 and couldn't feel any vibes, no noises or a puff of smoke out back. Ran silky smooth the entire time. So I know it's not a perfectly healthy engine but I'm thinking it's not a dead one either now. What's my next steps. Do a cylinder drop test on 1. Run it another season or more. Or my least favorite option rebuild/replace. I'm assuming since the pressure came up with the oil it's probably the rings and not valves so a doing the heads would fix it.

I will say we are looking to upgrade boats in the hopefully near future so if we need to replace the motor it becomes a real question cause that money could be going to a new purchase and know I won't get the cost back out.

Put a can of Marvel Mystery oil in the motor. Trust me!!!!! Go boating and later on check compression again. Check Shrader valve on old gauge. Mine was just loose and caused me grief!!!! However, my guess is that rings have worn out. If you have TB 5 ignition with a knock sensor it is more forgiving to rebuild your motor and have a little more higher compression. More HP... If you run cheap ass or old gas and you start to develop engine knocks, it will step down to save your motor. At least that is the way it has been explained to me. TB 4 is a good ignition system but to me TB 5 with a knock sensor is better.

 

[Rick Stephens]

I'd go boating until something made me think otherwise. Yes, a rebuild is indicated. No, it is not gonna blow up on you. Very well could be a valve problem that loosens up as you run it.. That won't show on oil pressure, just on compression numbers. A little water in an engine can do a bunch of different things.

 

[Boater31]

That sounds like the plan then. Gonna finish up the rest of the pm tomorrow and hopefully be back on the water by the end of the week. I might try the MMO, really got nothing to loose but it's cost to try it. As always thanks.

 

[achris]

...During summerizing today I ran a compression test ...

How long has this engine been asleep? Any more than a couple of weeks and the rings and valves will be a bit sticky. I'd be surprised if they weren't.

And yes I read the bulletin. That's for GM engines (automotive), that are run on a regular basis, and not an engine that's been hibernating for 6 months!

Did you do a compression test after you ran it? I was taught, and have always warmed an engine before a compression test. That's just the way it should be done.

 

[Boater31]

It's been since late September/early October, 8 to 9 months. I've always known to do it on a warm engine also but was trying to do it verbatim to the bulletin. I'm finishing up my drive pm and when it gets back on I'll run it again and do a test after it's warm. I'd imagine all the numbers go up, still probably not fully healthy but probably better.

 

[achris]

It's been since late September/early October, 8 to 9 months. I've always known to do it on a warm engine also but was trying to do it verbatim to the bulletin. I'm finishing up my drive pm and when it gets back on I'll run it again and do a test after it's warm. I'd imagine all the numbers go up, still probably not fully healthy but probably better.

Take it out on the water and run it like you stole it for 20 minutes. Then check the compressions, they'll be fine.

Chris..

 

[Boater31]

That's a answer I can get behind. Thanks