OMC 3.0 HO Engine Knock

[Rookie 132]

I have a 93 OMC that sounds like the rod is knocking. This boat has been out of the water for two seasons covered and did not have this issue when parked. It is also slowly overheating. Gets up to about 210 - 220 after running at 1200 - 1500 RPM's after 10 minutes or so, whether it is idling or in gear. Any ideas on this? I have heard everything from spark plugs to distributer to cables sticking not hitting micro sensors and of course the possible internal engine issues. Where is a good place to start on this? Spark plugs and compression first? I need some help guys, have never had to work on an inboard before. Any help would be appreciated.

 

[HT32BSX115]

Re: OMC 3.0 HO Engine Knock

Howdy,


Well, your over heat problem is very likely the raw water pump located in the drive. If it's in unknown condition and you have never replaced it it's probably the source of your overheating problem.....

I cannot speak to the knocking sound not being there. Loud engine knock is usually mechanical. If it's not all that loud or it's not actually in the engine, It could be in the drive....


Pull the drive and see if it still makes the noise (unless a stethoscope positively identifies it as in the engine) If it doesn't, it may be in the gimble bearing/yoke/U-joints etc......

If you have never pulled the drive then it needs to be pulled anyway to inspect the U-joints, bellows etc...



Cheers,


Rick

 

[Rookie 132]

Re: OMC 3.0 HO Engine Knock

Appreciate the start. I did manage to check the spark plugs a minute ago and noticed 3 of 4 had been a little burnt up. The weather turned south quick and I have not had a chance to pull the lower unit yet. I'll change the plugs and see if it quites down. I'm assuming I may have scored a piston. I'll check and let you know what I find tomorrow. Thanks!

 

[Gary H NC]

Re: OMC 3.0 HO Engine Knock

Spark plugs will not cause it to run at 210* or more.
Thats a major over heat issue!

Whats the oil look like? Was it changed after sitting over 2 years?
Are you running it on the hose or is the boat in the water?

 

[Rookie 132]

Re: OMC 3.0 HO Engine Knock

The oil looked pretty clean and I changed it after the first time I fired it back up. I stuck the boat in the lake when I cranked it up. It seemed to be pulling water up ok, so I was going to try the thermostadt and then the impeller. The knock seemed pretty bad at idle around 1000RPM's and not near as bad at 1500 RPM. I turned up the idle adjustment screw on the carb to 1200 and I could still hear it thumping. I am believing the knock is mechanical and was going to see if the spark plug change helped that situation any. I will pull the lower unit to change the impeller out regardless and will check it then also. Any ideas on the knock? Piston?

 

[Silvertip]

Re: OMC 3.0 HO Engine Knock

Does this boat have an oil pressure gauge? That might provide a clue. Does the knock go away at higher rpm or is it present only below 1500. If oil pressure is ok, you may have a piston skirt that has been worn or collapsed from the overheat and the knock is really piston slap.

 

[Bondo]

Re: OMC 3.0 HO Engine Knock

Ayuh,... Sounds like the long block otta be headin' to the Machine Shop...

 

[Rookie 132]

Re: OMC 3.0 HO Engine Knock

I have not run it again yet since I pulled out the spark plugs. The plugs in there were the wrong size. They were 15's instead of 12's. I've picked up some new ones and will put them in today after work I hope if it stops raining. The oil pressure gauge does read normal while it is knocking. If it is the piston sleeve, how would you go about fixing this? Can you replace it or does it need to go into the shop? Dollarwise, how nasty is that?

 

[Gary H NC]

Re: OMC 3.0 HO Engine Knock

If its a piston sleeve you would be better off buying a new long block..or spend a ton with a machine shop for a rebuild.

 

[HT32BSX115]

Re: OMC 3.0 HO Engine Knock

If it is the piston sleeve, how would you go about fixing this?

If it is in fact piston slap, from a broken skirt, severe scoring, etc.

The engine must be disassembled and rebuilt or replaced. This is not something that is fixed with a simple part replacement or dumping in a can of "engine fix"


Sorry about that,



Rick

 

[Rookie 132]

Re: OMC 3.0 HO Engine Knock

Alright, so I changed the plugs and fired it back up. I noticed the knock seemed to quite down a lot in mid range, but still noisy on low or high range. I will be putting in the new impeller housing tomorrow to see if I can stop the overheating issue to troubleshoot the knock better. Interestingly I ran into a boat mechanic whose brother supposedly was an OMC sales rep fro 20 years. I described the issue to him and he said that it sounded like piston slap to him. He stated the OMC 3.0 was notorious for developing piston slap and there were a couple ways to deal with it. He said that these engines can run for hundreds of hours with pistons slapping and that it is just irritating to most owners, which is what eventually brings them in to have it fixed. He also said that if you replaced all the pistons with new ones the slap would come back in a matter of weeks. He said there was one tried and true way to fix it and that was to replce the pistons with Chevy 5.7 pistons for their standard V8 engines. He said the difference between the two was .032 in the height of the piston and that they have been fixing it that way for years. He said it runs 700 - 900 to have this done. Has anyone else ever heard of this????

 

[Rookie 132]

Re: OMC 3.0 HO Engine Knock

OK, I replaced the impeller and housing yesterday evening and it was chewed up and the housing had shifted over, so I assumed this would fix the overheating issue for me to troubleshoot the engine knock. New issue..... now I can't get the boat to turn over when trying to start it. At this point, I have only changed the oil, spark plugs, impeller and housing... Any ideas on what would cause this issue???? I'm at wits end now and ready to take it in to the shop.