When to stop

[Norman Buus]

This is going to be a long one. I took my 1986 4.3 OMC to the repair shop because it is cutting out at about 3/4 throttle. The first thing they did was to run a compression check and found #1 and 3 had low compression (100 psi for 1 and 3 vs. 140 for the rest of them). Numbers 1 and 3 are next to each other in a 4.3 OMC. They didn't see any improvement by squirting oil into the cylinder. They called me and I decided to pull the heads myself.<br /><br />I have now pulled the heads and pulled the valves. The valves and seats look just shy of great. There is a small amount of pitting, but I find it hard to believe I am losing much air through this. The head gasket is in great condition. There are no visible cracks or holes in the heads.<br /><br />When I ran a compression test a year ago, I had slightly low compression on the same two cylinders. When I squirted oil into the cylinders, the compression came up to near normal. So I am guessing the shop didn't get enough oil onto the right part of the piston and I have some frozen rings.<br /><br />I can feel a small amount of taper in the cylinder walls on one side of #1 cylinder, but there are no visible vertical gouges in it. There is not much carbon deposited on top of the piston or on the heads. I do have corrosion problems on the outside of the engine. I am guessing part of at least one ring is corroded to the piston.<br /><br />Question 1: is it often possible to repair a piston that has had a ring frozen by corrosion?<br />Question 2: if not, is it possible to buy a piston or two for my engine without paying a machine shop to balance all the pistons?<br /><br />I hate to buy a new set of pistons without having the cylinders bored. Now I am into rebuilding the engine. But the water passageways are corroded and probably have gunk in them (open cooling system) so it would be better to replace the engine. But now I am left with the old corroded headers etc. (the old carburetor is probably why I took the boat to the repair shop in the first place).<br /><br />At this point, unless someone has much better ideas, I am thinking I am going to find a way to get some new rings in the engine, settle for less than perfect balance, run the boat for a year or two and get rid of it. Any input?

 

[Bondo]

Re: When to stop

Well,........ It Sounds like you have Very Little to Work with........<br /><br />If you Cob it up,..... I wouldn't Wait a year or 2 to Sell it.......<br />I'd Sell it as Soon as it's Running.........<br /><br />Either That,..... Or bite the bullet,+ Do it Up Right............

 

[rodbolt]

Re: When to stop

wow<br /> a leakdown test would have told you quickly if you had a sealing issue and EXACTLY where the issue was.<br /> I guesss its easier to poke and hope than test and go.<br />cheaper too.

 

[Norman Buus]

Re: When to stop

The shade-tree garage I learned in 40 years ago did not do leak down tests and I don't see anything about them in my manual. Pardon my ignorance, but what is a leak down test?

 

[rodbolt]

Re: When to stop

a leakdown test inserts a known pressure of air into a cyl that is monitored on a PSI gauge, the second gauge on the manifold measures the amount of air (make up air) required to maintain the pressure in a percentage reading, 0% equals a perfect seal and is rarely obtained, 5% means its real good, no valve,head gasket or ring issues. a reading of 10% is acceptable but myself I would hunt for the sealing failure. 15% and above is not acceptable and the cause must be found and corrected. however as your injecting this 100psi plus air it has the added bennie of it has to go someplace after escaping the cylinder. past the rings and it escapes from the crankcase, easy to find. past a head gasket and its either escaping from the cooling system or another cylinder. out the exhaust will indicate a bad ex valve,same with the intake.<br />so after a compression test tells me I have a mechanical issue the next step is a leakdown to further isolate it.<br /> I have seen low compression due to valve train issues that had nothing to do with cyl head and piston issues.<br /> just saves a bit of time and allows me to isolate problems.<br /> I have nothing agin shade trees but thats why they tend to tear up more than they fix, the days of the terbacky chawin redneck grease monkyboy are long long gone.<br /> todays technician has to have training and proper tooling for diagnostics. with ECU costing more than 1K and sensors in the 300 range you must be correct on the diagnoses or you can spend a lot of beer money quickly and still be walking.<br /> however nice thing about it its all printed in a book all you have to do is read it.<br /> most wont but that does not negate the fact its is written on paper.<br />just dont forget to remove the heatexchanger cap when doing a leakdown test

 

[Norman Buus]

Re: When to stop

That's a lot of good information. Thanks, rodbolt.<br /><br />Just for the record, this engine has a lot more in common with a 55 Chevy than a modern automobile. It has a carburetor, and a distributor with points. It doesn't even have a ballast resistor or vacuum advance.

