Port Bank Cylinders Low Compression

[crashkaloop]

Different Compression on each Bank

I know the possible causes of one cylinder being low on compression, but what about the entire bank being low? My port 3 cylinders are all lower than the starboard bank. These test were completed at 6,000 foot altitude, so they may all appear to be low compared to what would be expected at sea level. Here are the testing conditions.

6,000 altitude
Seafoamed all three port cylinders day before test
New plugs
Engine warmed up run hard for ? hour
All plugs removed
Kill switch clip removed
Throttle wide open
Compression tests done twice

And here are the results:

Port
Cyl/Lbs
2 - 100
4 - 88
6 - 95

Starboard
Cyl/Lbs
1 - 110
3 - 105
5 - 110

Ok, this is showing up nothing in the post like it does in the posting window.:blue: Sorry, trying to fix it.

Does this point to some common problem on the port bank? I realize that all three of the cylinders could have a problem each, but does this point to a head problem or head gasket or something? Are there any other test I can do to isolate what may be the problem? The engine starts and runs just fine.

Thanks for any possible solutions.

 

[Bosunsmate]

Re: Port Bank Cylinders Low Compression

Have you looked into cylinder and inspected walls and piston head for signs of ring failure?
Might be head gasket failing.
Might be worn/stuck rings
Have you tried a drop test on cylinder 4?

 

[boobie]

Re: Port Bank Cylinders Low Compression

How does the mtr run and what's the model # ??

 

[boobie]

Re: Port Bank Cylinders Low Compression

Edit....Double post.

 

[crashkaloop]

Re: Port Bank Cylinders Low Compression

What a dope I am. I somehow missed pasting in the engine model.

1989 GT175 The motor seems to run great?

I have looked into the cylinders through the plug holes, and all 6 cylinders look exactly the same. I also got no carbon on the plugs when I soaked and ran the Seafoam though it, so I don't think it has much carbon build-up.

If the drop test is a bleed down test, no I haven't. I have squirted oil into cylinder 4, but it did not increase the compression. So on that cylinder, I don't think it's rings.

 

[boobie]

Re: Port Bank Cylinders Low Compression

Another thing to try is get some BRP Engine Tuner and follow the directions on the can. I've used it and had good luck with it.

 

[Bosunsmate]

Re: Port Bank Cylinders Low Compression

What a dope I am. I somehow missed pasting in the engine model.

1989 GT175 The motor seems to run great?

I have looked into the cylinders through the plug holes, and all 6 cylinders look exactly the same. I also got no carbon on the plugs when I soaked and ran the Seafoam though it, so I don't think it has much carbon build-up.

If the drop test is a bleed down test, no I haven't. I have squirted oil into cylinder 4, but it did not increase the compression. So on that cylinder, I don't think it's rings.

Drop test is just removing the ht plug lead while engines running and check it drops engine rpm by an amount similar to other cylinders when they have their plugs removed.
It checks that cylinders contributing properly to engine power.
You may find its running dry or something else

 

[crashkaloop]

Re: Port Bank Cylinders Low Compression

Drop test is just removing the ht plug lead while engines running and check it drops engine rpm by an amount similar to other cylinders when they have their plugs removed.
It checks that cylinders contributing properly to engine power.
You may find its running dry or something else

Sure, I have done that with cars before. It never even occurred to me to do it on this motor. However, I am hitting peak RPM with the motor, and getting close to top speed for my area, so I think that would not be possible with a cylinder not working well. But it's a great idea, and I will do it first thing tomorrow morning.

Thanks Bosunsmate!

 

[emdsapmgr]

Re: Port Bank Cylinders Low Compression

It's not unusual for compression to vary maybe 5-7 lbs between heads, due to the way the blocks are bored. Your compression variance number are hard to explain, due to the variance within the low head itself. If the head were ever overheated, possible the head gasket seal rings are shot. That would be a good case scenario-an easy and cheap fix. Boobie has a good suggestion-run some Engine Tuner through it first.

