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2003 F90 Compression Test Results ??

N

newboater67

Contributing Member
I got compression results of 210, 210, 210, and 175 PSI, on a 2003 Yamaha F90TLRB (61P L 1000850). The service manual says compression should be a minimum of 135 PSI. Do I have a problem with cylinder #4? I'm new to this but everything I've read says they should be within 10% of each other.
 
you do have a problem...the motor will run but the time to fix it is now to prevent heavy expense later..take a reading with a cold engine...take a reading with a warm/hot engine..take a reading after adding a little oil to the low piston...record all these readings and get back to us...
 
couple of squirts from a small oil can..you want enough to get to the edges of the piston...you are actually checking for a worn ring or cylinder..was the #4 the last cylinder checked?the reason i ask was the battery turning the motor strongly and at the same rpm as the others?retake the readings with a strong battery and do #4 first...if the reading are the same you are going to have to pull the head and see whats going on if you decide to address the problem now.......
 
I retested cylinder 4. It was 175 cold, 175 hot, and 185 with a couple squirts of oil. I didn't recheck the other 3 cylinders yet since they all read the same in the initial tests. I haven't done a leak down test yet. It'll probably have to wait for this weekend.
 
I don't really know. I was going to check the valves this weekend before I did a leak test.

I just bought the pontoon boat/motor from a neighbor a few weeks ago before family came to visit so I'd have something big enough to take out the whole family. Was running a 16' CC with a 50hp before that. My neighbor had the boat serviced at a local marina about 1-1/2 years ago and put 52 hours on it since then. They did all the fluids, impeller, etc. Don't know if they checked the valves though. The motor has a total of 509 hours on it. The last several months he put it in the water long enough to run and warm up the motor, then flushed it.

I took the boat out for a full days fishing while family was here and it ran fine and sounded good. Went night fishing one night and smelled gas when I got back to the dock. It was spraying gas from behind the carbs. Pulled carb assembly, found two hose leaks and replaced all the fuel hoses. Put the carbs back on, and it sounded different to me, almost like it was missing. Thought I might have to clean the carbs, so I decided to make sure everything else was sound before tearing the carbs apart, and got the funky compression numbers.
 
I had hoped the valves would need adjusting but they were actually pretty tight. All intake valves measured 007 or 008 and all the exhaust valves measured 011. Didn't get to do the leak down yet.
 
From what I've read, the 10 psi increase from 175 to 185 psi on cylinder 4 indicates a ring or valve issue in that cylinder. Could decarbonizing the motor possibly help this situation?
 
Now I'm really confused. My compression readings show 210,210,210,175 for cylinders,1,2,3,& 4. Cylinder 4 checked at 175 psi cold, 175 psi warm and increased to 185 psi after I added 2 squirts of oil to the cylinder, which I believe indicates a ring or valve issue on cylinder 4. The leak down test shows leakage around 2-4% for cylinders 1,2 and 4 and I can hear a slight his through the oil filler cap. I can NOT pressurize cylinder 3. Cylinder 3 will not hold any pressure and it's leaking 100% through the carbs. I can't get any closer to the motor to isolate it since this motor is on a pontoon boat on a lift and I don't have a trailer.

How am I getting 210 PSI of compression on cylinder 3 when I can't get it to hold any pressure for the leak down test, not even 10-15 psi? I've read a leak through the carbs would mean something bad on the intake side, intake valve, seat, etc.

The motor actually sounds like it's running good, but I have not taken it out on the water. I wanted to try to figure out was possibly wrong with the motor I case it was bad enough that I should quit running it and work on fixing it instead.
 
The answer is the idiot with the gauges. I rechecked the results above twice, with two different sets of leak-down gauges. Decided this still didn't make any sense so went to redo all the readings one more time and noticed while I was pressurizing the cylinder that the flywheel moved, taking the cylinder off TDC. So this time I locked down the flywheel before I pressurized the cylinder.

So now the leak down results are less than 5% on all four cylinders. I expected to see a higher leak down rate on cylinder 4.

What does it mean to have low compression but good leak down numbers?
 
newboater67 said:
What does it mean to have low compression but good leak down numbers?
Click to expand...

Could be that a cylinder is scored but the scoring is below the piston rings when the piston is at top dead center. When the piston is at top dead center for a differential pressure check the piston rings are preventing any leakage that would be apparent below the piston rings. A score will cause less compression build up when a piston is dynamically tested versus when it is tested statically via a differential pressure check.