 

[rodbolt]

Re: When to stop

however it does have a resistor wire in the harness and the reason for no vacum advance is because its designed to run at or near WOT and full load, auto shop 101, even in 55, says at or near WOT with a full load intake pressure is about 14.7 psi or 0"Hg. means there aint no vacum to operate the advance mechanism so why bother.<br /> so while your engine may look similar to that 55 chevy just think how much info you could have gained with a 10 min test.<br /> the last referenced issue was a bad intake cam lobe causing about a 25 PSI drop, had the tech done the correct tests the camshaft would have been diagnosed with a tiny bit of thought. in the above referenced cam issue the customer got charged for about 14 hours of labor and about 500 in parts for work that was not needed. could have been avoided with a bit of thought about how each engine subsystem affects the rest.<br />a leakdown test that is in specification coupled with low compression eliminates,valves,headgaskets and rings. what does that leave?<br />if i cant put air in I cant compress it.<br /> my cousins 56 chevy we could change the heads and cam in about 6 hours, did it a few times. on some boats it takes 6 hours just to get the manifolds off and at 75-100 an hour its better if you have a plan rather than shotgun the procedure. most boats will require engine removal to replace a cam so if I suspect a cam I really need to be correct.<br /> and while it looks simple, actually its simpler than it looks, its best if you read some books on basic engine desighn and how each subsystem ties into the reciprocating assy. the major subsystems are then broken down to subsystem sub assemblies. makes it very very simple then.<br /> by the way, how long would that 265 in that 55 chebby last with a 5000 pound trailer at 140 MPH for say 1 hour each weekend ?<br /> full throttle,full load for 1 hour every weekend and spraying water at it each run.

 

[Norman Buus]

Re: When to stop

Rodbolt, your points from both posts are well taken. It is certainly worth time and effort to develop a good plan in any undertaking.<br /><br />Next question. Do you ever recommend rebuilding a 4.3 OMC Cobra engine? I can’t see mud and sand in the coolant channels, but I would guess there is some. There is some corrosion in the coolant channels. Other than this, my engine seems a good candidate for rebuilding.<br /><br />If I purchase a new marine long block, I presume some are better than others. Can you recommend manufacturers names or give guidelines?

 

[rodbolt]

Re: When to stop

dude<br /> the blocks were all the same for the same engine family, as were the crank and the rods.<br /> some had minor piston differences for the expected duty cycle as the valvetrain differed for a different duty cycle.<br />that being said the truck block assy's were almost identical and in some cases identical to the marine. the big difference starts with the bolt ons.<br /> when you buy a bilge pump or a solinoid and the specs refer to "ignition protected" its not refering to the ignition system its refering to the fact its not supposed to be a source of ignition ie FLAMES.<br /> as far as rebuilding what you have I cant see it.<br /> minor corrosion is easily delt with. a saltwater motor of that age is normally junk.<br /> so lets start with a block assy.<br /> first we want to put it back together with the same marine/truck/industrial reciprocating assy and heads and valvetrain.<br />forget the article in popular hotrod.<br /> now we want to insure the stock ign system is working correctly. a well maintained point system is more than capeable of extended runs above 4500.<br />now we want to move to the cooling/exhuast syatem.<br /> my reccomendation, if you plan to keep and run it in the salt pond, is freshwater cooling to include the manifolds. then constant maint on both the fuel and cooling systems to include any saltwater cooled exhaust parts.<br />I have a few of the 4.3 cobra packages that have been crabbing since 95 and are still running.<br />its very rare to see an engine wear out.<br /> most die from lack of preventative maint, as in they waited till it broke to fix it.<br />works kinda with cars but not at all with boats.<br />myself if I have a good set of castings I prefer to build my own, then I know what I have. and I know who will warrenty what.<br />the biggest key is a good machine shop.<br /> they are getting harder and harder to find.<br />trying to find a shop that can rebore and hone and place it back to new specs is tough.<br /> but Ill assume you have the long block assy redone to the OMC specs, then bring the fuel and ign and cooling system to spec, freshwater cool the block and manifolds and if avalible change the risers to cast stainless and your done, from there it can you do the maint on the seawater pump,heat exchanger,fuel and ign system?<br />the biggest weak point is the drive, some I just cant get past about 5000 hours no matter what I try.<br />but we found if you change the drive oil once a month or every 100 hours regardless of which comes first or how inconvient, the dang thangs live.<br /> if you get that far Ill tell you a trick that takes 30 min but you will never have a stuck exhaust valve issue.

