1991 Omc 4.3 Cobra, Compression test results.....?

[mtconcreteco@gmail.com]

Hello there,
In need of help. I Have OEM 1991 OMC Service manual and i can not find the info i need.
Just got this boat a couple weeks ago, ran though it doing and checking all the things you would before taking it on the lake.
Oil looked good before i started it. actually everything looked good. Ran boat for 15 mins on paddles in the drive way started right up sounded good water cycled. Turned off. Did not check oil again.(stupid i dont know how i did not check oil at this point.) took boat to lake. ran it for 2 hours max around 2800 rpm everything seemed good guages where good sounded great i never tried to push throttle past 3000rpm.(Seemed to go through 4-5 Gallons of gas in that amount of time?) temp never hit above mid. Got home was going to give it an oil change and OIL is Milky. smells like fuel as well.
So being a boat i was told that the heads dont break like a car does and i shouldnt just tear into the heads thinking that is the problem.
So i ran compression test on all cyl. 125 PSI on cyl. 1-5 number 6 the rear port side was slightly higher like 140psi and i noticed more gas coming out of that spark as well probably why the compression was a little higher...(sparks all look a little more black then i would want, running lean?)
I also removed and inspected both fuel pump and carb. Both do not seem to be leaking fuel and float looks to be in working order.
My main question is does anyone know what the compression should be on this OMC 4.3 v6 cobra.
What can i do to test where water might be coming from?
crossing my fingers that because boat was sitting for a while that maybe water has been leaking in and was sitting on top of oil and then when i ran it i mixed everything but no more water was getting in. that is my hope at least i was going to flush oil and try running on paddles again to conclude that water is getting ln oil.Assuming it get milky again is there any chance that water comes in though a broken exhaust paddle or anything?
short of tearing the intake and exhuast manifolds off to inspect them.
Any advice i have rebuild a few car engines and I am mechanical but boats are relatively new to me.
Motor does look like it was taken care of for the most part 300hrs. and where it was sitting does not get below 30 degress in the winter.
Thanks for any help.

 

[mtconcreteco@gmail.com]

I should clarify when i was on the lake. the total time i was there was 2 hours all boat did was run about the lake for 15 mins or so and then we stoped and i would kill it and then we would do it again the boat maybe ran a total of an hour. went through 4 gallons in that amound of time.)

 

[Scott Danforth]

Cranking compression should be about 150 psi

 

[mtconcreteco@gmail.com]

thanks scott,
well with that info i went and retested, after having a hand this time and 4 eyes we came to the conclusion that the Compression test for all 6 cyl is right at 141-142 psi....what should be my next course of action? Still was planning on new oil and run on hose paddles.... dont really want to waste the oil...but not sure how to test or if water can even get to the oil through the intake manifold and i know water can get through the exhaust manifold into the oil but not sure how to test that short of taking it off and inspecting it....any advice out there short of taking everything apart..

 

[Scott Danforth]

regarding the fuel and fuel in the oil. first thing, if the clear line from the fuel pump to the spark arrestor has fuel in it, your fuel pump is shot. check the stickies on pressure testing your water side. has nothing to do with compression. this is your first test. if it fails to hold pressure, do a leak down test to find the issue.

you may have to take things apart

oil is cheap, probably the cheapest thing about boat ownership.

oil goes nowhere near the exhaust manifold, unless you have an oil cooler and its leaking and pushing oil into the cooling water which eventually makes it out he exhaust.

by the way, if you have the original "bat-wing" exhaust manifolds. they have been known to fail and take out motors. pressure test them and acetone test them....... better yet replace them with the two-piece retrofit kit.

 

[HT32BSX115]

Oil looked good before i started it. actually everything looked good. Ran boat for 15 mins on paddles in the drive way started right up sounded good water cycled. Turned off. Did not check oil again.(stupid i dont know how i did not check oil at this point.) took boat to lake. ran it for 2 hours max around 2800 rpm everything seemed good guages where good sounded great i never tried to push throttle past 3000rpm.(Seemed to go through 4-5 Gallons of gas in that amount of time?) temp never hit above mid. Got home was going to give it an oil change and OIL is Milky. smells like fuel as well.

Howdy,

Welcome aboard!

Well, first of all, It might be helpful if you tell us what you have.

91 boat, 4.3L could be any of following! AND knowing the correct model number might help with the Service manual because some OMC manuals specified which models they covered.

432APLRGD 1991
432APRRGD 1991
434APLRGD 1991
434APRRGD 1991

432AMLPWS 1990
432APRPWS 1990
434AMLPWS 1990
434APRPWS 1990

Now having said all that, "milky" oil is usually the result of a cracked block. Compression testing will reveal intake and exhaust valve leaks and leaks past the rings and in few cases head gasket leaks.

To find cooling system leaks, you MUST do a cooling system pressure check. Cracked V-8/V-6 blocks frequently leak "in the valley" so they're not always obvious. (pull the intake manifold and you might see rust around the push-rods)

The best method though is to pressure check the cooling to 15-20 psi and listen. If the block is cracked, you'll hear it.

If you have oil that smells like gasoline, it's frequently from the fuel pump leaking into the crankcase ( crankcase mounted) . The fuel pump has failed and must be replaced....you can usually tell by seeing fuel in the plastic line from the pump to the carb intake.

It appears that the pump is the same for all the above models and you can get one online all over the net..............iBoats will match all the prices. So buy it here and support the site if you actually need one!

https://www.iboats.com/shop/sierra-f...p-18-7289.html


Cracked exhaust manifolds or leaking riser gaskets (in a 2-piece manifold/riser) cannot leak enough water into the engine to cause milky oil and if you start out with clean oil, even if both manifolds are leaking water into the exhaust, it'll be blown out with the exhaust. It only gets into the engine via an open exhaust valve after shutdown.