I personally don't think that having one cylinder at 175 psi while the others are at 210 constitutes a major issue. How does the motor run? Is there blowby or high oil consumption? If not, the motor may be just fine. I think the three cylinders having 210 psi (seems too high) is a bigger issue than one cylinder having 175 psi (seems more normal). Was a calibrated gauge used for the compression check?
 
Thanks for the reply boscoe.

The motor seems to run fine. No blowby and doesn't consume oil. The gauges are not calibrated. I use an OTC 5605 compression test kit and an OTC 5609 leakage test kit which are DIY test kits.

I decided to pop the flywheel to check the crankshaft timing mark against the cam pulleys and it was off a couple teeth. I made sure they were set correctly and checked the carb sync which was off, so I resync'ed the carbs. Then I set the TPS to the correct voltage at 900 RPM and took it out for a run.

It runs good, accelerates and decels properly. It's on a 25ft pontoon boat. It was too windy and choppy to open it up to full throttle, but ran without any issues to around 4000-4500 RPM. Have to wait for a better day to give it a good long run at different RPMs and see how it performs.

I just bought the boat a month ago. I know the previous owner only used regular Sta-bil fuel stabilizer. I was thinking of putting some hours on it with a tank of enriched ring free plus and stabil marine, and then rechecking the compression and leakage. It may be okay. I just didn't want to risk blowing the motor if there was something that need to be addressed now.

Thanks Again,
Bob
 
newboater67 said:
I just bought the boat a month ago. I know the previous owner only used regular Sta-bil fuel stabilizer. I was thinking of putting some hours on it with a tank of enriched ring free plus and stabil marine, and then rechecking the compression and leakage. It may be okay. I just didn't want to risk blowing the motor if there was something that need to be addressed now.

Thanks Again,
Bob
Click to expand...

Good idea.

Low compression from wear will seldom blow a motor. The wear obviously will increase with time and the compression will get worse to the point that the motor may not start easily, idle properly or run well. I ran a 1962 Ford Falcon with low low compression for a number of years. Smoked like hell from the breather tube because of oil getting past worn piston rings and into the combustion chamber. Other than the smoking, and the use of more oil consumption than normal, it ran well. It never did blow. I finally scrapped the car when I got the money to upgrade to a 1965 Ford Falcon. Those were the days.

Test again after you have fifty or so hours on the motor and let us know the results.
 
That's funny. I had a blue Ford Falcon convertible in my JR year of high school. Can't remember the year though (of the car). That motor blew a year later and I bought a 65 MG Midget for $75 that wouldn't start because the starter was stuck in the flywheel.
 
I'll toss my simple 2 cents in... If the engine is running and performing well from idle throughout the entire RPM range and you aren't losing or "making" oil, I wouldn't start taking things apart right away as you could do more harm than good by breaking seals, unseating the head gasket, etc. that are all still unbroken from the factory. With 509 hours on the engine, it should still be a strong engine as these engines are built like car engines these days usually good for at least 2000 hours with proper treatment and maintenance. One old mechanic told me to think about it this way... Take your hours and multiply by 40 as in 40 MPH average speed you use you car between city/hwy. That would give you the equivalent of a car engine with 20K miles on it. Take that and multiply by 2 because unlike a car that rolls on wheels, an outboard pushing a boat through water is like driving a car up Mount Everest all the time. So you have about 40K "car" miles on the outboard. Most of these engines have a life of 2000 hours which puts you at 160,000 "car" miles or about the life of a car engine before it's time to consider stopping repairs and buying a new one. Seemed to make sense to me!
 
Thanks to everyone for your replies. It would seem I'm back to a fuel leak, where this all started to begin with. I thought I was going to have to tear down the carbs, so did all the mechanical checks first and had the compression/leak down results.

Originally had leaking carbs and found two holes in the check valve hoses to the back of carbs 3 & 4. Reterminated the check valve hoses and replaced all the fuel hoses since they were showing their age on the inside of the hoses.

Took it out last night and it ran fine but it was too choppy to get over 4000-4500 RPM. Took it out again this morning and it ran fine for awhile, then I could hear and see the RPMs dropping off on the tach, then pick up, drop off, etc.

Got back to the dock and have fuel leaking from the carbs again. Have the boat on a lift and no trailer so I can't get really close to the motor to tell exactly where it's leaking from, but it looks like it's down the backside of carbs 3 and/or 4 again. Didn't really feel like popping the carbs again today, so will leave it for another day and come back with a better attitude.

Maybe I'll get lucky and it'll be the black hoses from the check valve again.
 
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