 

[Norman Buus]

Re: When to stop

rodbolt - if you are going to be at work at 8:00 Monday, you are putting in some long hours. I'll have a further reply Tuesday - I'm going fishing tomorrow.

 

[Norman Buus]

Re: When to stop

I think I understand most of what you said in your last reply. Let me say it back to you and see if I am getting it right.<br /><br />“…some had minor piston differences for the expected duty cycle as the valvetrain differed for a different duty cycle” a truck or marine engine usually has a lower compression ratio which slows the generation of heat. Since a car typically only operates at full throttle for short periods of time, it can get by with the higher compression ratio. <br /><br />"when you buy a bilge pump or a solinoid and the specs refer to "ignition protected" its not refering to the ignition system its refering to the fact its not supposed to be a source of ignition ie FLAMES." The shell of the unit is supposed to contain any ignition sources and not allow anything that happens inside ignite any vapors that could be on the outside.<br /><br />"minor corrosion is easily delt with. a saltwater motor of that age is normally junk." My engine has been in salt water at least a time or two. Is this a deal breaker? It has been 20 years since I last rebuilt an engine. Do the machine shops still hot tank the engine and does this remove the rust? I’ve seen automotive engines with more apparent corrosion damage than this engine has. I am thinking I will take the engine to the machine shop, let them hot tank it, and then get their advice as to whether to abort the project because of corrosion damage.<br /><br />"forget the article in popular hotrod." Racing pistons, cams, or other tweaks will increase heat production and be very destructive to the engine if I try to run it wide open throttle for more than a moment at a time. The thing is also supposed to idle at 500 rpm if it is to shift properly. I also want to be able to troll with it – it works well now using a trolling plate. I might be tempted to tweak the engine, knowing I would not want to run it wide open throttle more than momentarily, but I have not seen a high performance engine that would idle that slow.<br /><br />One other thing in your reply I am probably short on – maintenance. I’ll have to ask you more about that later. One thing at a time. As I write this reply, I am getting more enthused about rebuilding the engine. One thing a mechanic I talked to makes some sense – get the engine core into good condition now, then deal with the bolt-ons as repair issues later as necessary.

 

[rodbolt]

Re: When to stop

most the readbacks are about right.<br /> as far as the internal cooling passage corrosion its hard to say, tank it then discuss it with your machinist.<br />nice thing is blocks are easily obtaiable.<br /> the major failure cause on sterndrive engines is cooling system failure, most common is overheat due to seawater pumps, second is manifold/riser failures that leak water into the cyl and/or stick an exhaust valve.<br /> its a straight fwd rebuild you just have to keep in mind the duty cycle its going to be expected to perform.

 

[Norman Buus]

Re: When to stop

rodbolt - thank you again for all the info. I was hoping to postpone the purchase of exhaust manifolds, but after your last post, that does not sound wise. By the way, is that trick to prevent a stuck exhaust valve something to be done while the engine is out?