It's usually only enough to cause a brief hydrolock if you try to start it within several minutes after shutdown.

If it sits all night, the water will usually leak past the rings and then you can start it right up. The water is such a small amount, that it will evaporate right out after running the engine at normal operating temp for an hour or so.

If you have milky oil due to water after running for a long time, not only is it not due to an exhaust manifold, but it's running in continuously (faster than it can evap) I.E. cracked block cracked head(s), cracked intake manifold in some cases.......

A cooling system pressure check will reveal it.

Cheers,


Rick

Oh and by the way, Take Scotts advice and replace your manifold (if they're "Bat-Wing" types!!!)

 

[mtconcreteco@gmail.com]

Thanks for all the feed back. I have a 1991 432APRRGD / MOD#986698
I read all the info that was said. The motor does not have the Bat Wing. It also appears that this 4.3 does not come with a flapper valve..is this correct?
I did not see any evidence of water on the pistons nor on the spark plug.On Lake i did not notice any trouble restarting the motor once stopped in the lake no sign of any sort of water entering from the exhaust, but that would be an easier fix then a cracked block or head. The Fuel pump checked out.
I will go get the things to modify my thermostat to hold 15-20 psi. I will post my results.
Thanks

 

[HT32BSX115]

I did not see any evidence of water on the pistons nor on the spark plug.On Lake i did not notice any trouble restarting the motor once stopped in the lake no sign of any sort of water entering from the exhaust, but that would be an easier fix then a cracked block or head. The Fuel pump checked out.

Ok. Missing "Flappers" are not a particular problem unless they're stuck down in the bottom of the exhaust Y-tube.

Treat the block like an air tank with multiple openings. Close them all off with whatever you can find at Home Depot. Connect an air hookup, gage and a valve so you can pressure it up and turn the valve off and observe the gage.

It should hold 15-20 psi for quite some time. Turn off the compressor so you can listen closely. If you hear hissing in the oil fill, it's likely cracked internally.

You can splash a little soapy water on the outside of the block to see hairline cracks there. (but outside cracks don't really "hurt" anything and some people use epoxy to seal them for years)

Make sure you replace the riser gaskets and check for flat riser/manifold surfaces. You can even (carefully) use a belt sander to "true" them up if needed.

 

[whitney]

Great advice so far ... let me offer a couple ideas and comments (I'm not a pro, but I've been wrenching on my 1992 OMC 4.3 for 8 years)...

- Black plugs would indicate rich not lean. I'd guess you have a leak or some other issue in your carb, time for a carb rebuild
- If I were you I would not worry about the fuel smell issue yet. First focus on the milky oil.
- Fuel smell could be from the carb leaking into the ports, past the rings, and into the oil
- You said the oil looked fine before your run test. I think it's possible that, if it had been sitting a LONG time before you checked the dipstick, that the water had settled out of the oil into the bottom of the pan, and you didn't see it. This opens the possibility that, as you mentioned, the engine accumulated some water in the oil when sitting, and you saw milky after running
- Oil is cheap, replace and run another test before going any further. If the milkyness gets better you may actually not have a problem.
- I did not see anyone mention head gasket issues, not sure if the 4.3 is susceptible, but that's another way for water to get into the oil.

Good luck and let us know what you find out!

 

[whitney]

Oh and regarding the flappers -- your motor does (did?) have them. Volvo phased them out in 1999 and from what I have been reading, it's not much risk to remove them from earlier engines provided your risers are an adequate height above potential stern waves. I tossed them on my 92.

 

[mtconcreteco@gmail.com]

Well, I did the first step, new oil & filter, ran on hose for 45 mins at 1800 rpm. temp never really got that high specially if the t-stat opens at 150-160 boat never got to 150 sat at 120-130 the whole time(t-stat stuck open?). Checking oil and water flow....no water in oil.
Soooo... do i still do a pressure test?
(ps the first lake trip for the boat the temp did get up to 160-170 at idle and then when we started to move it dropped back to 130 ish) Again i say the oil is nice and clean still. I like the idea of knowing if everything is sound with the pressure test though. Thanks for the feedback.

 

[HT32BSX115]

Well, I did the first step, new oil & filter, ran on hose for 45 mins at 1800 rpm. temp never really got that high specially if the t-stat opens at 150-160 boat never got to 150 sat at 120-130 the whole time(t-stat stuck open?). Checking oil and water flow....no water in oil.
Soooo... do i still do a pressure test?

Running an engine on a flush adapter will not usually load the engine enough to even get it to normal running temp.

You really need to run it on the lake at maximum cruise power for long enough to fully heat the oil. That usually takes a more than 30 min at planing speed.

If there was water in the oil (sitting at the bottom of the pan the first time ) it would have looked "OK" before running it.....and then would have started mixing once the oil got hot and at high RPM was sucked into the oil pickup.

I would run it on the lake for an hour or so and check it. If there's a crack, you'll know it when you check the oil after a good hard run.

It's also possible that you still have a little bit of water in the bottom of the pan that didn't get sucked out or drain out. If it milks up again after running it a while, you may have to change it again to get the last little bit out.

So don't be completely discouraged if you get a little milkshake again......but if it's NOT leaking continuously, it will be LESS of a milkshake compared to the last time.

 

[mtconcreteco@gmail.com]

Awesome thanks, that was kinda what i was thinking. Off to the lake i go, crossing fingers that the fluids stay the same level and clear. I'll post my results. Thanks

 

[mtconcreteco@gmail.com]

Finally had time to get the boat out on the lake. No Oil in Water...I think i am in the clear on that one. Other problems of course on an older boat but i think i dodged the bullet on the major one...

 

[HT32BSX115]

i think i dodged the bullet on the major one...

I think you did too! :thumb: