No Compression

[Jerry San Diego]

I have a 21 foot 1989 Bayliner Trophy. Powered by a 3.0 L GM engine and a OMC Corba outdrive. There is no compression in the number 4 cylinder. The engine runs but a little rough. When I bought the boat the previous owner had just completed a valve job. It ran good. The plugs all four of them have a black soot on them but they appear to be in good condition I replaced them about 30 hours ago alongs with the points and condenser. I pulled the rocker arm cover and all the rocker arms are going up and down there is no unusual noise coming from the engine. The compression on the cylinders are #1- 120--#2 125-- #3-125- #4-0-<br />It looks like I will probably will have to putt the head to see whats going on. I squirted oil in the Number 4 cyl and then checked compression again with no improvement. Does anyone have any suggestions for me that will give me some idea what may be going on I don't want to have to pull the head if there is some other way to tell what going on. and If I pull the head and all looks Ok then what do I do? Thanks for your help<br />Jerry

 

[djvan]

Re: No Compression

Try to look down the spark plug hole and see if the piston is going up and down. You probably shouldn't use the starter to crank it at this point (in case the rod is broken). There is a chance of causing more damage by using the starter.<br /><br />DougV>

 

[dick]

Re: No Compression

Well you said the last owner did a valve job and now you got 0 on 4. You have done all the right checks now you have 2 options. <br />1-if you have a air compresser bring #4 to top dead center(valves closed piston all the way up apply 15-20 psi through the plug hole and listen where the air is coming from,from carb,bad intake valve, from exhaust ,bad exhaust valve ,from crank case ,bad piston rings or worse.<br />2-ignore 1 option and pull the head ,I would do this simply because 0 compresion on a hole is very bad and to fix any problem that would give you 0 on a hole will require you to atleast remove the head to inspect the valves and/or cylinders.<br />good luck.

 

[Ryoken]

Re: No Compression

sounds like you maybe sucked a valve in.

 

[Lou C]

Re: No Compression

If you have one of those Sears compression testers with the flex hose on it, make sure it still works, I had the check valve go bad on mine and it had me thinking I had 0 on 2 cyls of my 4.3, fixed it and got 180 on all 6. If you had 0 on 1 cyl on a 4 cyl it would run rough as all get out and have no power.

 

[Walt T]

Re: No Compression

I would think a bad tester would read bad on the other ones too. No harm in verifying though. Try taking a reading while it is running. If it comes up then thats why it runs okay. I'd do a leak down test first of all. It could be a misadjusted valve, but you would be able to hear where the air is getting pushed out while it is running.<br />Otherwise pull the head and have a lok see. If you cant see anything then have the head tested by someone other than the rebuilder. If it is okay, then it's time to pull the engine.

 

[Bhamil]

Re: No Compression

I have the same engine and experienced the same problem in the spring. If the wrong grade gas was used (anything less than 93 octane) you will burn a piston in minutes, not hours. You cannot use unleaded 87 octane in this engine, not ever! I can assure you that #4 will look like a cinder cone on the side away from the spark plug, or the port side of the engine. If #4 is bad, the rest aren't far from burning up, and they will. Pull that head and check to see if that isn't the problem. I hope it isn't but, if your luck is like mine, well, enough said. It's easy to do, though. If you want some help, you can e-mail me. Been there, done that. Oh, and I might add that being in California, you really have to check the octane of the gas. It has to be a minimum of 93.

 

[Walt T]

Re: No Compression

Gee Ben, then what about the several million owners using the recommended 87 octane?

 

[Northern Eclipse]

Re: No Compression

I would suggest that you pull valve cover, back off rocker nuts for #4 and re-adjust the valves, If you cannot get a compression reading after, then you'll need a leakdown test, I always go back to the last repair when scoping a problem first before looking for a new one, you'll be surprized how often 'the last repair' is the problem, Good luck and Happy holiday..

 

[jeremyp111]

Re: No Compression

Pull the head and take a look. As stated above, anytime you get a 0 reading on the compression test you know it's very bad. Do you have a manual for your engine? If not, then get one.

 

[Lou C]

Re: No Compression

But before you do all that, go and check the tester. I did a comp test on the 4.3, got 180 or so on the 1st 4, close to 0 on the last 2. Now this is an engine that runs fine. First I had a fit, oh no rebuilt engine time, then I went in the garage and tried it on the lawnmower and got zero too. Went back the the boat and did the same cyls that has 180 before, got zero too. Replaced check valve in tester got 180 in all 6. The valve must have went bad when I got to the 5th cyl.

 

[Bhamil]

Re: No Compression

Th dieselwalt; OMC doesn't recommend 87 octane for that year engine. Later models yes, but not those older ones. Timing is part of the problem. To those who believe you can burn less than 93 octane in a 3.0L GM (circa 1988 +/- a few years) engine, I am telling you you'll burn up your pistons, and it will burn through to the compression ring. I have seen pistons burned so bad you would think you were looking at a clinker from an old coal burning furnace. If you burn less tha 93 octane you have to do some serious timing adjustments and even then I doubt you'll save them. Read any of the manuals on OMC engines and their recommended fuels and you'll see I'm right. More than one older engine has required overhaul because of low octane gas. But, don't take my word for it; pull the head and see for yourself. I saw a newly rebuilt 350 (5.7L)engine burned up in less than 2 hours while the owner tuned the engine because he was using the fuel left in his tank, which was 87 octane. He burned it up once, had it rebuilt and didn't know any different than to at least add an octane booster to the fuel. I have done it myself.

