Is 60 PSI to low?

Finally got to borrow compression tester today, rushed home let the dogs out and started pulling plugs. Here is what i found, all 4 cylinders had 60 psi +/- 1 or 2 psi. So can this be or do i need to retest with a diffrent tester, this one belongs to a bus mechanic where i work and looks ok! Motor is 1983 j90tlcte. Last time out it ran 44.1 mph at 5600 rpm with 13.25 x 17p alum prop, i have since raised motor one hole but have not ran it. Thanks 4 all comments!

Re: Is 60 PSI to low?

how many time did you turn the motor over in each hole. turn it over till the guage quits going up. 60 psi is horrible.

Re: Is 60 PSI to low?

What made you want to test compression in the first place?

Re: Is 60 PSI to low?

I don't know much about those big motors, but it sounds odd that both banks would have 60 psi in each jug. I suspect you need to revist either your testing protocol or your compression tester.

Also, if it ran good prior to testing I think that in its self says the results are wrong. I doubt 60 psi is enough when most would recommend a rebuild with anything less than 100 psi.

Re: Is 60 PSI to low?

Folks get paranoid about psi in a compression test when the REAL objective is to detect a severe difference in compression among the cylinders. It must be a form of pre-season hypochondria.

I think the chances of all four cylinders going bad by equal amounts at once are just about zilch, and on an engine that obviously performs well less than zilch.

60psi sounds horrible, but 60-60-60-60 sounds like a healthy engine to me.

Re: Is 60 PSI to low?

For a motor that is showing only 60 psi in all cylinders to run under any sort of a load at 5600 rpm is doubtful. To me, the speed is almost unimportant because with that little compression, it would be hard to get a motor to turn up to max rpm unless the boat was unusually small for the motor size.

Compression testers are inexpensive - go buy one and make sure that the thread length is not too long for your engine. On some OMC motors, if the thread length on the tester is too long, it will bottom out againt the piston crown at TDC.

When testing, make sure the motor turns over long enough to get a decent reading. I usually do at least two tests per cylinder to insure an accurate reading. My guess is that you will get values at or over 100 psi.

Re: Is 60 PSI to low?

I'm with JB.

Testing for compression and then panicking when some mythical number isn't met is almost epidemic on the forum right now.

Tell me, when you punch the throttle, do you want your motor to propel the boat forward or post certain psi readings?


Baseline a healthy engine and write down the numbers - don't worry about the actual values. If it starts behaving badly take a reading and compare against YOUR previous numbers.

Re: Is 60 PSI to low?

While I agree that psi differential is a critical issue, having even pressure at a very low value isn't good either. If you have a motor that should be showing numbers over 100 to 110 psi, having compression at nearly half those values is not a good thing and definately means that something is wrong. That said, and as I just wrote above, I doubt his numbers really are that low.

Re: Is 60 PSI to low?

Used the shortest end that fit the threaded holes, reason for checking just to make sure things are at least close to what is normal,starts and runs pretty well for 25yr old motor. Turned engine over for 5 to 6 sec on each test, gauge had stopped moving. So i take it that testing with new gauge should be next step, just wasnt sure if it would run at 60psi GUESS NOT!

Re: Is 60 PSI to low?

JB said:

Folks get paranoid about psi in a compression test when the REAL objective is to detect a severe difference in compression among the cylinders. It must be a form of pre-season hypochondria.

I think the chances of all four cylinders going bad by equal amounts at once are just about zilch, and on an engine that obviously performs well less than zilch.

60psi sounds horrible, but 60-60-60-60 sounds like a healthy engine to me.

Click to expand...

What JB said.

Re: Is 60 PSI to low?

A compression tester has a valve stem, or a shrader valve same as a tire, or a inner tube has. This valve stem can be removed by the same tool as used on a tire, or inner tube valve. Change this stem, and you will get a different reading, and if you had a handful of stem's, try to get two, to agree. When you buy a brand name tester, such as Mac, or Snap-ON, replacement stems' are included. Change the stem, change the reading. Throw the sensitivity of the quage in there to while your at it. If you look at a catalog from a machine tool supplier, you will see pressure guages can run from less than $10.00, to well over a $100. Which one do they put on your Snap-On, Mac, Sears, Wal-Mart compression tester?

Re: Is 60 PSI to low?

I guess the question that I would ask about this is "too low for what?" For the engine to run at all ... to run smoothly ... to develop its rated horsepower ... to pull water skiers ... to push a good sized boat?

If you are getting even values, no matter what they are, and the motor seems to run ok, and the boat is small enough to accept a motor that is running way under its design compression values, maybe all is fine. That doesn't mean the motor is "ok" in general.