 

[crashkaloop]

Re: Port Bank Cylinders Low Compression

Some sort of problem with the head, or gasket makes perfect sense to me too. I would just like to somehow confirm it. Is the Engine Tuner basically the same as Seafoam, because I have run a bunch of that through in several different ways? Or is the Engine Tuner significantly different?

I may have this part confused, but I think I have read that shaving the heads increases compression. I have also heard the heads are adjusted somehow at the time of assembly by having shims, or different thickness head gaskets by the factory. Can anyone comment on that?

 

[racerone]

Re: Port Bank Cylinders Low Compression

Simple----Take the 3 bypass covers off the port side and take a look at the pistons and rings.----Cost you 3 gaskets at say 99 cents each.-------Picked up a GT 175 some years ago along with a box of spare electronic parts like a stator and powerpacks.----Pistons were scored on it , but many folks like to throw parts at motors instead of applying trouble shooting.

 

[Faztbullet]

Re: Port Bank Cylinders Low Compression

It not unusual for this problem on the 3.625 bore motors as the cylinder sleeves are so thin. A defective t-stat will cause the bank to run hotter and piston will scuff the skirts and ring lands.

 

[crashkaloop]

Re: Port Bank Cylinders Low Compression

Interesting Faztbullet, because when I first bought the boat a year ago, we noticed the port side running hot. I used a thermal gun on it, and it was about 190, while the starboard was at 170. So this could be the problem. I replace both thermostats because they were both in bad shape.

Removing the bypass covers is a good idea, and certainly cheap enough. I will order the gaskets and take a look. Thanks racerone!

 

[emdsapmgr]

Re: Port Bank Cylinders Low Compression

The heads are just bolted in place. No shims. These heads on the crossflows are sensitive to being cut, due to the piston clearances in a crossfflow. (They are domed.) Running hot could contribute to low compression problems-long term. That engine should idle between 143 and 155. The overheat horn comes on at 212, and shuts off at 175, which is still way too hot. When planing, the pressure relief valves open, flooding the engine with extra cooling water. Temps could drop 20 degrees (or more) when planing. So, it's good that you've changed the stats. Recheck the temps again-esp when on plane.

 

[crashkaloop]

Re: Port Bank Cylinders Low Compression

OK emdsapmgr , the temps I mentioned earlier were at about 3500 RPM on plane. My son-in-law measured on the heads while I drove the boat. Kinda awkward! I just re-measured on the muffs with the boat in my driveway. My engine idles pretty rough in the 900 RPM range, so I measured the temps at 1400 RPM. The port side was at about 125, while the starboard was at 115, so about 10 degrees hotter on the port side.

After replacing both thermostats last year, we have not re-checked the temps on plane, but I do believe the engine is operating within the proper range. The high temp alarms are working on both port and starboard banks, and I have never had the alarm come on while operating the boat.

Bosunsmate, I did the drop test as you suggested. Again, because my engine is not idling very smoothly at 900, I did the test at 1400 RPM. All 6 cylinders had a similar drop in RPM.

I think the next thing I will do is to take racerone’s suggestion, and remove the bypass covers. I already ordered the gaskets.

 

[Bosunsmate]

Re: Port Bank Cylinders Low Compression

drop test is encouraging, bad idlings a concern
to test the rings just press on them with a screwdriver, careful not to mark them, they should be springy.

 

[Faztbullet]

Re: Port Bank Cylinders Low Compression

Its not going to idle good with 2 cylinders low on compression.....

 

[emdsapmgr]

Re: Port Bank Cylinders Low Compression

The cyl head temps should noticeably decrease when planning. You said you had not retested the cyl head temps on plane since you installed the new thermostats. Sounds like it is a good time for a planning re-test. Hopefully, you will find that the stats were the answer and that the normal running (on-plane) temps are back to normal. (Significantly under what the idle temps are....)