 

[Boatin Bob]

Re: No Compression

Well Ben...I'm looking in my 1988 OMC factory manual and it says under the tuneup spec section for the 3.0 ltr that for 89 octane set the timing to 4 BTDC and even gives it for 86 octane to be 1 ATDC. I've been running a pair of OMC 460's on 87 for the last 10 years, still have tons of compression and power. As DieselWalt says "what about the millions of others that run 87 octane'?

 

[Bhamil]

Re: No Compression

Well, I'm not trying to start a squabble, but, OMC does not recommend 89 octane or lower for a 3.0L engine circa 1988/89, 93 being the minimum. But, as I said before, do not take my word for it. Pull the head. The reason is the time of burn; higher octane will burn more slowly and evenly. Lower octane burne close to the intake valve, and that's where the piston gets burned. However, the manufacturer specifies fuel for each engine and we're talking about a 1989 3.0L, not 460.

 

[Bhamil]

Re: No Compression

One final thought for the non-believer, those who don't think low octane is the bane to old engines, I have an article I am pasting to this post.<br /><br />From an article on octane and internal combustion engines<br /><br />WHAT DOES A LOW OCTANE VALUE MEAN, TO ME?<br /><br />In the absolute worst case, if the fuel is too low octane, it may spontaneously ignite before the spark plug fires <br />due to thermal rises from the heat of compression or from hot spots in the cylinder itself. This kind of ignition is <br />called pre-ignition (as opposed to knocking) and is a pathological case which will just turn an engine to scrap <br />(Pre-ignition can damage an engine before you finish reading this sentence).<br />Diesel fuel is low enough octane that mixing it with gasoline can cause pre-ignition! What usually happens, and what <br />we usually call knocking or pinging is that the fuel/air mixture does not ignite before the spark plug fires but does <br />ignite spontaneously after that. The sparkplug fires and this causes an immediate, rapid, rise in combustion chamber <br />pressure. This causes fuel on the other side of the flame-front to ignite before the flame-front reaches it. <br />In turn, this causes combustion chamber pressure to rise even more rapidly. The result is an explosion inside the <br />combustion chamber as opposed to the desired rapid burning. <br /><br />WHAT DOES A HIGH OCTANE VALUE MEAN, TO ME?<br /><br />A high octane rating ensures that it takes a REALLY hot ignition source to ignite the fuel<br />(such as a spark plug or the flame-front itself) and not just the rise in pressure & temperature that's a result of <br />normal combustion. Note that the thermal rises in the cylinder are in direct proportion to the compression ratio of the <br />engine (more below). The higher the compression ratio, the higher the octane of the fuel that's needed. <br /><br />Again, if the mixture in a gasoline engine ignites before the spark plug fires, we call that "pre-ignition."<br />Pre-ignition can damage an engine before you finish reading this sentence. To reiterate, what we're really concerned <br />with is called "knock" and that's the spontaneous ignition of the fuel-air mixure ahead of the flame-front as a result <br />of the rise in cylinder pressure caused by the onset of ignition (caused by the firing of the spark plug).

 

[Walt T]

Re: No Compression

What Octane method does OMC mention?

 

[Boatin Bob]

Re: No Compression

Ben...I'm not trying to start a squabble either and I know we are talking about a 3.0 ltr as I mentioned it in my post. I've read my 1988 and 1989 OMC factory manuals about the 3.0 ltr and there is nothing in there to suggest that OMC says not to use anything below 93 octane. Where do you get this from? A lot of the marinas in my area don't even carry 93 most only have 87 or 89.

 

[Walt T]

Re: No Compression

Around here they don't even carry anything higher than 91 at gas stations. 'Course we're way up high in Colorado breathin this rarified air which probably explains why there's so many weirdos here.

 

[Bhamil]

Re: No Compression

Hey Guy's. Well, actually everyone is right; In Canada I think the octane formula is one used in Europe which is not the same as what is used in the U.S. and is called RON (that may or may not be the case). OMC recommends 93 octane RON which equates to 89 octane by U.S. standards. That being said, there's a problem; the recommendations of engine mfg's comes from an ASTM chart that gives the compression ratios and suggested octane per ratio. The 3.0L has two ratios available, 8.2:1 and 9.25:1. Please note the chart below:<br /><br />Compression Octane Number <br /> Ratio Requirement <br /> 5:1 72 <br /> 6:1 81 <br /> 7:1 87 <br /> 8:1 92 <br /> 9:1 96 <br /> 10:1 100 <br /> 11:1 104 <br /> 12:1 108 <br /><br />So, the engine in question clearly requires an octane of at least 92. Newer engines don't require as high an octane because, among other things, they have monitoring systems that adjust timing, etc. The older carbeuretor and point systems were much more sensitive and unforgiving to timing issues. And in the final analysis, that's where the problem occurs. So, if a guy keeps his engine timed properly, he might get away with the lower octane. You guy's sound like your pretty savvy with engines so I'll bet you keep them running top notch. Others aren't as mechanically inclined as you and maybe don't understand the importance of tune-ups. But, from the bottom of my heart, I promise you that the engine that started this discussion has either a burned piston or, as someone else pointed out, he's dropped a valve. Oh, and we're not squabbling, just having a good, old fashioned arguement, the kind we old-timer's have that keeps us going when it's too cold to fish.

 

[DJ]

Re: No Compression

All this talk of octane and rating differences is a little bit academic. We still do not know why # 4 is dead.<br /><br />Other things can cause lack of compression. In this case, my money is on a holed piston. <br /><br />One should also look at: plug wires, and dist. cap. A cross fire condition will cause a holed piston, in a heartbeat.