Simply put, an outboard that would be expected to have readings in the neighborhood of 120 psi if it were in better than average condition, isn't operating efficiently at 60 psi. That is true simply because it can't develop the power that it was designed to put out. It can't compress the fuel/air mixture well and it can't contain the expansion of gases on the power stroke very well. To me, what you have is a 4 cylinder engine of 65 to 70 hp.

Your motor is also telling you something if it has such low compression values. It may need a decarb or it may just be worn. The most obvious possibility when the values are uniformly low is worn rings or cylinder walls. You may want to do a "wet compression test" by putting a little oil in the cylinders before doing another test. If the numbers increase, its a pretty good bet that one or both of those things is/are an issue. There could be other issues as well, such as cylinder head gaskets that need to be replaced or heads that need to be re-torqued.

What you do about all of this really depends on your tolerence for both the motor's present performance on the boat it is powering, and on your tolerence for getting into engine projects. Some people almost look for an excuse to tear a motor down to get better performance and some would rather just keep using it, as long as it is doing what they want it to. I don't think there is a "right" or a "wrong" in this regard but I would take the information for what it is - a sign that the engine has anything from a minor problem to the fact that it is approaching the end of its useful life.

For now, you might be just as well off to check out the simple possibilities or just use the motor as it is, until it becomes time to replace it or rebuild it. Whether that time is a few seasons or many seasons is hard to tell. As the saying goes, "it depends."

Re: Is 60 PSI to low?

I have found that various different compression testers will give different readings. Cranking RPM will also affect the readings as will a cold or hot motor.
A compression check should be done with the throttle held open, the engine cranking at the recommended cranking speed, and with the engine at operating temperature. A motor that has only 60psi in all 4 cylinders would exhibit other concerns and wouldn't likely ever reach maximum RPM due to a lack of engine vacuum and inability to draw in fuel properly.

That said, most V4 motors run in the 110 to 125 psi range with a proper compression test, with Merc and Mariner being slightly higher than others.

My question would be what type of gauge are you using and has it been tested against a known good gauge?
I have 4 Snap On gauges here, as well as two other major brand gauges and a few OEM compression gauges, all read different. My most accurate is my Snap On small engine gauge and next is one from an outboard tool supplier.

Why did you feel that you had a compression problem or needed to check?

Are there other signs of engine wear? (Knocking, hard to start, excessive smoke or fuel usage?)

Step one is to verify your gauge, second is to know what type of problem your chasing.

If you find that compression is really low or suspect the next step would be to do a leak down test and see for certain that you have ring leakage or a head gasket problem.

The fact that you mentioned that you raised the motor one hole makes me wonder what you are really chasing?

I would expect a motor with only 60psi to be quite hard to start if it ran at all and it wouldn't likely achieve maximum rpm.

Re: Is 60 PSI to low?

JB hit he nail right on the head(s).

Grits

Re: Is 60 PSI to low?

As has been well stated in previous posts, compression gauges are different.

I have been using a new compression gauge for my readings. MY 85hp read 125 on all 4 plus or minus 2 when I purchased it.


I got it running well and took it our a couple times, de carbed and then did another compression check, using the 125lbs as baseline.

All 4 cyl read 90, even on the board. Same gauge. Engine runs great.
I would not worry abut the readings until you get a chance to check it out under load on te water. You will probaly get satisfactory performance.

Re: Is 60 PSI to low?

Yes, compression gauges vary somewhat. If the gauge is off by a wide margin, however, it is either cheap junk or is not being used properly.

I have to say that I find the attempts to explain away the role of compression values in overall engine health to be curious. This is pretty straight forward - if an engine is designed to produce compression in the 120 psi range, and it is only doing so to the tune of 60 psi, there is a problem. The only question from that point is how big a problem it is to the owner. If the motor were intended to be used on a race boat, you wouldn't hear a bunch of mechanics saying "well, its ok as long as the compression differential isn't wide." On the other hand, if the motor isn't going to be worked very hard, there is a great deal more ability to just "roll with it" and keep using the motor until it just stops running at all.

Re: Is 60 PSI to low?

I don't really disagree with what you say, Jay, but the 4 cylinders reading almost the same psi, even a low psi, is a near impossibility if there is a problem. Four cylinders just don't experience the same failures simultaneously.

This is a cold engine that has been sitting out the winter. If it was a warm test with a guage proven on other engines, the battery was confirmed at full charge and the technique was proven on other engines I might (might) be a bit concerned and suggest a decarb and retest. Because the engine had no history of performance loss before being laid up for the winter I would give the compression a pass.

Re: Is 60 PSI to low?

Let's just try a different guage, or better still, a leak-down test.As for a guage being cheap junk, it all depend's on the quality of the guage you want to screw into the assembly.

Re: Is 60 PSI to low?

I think you're almost better off lifting the glass on the gauge and scratching off the numbers. Just pay attention to the tick-marks and make sure all the cylinders under one head gasket hit roughly the same tick-mark.